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Glossary

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Oct 22, 2023
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Example and Directions
Words (or words that have the same definition)	The definition is case sensitive	(Optional) Image to display with the definition [Not displayed in Glossary, only in pop-up on pages]	(Optional) Caption for Image	(Optional) External or Internal Link	(Optional) Source for Definition
(Eg. "Genetic, Hereditary, DNA ...")	(Eg. "Relating to genes or heredity")		The infamous double helix	https://bio.libretexts.org/	CC-BY-SA; Delmar Larsen

Glossary Entries
Word(s)	Definition	Image	Caption	Link	Source
FR	free radical
LP	lipid peroxide
NO	nitric oxide
O₂	super oxide free radical
OH	hydroxyl free radical
VO₂ max	the amount of oxygen used per kilogram of body weight per minute while engaging in exercise at the fastest rate attainable by the person performing the exercise; aerobic capacity
β-A, beta-amyloid	a protein of unknown function that is abundant in senile plaques
Absorption	the passage of materials contained within the GI tract into the circulatory system
Accommodation	the process of adjusting the curvature of the lens to focus light from – objects located at different distances from the eye
Acetylcholine	a neurotransmitter used in numerous activities (e.g., memory; controlling skeletal muscle, cardiac muscle, smooth muscle, and certain glands)
Acquired active immunity	the ability to produce a secondary immune response that is strong enough to prevent significant adverse effects from any subsequent encounter with the antigen
Action potential	the process by which an impulse travels along a neuron or a muscle cell
Acute gastritis	an abnormal condition consisting of short term inflammation of the stomach
AD	Alzheimer’s disease
Ad libitum, AL	with reference to diet, being able to eat as much as desired at any time
ADH	antidiuretic hormone
Advanced glycation end-product, AGE	a protein chain containing glucose cross-links
Afterimage	a lingering perception that an image is present after the actual image in the eye has changed or disappeared
AGE	advanced glycation end product
Age changes	developmental changes that occur in the later years of life
Age-related macular degeneration, AMD	a disease of the eye that reduces vision by causing degeneration of the cones in the macular region of the retina
AL (ad libitum)	with reference to diet, being able to eat as much as desired at any time
Allergic response	an excessive undesirable immune response against a foreign antigen
Alveolus (pl., alveoli)	a small cup-shaped outpocketing in the lung where gas exchange occurs
Alzheimer's disease , AD	a form of dementia characterized by memory loss and by the presence of excessive senile plaques and neurofibrillar tangles in the brain
Amino acid	a molecular unit that makes up protein molecules
Amyloid	a specific type of protein found within or between cells
Amyloid precursor protein, APP	a protein in cell membranes whose breakdown yields beta-amyloid and other products
Anemia	an abnormal condition consisting of a low concentration of red blood cells in the blood
Aneurysm	an abnormal outpocketing of either a blood vessel or the wall of the heart
Antagonistic pleiotropy	having several effects that oppose one another
Antibody	a protein molecule that is produced by immune system plasma cells and that adheres to antigens and assists in combating antigens
Antigen	a substance that causes an immune response
Antigen-specific receptor	a type of surface receptor on a T-cell that allows that T-cell to bind to one and only one substance capable of initiating an immune response
Antioxidant, AOX	a substance that converts free radicals into substances that are not free radicals by donating one or more electrons to other molecules
AOX (Antioxidant)	a substance that converts free radicals into substances that are not free radicals by donating one or more electrons to other molecules
APO	apolipoprotein
APOE	apolipoprotein E
Apolipoprotein E , APOE	a lipoprotein that helps move cholesterol and other lipoproteins from cell to cell and through cell membranes, and also seems to help in neuron development and repair
Apoptosis	programmed cell death
APP	amyloid precursor protein
Appendicitis	an abnormal condition involving inflammation of the appendix
Aqueous humor	the liquid that fills the region in the eye behind the cornea and in front of the lens
Arthritis	an abnormal condition that involves inflammation of joints between bones
Ascites	the presence of extra fluid in the abdominal cavity
Atherosclerosis	a disease involving the formation and enlargement of a weak scar-like material in the walls of arteries
ATP, adenosine triphosphate	a nucleotide used for transferring energy within a cell
Atrophic gastritis	an abnormal condition resulting in excessive thinning of the lining of the stomach
Auerbach's plexus	a network of nerve cells in the wall of the GI tract that helps to control functions of the GI tract
Auto-antibody	an antibody that can act against a native body material
Autoimmune reaction	an immune response against a normal body part
Autonomic motor neuron	a motor neuron involved in controlling either cardiac muscle, smooth muscle, or a gland
Axon	a neuron projection that conducts impulses away from the nerve cell body of that neuron
Basement membrane	a non-cellular layer between a layer of cells and the material under the layer of cells
Basilar membrane	a flexible membrane within the cochlea of the ear that supports the organ of corti
B-cell	a type of specialized lymphocyte that can produce antibody-secreting cells as part of an immune response; B-lymphocyte
Bed sore	a patch of skin that has died because it received insufficient blood flow; a decubitus ulcer
Benign neoplasm	a neoplasm that does not spread to other areas
Benign prostatic hypertrophy, BPH	an abnormal non-cancerous condition in which the prostate gland grows and becomes excessively enlarged
Bile	a liquid secreted by the liver and stored in the gall bladder and that contains bilirubin, cholesterol and other waste materials
Bilirubin	a substance consisting of remnants of hemoglobin molecules from red blood cells
Biological aging	aging in the physical structures and functioning of the body that affects either the body’s ability to survive or its appearance
Biorhythm	a cyclic fluctuation or oscillation of a biological process
BMR, basal metabolic rate	the rate of energy use needed to sustain body functions when a person is awake and in a state of complete rest
Bone marrow	the specialized tissue that is housed within bones and that produces blood platelets and some types of blood cells
Bone marrow	the specialized tissue that is housed within bones and that produces blood platelets and some types of blood cells
Bone matrix	the material between bone cells in bone tissue
BPH, benign prostatic hypertrophy	an abnormal non-cancerous condition in which the prostate gland grows and becomes excessively enlarged
Bruch's membrane	a non-cellular membrane that lies between the pigmented epithelium of the retina and the choroid layer
Caloric restriction, CR	receiving a diet that is limited in the number of calories it contains
Cancer	a disease consisting of the uncontrolled reproduction and spreading of cells; a neoplasm that spreads to other areas; a malignant neoplasm
Carbohydrate	a molecule consisting of one or more simple sugar molecules
Cardiac muscle	the type of muscle that is found only in the heart
Cardiac output, CO	the amount of blood pumped by each ventricle of the heart in one minute
Cataracts	the condition of having so many opacities close to the center of the lens of the eye that vision is greatly impaired
Cavity	a spot of decay on a tooth
CCK, cholecystokinin	a hormone from the small intestine that stimulates emptying of the gall bladder
Cells	the living structural and functional units of the body
Central nervous system , CNS	the combination of the brain and the spinal cord
Cerumen	a semi-solid waxy material that is secreted into the ear canal by ceruminous glands; ear wax
Cervix	the narrow lower portion of the uterus
Chain reaction	with free radicals, a sequence of chemical reactions where free radicals react with substances that are not free radicals and form new free radicals
Chronological aging	the passage of time since birth
Cilia	microscopic hair-like structures that project from cells and that swing back and forth
Circadian rhythm	a cyclic fluctuation or oscillation that repeats itself approximately every 24 hours
Cirrhosis	an abnormal condition characterized by the presence of much scar tissue within the liver because of chronic liver damage
Clonal selection	the process of destroying lymphocytes capable of binding to native body materials and initiating an immune response against native body materials
CNS	central nervous system
Collagen	a tough non-elastic fibrous structural protein that is found outside of cells
Colorectal cancer	cancer of the large intestine
Compact bone	the type of bone tissue that is dense and that forms the outer layer of bone; cortical bone
Compliance	the ability to increase or decrease in volume as a result of a change in pressure
Conduction	the process by which an impulse travels along a neuron
Cones	photoreceptors in the eye that have different responses to different colors of light
Coronary artery	a blood vessel that transports blood to capillaries within heart muscle
Cortical bone	the type of bone tissue that is dense and that forms the outer layer of bone; compact bone
Cosmetic aging	changes in outward appearance with advancing age
Cough reflex	a reflex that causes bursts of air to be expired rapidly to force materials out of airways located below the pharynx
CR	caloric restriction
Creatinine	a waste material from muscle cells
Cross-link	a chemical connection between molecules
Cross-sectional study	a study that places individuals with similar ages into groups and evaluates them one time
Cross-sequential study	a study method that combines the cross-sectional method and the longitudinal method. Cross-sectional studies on age categories are repeated on the same population as time passes.
Crush fracture	a type of fracture caused by compression forces acting on a bone that result in collapse of the bone
Crystallized intelligence	using cognitive skills with familiar learned activities
cT-cells	cytotoxic T-cell
Cytoskeleton	a network of microfilaments and microtubules that provides support within a cell
Dead space	the volume of air in the lower airways that does not reach the alveoli during ventilation but that remains in the lower airways during ventilation
Declarative memory	remembering specific facts that a person tried to learn so they could be
Defecation	the elimination of feces through the anus; a bowel movement
Dementia	an abnormal condition that involves a serious decline in memory functioning accompanied by a major decline in at least one other mental function
Dementia with Lewy bodies	a type of age-related dementia characterized by the presence of excess round masses of clumped microfilaments in neurons
Dendrite	a neuron projection that conducts impulses toward the nerve cell body of that neuron
Dentin	a firm layer located internal to the enamel of a tooth
Depth perception	the perception of the distance from the eyes to an object being viewed; binocular vision
Dermis	the layer of the skin underlying the epidermis
Development	developmental changes that occur before birth or during childhood
Developmental changes	irreversible normal changes in a living organism that occur as time passes
DHEA	dehydroepiandrosterone; a steroid hormone of unknown function
DHT	a principal sex hormone in men, much of which is produced by the testes and the prostate gland; 5-alpha-dihydrotestosterone
Diabetes mellitus	a disease that involves inadequate insulin production or inadequate sensitivity of cells to insulin and that results in inhibition of the regulation of blood glucose levels
Diabetic retinopathy (DR)	a disease of the eye that is associated with diabetes mellitus and that reduces vision because of sorbitol accumulation in the retina
Diarrhea	the condition of having more than three relatively liquid and voluminous bowel movements in one day
Diastolic pressure	the lowest blood pressure attained between contractions of the ventricles of the heart
Dietary Reference Intakes, DRIs	a comprehensive method for establishing and evaluating recommended dietary intake recommendation
Dietary restriction, DR	receiving a diet that is limited in the amount of food
Diffusion	the movement of materials from an area where they are in high concentration to an area where they are in lower concentration
Diglyceride	a lipid molecule composed of one glycerol and two fatty acid molecules
Disaccharide	a molecule consisting of two simple sugar molecules
Diurnal rhythm	a repetitive cycle of events that occurs once in each 2-hour period; circadian rhythm
Diverticula	deep outpocketings in the wall of the large intestine
Diverticulitis	the abnormal condition of having inflamed diverticula
Diverticulosis	a disease condition consisting of the presence of deep outpocketings in the wall of the large intestine
Dizziness	the sensation that the body is unstable
DM	diabetes mellitus
DNA, deoxyribonucleic acid	the nucleic acid that makes up genes
DR	dietary restriction
dT-cells	delayed hypersensitivity T-cell
Duodenal peptic ulcer	a peptic ulcer in the duodenum
Early onset AD	a form of Alzheimer's disease with onset before age 65, usually during the sixth decade of life
Economic aging	age changes in financial status
Edema	the presence of extra fluid between cells
Ejaculation	the expulsion of semen from the penis caused by reflexive rhythmic peristaltic contractions of the urethra, other male reproductive ducts, and muscles at the base of the penis
Elastase	an enzyme that breaks down elastic fibers into elastin peptides
Elastin	a resilient structural protein that is found outside of cells
Elastin peptides	short pieces of the protein chain that composed elastin
Electron	small negatively charged particle that moves about at a distance from the nucleus of an atom
Electron transport	a series of steps where electrons lose energy
Embolus, emboli	a mass (usually a blood clot) that is transported within a blood vessel
Enamel	a hard substance that covers the exposed surface a tooth
Endocrine gland	a structure that has hormone secretion as its primary function
Endolymph	the fluid that fills the membranous chambers of the inner ear
Endometrium	the innermost layer of the uterus
Endothelium	the innermost layer of a blood vessel
Endurance	the length of time that vigorous activities can be performed without stopping
Energy	the power that makes things happen; the ability to do work
Energy balance	the condition of having the amount of energy taken in over a period of time equal the amount of energy used during the same period
Enzyme	a protein substance that increases the rate of chemical reactions in a regulated manner
Epidermis	the outer layer of the skin
Episodic memory	remembering the times and places at which events happened
Erectile tissue	the spongy tissue in the penis and the clitoris that fills with blood and becomes firm during sexual arousal
Erection	the swelling, elongating, and stiffening of the penis due to engorgement of the erectile tissue with blood
ERT, estrogen replacement therapy	the administration of estrogen-like substances to bolster the effects of estrogen in the body when levels of endogenous estrogen become low
Estrogen	a principal sex hormone, most of which is produced by the ovaries
Evolutionary theory	a theory that attempts to explain how aging developed in concert with natural selection
Expiration	the movement of air out of the respiratory system; exhalation
Expiratory reserve volume, ERV	the amount of air that can be forcefully expired after expiring the tidal volume
Explicit memory	remembering specific facts that a person tried to learn so they could be remembered
External ear	the region of the ear external to the eardrum
Familial AD, FAD	a form of Alzheimer's disease with onset before age 65, usually during the sixth decade of life
Fat	lipid molecules composed of one glycerol and three fatty acid molecules; triglycerides
Fecal incontinence	the abnormal condition of eliminating feces at inappropriate times at least once per month
Feces	a mixture consisting largely of undigested materials, waste products, and bacteria that is located in the large intestine
Fiber	dietary polysaccharides that cannot be broken down by digestive enzymes
Fibroblast	a cell that can produce and secrete proteins that form collagen and elastin fibers
Fibrocartilage	the type of cartilage that consists of a smooth, slippery, and resilient substance that contains many thick collagen fibers and that has the consistency of hard rubber
Filtration	the separation of small substances from large ones when fluid pressure forces the small substances through the pores in a membrane
Fluid intelligence	using cognitive skills in new situations to learning novel problem-solving, motor activities, or reasoning
Foam cell	a fat-filled macrophage in a vessel wall
Follicle	in the ovary, a cluster of cells that surrounds a developing egg cell
Fovea centralis	the central region of the macula lutea that contains the highest concentration of cones
Free radical, FR	an atom or molecule with an unpaired electron (* = unpaired electron)
FSH	follicle stimulating hormone
Gag reflex	a reflex that causes closure of the openings into the larynx and the esophagus when irritating materials enter the pharynx
Gall stone	a solid mass formed from materials in bile
Gas exchange	the movement of gases between the atmosphere and the blood
Gastric peptic ulcer	a peptic ulcer in the stomach
Gastric refluxing	the movement of stomach contents upward and into the esophagus
Gene	a length of DNA used to direct the formation of an amino acid chain
Genitalia	external reproductive structures
Gerontology	the study of aging
GFR, glomerular filtration rate	the rate of filtration through the glomeruli
GH	growth hormone
GI tract, gastrointestinal tract	the portion of the digestive system that consists of a tube extending from the mouth to the anus
Glare	seeing bright areas in the wrong places in the field of view because of scattered light striking the retina in the wrong places and in a disorganized way
Glaucoma	a disease of the eye that reduces vision by causing an increase in pressure within the eye due to an accumulation of aqueous humor
Glomerulus	a tuft of capillaries in each nephron in the kidney
Glucagon	a hormone from the pancreas that tends to increase blood sugar levels
Glucose	a simple sugar that is found in abundance in the body and that provides major source of energy for cells
Glycation	using glucose to form cross-links between protein chains
Glycoprotein	a molecule consisting of a combination of carbohydrate and protein
Glycosylation	using glucose to form cross-links between protein chains
Glycoxydation	using glucose to form cross-links between protein chains
Gonads	the primary reproductive organs (i.e., testes in males, ovaries in females), which produce hormones and sperm cells or egg cells
H₂O₂	hydrogen peroxide
Hair cells	neurons in the inner ear that respond to bending caused by different stimuli (e.g., vibrations, gravity, rotation of the head)
Hayflick limit	the maximum number of divisions a cell can undergo
HCl	an acid secreted by the stomach; hydrochloric acid
HDL	high density lipoprotein
Hearing	the conscious perception of sound vibrations
Heart rate (HR)	the number of times the heart beats in one minute
Heat shock proteins	proteins produced by cells when they are stressed. Heat shock proteins have a protective effect.
Hemorrhoid	a varicose vein in the rectum or the anal canal
Heterochromatin	masses of tightly wound DNA and protein
Histamine	a substance from cells that initiates inflammation
HLA protein	a type of protein molecule on a cell surface that serves to identify that cell as belonging to a specific person
HLA receptor	a surface receptor molecule on a T-cell that is able to bind to specific identification molecules (i.e., HLA protein) found on other body cells
Homeostasis	the state of having proper and fairly steady conditions
Hormone	a substance that result from manufacturing processes in cells, that is secreted into the blood, and that alters the activities of cells in other parts of the body
hT-cell	helper T-cell
Hyaline cartilage	the type of cartilage that consists of a smooth, slippery, white substance having the consistency of hard rubber
Hydrogen peroxide (H₂O₂)	a reactive oxygen species containing two hydrogen atoms and two oxygen atoms
Hydroxyl radical, OH	a free radical containing hydrogen atom and one oxygen atom and having 17 electrons
IDDM, insulin-dependent diabetes mellitus	the type of diabetes mellitus wherein death ensues unless insulin therapy is provided
IL-2, interleukin-2	a substance secreted by hT-cells that enhances an immune response by stimulating macrophages to phagocytize more antigen, stimulating the production of more of the hT-cells and cT-cells, and stimulating the proliferation and activity of B cells
IL-1, interleukin-1	a secretion that stimulates T-cells to produce more identical T-cells during an immune response
IL-6, interleukin-6	an interleukin that stimulates inflammation, macrophages, osteoclasts, and other activities
Immune response	the activities of the immune system whereby the system combats an antigen
Implicit memory	remembering specific facts that a person did not try intentionally to learn so they could be remembered
Impotence	the inability of a man to engage in sexual intercourse because the penis is not sufficiently erect to be inserted into the vagina
Incidental memory	remembering information or skills that were self-taught
Inhibin	a sex hormone from gonads that helps regulate sex hormone production by the
Initiation	with free radicals, a chemical that converts a molecule to a free-radical
Inner ear	the region of the ear internal to the oval window
Inspiration	the movement of air into the respiratory system; inhalation
Inspiratory reserve volume, IRV	the amount of air that can be forcefully inspired after inspiring the tidal volume
Insulin	a hormone from the pancreas that tends to decrease blood sugar levels
Insulin-like growth factor, IGF	signaling substances from cells that stimulate growth and regulate other cell activities
Intercellular material	substances located between cells
Interleukin, IL	a signaling substance that helps regulate immune responses and other body functions
Interneuron	a neuron that carries impulses from one neuron to another neuron
Interstitial cells of Leydig	the cells among the seminiferous tubules in the testes that produce sex hormones
Intervertebral disc	a pad of fibrocartilage located between the vertebrae in the spinal column
Intrinsic factor	a substance secreted by the stomach that promotes the absorption of vitamin B12 by the small intestine
Ion	an atom that has lost or gained one or more electrons
Ischemia	the condition of having inadequate blood flow
Jaundice	the condition wherein parts of the body have a yellow or brown color because of the accumulation of bilirubin
Joint capsule	the thick layer of fibrous material that encases a synovial membrane and that helps to bind bones together
Kcal, kilocalorie	the unit of energy most often used in measuring the energy content in food
Keratin	a structural protein found in the outermost layer of the epidermis, in air, and in the nails of the fingers and toes
Keratinocyte	a cell that can make keratin
Ketoacid	a small molecule resulting from the breakdown of a fatty acid or an amino acid; ketone
Ketoacidosis	the abnormal condition of having excess ketoacids in the body
Ketone	a small molecule resulting from the breakdown of a fatty acid or an amino acid; ketoacid
Lacrimal fluid	a liquid secreted by the lacrimal glands that bathes the front surface of the eye
Lactase	an enzyme that assists in the breakdown of lactose
Lactic acid	a substance produced by the partial breakdown of glucose in cells that have a low amount of oxygen
Lactose	a type of sugar that consists of one glucose molecule joined to one galactose molecule; milk sugar
Lactose intolerance	the condition of producing an insufficient quantity of lactase to properly digest dietary lactose
Langerhans cell	a cell in the epidermis of the skin that carries out phagocytosis and assists in initiating an immune response
Late onset AD	a form of Alzheimer's disease with onset usually after age 60
LDL	low density lipoprotein
Lewy body	a round mass of clumped microfilaments in neurons found in excess numbers in types of dementia including Parkinson's disease
LH	luteinizing hormone
Life expectancy of a population	the average age at which death occurs for the members of a population
Ligament	a cable-like structure composed largely of collagen fibers that binds one bone to another bone
Lipid peroxide, LP	a lipid molecule that is a free radical
Lipofuscin	a mixture of certain chemical waste products from normal cell activities
Lipoprotein	a molecule consisting of a combination of lipid and protein
Localization of sound	the perception of the direction from which a sound originates
Longitudinal study	a study that evaluates individuals over a period of time
Lower airways	air passages within the lungs that conduct air to and from alveoli
Lymphocyte	a type of white blood cell that functions as part of the immune system
Lymphokines	a group of substances from lymphocytes that augments the defensive effects of an immune response in several ways including by activating macrophages and certain lymphocytes, by causing inflammation, and by protecting normal body cells from viruses
Macrophage	a cell outside of the blood that performs phagocytosis and that assists in immune responses
Macula lutea	the region of the retina that is in line with the center of the cornea and the lens and that has a high concentration of cones
Maillard reaction	a reaction that forms glucose cross-links between protein chains
Malignant neoplasm	a neoplasm that spreads to other areas, cancer
Malnutrition	the condition of lacking nutritional homeostasis
Maltose	a molecule that consists of two glucose molecules linked together
Maturation	developmental changes that result in the transformation of a child into an adult
Maximum longevity (XL)	the greatest age ever achieved by any member of a species
mB-cell	memory B-cell
Mean longevity (ML)	the average age at which death occurs for the members of a population
Melanin	a brown pigment found in the epidermis, in hair, and in other places
Melanocyte	a cell that can make melanin
Memory	with regard to the immune system, the ability of the system to respond to an antigen more quickly and more intensely when the antigen is encountered a second or subsequent time
Menopause	the time when age changes in the ovaries cause menstrual cycles to cease for at least one year
Menstrual cycle	a sequence of events in the uterus that repeats itself and that involves thickening, deterioration, and shedding of the endometrium; uterine cycle
Menstruation	the shedding and passage of the endometrium and blood through the vagina
Metastasis	the spreading of cancer from one location to another
Metastatic cancer	cancer that is transported to other parts of the body through blood, lymph, or other body fluids
Middle ear	the region of the ear between the eardrum and the oval window
Minute volume	the volume of air that is inspired or expired per minute
Mitochondria	organelles that release useful energy for cells
Mitochondrial DNA, mtDNA	DNA in mitochondria
ML	mean longevity
Molecule	a group of atoms or ions bound together in a specific ratio
Monoglyceride	a lipid molecule composed of one glycerol and one fatty acid molecule
Monosaccharide	a single simple sugar molecule
Motor end plate	the modified area of a muscle cell membrane that contains receptor molecules to receive and respond to neurotransmitter molecules from a motor neuron
Motor neuron	a nerve cell that conducts impulses that control the actions of a muscle or a gland
Motor unit	the combination of one motor neuron and all the muscle cell it controls
mRNA (messenger RNA)	a nucleic acid that carries information to determine the sequence of amino acids in a protein molecule
mT-cell	memory T-cell
mtDNA	mitochondrial DNA
Myelin	a substance that coats axons and dendrites and that allows impulses to travel faster along these structures
Myocardium	the layer of muscle in the heart, which is the middle layer of the heart
Myofilament	a protein filament in a muscle cell that is involved in producing contraction of the cell
Myoglobin	a type of protein in muscle cells that attracts oxygen from the blood into muscle cells and stores oxygen
Myometrium	the muscular middle layer of the uterus
Near point (of accommodation)	the smallest distance from the eye that an object can be located and still be seen clearly
Negative energy imbalance	the condition wherein the amount of energy taken in over a period of time is less than the amount of energy used during the same period
Negative feedback	a process that prevents or reverses changes
Neoplasm	a group of cells that continue cell reproduction in an uncontrolled fashion
Nephron	a unit of structure and function in the kidney consisting of a glomerulus, Bowman's capsule, and a renal tubule
Neurofibrillar tangle (NTs)	a network of fibers composed of one or two protein fibers in a neuron twisted into a helix
Neuromuscular junction	the site of transmission of an impulse from a neuron to a muscle cell
Neuron	a nerve cell
Neurotransmitter	a substance that transmits an impulse from a neuron to another structure
NIDDM (non-insulin dependent diabetes mellitus)	the type of diabetes mellitus wherein survival is not dependent upon the administration of insulin therapy
Nitric oxide (*NO)	a free radical that serves as a signaling substance in the body
NK cell (natural killer cell)	a non-specific lymphocyte that destroys cancer cells
Non-enzymatic glycation	reactions forming glucose cross-links between protein chains without the use of enzymes
Norepinephrine	a substance that is used as a neurotransmitter and as a hormone and the causes effects of the sympathetic nervous system other than those in the skin
NT	neurofibrillar tangle
Obesity	having a body weight that is greater than 20 percent more than the desirable body weight and a percent body fat that exceeds 25 percent (men)or 30 percent (women), or having a body mass index of more than 30
Opacities	opaque spots in the lens of the eye
Optic disc	the region of the retina to which the optic nerve is attached
Oral mucosa	the lining of the oral cavity
Organ	an organized group of different tissues that performs certain functions
Organ of Corti	the rows of neurons on the basilar membrane of the inner ear that respond to vibrations
Organelle	a structural component of a cell that is composed of several or many molecules
Osmotic pressure	a measure of the total concentration of dissolved materials in a liquid
Osteoarthritis	a type of arthritis that is characterized by the deterioration of cartilage and the formation of extra bone
Osteoblast	the type of cell in bone tissue that produces bone matrix
Osteoclast	the type of bone cell that removes bone matrix from a bone
Osteocyte	the type of bone cell that remains quiescent unless some severe condition develops in the bone
Osteon	a long thick tube of bone matrix which, when fused with other osteons, composes compact bone tissue
Osteoporosis	a disease of the skeletal system that causes substantial reductions in the quantity and strength of bone matrix
Otolith	a heavy crystal that is embedded in the gel attached to the ends of the protruding hair cells in the utricle and the saccule of the inner ear
Ovarian cycle	a sequence of events in the ovaries that repeats itself and that results in the production of hormones and ova
Overweight	having a body weight that is 10=20 percent greater than the desirable body weight
Ovulation	the release of an ovum from an ovary
Oxidation	removing one or more electrons from an atom or molecule
Oxidative phosphorylation	chemical reactions where phosphate groups and the energy from electrons are transferred to molecules (e.g., ATP)
Pancreatitis	inflammation of the pancreas
Pentosidine	glucose cross-links between certain specific amino acids in protein chains
Peptic ulcer	an abnormal condition consisting of a pit created by the death and pealing away of cells because of the presence of secretions from the stomach
Perfusion	the passage of blood through the vessels in a body part
Perilymph	the fluid that fills the bony chambers of the inner ear
Periodontal disease	a disease at the base of teeth
Peripheral nervous system (PNS)	all parts of the nervous system outside of the brain and the spinal cord, all nerves
Peristalsis	a wave of contraction that travels along a tube-shaped structure and that causes material contained within the tube to move along progressively
Peroxyl radical (*ROO)	a free radical containing an organic molecule joined to two oxygen atoms
Peroxynitrite anion (ONOO^-)	a reactive oxygen species that is toxic to cells
Phagocytosis	the process by which a cell engulfs a particle and takes it into the cell
Photoreceptor	a neuron in the sensory retina that responds to light by starting impulses in the form of action potentials
Physiological theory	a theory that attempts to explain how aging occurs in a living animal
Pigmented epithelium	the outer layer of the retina
Plasma	the liquid portion of blood
Plasma cells	a B-cell that produces antibodies
PMN	polymorphonuclear leukocyte (a type of white blood cell)
PNS	peripheral nervous system
Polysaccharide	a molecule consisting of many simple sugar molecules
Polyunsaturated fatty acid, PUFA	a fatty acid molecule with more than one double bond in the carbon chain
Positive energy imbalance	the condition wherein the amount of energy taken in over a period of time is greater than the amount of energy used during the same period
Post-menopausal osteoporosis	the type of osteoporosis that occurs most commonly in women during the years following menopause; Type I osteoporosis
Presbycusis	the ear disorder consisting of a significant decrease in the ability to hear due to aging
Presbyopia	farsightedness caused by age-related stiffening of the lens
Presenilin	a protein which when mutated is correlated with increased risk of Alzheimer’s disease
Primary immune response	the immune response initiated by the first encounter with an antigen
Procedural memory	remembering how to perform a process or series of steps
Progeroid syndrome	a series of manifestations of aging that occur prematurely
Progesterone	a principal sex hormone, most of which is produced by the ovaries
Programmed theory	a theory based on the premise that aging occurs in a directed manner
Propagation	with free radicals, a sequence of chemical reactions where free radical convert other molecules to a free radicals, leaving the original free radicals in an abnormal shape
Prostate gland	a donut-shaped gland in men that surrounds the urethra below its point of connection to the bottom of the urinary bladder
Protein	a molecule composed of one or more chains of amino acids
Psychological aging	age changes that affect the way a person thinks and behaves
PUFA	polyunsaturated fatty acid
Pulmonary congestion	an accumulation of blood in pulmonary vessels
Pulmonary edema	an accumulation of fluid in the lungs from blood vessels in the lungs
Pulmonary embolism	the disease condition of having one or more emboli move into the lungs
Pulmonary vessel	a blood vessel involved in perfusion of the lungs (e.g., pulmonary artery, pulmonary capillary, pulmonary vein)
Pulp	the nerves, blood vessels, and other soft materials in the innermost region of a tooth
RBC	red blood cell
RBF, renal blood flow	the amount of blood flow through the kidneys per unit time
RDA, Recommended Dietary Allowance	the recommended dietary intake of a nutrient as established by the Food and Nutrition Board of the National Academy of Science
Reaction time	the time needed to begin a voluntary motion in response to a stimulus
Reactive oxygen species, ROS	highly reactive substances that contain oxygen
Reception	the process in a neuron of having an impulse generated in response to environmental conditions or to messages from other neurons
Receptor for advanced glycation end product, RAGE	a receptor molecule on a cell that bonds to an advanced glycation end product (AGE)
Receptor molecule	a molecule in a cell or on a cell membrane that binds to a substance that is carrying a message to the cell
Reflex	an involuntary response to a stimulus
Reinitiation	chemical reactions where molecules that were formerly free radical interact and form one or more new free radicals
Renin	a substance produced by the kidneys that helps to regulate blood pressure, osmotic pressure, and sodium concentration in the body
Replicative senescence, RS	the cessation of cell division by a cell as it ages
Residual volume, RV	the amount of air left in the lungs after expiring as much as possible
Respiratory rate	the number of breaths per minute
Rheumatoid arthritis	a type of arthritis characterized by the destruction of cartilage and bone in a joint
RNA, ribonucleic acid	the nucleic acid used to guide the synthesis of protein
Rods	photoreceptors in the eye that can respond to dim light but that cannot respond differently to different colors of light
ROS	reactive oxygen species
Sarcolemma	the cell membrane of a muscle cell
Sarcomere	a cluster of myofilaments in a muscle cell
Sarcopenia	gradual loss of muscle mass by shrinkage of muscle
Sarcoplasmic reticulum	the membranes within a muscle cell that constitutes its endoplasmic reticulum
Saturated fat	a molecule of fat having fatty acids containing the maximum number of hydrogen atoms
Schwann cells	cells in the peripheral nervous system that form myelin
SDAT	Senile dementia of the Alzheimer's type
Sebaceous gland	a skin gland that secretes sebum
Sebum	an oily substance secreted by sebaceous glands
Secondary immune response	an immune response produced by the second or subsequent encounter with an antigen
Self-antigen	a native body material that can bind to an antigen-specific receptor and therefore can initiate an immune response against a body component
Semen	the mixture of sperm cells and secretions that is released from a man's body during sexual activity
Seminiferous tubule	a long and highly coiled tube that produces sperm cells in the testes
Senescence	age changes that have detrimental effects
Senile dementia of the Alzheimer's type, SDAT	a form of Alzheimer's disease with onset usually after age 60
Senile osteoporosis	the type of osteoporosis that occurs most commonly in people of more advanced age, especially those over age 60; Type II osteoporosis
Senile plaque, SPs	a round microscopic masses found near brain synapses and having various mixtures and densities of materials
Sensory neuron	a nerve cell that monitors conditions and conducts impulses toward the brain or spinal cord
Sensory retina	the layer of the retina that is closest to the vitreous humor and that contains several layers of neurons including rods and cones
Skeletal muscle	the type of muscle that is almost always attached to bones of the skeletal system
Sleep apnea, SA	a condition consisting of having least five temporary cessation of ventilation per hour or exhibiting at least 10 occasions of depressed ventilation and cessation of ventilation per hour when asleep
Sliding hiatal hernia	a structural abnormality wherein a portion of the stomach is situated above the diaphragm and is between the diaphragm and the lower end of the esophagus
Smooth muscle	a type of muscle that lacks striations and that is under involuntary control by the nervous system and the endocrine system; visceral muscle
Social aging	age changes in the interactions people have with others
Somatic motor neuron	a motor neuron that controls skeletal muscle
Sorbitol	a type of sugar that is derived from glucose and that tends to accumulate in certain
SP	senile plaque
Speed of movement	the time from the beginning of a motion to the end of that motion
Spongy bone	the type of bone tissue that consists of an open network of trabeculae that are fused together; trabecular bone
Sporadic Alzheimer's disease	a form of Alzheimer's disease with onset usually after age
Stamina	the ability to perform vigorous activity continuously for more than a few seconds
sT-cells	suppressor T-cell
Stochastic theory	a theory based on the premise that aging occurs by random chance events that occur with no direction or control
Stricture	a ring of scar tissue that inhibits the movement of materials through a tube-shaped structure by causing a narrowing of the structure
Stroke	a disease condition involving an injury to or death of brain cells caused by low blood flow or bleeding in an area of the brain
Stroke volume, SV	the amount of blood pumped by one contraction of a ventricle of the heart-
Subcutaneous layer	the layer of the integumentary system that underlies the skin
Superoxide radical	an oxygen molecule containing two oxygen atoms and having 17 electrons
Suprachiasmatic nucleus, SCN	a region in the hypothalamus of the brain that regulates circadian rhythms
Surface tension	the force of attraction between substances on a surface
Surfactant	a substance that reduces surface tension
Survival curve	a graph depicting the percentage of a population that is still alive as time passes from a reference point
Suspensory ligaments	fibers within the eye that radiate outward from the lens and attach to the ciliary body
Sustentacular cells	cells in the wall of seminiferous tubules that promote sperm production; Sertoli cells
SV	the amount of blood pumped by one contraction of a ventricle of the heart
Swallowing reflex	a reflex that clears the pharynx by pushing materials down into the esophagus
Synapse	the site of transmission of an impulse from one neuron to another neuron
Synovial cavity	the space between bones joined by a freely movable joint
Synovial fluid	the fluid that lubricates and cushions the ends of the bones joined by freely movable joint
Synovial membrane	the membrane that surrounds a synovial cavity and that produces and removes synovial fluid
Systemic vessel	a blood vessel other than one involved in perfusion of the lungs (i.e., other than a pulmonary vessel)
Systolic pressure	the peak blood pressure attained by contraction of the ventricles of the heart
Target	with regard to a hormone, a structure that responds to the hormone
Tau protein, τ-protein	a protein that seems to promote microtubule formation and that is abundant in neurofibrillar tangles
T-cell	a type of immune system cell formed from an unspecialized lymphocyte because of the influence of the thymus on the lymphocyte; T-lymphocyte
Telomerase	an enzyme that repairs and replaces telomeres on chromosomes during DNA replication)
Telomere	a strand of DNA on the end of a chromosome
Termination	a chemical reaction where a free radical is converted to a substance that is not a free radical without forming another free radical
Testosterone	a principal sex hormone, most of which is produced by the testes
Thrombus, thrombi	a stationary blood clot inside a vessel
Tidal volume, TV	the volume of air that is inspired or expired per breath when a person is at rest and breathing quietly
Time-lag study	a study method that uses a cross-sectional study procedure more than one time. The cross-sectional procedure is repeated on different groups that have the same age at different times in the study.
Tinnitus	the perception of sound by a person when there is no sound external to the person
Tissue	an organized group of similar cells and associated intercellular material that performs certain functions
TMJ, temporomandibular joint	the joint that attaches the bone of the lower jaw to the skull
Total lung capacity, TLC	the total amount of air that the lungs can hold
Trabeculae	small pieces of bone tissue which, when fused together, compose trabecular bone
Trabecular bone	the type of bone tissue that consists of an open network of trabeculae that are fused together; spongy bone
Transmission	the process by which an impulse is passed from a neuron to another structure
Triglyceride	a lipid molecule composed of one glycerol and three fatty acid molecules; a molecule of fat
Type I fiber	a muscle cell that contracts slowly and can work longer than other types of muscle cells before becoming fatigued
Type IIA fiber	a type of muscle cell that contracts quickly and resists becoming fatigued
Type IIB fiber	a type of muscle cell that contracts quickly and becomes fatigued quickly
Type IIC fiber	a type of muscle cell that contracts at a moderate speed and that becomes fatigued at a moderate period of contraction
Underweight	having a body weight that is below the range for desirable body weight
Unsaturated fat	a molecule of fat having one or more fatty acids that can contain additional hydrogen atoms linked to their carbon atoms
Upper airways	air passages that conduct air into and out of the lungs
Urea	a waste product resulting from the detoxification of ammonia by the liver or from the breakdown of proteins
Urinary incontinence	the elimination of urine at inappropriate times
Uterine fibroid	an abnormal growth consisting of a spherical mass of smooth muscle within
Vaccine	a substance that contains an antigen and that is administered to produce an acquired active immunity to that antigen or to antigens that are very similar to it
Varicose vein	a vein that has developed and retains an abnormally large diameter
Ventilation	the movement of air into and out of the respiratory system; breathing
Vertigo	the sensation that either the body or the surrounding environment is pinning when no spinning is actually occurring
Vision	the conscious perception of images formed on the retina of the eye
Visual acuity	the amount of detail that can be seen
Vital capacity, VC	the maximum volume of air that a person can expire after taking the
Vitamin D	a vitamin that assists in the absorption of calcium by the small intestine
Vitreous humor	the transparent soft gel that fills the region of the eye behind the lens and in
Voiding	releasing urine from the body
Work of breathing	the amount of energy used or the amount of oxygen consumed to perform ventilation
Working memory	remembering information at or close to the level of consciousness so it can be used in cognitive processing
XL	maximum longevity

Absorption | the passage of materials contained within the GI tract into the circulatory system

Accommodation | the process of adjusting the curvature of the lens to focus light from – objects located at different distances from the eye

Acetylcholine | a neurotransmitter used in numerous activities (e.g., memory; controlling skeletal muscle, cardiac muscle, smooth muscle, and certain glands)

Acquired active immunity | the ability to produce a secondary immune response that is strong enough to prevent significant adverse effects from any subsequent encounter with the antigen

Action potential | the process by which an impulse travels along a neuron or a muscle cell

Acute gastritis | an abnormal condition consisting of short term inflammation of the stomach

AD | Alzheimer’s disease

Ad libitum | with reference to diet, being able to eat as much as desired at any time

ADH | antidiuretic hormone

Advanced glycation end-product | a protein chain containing glucose cross-links

Afterimage | a lingering perception that an image is present after the actual image in the eye has changed or disappeared

AGE | advanced glycation end product

Age changes | developmental changes that occur in the later years of life

Age-related macular degeneration | a disease of the eye that reduces vision by causing degeneration of the cones in the macular region of the retina

AL (ad libitum) | with reference to diet, being able to eat as much as desired at any time

Allergic response | an excessive undesirable immune response against a foreign antigen

Alveolus (pl. | a small cup-shaped outpocketing in the lung where gas exchange occurs

Alzheimer's disease | a form of dementia characterized by memory loss and by the presence of excessive senile plaques and neurofibrillar tangles in the brain

Amino acid | a molecular unit that makes up protein molecules

Amyloid | a specific type of protein found within or between cells

Amyloid precursor protein | a protein in cell membranes whose breakdown yields beta-amyloid and other products

Anemia | an abnormal condition consisting of a low concentration of red blood cells in the blood

Aneurysm | an abnormal outpocketing of either a blood vessel or the wall of the heart

Antagonistic pleiotropy | having several effects that oppose one another

Antibody | a protein molecule that is produced by immune system plasma cells and that adheres to antigens and assists in combating antigens

Antigen | a substance that causes an immune response

Antigen-specific receptor | a type of surface receptor on a T-cell that allows that T-cell to bind to one and only one substance capable of initiating an immune response

Antioxidant | a substance that converts free radicals into substances that are not free radicals by donating one or more electrons to other molecules

AOX (Antioxidant) | a substance that converts free radicals into substances that are not free radicals by donating one or more electrons to other molecules

APO | apolipoprotein

APOE | apolipoprotein E

Apolipoprotein E | a lipoprotein that helps move cholesterol and other lipoproteins from cell to cell and through cell membranes, and also seems to help in neuron development and repair

Apoptosis | programmed cell death

APP | amyloid precursor protein

Appendicitis | an abnormal condition involving inflammation of the appendix

Aqueous humor | the liquid that fills the region in the eye behind the cornea and in front of the lens

Arthritis | an abnormal condition that involves inflammation of joints between bones

Ascites | the presence of extra fluid in the abdominal cavity

Atherosclerosis | a disease involving the formation and enlargement of a weak scar-like material in the walls of arteries

ATP | a nucleotide used for transferring energy within a cell

Atrophic gastritis | an abnormal condition resulting in excessive thinning of the lining of the stomach

Auerbach's plexus | a network of nerve cells in the wall of the GI tract that helps to control functions of the GI tract

Auto-antibody | an antibody that can act against a native body material

Autoimmune reaction | an immune response against a normal body part

Autonomic motor neuron | a motor neuron involved in controlling either cardiac muscle, smooth muscle, or a gland

Axon | a neuron projection that conducts impulses away from the nerve cell body of that neuron

B-cell | a type of specialized lymphocyte that can produce antibody-secreting cells as part of an immune response; B-lymphocyte

Basement membrane | a non-cellular layer between a layer of cells and the material under the layer of cells

Basilar membrane | a flexible membrane within the cochlea of the ear that supports the organ of corti

Bed sore | a patch of skin that has died because it received insufficient blood flow; a decubitus ulcer

Benign neoplasm | a neoplasm that does not spread to other areas

Benign prostatic hypertrophy | an abnormal non-cancerous condition in which the prostate gland grows and becomes excessively enlarged

Bile | a liquid secreted by the liver and stored in the gall bladder and that contains bilirubin, cholesterol and other waste materials

Bilirubin | a substance consisting of remnants of hemoglobin molecules from red blood cells

Biological aging | aging in the physical structures and functioning of the body that affects either the body’s ability to survive or its appearance

Biorhythm | a cyclic fluctuation or oscillation of a biological process

BMR | the rate of energy use needed to sustain body functions when a person is awake and in a state of complete rest

Bone marrow | the specialized tissue that is housed within bones and that produces blood platelets and some types of blood cells

Bone matrix | the material between bone cells in bone tissue

BPH | an abnormal non-cancerous condition in which the prostate gland grows and becomes excessively enlarged

Bruch's membrane | a non-cellular membrane that lies between the pigmented epithelium of the retina and the choroid layer

Caloric restriction | receiving a diet that is limited in the number of calories it contains

Cancer | a disease consisting of the uncontrolled reproduction and spreading of cells; a neoplasm that spreads to other areas; a malignant neoplasm

Carbohydrate | a molecule consisting of one or more simple sugar molecules

Cardiac muscle | the type of muscle that is found only in the heart

Cardiac output | the amount of blood pumped by each ventricle of the heart in one minute

Cataracts | the condition of having so many opacities close to the center of the lens of the eye that vision is greatly impaired

Cavity | a spot of decay on a tooth

CCK | a hormone from the small intestine that stimulates emptying of the gall bladder

Cells | the living structural and functional units of the body

Central nervous system | the combination of the brain and the spinal cord

Cerumen | a semi-solid waxy material that is secreted into the ear canal by ceruminous glands; ear wax

Cervix | the narrow lower portion of the uterus

Chain reaction | with free radicals, a sequence of chemical reactions where free radicals react with substances that are not free radicals and form new free radicals

Chronological aging | the passage of time since birth

Cilia | microscopic hair-like structures that project from cells and that swing back and forth

Circadian rhythm | a cyclic fluctuation or oscillation that repeats itself approximately every 24 hours

Cirrhosis | an abnormal condition characterized by the presence of much scar tissue within the liver because of chronic liver damage

Clonal selection | the process of destroying lymphocytes capable of binding to native body materials and initiating an immune response against native body materials

CNS | central nervous system

Collagen | a tough non-elastic fibrous structural protein that is found outside of cells

Colorectal cancer | cancer of the large intestine

Compact bone | the type of bone tissue that is dense and that forms the outer layer of bone; cortical bone

Compliance | the ability to increase or decrease in volume as a result of a change in pressure

Conduction | the process by which an impulse travels along a neuron

Cones | photoreceptors in the eye that have different responses to different colors of light

Coronary artery | a blood vessel that transports blood to capillaries within heart muscle

Cortical bone | the type of bone tissue that is dense and that forms the outer layer of bone; compact bone

Cosmetic aging | changes in outward appearance with advancing age

Cough reflex | a reflex that causes bursts of air to be expired rapidly to force materials out of airways located below the pharynx

CR | caloric restriction

Creatinine | a waste material from muscle cells

Cross-link | a chemical connection between molecules

Cross-sectional study | a study that places individuals with similar ages into groups and evaluates them one time

Cross-sequential study | a study method that combines the cross-sectional method and the longitudinal method. Cross-sectional studies on age categories are repeated on the same population as time passes.

Crush fracture | a type of fracture caused by compression forces acting on a bone that result in collapse of the bone

Crystallized intelligence | using cognitive skills with familiar learned activities

cT-cells | cytotoxic T-cell

Cytoskeleton | a network of microfilaments and microtubules that provides support within a cell

Dead space | the volume of air in the lower airways that does not reach the alveoli during ventilation but that remains in the lower airways during ventilation

Declarative memory | remembering specific facts that a person tried to learn so they could be

Defecation | the elimination of feces through the anus; a bowel movement

Dementia | an abnormal condition that involves a serious decline in memory functioning accompanied by a major decline in at least one other mental function

Dementia with Lewy bodies | a type of age-related dementia characterized by the presence of excess round masses of clumped microfilaments in neurons

Dendrite | a neuron projection that conducts impulses toward the nerve cell body of that neuron

Dentin | a firm layer located internal to the enamel of a tooth

Depth perception | the perception of the distance from the eyes to an object being viewed; binocular vision

Dermis | the layer of the skin underlying the epidermis

Development | developmental changes that occur before birth or during childhood

Developmental changes | irreversible normal changes in a living organism that occur as time passes

DHEA | dehydroepiandrosterone; a steroid hormone of unknown function

DHT | a principal sex hormone in men, much of which is produced by the testes and the prostate gland; 5-alpha-dihydrotestosterone

Diabetes mellitus | a disease that involves inadequate insulin production or inadequate sensitivity of cells to insulin and that results in inhibition of the regulation of blood glucose levels

Diabetic retinopathy (DR) | a disease of the eye that is associated with diabetes mellitus and that reduces vision because of sorbitol accumulation in the retina

Diarrhea | the condition of having more than three relatively liquid and voluminous bowel movements in one day

Diastolic pressure | the lowest blood pressure attained between contractions of the ventricles of the heart

Dietary Reference Intakes | a comprehensive method for establishing and evaluating recommended dietary intake recommendation

Dietary restriction | receiving a diet that is limited in the amount of food

Diffusion | the movement of materials from an area where they are in high concentration to an area where they are in lower concentration

Diglyceride | a lipid molecule composed of one glycerol and two fatty acid molecules

Disaccharide | a molecule consisting of two simple sugar molecules

Diurnal rhythm | a repetitive cycle of events that occurs once in each 2-hour period; circadian rhythm

Diverticula | deep outpocketings in the wall of the large intestine

Diverticulitis | the abnormal condition of having inflamed diverticula

Diverticulosis | a disease condition consisting of the presence of deep outpocketings in the wall of the large intestine

Dizziness | the sensation that the body is unstable

DM | diabetes mellitus

DNA | the nucleic acid that makes up genes

DR | dietary restriction

dT-cells | delayed hypersensitivity T-cell

Duodenal peptic ulcer | a peptic ulcer in the duodenum

Early onset AD | a form of Alzheimer's disease with onset before age 65, usually during the sixth decade of life

Economic aging | age changes in financial status

Edema | the presence of extra fluid between cells

Ejaculation | the expulsion of semen from the penis caused by reflexive rhythmic peristaltic contractions of the urethra, other male reproductive ducts, and muscles at the base of the penis

Elastase | an enzyme that breaks down elastic fibers into elastin peptides

Elastin | a resilient structural protein that is found outside of cells

Elastin peptides | short pieces of the protein chain that composed elastin

Electron | small negatively charged particle that moves about at a distance from the nucleus of an atom

Electron transport | a series of steps where electrons lose energy

Embolus | a mass (usually a blood clot) that is transported within a blood vessel

Enamel | a hard substance that covers the exposed surface a tooth

Endocrine gland | a structure that has hormone secretion as its primary function

Endolymph | the fluid that fills the membranous chambers of the inner ear

Endometrium | the innermost layer of the uterus

Endothelium | the innermost layer of a blood vessel

Endurance | the length of time that vigorous activities can be performed without stopping

Energy | the power that makes things happen; the ability to do work

Energy balance | the condition of having the amount of energy taken in over a period of time equal the amount of energy used during the same period

Enzyme | a protein substance that increases the rate of chemical reactions in a regulated manner

Epidermis | the outer layer of the skin

Episodic memory | remembering the times and places at which events happened

Erectile tissue | the spongy tissue in the penis and the clitoris that fills with blood and becomes firm during sexual arousal

Erection | the swelling, elongating, and stiffening of the penis due to engorgement of the erectile tissue with blood

ERT | the administration of estrogen-like substances to bolster the effects of estrogen in the body when levels of endogenous estrogen become low

Estrogen | a principal sex hormone, most of which is produced by the ovaries

Evolutionary theory | a theory that attempts to explain how aging developed in concert with natural selection

Expiration | the movement of air out of the respiratory system; exhalation

Expiratory reserve volume | the amount of air that can be forcefully expired after expiring the tidal volume

Explicit memory | remembering specific facts that a person tried to learn so they could be remembered

External ear | the region of the ear external to the eardrum

Familial AD | a form of Alzheimer's disease with onset before age 65, usually during the sixth decade of life

Fat | lipid molecules composed of one glycerol and three fatty acid molecules; triglycerides

Fecal incontinence | the abnormal condition of eliminating feces at inappropriate times at least once per month

Feces | a mixture consisting largely of undigested materials, waste products, and bacteria that is located in the large intestine

Fiber | dietary polysaccharides that cannot be broken down by digestive enzymes

Fibroblast | a cell that can produce and secrete proteins that form collagen and elastin fibers

Fibrocartilage | the type of cartilage that consists of a smooth, slippery, and resilient substance that contains many thick collagen fibers and that has the consistency of hard rubber

Filtration | the separation of small substances from large ones when fluid pressure forces the small substances through the pores in a membrane

Fluid intelligence | using cognitive skills in new situations to learning novel problem-solving, motor activities, or reasoning

Foam cell | a fat-filled macrophage in a vessel wall

Follicle | in the ovary, a cluster of cells that surrounds a developing egg cell

Fovea centralis | the central region of the macula lutea that contains the highest concentration of cones

FR | free radical

Free radical | an atom or molecule with an unpaired electron (* = unpaired electron)

FSH | follicle stimulating hormone

Gag reflex | a reflex that causes closure of the openings into the larynx and the esophagus when irritating materials enter the pharynx

Gall stone | a solid mass formed from materials in bile

Gas exchange | the movement of gases between the atmosphere and the blood

Gastric peptic ulcer | a peptic ulcer in the stomach

Gastric refluxing | the movement of stomach contents upward and into the esophagus

Gene | a length of DNA used to direct the formation of an amino acid chain

Genitalia | external reproductive structures

Gerontology | the study of aging

GFR | the rate of filtration through the glomeruli

GH | growth hormone

GI tract | the portion of the digestive system that consists of a tube extending from the mouth to the anus

Glare | seeing bright areas in the wrong places in the field of view because of scattered light striking the retina in the wrong places and in a disorganized way

Glaucoma | a disease of the eye that reduces vision by causing an increase in pressure within the eye due to an accumulation of aqueous humor

Glomerulus | a tuft of capillaries in each nephron in the kidney

Glucagon | a hormone from the pancreas that tends to increase blood sugar levels

Glucose | a simple sugar that is found in abundance in the body and that provides major source of energy for cells

Glycation | using glucose to form cross-links between protein chains

Glycoprotein | a molecule consisting of a combination of carbohydrate and protein

Glycosylation | using glucose to form cross-links between protein chains

Glycoxydation | using glucose to form cross-links between protein chains

Gonads | the primary reproductive organs (i.e., testes in males, ovaries in females), which produce hormones and sperm cells or egg cells

H₂O₂ | hydrogen peroxide

Hair cells | neurons in the inner ear that respond to bending caused by different stimuli (e.g., vibrations, gravity, rotation of the head)

Hayflick limit | the maximum number of divisions a cell can undergo

HCl | an acid secreted by the stomach; hydrochloric acid

HDL | high density lipoprotein

Hearing | the conscious perception of sound vibrations

Heart rate (HR) | the number of times the heart beats in one minute

Heat shock proteins | proteins produced by cells when they are stressed. Heat shock proteins have a protective effect.

Hemorrhoid | a varicose vein in the rectum or the anal canal

Heterochromatin | masses of tightly wound DNA and protein

Histamine | a substance from cells that initiates inflammation

HLA protein | a type of protein molecule on a cell surface that serves to identify that cell as belonging to a specific person

HLA receptor | a surface receptor molecule on a T-cell that is able to bind to specific identification molecules (i.e., HLA protein) found on other body cells

Homeostasis | the state of having proper and fairly steady conditions

Hormone | a substance that result from manufacturing processes in cells, that is secreted into the blood, and that alters the activities of cells in other parts of the body

hT-cell | helper T-cell

Hyaline cartilage | the type of cartilage that consists of a smooth, slippery, white substance having the consistency of hard rubber

Hydrogen peroxide (H₂O₂) | a reactive oxygen species containing two hydrogen atoms and two oxygen atoms

Hydroxyl radical | a free radical containing hydrogen atom and one oxygen atom and having 17 electrons

IDDM | the type of diabetes mellitus wherein death ensues unless insulin therapy is provided

IL-1 | a secretion that stimulates T-cells to produce more identical T-cells during an immune response

IL-2 | a substance secreted by hT-cells that enhances an immune response by stimulating macrophages to phagocytize more antigen, stimulating the production of more of the hT-cells and cT-cells, and stimulating the proliferation and activity of B cells

IL-6 | an interleukin that stimulates inflammation, macrophages, osteoclasts, and other activities

Immune response | the activities of the immune system whereby the system combats an antigen

Implicit memory | remembering specific facts that a person did not try intentionally to learn so they could be remembered

Impotence | the inability of a man to engage in sexual intercourse because the penis is not sufficiently erect to be inserted into the vagina

Incidental memory | remembering information or skills that were self-taught

Inhibin | a sex hormone from gonads that helps regulate sex hormone production by the

Initiation | with free radicals, a chemical that converts a molecule to a free-radical

Inner ear | the region of the ear internal to the oval window

Inspiration | the movement of air into the respiratory system; inhalation

Inspiratory reserve volume | the amount of air that can be forcefully inspired after inspiring the tidal volume

Insulin | a hormone from the pancreas that tends to decrease blood sugar levels

Insulin-like growth factor | signaling substances from cells that stimulate growth and regulate other cell activities

Intercellular material | substances located between cells

Interleukin | a signaling substance that helps regulate immune responses and other body functions

Interneuron | a neuron that carries impulses from one neuron to another neuron

Interstitial cells of Leydig | the cells among the seminiferous tubules in the testes that produce sex hormones

Intervertebral disc | a pad of fibrocartilage located between the vertebrae in the spinal column

Intrinsic factor | a substance secreted by the stomach that promotes the absorption of vitamin B12 by the small intestine

Ion | an atom that has lost or gained one or more electrons

Ischemia | the condition of having inadequate blood flow

Jaundice | the condition wherein parts of the body have a yellow or brown color because of the accumulation of bilirubin

Joint capsule | the thick layer of fibrous material that encases a synovial membrane and that helps to bind bones together

Kcal | the unit of energy most often used in measuring the energy content in food

Keratin | a structural protein found in the outermost layer of the epidermis, in air, and in the nails of the fingers and toes

Keratinocyte | a cell that can make keratin

Ketoacid | a small molecule resulting from the breakdown of a fatty acid or an amino acid; ketone

Ketoacidosis | the abnormal condition of having excess ketoacids in the body

Ketone | a small molecule resulting from the breakdown of a fatty acid or an amino acid; ketoacid

Lacrimal fluid | a liquid secreted by the lacrimal glands that bathes the front surface of the eye

Lactase | an enzyme that assists in the breakdown of lactose

Lactic acid | a substance produced by the partial breakdown of glucose in cells that have a low amount of oxygen

Lactose | a type of sugar that consists of one glucose molecule joined to one galactose molecule; milk sugar

Lactose intolerance | the condition of producing an insufficient quantity of lactase to properly digest dietary lactose

Langerhans cell | a cell in the epidermis of the skin that carries out phagocytosis and assists in initiating an immune response

Late onset AD | a form of Alzheimer's disease with onset usually after age 60

LDL | low density lipoprotein

Lewy body | a round mass of clumped microfilaments in neurons found in excess numbers in types of dementia including Parkinson's disease

LH | luteinizing hormone

Life expectancy of a population | the average age at which death occurs for the members of a population

Ligament | a cable-like structure composed largely of collagen fibers that binds one bone to another bone

Lipid peroxide | a lipid molecule that is a free radical

Lipofuscin | a mixture of certain chemical waste products from normal cell activities

Lipoprotein | a molecule consisting of a combination of lipid and protein

Localization of sound | the perception of the direction from which a sound originates

Longitudinal study | a study that evaluates individuals over a period of time

Lower airways | air passages within the lungs that conduct air to and from alveoli

LP | lipid peroxide

Lymphocyte | a type of white blood cell that functions as part of the immune system

Lymphokines | a group of substances from lymphocytes that augments the defensive effects of an immune response in several ways including by activating macrophages and certain lymphocytes, by causing inflammation, and by protecting normal body cells from viruses

Macrophage | a cell outside of the blood that performs phagocytosis and that assists in immune responses

Macula lutea | the region of the retina that is in line with the center of the cornea and the lens and that has a high concentration of cones

Maillard reaction | a reaction that forms glucose cross-links between protein chains

Malignant neoplasm | a neoplasm that spreads to other areas, cancer

Malnutrition | the condition of lacking nutritional homeostasis

Maltose | a molecule that consists of two glucose molecules linked together

Maturation | developmental changes that result in the transformation of a child into an adult

Maximum longevity (XL) | the greatest age ever achieved by any member of a species

mB-cell | memory B-cell

Mean longevity (ML) | the average age at which death occurs for the members of a population

Melanin | a brown pigment found in the epidermis, in hair, and in other places

Melanocyte | a cell that can make melanin

Memory | with regard to the immune system, the ability of the system to respond to an antigen more quickly and more intensely when the antigen is encountered a second or subsequent time

Menopause | the time when age changes in the ovaries cause menstrual cycles to cease for at least one year

Menstrual cycle | a sequence of events in the uterus that repeats itself and that involves thickening, deterioration, and shedding of the endometrium; uterine cycle

Menstruation | the shedding and passage of the endometrium and blood through the vagina

Metastasis | the spreading of cancer from one location to another

Metastatic cancer | cancer that is transported to other parts of the body through blood, lymph, or other body fluids

Middle ear | the region of the ear between the eardrum and the oval window

Minute volume | the volume of air that is inspired or expired per minute

Mitochondria | organelles that release useful energy for cells

Mitochondrial DNA | DNA in mitochondria

ML | mean longevity

Molecule | a group of atoms or ions bound together in a specific ratio

Monoglyceride | a lipid molecule composed of one glycerol and one fatty acid molecule

Monosaccharide | a single simple sugar molecule

Motor end plate | the modified area of a muscle cell membrane that contains receptor molecules to receive and respond to neurotransmitter molecules from a motor neuron

Motor neuron | a nerve cell that conducts impulses that control the actions of a muscle or a gland

Motor unit | the combination of one motor neuron and all the muscle cell it controls

mRNA (messenger RNA) | a nucleic acid that carries information to determine the sequence of amino acids in a protein molecule

mT-cell | memory T-cell

mtDNA | mitochondrial DNA

Myelin | a substance that coats axons and dendrites and that allows impulses to travel faster along these structures

Myocardium | the layer of muscle in the heart, which is the middle layer of the heart

Myofilament | a protein filament in a muscle cell that is involved in producing contraction of the cell

Myoglobin | a type of protein in muscle cells that attracts oxygen from the blood into muscle cells and stores oxygen

Myometrium | the muscular middle layer of the uterus

Near point (of accommodation) | the smallest distance from the eye that an object can be located and still be seen clearly

Negative energy imbalance | the condition wherein the amount of energy taken in over a period of time is less than the amount of energy used during the same period

Negative feedback | a process that prevents or reverses changes

Neoplasm | a group of cells that continue cell reproduction in an uncontrolled fashion

Nephron | a unit of structure and function in the kidney consisting of a glomerulus, Bowman's capsule, and a renal tubule

Neurofibrillar tangle (NTs) | a network of fibers composed of one or two protein fibers in a neuron twisted into a helix

Neuromuscular junction | the site of transmission of an impulse from a neuron to a muscle cell

Neuron | a nerve cell

Neurotransmitter | a substance that transmits an impulse from a neuron to another structure

NIDDM (non-insulin dependent diabetes mellitus) | the type of diabetes mellitus wherein survival is not dependent upon the administration of insulin therapy

Nitric oxide (*NO) | a free radical that serves as a signaling substance in the body

NK cell (natural killer cell) | a non-specific lymphocyte that destroys cancer cells

NO | nitric oxide

Non-enzymatic glycation | reactions forming glucose cross-links between protein chains without the use of enzymes

Norepinephrine | a substance that is used as a neurotransmitter and as a hormone and the causes effects of the sympathetic nervous system other than those in the skin

NT | neurofibrillar tangle

O₂ | super oxide free radical

Obesity | having a body weight that is greater than 20 percent more than the desirable body weight and a percent body fat that exceeds 25 percent (men)or 30 percent (women), or having a body mass index of more than 30

OH | hydroxyl free radical

Opacities | opaque spots in the lens of the eye

Optic disc | the region of the retina to which the optic nerve is attached

Oral mucosa | the lining of the oral cavity

Organ | an organized group of different tissues that performs certain functions

Organ of Corti | the rows of neurons on the basilar membrane of the inner ear that respond to vibrations

Organelle | a structural component of a cell that is composed of several or many molecules

Osmotic pressure | a measure of the total concentration of dissolved materials in a liquid

Osteoarthritis | a type of arthritis that is characterized by the deterioration of cartilage and the formation of extra bone

Osteoblast | the type of cell in bone tissue that produces bone matrix

Osteoclast | the type of bone cell that removes bone matrix from a bone

Osteocyte | the type of bone cell that remains quiescent unless some severe condition develops in the bone

Osteon | a long thick tube of bone matrix which, when fused with other osteons, composes compact bone tissue

Osteoporosis | a disease of the skeletal system that causes substantial reductions in the quantity and strength of bone matrix

Otolith | a heavy crystal that is embedded in the gel attached to the ends of the protruding hair cells in the utricle and the saccule of the inner ear

Ovarian cycle | a sequence of events in the ovaries that repeats itself and that results in the production of hormones and ova

Overweight | having a body weight that is 10=20 percent greater than the desirable body weight

Ovulation | the release of an ovum from an ovary

Oxidation | removing one or more electrons from an atom or molecule

Oxidative phosphorylation | chemical reactions where phosphate groups and the energy from electrons are transferred to molecules (e.g., ATP)

Pancreatitis | inflammation of the pancreas

Pentosidine | glucose cross-links between certain specific amino acids in protein chains

Peptic ulcer | an abnormal condition consisting of a pit created by the death and pealing away of cells because of the presence of secretions from the stomach

Perfusion | the passage of blood through the vessels in a body part

Perilymph | the fluid that fills the bony chambers of the inner ear

Periodontal disease | a disease at the base of teeth

Peripheral nervous system (PNS) | all parts of the nervous system outside of the brain and the spinal cord, all nerves

Peristalsis | a wave of contraction that travels along a tube-shaped structure and that causes material contained within the tube to move along progressively

Peroxyl radical (*ROO) | a free radical containing an organic molecule joined to two oxygen atoms

Peroxynitrite anion (ONOO^-) | a reactive oxygen species that is toxic to cells

Phagocytosis | the process by which a cell engulfs a particle and takes it into the cell

Photoreceptor | a neuron in the sensory retina that responds to light by starting impulses in the form of action potentials

Physiological theory | a theory that attempts to explain how aging occurs in a living animal

Pigmented epithelium | the outer layer of the retina

Plasma | the liquid portion of blood

Plasma cells | a B-cell that produces antibodies

PMN | polymorphonuclear leukocyte (a type of white blood cell)

PNS | peripheral nervous system

Polysaccharide | a molecule consisting of many simple sugar molecules

Polyunsaturated fatty acid | a fatty acid molecule with more than one double bond in the carbon chain

Positive energy imbalance | the condition wherein the amount of energy taken in over a period of time is greater than the amount of energy used during the same period

Post-menopausal osteoporosis | the type of osteoporosis that occurs most commonly in women during the years following menopause; Type I osteoporosis

Presbycusis | the ear disorder consisting of a significant decrease in the ability to hear due to aging

Presbyopia | farsightedness caused by age-related stiffening of the lens

Presenilin | a protein which when mutated is correlated with increased risk of Alzheimer’s disease

Primary immune response | the immune response initiated by the first encounter with an antigen

Procedural memory | remembering how to perform a process or series of steps

Progeroid syndrome | a series of manifestations of aging that occur prematurely

Progesterone | a principal sex hormone, most of which is produced by the ovaries

Programmed theory | a theory based on the premise that aging occurs in a directed manner

Propagation | with free radicals, a sequence of chemical reactions where free radical convert other molecules to a free radicals, leaving the original free radicals in an abnormal shape

Prostate gland | a donut-shaped gland in men that surrounds the urethra below its point of connection to the bottom of the urinary bladder

Protein | a molecule composed of one or more chains of amino acids

Psychological aging | age changes that affect the way a person thinks and behaves

PUFA | polyunsaturated fatty acid

Pulmonary congestion | an accumulation of blood in pulmonary vessels

Pulmonary edema | an accumulation of fluid in the lungs from blood vessels in the lungs

Pulmonary embolism | the disease condition of having one or more emboli move into the lungs

Pulmonary vessel | a blood vessel involved in perfusion of the lungs (e.g., pulmonary artery, pulmonary capillary, pulmonary vein)

Pulp | the nerves, blood vessels, and other soft materials in the innermost region of a tooth

RBC | red blood cell

RBF | the amount of blood flow through the kidneys per unit time

RDA | the recommended dietary intake of a nutrient as established by the Food and Nutrition Board of the National Academy of Science

Reaction time | the time needed to begin a voluntary motion in response to a stimulus

Reactive oxygen species | highly reactive substances that contain oxygen

Reception | the process in a neuron of having an impulse generated in response to environmental conditions or to messages from other neurons

Receptor for advanced glycation end product | a receptor molecule on a cell that bonds to an advanced glycation end product (AGE)

Receptor molecule | a molecule in a cell or on a cell membrane that binds to a substance that is carrying a message to the cell

Reflex | an involuntary response to a stimulus

Reinitiation | chemical reactions where molecules that were formerly free radical interact and form one or more new free radicals

Renin | a substance produced by the kidneys that helps to regulate blood pressure, osmotic pressure, and sodium concentration in the body

Replicative senescence | the cessation of cell division by a cell as it ages

Residual volume | the amount of air left in the lungs after expiring as much as possible

Respiratory rate | the number of breaths per minute

Rheumatoid arthritis | a type of arthritis characterized by the destruction of cartilage and bone in a joint

RNA | the nucleic acid used to guide the synthesis of protein

Rods | photoreceptors in the eye that can respond to dim light but that cannot respond differently to different colors of light

ROS | reactive oxygen species

Sarcolemma | the cell membrane of a muscle cell

Sarcomere | a cluster of myofilaments in a muscle cell

Sarcopenia | gradual loss of muscle mass by shrinkage of muscle

Sarcoplasmic reticulum | the membranes within a muscle cell that constitutes its endoplasmic reticulum

Saturated fat | a molecule of fat having fatty acids containing the maximum number of hydrogen atoms

Schwann cells | cells in the peripheral nervous system that form myelin

SDAT | Senile dementia of the Alzheimer's type

Sebaceous gland | a skin gland that secretes sebum

Sebum | an oily substance secreted by sebaceous glands

Secondary immune response | an immune response produced by the second or subsequent encounter with an antigen

Self-antigen | a native body material that can bind to an antigen-specific receptor and therefore can initiate an immune response against a body component

Semen | the mixture of sperm cells and secretions that is released from a man's body during sexual activity

Seminiferous tubule | a long and highly coiled tube that produces sperm cells in the testes

Senescence | age changes that have detrimental effects

Senile dementia of the Alzheimer's type | a form of Alzheimer's disease with onset usually after age 60

Senile osteoporosis | the type of osteoporosis that occurs most commonly in people of more advanced age, especially those over age 60; Type II osteoporosis

Senile plaque | a round microscopic masses found near brain synapses and having various mixtures and densities of materials

Sensory neuron | a nerve cell that monitors conditions and conducts impulses toward the brain or spinal cord

Sensory retina | the layer of the retina that is closest to the vitreous humor and that contains several layers of neurons including rods and cones

Skeletal muscle | the type of muscle that is almost always attached to bones of the skeletal system

Sleep apnea | a condition consisting of having least five temporary cessation of ventilation per hour or exhibiting at least 10 occasions of depressed ventilation and cessation of ventilation per hour when asleep

Sliding hiatal hernia | a structural abnormality wherein a portion of the stomach is situated above the diaphragm and is between the diaphragm and the lower end of the esophagus

Smooth muscle | a type of muscle that lacks striations and that is under involuntary control by the nervous system and the endocrine system; visceral muscle

Social aging | age changes in the interactions people have with others

Somatic motor neuron | a motor neuron that controls skeletal muscle

Sorbitol | a type of sugar that is derived from glucose and that tends to accumulate in certain

SP | senile plaque

Speed of movement | the time from the beginning of a motion to the end of that motion

Spongy bone | the type of bone tissue that consists of an open network of trabeculae that are fused together; trabecular bone

Sporadic Alzheimer's disease | a form of Alzheimer's disease with onset usually after age

sT-cells | suppressor T-cell

Stamina | the ability to perform vigorous activity continuously for more than a few seconds

Stochastic theory | a theory based on the premise that aging occurs by random chance events that occur with no direction or control

Stricture | a ring of scar tissue that inhibits the movement of materials through a tube-shaped structure by causing a narrowing of the structure

Stroke | a disease condition involving an injury to or death of brain cells caused by low blood flow or bleeding in an area of the brain

Stroke volume | the amount of blood pumped by one contraction of a ventricle of the heart-

Subcutaneous layer | the layer of the integumentary system that underlies the skin

Superoxide radical | an oxygen molecule containing two oxygen atoms and having 17 electrons

Suprachiasmatic nucleus | a region in the hypothalamus of the brain that regulates circadian rhythms

Surface tension | the force of attraction between substances on a surface

Surfactant | a substance that reduces surface tension

Survival curve | a graph depicting the percentage of a population that is still alive as time passes from a reference point

Suspensory ligaments | fibers within the eye that radiate outward from the lens and attach to the ciliary body

Sustentacular cells | cells in the wall of seminiferous tubules that promote sperm production; Sertoli cells

SV | the amount of blood pumped by one contraction of a ventricle of the heart

Swallowing reflex | a reflex that clears the pharynx by pushing materials down into the esophagus

Synapse | the site of transmission of an impulse from one neuron to another neuron

Synovial cavity | the space between bones joined by a freely movable joint

Synovial fluid | the fluid that lubricates and cushions the ends of the bones joined by freely movable joint

Synovial membrane | the membrane that surrounds a synovial cavity and that produces and removes synovial fluid

Systemic vessel | a blood vessel other than one involved in perfusion of the lungs (i.e., other than a pulmonary vessel)

Systolic pressure | the peak blood pressure attained by contraction of the ventricles of the heart

T-cell | a type of immune system cell formed from an unspecialized lymphocyte because of the influence of the thymus on the lymphocyte; T-lymphocyte

Target | with regard to a hormone, a structure that responds to the hormone

Tau protein | a protein that seems to promote microtubule formation and that is abundant in neurofibrillar tangles

Telomerase | an enzyme that repairs and replaces telomeres on chromosomes during DNA replication)

Telomere | a strand of DNA on the end of a chromosome

Termination | a chemical reaction where a free radical is converted to a substance that is not a free radical without forming another free radical

Testosterone | a principal sex hormone, most of which is produced by the testes

Thrombus | a stationary blood clot inside a vessel

Tidal volume | the volume of air that is inspired or expired per breath when a person is at rest and breathing quietly

Time-lag study | a study method that uses a cross-sectional study procedure more than one time. The cross-sectional procedure is repeated on different groups that have the same age at different times in the study.

Tinnitus | the perception of sound by a person when there is no sound external to the person

Tissue | an organized group of similar cells and associated intercellular material that performs certain functions

TMJ | the joint that attaches the bone of the lower jaw to the skull

Total lung capacity | the total amount of air that the lungs can hold

Trabeculae | small pieces of bone tissue which, when fused together, compose trabecular bone

Trabecular bone | the type of bone tissue that consists of an open network of trabeculae that are fused together; spongy bone

Transmission | the process by which an impulse is passed from a neuron to another structure

Triglyceride | a lipid molecule composed of one glycerol and three fatty acid molecules; a molecule of fat

Type I fiber | a muscle cell that contracts slowly and can work longer than other types of muscle cells before becoming fatigued

Type IIA fiber | a type of muscle cell that contracts quickly and resists becoming fatigued

Type IIB fiber | a type of muscle cell that contracts quickly and becomes fatigued quickly

Type IIC fiber | a type of muscle cell that contracts at a moderate speed and that becomes fatigued at a moderate period of contraction

Underweight | having a body weight that is below the range for desirable body weight

Unsaturated fat | a molecule of fat having one or more fatty acids that can contain additional hydrogen atoms linked to their carbon atoms

Upper airways | air passages that conduct air into and out of the lungs

Urea | a waste product resulting from the detoxification of ammonia by the liver or from the breakdown of proteins

Urinary incontinence | the elimination of urine at inappropriate times

Uterine fibroid | an abnormal growth consisting of a spherical mass of smooth muscle within

Vaccine | a substance that contains an antigen and that is administered to produce an acquired active immunity to that antigen or to antigens that are very similar to it

Varicose vein | a vein that has developed and retains an abnormally large diameter

Ventilation | the movement of air into and out of the respiratory system; breathing

Vertigo | the sensation that either the body or the surrounding environment is pinning when no spinning is actually occurring

Vision | the conscious perception of images formed on the retina of the eye

Visual acuity | the amount of detail that can be seen

Vital capacity | the maximum volume of air that a person can expire after taking the

Vitamin D | a vitamin that assists in the absorption of calcium by the small intestine

Vitreous humor | the transparent soft gel that fills the region of the eye behind the lens and in

VO₂ max | the amount of oxygen used per kilogram of body weight per minute while engaging in exercise at the fastest rate attainable by the person performing the exercise; aerobic capacity

Voiding | releasing urine from the body

Work of breathing | the amount of energy used or the amount of oxygen consumed to perform ventilation

Working memory | remembering information at or close to the level of consciousness so it can be used in cognitive processing

XL | maximum longevity

β-A | a protein of unknown function that is abundant in senile plaques

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