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16.2: An Overview of Blood

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  • By the end of this section, you will be able to:

    • Identify the primary functions of blood in transportation, defense, and maintenance of homeostasis
    • Name the fluid component of blood and the three major types of formed elements, and identify their relative proportions in a blood sample
    • Discuss the unique physical characteristics of blood
    • Identify the composition of blood plasma, including its most important solutes and plasma proteins

    Recall that blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular matrix. The cellular elements—referred to as the formed elements—include erythrocytes (aka red blood cells or RBCs), leukocytes (aka white blood cells or WBCs), and cell fragments called platelets (aka thrombocytes). The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system.

    Functions of Blood

    The primary function of blood is to deliver oxygen and nutrients to and remove wastes from body cells, but that is only the beginning of the story. The specific functions of blood also include defense, distribution of heat, and maintenance of homeostasis.


    Nutrients from the foods you eat are absorbed in the digestive tract. Most of these travel in the bloodstream directly to the liver, where they are processed and released back into the bloodstream for delivery to body cells. Oxygen from the air you breathe diffuses into the blood, moving it from the lungs to the heart, which in turn pumps it out to the rest of the body. Moreover, endocrine glands scattered throughout the body release their products, called hormones, into the bloodstream so they may be delivered to distant target cells to regulate body processes like growth, repair, and the reproductive cycles. Blood also picks up cellular wastes and byproducts and transports them to various organs for removal. For instance, blood moves carbon dioxide to the lungs for exhalation from the body, and various waste products are transported to the kidneys and liver for excretion from the body in the form of urine or bile.


    Many types of leukocytes traveling in blood protect the body from external threats, such as disease-causing bacteria that have entered the bloodstream in a wound. Other leukocytes seek out and destroy internal threats, such as cells with mutated DNA that could multiply to become cancerous or body cells infected with viruses.

    When damage to the vessels results in bleeding, blood platelets and certain proteins dissolved in blood interact to clot blood in the area and block the ruptured areas of the blood vessels. This protects the body from further blood loss.

    Maintenance of Homeostasis

    Body temperature is regulated via a classic negative-feedback loop. If you were exercising on a warm day, your rising core body temperature would trigger several homeostatic mechanisms to cool it back down, including increased transport of blood from your core to your body periphery, which is typically cooler. As blood passes through the vessels of the skin, heat would be dissipated to the environment, and the blood returning to your body core would be cooler. In contrast, on a cold day, blood is diverted away from the skin to maintain a warmer body core. In extreme cases, this may result in frostbite.

    Blood also helps to maintain the chemical balance of the body. Proteins and other compounds in blood act as buffers, which thereby help to regulate the pH of body tissues. Blood also helps to regulate the water content of body cells.

    Composition of Blood

    You have probably had blood drawn from a superficial vein in your arm, which was then sent to a lab for analysis. Some of the most common blood tests—for instance, those measuring lipid or glucose levels in plasma—determine which substances are present within blood and in what quantities. Other blood tests check for the composition of the blood itself, including the quantities and types of formed elements.

    One such test, called a hematocrit, measures the percentage of erythrocytes in a blood sample. Erythrocytes are the specific cells that carry oxygen in the blood. The hematocrit test is performed by spinning the blood sample in a specialized centrifuge, a process that causes the heavier elements suspended within the blood sample to separate from the lightweight, liquid plasma (Figure \(\PageIndex{1}\)). Because the heaviest elements in blood are the iron-containing erythrocytes, these settle at the very bottom of the hematocrit tube. Located above the erythrocytes is a pale, thin layer composed of the remaining formed elements of blood: leukocytes and platelets. The layer containing the leukocytes and platelets is referred to as the buffy coat because of its color; it normally constitutes less than 1 percent of a blood sample. Above the buffy coat is the blood plasma, normally a pale, straw-colored fluid matrix, that constitutes the remainder of the sample.

    The hematocrit test allows one to measure the volume of erythrocytes after centrifugation and the result is also commonly referred to as packed cell volume (PCV). In normal blood, about 45 percent of a sample is erythrocytes. The hematocrit of any one sample can vary significantly, ranging from 37–52 percent based on sex and other factors. Normal hematocrit values for females range from 37 to 47 percent, with a mean value of 41 percent; for males, hematocrit values range from 42 to 52 percent, with a mean of 47 percent. The percentage of other formed elements, the leukocytes and platelets, is extremely small so it is not normally considered with the hematocrit. Thus the mean plasma percentage is the percent of blood that is not erythrocytes: for females, it is approximately 59 percent (or 100 minus 41 percent), and for males, it is approximately 53 percent (or 100 minus 47 percent).

    Whole blood separates by centrifugation into red blood cells comprising just under half the sample at the bottom, a thin line called the buffy coat containing the white blood cells and platelets in the middle, and the plasma comprising just over half at the top of the sample.  Also depicted are a sample with significantly fewer red blood cells than is considered normal (indicating anemia) and a sample with significantly more red blood cells than is considered normal (indicating polycythemia).
    Figure \(\PageIndex{1}\): Composition of Blood. The cellular elements of blood include a vast number of erythrocytes and comparatively fewer leukocytes and platelets. Plasma is the fluid in which the formed elements are suspended. A sample of blood spun in a centrifuge reveals a straw-colored plasma as the lightest component. It floats at the top of the tube separated from the heaviest elements, the erythrocytes, by a buffy coat of leukocytes and platelets. Hematocrit is the percentage of the total sample that is comprised of erythrocytes. A normal hematocit (shown just above 40 percent) is compared with a depressed hematocrit (shown just above 20 percent), which indicates anemia, and an elevated hematocrit (shown above 60 percent), which indicates polycythemia. (Image credit: "Composition of Blood" by OpenStax is licensed under CC BY 3.0)

    Characteristics of Blood

    When you think about blood, the first characteristic that probably comes to mind is its color. Blood that has just taken up oxygen in the lungs is bright red, and blood that has released oxygen in the tissues is a more dusky red. This is because hemoglobin is a pigment that changes color depending upon the degree of oxygen saturation.

    Two drops of blood are shown side-by-side: a bright red drop of oxygenated blood on the left and a darker red drop of deoxygenated blood on the right.
    Figure \(\PageIndex{2}\): Shades of Red Blood. The bright red hue of oxygenated blood is shown on the left while the deeper red hue of deoxygenated blood is shown on the right. ("Drops of Blood Medium" by unknown author is licensed under CC BY 3.0)

    Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water. Viscosity is a measure of a fluid’s thickness or resistance to flow, and is influenced by the presence of the plasma proteins and formed elements within the blood. The viscosity of blood has a dramatic impact on blood pressure and flow. Consider the difference in flow between water and honey. The more viscous honey would demonstrate a greater resistance to flow than the less viscous water. The same principle applies to blood.

    The normal temperature of blood is slightly higher than normal body temperature—about 38 °C (or 100.4 °F), compared to 37 °C (or 98.6 °F) for an internal body temperature reading, although daily variations of 0.5 °C are normal. Although the surface of blood vessels is relatively smooth, as blood flows through them, it experiences some friction and resistance, especially as vessels age and lose their elasticity, thereby producing heat. This accounts for its slightly higher temperature.

    The pH of blood averages about 7.4; it is maintained within an extremely tight range from 7.35 to 7.45 in a healthy person. At an average pH of 7.4, blood is somewhat more basic (alkaline) on a chemical scale than pure water, which has a pH of 7.0. Blood contains numerous buffers that contribute to the regulation of pH.

    Blood constitutes approximately 8 percent of adult body weight. Adult males typically average about 5 to 6 liters of blood. Females average 4 to 5 liters.

    Blood Plasma

    Like other fluids in the body, plasma is composed primarily of water; in fact, it is about 92 percent water. Dissolved or suspended within this water is a mixture of substances, most of which are proteins. There are literally hundreds of substances dissolved or suspended in the plasma, although many of them are found only in very small quantities.

    Plasma Proteins

    About 7 percent of the volume of plasma—nearly all that is not water—is made of proteins. These include several plasma proteins (proteins that are unique to the plasma), plus a much smaller number of regulatory proteins, including enzymes and some hormones. The major components of plasma are summarized in Table \(\PageIndex{1}\).

    The three major groups of plasma proteins are as follows:

    • Albumin is the most abundant of the plasma proteins. Manufactured by the liver, albumin molecules serve as binding proteins—transport vehicles for lipids such as fatty acids and steroid hormones like testosterone. Recall that lipids are hydrophobic; however, their binding to albumin enables them to be transported in the watery plasma. Albumin is also the most significant contributor to the osmotic pressure of blood; that is, its presence holds water inside the blood vessels and draws water from the tissues, across blood vessel walls, and into the bloodstream. This in turn helps to maintain both blood volume and blood pressure. Albumin normally accounts for approximately 54 percent of the total plasma protein content; clinically, this measures as 3.5–5.0 g/dL blood.
    • The second most common plasma proteins are the globulins. A heterogeneous group, there are three main subgroups known as alpha, beta, and gamma globulins. The alpha and beta globulins transport iron, lipids, and the fat-soluble vitamins A, D, E, and K to the cells; like albumin, they also contribute to osmotic pressure. The gamma globulins are proteins involved in immunity and are better known as an antibodies or immunoglobulins. Although other plasma proteins are produced by the liver, immunoglobulins are produced by specialized leukocytes known as plasma cells. Globulins make up approximately 38 percent of the total plasma protein volume; clinically, this measures as 1.0–1.5 g/dL blood.
    • The least abundant plasma protein is fibrinogen. Like albumin and the alpha and beta globulins, fibrinogen is produced by the liver. It is essential for blood clotting, a process described later in this chapter. Fibrinogen accounts for about 7 percent of the total plasma protein volume; clinically, this measures as 0.2–0.45 g/dL blood.

    Other Plasma Solutes

    In addition to proteins, plasma contains a wide variety of other substances. These include various electrolytes, such as sodium, potassium, and calcium ions; dissolved gases, such as oxygen, carbon dioxide, and nitrogen; various organic nutrients, such as vitamins, lipids, glucose, and amino acids; and metabolic wastes. All of these non-protein solutes combined contribute approximately 1 percent to the total volume of plasma.

    Table \(\PageIndex{1}\): Summary of Major Blood Components
    Component and % of blood Subcomponent and % of component Type and % (where appropriate) Site of production Major function(s)
    46-63 percent

    92 percent

    Fluid Absorbed by intestinal tract or produced by metabolism Transport medium
    Plasma proteins
    7 percent
    54-60 percent
    Liver Maintain osmotic concentration, transport lipid molecules
    35-38 percent
    Alpha globulins
    Transport, maintain osmotic concentration
    Beta globulins
    Transport, maintain osmotic concentration
    Gamma globulins (immunoglobulins)
    Plasma Cells
    Antibody immune responses
    4-7 percent
    Liver Blood clotting in hemostasis
    Regulatory proteins
    <1 percent
    Hormones and enzymes Various sources Regulate various body functions
    Other solutes
    1 percent
    Nutrients, gases, and wastes Absorbed by intestinal tract, exchanged in respiratory system, or produced by cells Numerous and varied
    Formed Elements
    37-54 percent
    99 percent
    Erythrocytes Red bone marrow Transports gases, primarily oxygen and some carbon dioxide
    <1 percent
    Granular leukocytes: neutrophils, eosinophils, basophils Red bone marrow Nonspecific immune responses
    Agranular leukocytes: lymphocytes, monocytes Lymphocytes: red bone marrow and lymphatic tissues Specific immune responses
    Monocytes: red bone marrow Nonspecific immune responses
    <1 percent
    Platelets Megakaryocytes: red bone marrow Hemostasis


    Phlebotomy and Medical Lab Technology

    Phlebotomists are professionals trained to draw blood (phleb- = “a blood vessel”; -tomy = “to cut”). When more than a few drops of blood are required, phlebotomists perform a venipuncture, typically of a surface vein in the arm. They perform a capillary stick on a finger, an earlobe, or the heel of an infant when only a small quantity of blood is required. An arterial stick is collected from an artery and used to analyze blood gases. After collection, the blood may be analyzed by medical laboratories or perhaps used for transfusions, donations, or research. While many allied health professionals practice phlebotomy, the American Society of Phlebotomy Technicians issues certificates to individuals passing a national examination, and some large labs and hospitals hire individuals expressly for their skill in phlebotomy.

    Medical or clinical laboratories employ a variety of individuals in technical positions:

    Medical technologists (MT), also known as clinical laboratory technologists (CLT), typically hold a bachelor’s degree and certification from an accredited training program. They perform a wide variety of tests on various body fluids, including blood. The information they provide is essential to the primary care providers in determining a diagnosis and in monitoring the course of a disease and response to treatment.
    Medical laboratory technicians (MLT) typically have an associate’s degree but may perform duties similar to those of an MT.
    Medical laboratory assistants (MLA) spend the majority of their time processing samples and carrying out routine assignments within the lab. Clinical training is required, but a degree may not be essential to obtaining a position.

    Formed Elements

    The formed elements include erythrocytes (RBCs), leukocytes (WBCs), and platelets (thrombocytes). They are formed mostly in the red bone marrow (not within the blood itself) and one of them, platelets, are circulating fragments of a much larger cell so referring to them as cells (even thrombocytes) is a misnomer. It is for this reason that this group is not labeled as cellular components of blood.

    Erythrocytes function to carry oxygen through the bloodstream so it may be delivered throughout the body. There are several different types of leukocytes that have unique functions contributing to the body's housekeeping and sickness fighting efforts. Platelets function in helping to maintain blood volume (a process called hemostasis) by forming blood clots to prevent blood loss.

    The formed elements are summarized in Table \(\PageIndex{2}\). Each category of formed element is covered in more detail in a dedicated section of this chapter.

    Table \(\PageIndex{2}\) Summary of Formed Elements in Blood

    Formed Element

    Major subtypes

    Numbers present per microliter (μL); mean (range)

    Appearance in a standard blood smear

    Summary of functions


    (red blood

    5.2 million
    (4.4-6.0 million)

    Flattened biconcave disc; no nucleus; pale red color

    Transport oxygen and some carbon dioxide between tissues and lungs

    Lifespan of approximately 120 days

    (white blood

    Granulocytes (neutrophils, eosinophils, and basophils)

    7000 (5000-10,000)

    Obvious dark-staining nucleus

    All function in body defenses

    Exit capillaries and move into tissues; lifespan of usually a few hours or days


    4150 (1800-7300)

    Nuclear lobes increase with age; pale lilac granules

    Phagocytic; particularly effective against bacteria; Release cytotoxic chemicals from granules

    Most common leukocyte; lifespan of minutes to days


    165 (0-700)

    Nucleus generally two-lobed; bright red-orange granules

    Phagocytic cells; effective with antigen-antibody complexes; release antihistamines; increase in allergies and parasitic infections.

    Lifespan of minutes to days.


    44 (0-150)

    Nucleus generally two-lobed but difficult to see due to presence of heavy, dense, dark purple granules

    Promotes inflammation

    Least common leukocyte; lifespan unknown

    Agranulocytes (lymphocytes and monocytes)

    2640 (1700-4950)

    Lack abundant granules in cytoplasm; have a simple-shaped nucleus that may be indented

    Body defenses

    Group consists of two major cell types from different lineages


    2185 (1500-4000)

    Spherical cells with a single often large nucleus occupying much of the cell’s volume; stains purple; seen in large (natural killer cells) and small (B and T cells) variants

    Primarily specific (adaptive) immunity: T cells directly attack other cells; B cells release antibodies; natural killer cells are like T cells but nonspecific

    Initial cells originate in bone marrow, but secondary production occurs in lymphatic tissue; several distinct subtypes; memory cells form after exposure to a pathogen and rapidly increase responses to subsequent exposure; lifespan of many years


    455 (200-950)

    Largest leukocyte with an indented or horseshoe-shaped nucleus

    Very effective phagocytic cells engulfing pathogens or worn-out cells; also serve as antigen- presenting cells (APCs) for other components of the immune system

    Produced in red bone marrow; referred to as macrophages after leaving circulation


    350,000 (150,000-500,000)

    Cellular fragments surrounded by a plasma membrane and containing granules; purple stain

    Hemostasis plus release growth factors for repair and healing of tissue

    Formed from megakaryocytes that remain in the red bone marrow and shed platelets into circulation

    In addition to measuring the hematocrit, a very common blood test is a complete blood count (or CBC) with differential. A small amount of blood is taken and smeared onto a gridded microscope slide and then stained to allow for proper examination under a microscope. A trained technician will then view the blood under the microscope and count the numbers of each different type of formed element visible. The report includes numbers for each specific type of leukocyte for the differential portion, along with some other information about the sizes, shapes, and characteristics of each element. The grid on the slide allows counts to be converted into concentrations of each formed element in the blood that can be compared with expected values such as those listed in Table \(\PageIndex{2}\). Elevated or low levels of each type of formed element can indicate specific health concerns.

    Concept Review

    Blood is a fluid connective tissue critical to the transportation of nutrients, gases, and wastes throughout the body; to defend the body against infection and other threats; and to the homeostatic regulation of pH, temperature, and other internal conditions. Blood is composed of formed elements—erythrocytes, leukocytes, and cell fragments called platelets—and a fluid extracellular matrix called plasma. More than 90 percent of plasma is water. The remainder is mostly plasma proteins—mainly albumin, globulins, and fibrinogen—and other dissolved solutes such as glucose, lipids, electrolytes, and dissolved gases. Because of the formed elements, the plasma proteins, and other solutes, blood is sticky and more viscous than water. It is also slightly alkaline, and its temperature is slightly higher than normal body temperature.

    Review Questions

    Q. Which of the following statements about blood is true?

    A. Blood is about 92 percent water.

    B. Blood is slightly more acidic than water.

    C. Blood is slightly more viscous than water.

    D. Blood is slightly more salty than seawater.


    Answer: C

    Q. Which of the following statements about albumin is true?

    A. It draws water out of the blood vessels and into the body’s tissues.

    B. It is the most abundant plasma protein.

    C. It is produced by specialized leukocytes called plasma cells.

    D. All of the above are true.


    Answer: B

    Q. Which of the following plasma proteins is not produced by the liver?

    A. fibrinogen

    B. alpha globulin

    C. beta globulin

    D. immunoglobulin


    Answer: D

    Critical Thinking Questions

    Q. A patient’s hematocrit is 42 percent. Approximately what percentage of the patient’s blood is plasma?


    A. The patient’s blood is approximately 58 percent plasma (since the buffy coat is less than 1 percent).

    Q. Why would it be incorrect to refer to all formed elements as cells?


    A. The formed elements include erythrocytes and leukocytes, which are cells (although mature erythrocytes do not have a nucleus); however, the formed elements also include platelets, which are not true cells but cell fragments.

    Q. True or false: The buffy coat is the portion of a blood sample that is made up of its proteins.


    A. False. The buffy coat is the portion of blood that is made up of its leukocytes and platelets.


    most abundant plasma protein, accounting for most of the osmotic pressure of plasma
    (also, immunoglobulins or gamma globulins) antigen-specific proteins produced by specialized B lymphocytes that protect the body by binding to foreign objects such as bacteria and viruses
    liquid connective tissue composed of formed elements—erythrocytes, leukocytes, and platelets—and a fluid extracellular matrix called plasma; component of the cardiovascular system
    buffy coat
    thin, pale layer of leukocytes and platelets that separates the erythrocytes from the plasma in a sample of centrifuged blood
    complete blood count (CBC) with differential
    a test in which a prepared microscope slide of a patient's blood is used to count the numbers of each type of formed element visible in a given volume of blood; observations about the size, shape, and characteristics of each type of formed element are also made
    one of the formed elements of blood that transports oxygen (also, red blood cells or RBCs)
    plasma protein produced in the liver and involved in blood clotting
    formed elements
    cell-derived components of blood; that is, erythrocytes, leukocytes, and platelets
    heterogeneous group of plasma proteins that includes transport proteins, clotting factors, immune proteins, and others
    (also, packed cell volume) volume percentage of erythrocytes in a sample of centrifuged blood
    (also, antibodies or gamma globulins) antigen-specific proteins produced by specialized B lymphocytes that protect the body by binding to foreign objects such as bacteria and viruses
    one of the formed elements of blood that provides defense against disease agents and foreign materials (also, white blood cells or WBCs)
    packed cell volume (PCV)
    (also, hematocrit) volume percentage of erythrocytes present in a sample of centrifuged blood
    in blood, the liquid extracellular matrix composed mostly of water that circulates the formed elements and dissolved materials throughout the cardiovascular system
    one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes (also, thrombocytes)

    Contributors and Attributions

    OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at