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1.2: What is Aging?

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    Exactly what is aging? How would you define it? Do all people use the same definition? Is aging different from other changes that occur as people get older? The term aging is difficult to define because it has diverse meanings for different people. However, one definition will be selected here to help in the study of aging. To understand this definition, we must first understand developmental changes.

    Developmental changes are irreversible normal changes in a living organism that occur as time passes. The same changes can be expected to occur in all members of a particular type of organism in a natural population (i.e., not genetically altered intentionally or living in a carefully controlled “artificial” environment). Developmental changes are neither accidental nor a result of abuse, misuse, disuse, or disease. They occur in humans from the moment of conception to the moment of death. Familiar examples include growth in height, sexual maturation, and graying of the hair. The field of biology in which developmental changes are studied is called developmental biology.

    Note that developmental changes are irreversible or at least rarely reversible. Conversely, bodily changes that occur in one direction for a while and then reverse direction are called physiological changes. Some physiological changes, such as increases and decreases in the rate of breathing, are rapidly reversible. Others, such as fluctuations in weight and physical fitness, are reversed more slowly.

    Developmental changes can be divided into three categories. The first consists of changes that occur before birth or during childhood. Examples include the formation of specialized organs from a single-celled fertilized egg and increases in muscle coordination. Collectively, these early changes are usually called development. Studies of development before birth constitute the segment of biology called embryology.

    The second category includes changes that result in the transformation of a child into an adult. These changes make up what is frequently called maturation. Puberty is an example of maturation. Both development and maturation consist of changes that usually improve the ability of a person to survive. Examples include the strengthening of muscles and bones and increases in intellectual ability.

    The third category - aging refers to the group of developmental changes that become most evident in the later years; these are also called age changes. Examples described in later chapters include stiffening of the lungs, thinning of the bones, and a declining sense of smell. For practical purposes, the later years of life are considered in this book to begin about age 50. However, no one knows when many age changes that become evident in the later years actually start. Many age changes, such as reductions in kidney function, begin as early as age 20. Unlike development and maturation, almost all age changes reduce a person’s ability to maintain healthy survival and a high quality of life. The term senescence includes only those age changes that have such detrimental effects. However, there are beneficial age changes, such as certain changes in the sweat glands, the heart, and the brain. These and other examples of positive age changes are described in Chaps. 3, 4, and 6.

    Types of Aging

    Biological Aging

    Aging includes several different kinds of changes. One group of changes - biological aging - involves aging in the physical structures and functioning of the body that affects a person’s ability to survive or a person’s appearance. Biological aging is the main topic of this book. To understand its significance, one must first understand what is required for the survival and well-being of the body.

    The human body, like most living things, is made up of small units called cells and materials that the cells produce. For example, muscles are made up mostly of muscle cells (Figure 1.8a).

    clipboard_ecd41e9a208479e6c41452414469da4d0.png
    Figure 1.8a The structural; basis of the body (a) Cross section of a muscle. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

    By contrast, bones contain some bone cells but consist mostly of materials that those cells secrete (Figure 1.8b).

    clipboard_e56b183ccc530309388c2f7dbdfcb72f3.png
    Figure 1.8b The structural basis of the body (b) Cross section of a bone. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

    The cells do more than furnish the substance of the body: They also perform all of its functions. Every thought and movement a person has actually results from nerve cells producing and carrying impulses and muscle cells moving. If the cells stopped working, there would be no bodily activity.

    The cells of the body must have just the right set of conditions virtually all the time to build and maintain the structure of the body and carry out its functions. The state of having proper and fairly steady conditions is called homeostasis. It involves many conditions, such as temperature, nutrient levels, water content, and other parameters measured in medical checkups and diagnoses. Each condition may change slightly from time to time; such small changes occur because being alive means doing things such as growing and moving, and doing things causes changes in body conditions (Figure 1.9). For example, an ordinary activity such as walking raises body temperature, burns nutrients for energy, and results in water loss by evaporation from breathing and perspiring. Even the environment surrounding the body tends to cause changes within the body. An example is the tendency of body temperature to drop when a person is in a cool room because warm objects lose heat to a cool environment.

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    Figure 1.9 Homeostasis and unacceptable alterations in body conditions (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

    For a person to stay alive and well, each condition must not be allowed to stray above or below an acceptable range. If one of them, such as temperature, deviates too far, the cells will be injured and begin to malfunction. This means that the body is malfunctioning. Its well-being, and perhaps its very survival, is then jeopardized. The greater the number of injured cells and the more severe the injury, the greater the decline in bodily functioning and well-being and the greater the danger to the body.

    If the errant condition is out of the acceptable range for only a brief period or to only a small degree, the cells can often recover once conditions are again favorable. However, if the deviation is present for an extended period, is extreme, or occurs frequently many times, some cells may be permanently altered or killed. The body has then lost the contributions which those cells should be making (Figure 1.3). Again, depending on the amount of injury, the result can range from barely noticeable discomfort to death.

    Consider what happens to a person whose body temperature is dropping. As body temperature falls, the heart cells and brain cells slow down. If the chill is not severe or long-lasting, the person will recover completely once the body is warmed again. However, if the temperature drops too far or if the person stays chilled for a long time, as can happen when a person falls into icy water, cell functioning becomes so slow that the person dies of hypothermia.

    Since many activities are occurring inside the body and many changes occur -in its surroundings, one might ask how conditions for the cells are kept proper and fairly stable. Part of the answer involves the ability of the body to provide materials and structures that tend to prevent changes in these conditions. For example, fat under the skin helps prevent cooling by slowing heat loss from the body. The other part of the answer is the process of negative feedback, which involves three steps. The first step is detecting the presence of deviations from homeostasis. The next is informing the parts of the body that some condition is unacceptable and telling them how to slow, stop, or correct the developing problem. The nervous system contributes to these steps by continuously monitoring conditions such as body temperature. For example, if the nervous system detects a drop in temperature, it sends impulses to several parts of the body (Figure 1.10). The brain is informed about the problem, and the person is warned of the danger by the feeling of being cold. The skin and the muscles are directed to compensate for the developing deviation from homeostasis.

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    Figure 1.10 Negative feedback for homeostasis. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

    The third step in negative feedback is making the necessary adjustment to slow or stop the deviation before it causes a loss of homeostasis, or to restore the condition to a normal level. Many body systems contribute to this process. For example, when the body becomes chilled, the person may use muscles and bones to turn on a heater or move to a warmer location. The blood vessels in the skin become narrow. Both actions would reduce the loss of heat, slowing or stopping the deviation from progressing. The muscles may then cause shivering as they contract and relax quickly and repeatedly to produce more heat to warm the body, thus compensating for heat loss and returning the body to a normal internal temperature. These and other activities maintain and restore normal body temperature before any cells are significantly affected. Homeostasis is maintained and the cells stays alive and well.

    A similar process that can help maintain homeostasis is positive feedback. This process also the same has three steps, except that the third step increases the change being made. To remain beneficial, the body provides a mechanism to stop the positive feedback system before it causes excess changes. Examples include inflammatory reactions when cells are injured or killed and immune reactions when the body detects foreign substances on or in the body. If an inflammatory or immune reaction is not stopped in time, the reaction becomes harmful (e.g., scar formation, allergic reaction). Other positive feedback systems that remain beneficial as long as they are stopped in time include blood clotting and impulse conduction by action potentials. (Chapters 3, 4 and 6) Harmful outcomes from these when unchecked include blood clots blocking vessels and seizures. Still other beneficial positive feedback systems include certain human sexual responses, ovulation, and uterine contractions during childbirth. (Chapters 13, 14, and 15)

    Having developed an appreciation for how the body keeps itself alive and well, one can understand the importance of biological aging. With few exceptions, biological aging reduces the ability of the body to maintain homeostasis and therefore to survive. This happens in two main ways.

    First, some biological age changes allow more rapid or extreme alterations in body conditions to occur. For example, thinning of the insulating layer of fat under the skin allows the body to chill faster.

    Second, other biological age changes reduce the functioning of negative feedback systems and positive feedback systems. For negative feedback systems, there is a decline in the ability of certain parts of the body to detect alterations in body conditions and notify other parts that the body is threatened. Age changes in the nervous system are among the most important in this category. With aging, there is a decrease in the number of nerve cells that monitor conditions, and the nerve cells that remain often function weakly. Thus, the detection of deviations from homeostasis, such as a lowering of body temperature, is reduced. The ability to notify and activate parts of the body that can reduce, eliminate, or correct the problem also declines. This is especially pronounced when several parts of the body must act in a coordinated fashion. For example, there is a decline in coordinating the many complex muscle contractions needed to maintain balance while one is standing on a moving surface such as a boat deck. Finally, the structures that should restore conditions to an acceptable level are less able to do so. For example, as aging causes a decrease in the amount of muscle, there is a reduced ability to produce heat to raise body temperature back to normal.

    Positive feedback systems also become weaker and less beneficial due to decreases in their detection, notification, activation, and control phases.

    In summary, most biological aging allows more of the conditions in the body to stray further from the acceptable range and to stay beyond the normal range longer or more frequently. This causes more cells to be injured and fail in their functions. When many cells are affected to a large degree, the person feels less well and does not function as well. When to many cells are no longer able to perform adequately, the person becomes ill and dies.

    Chronological Aging

    The simplest type of aging is chronological aging, which refers to the passage of time since birth. It is usually measured in years, though sometimes decades are used. While chronological age can be useful in estimating the average status of a large group of people, it is a poor indicator of an individual person’s status because there is tremendous variation among individuals in the rate biological age changes occur. For example, on the average, aging results in people losing much of their ability to perform strenuous activities, yet some elderly individuals are excellent marathon runners.

    Cosmetic Aging

    Cosmetic aging consists of changes in outward appearance with advancing age. This includes changes in the body and changes in other aspects of a person’s appearance, such as the style of hair and clothing, the type of eyeglasses worn, and the use of a hearing aid. Like chronological aging, it is frequently used to estimate the degree to which other types of aging have occurred. It is even used to guess a person’s chronological age. However, it is an inaccurate indicator for either purpose because of variation among individuals and because a person’s appearance is affected by many factors that are not part of aging, including illness, poor nutrition, and exposure to sunlight.

    Although cosmetic aging provides little evidence about other forms of aging, it can have profound effects on many aspects of life. For example, people who notice that their hair is turning gray may begin to think of themselves as old, and this may result in withdrawal from physically demanding activities, loss of appetite, depression, and subsequent declining health. Since people in this situation may lose interest in their appearance and may look worse because of ill health, they may be entering a vicious spiral of decline. The time, effort, and money that people spend trying to look young provides further evidence for the importance of appearance.

    Social Aging

    Another type of aging is social aging, which consists of age changes in the interactions people have with others. The birth of grandchildren, for example, can alter the ways in which the new parents interact with the new grandparents and even the ways in which the maternal and paternal grandparents relate to each other.

    As with chronological and cosmetic aging, social aging has an impact on other age changes. The death of a spouse, for example, may decrease a person’s interest in his or her own appearance, leading to cosmetic changes. The loneliness that often follows the loss of a spouse may cause stress, which in turn may result in a more rapid decline in the ability to fight off infection.

    Psychological Aging

    Psychological aging consists of age changes that affect the way people think and behave. It often results from other types of aging. For example, biological aging of the brain directly affects the speed of learning and the ability to remember some types of information. Examples involving other types of aging were mentioned above.

    Psychological aging also contributes to other types of aging. Memory loss can result in forgetting to keep an appointment with a friend or a physician. Slowed thinking can prevent a person from retaining certain types of employment, such as jobs requiring rapid decision-making involving many variables (e.g., flight controller, fighter pilot, emergency room staff, crisis situation manager), especially in unfamiliar situations (e.g., newly employed or promoted).

    Economic Aging

    Economic aging consists of age changes in a person’s financial status. Like psychological aging, economic aging can result from other types of aging. For example, in spite of laws against discrimination based on chronological age, some older people find it difficult to retain a job or obtain a new one simply because of their age. The resulting loss of income can cause difficulty in obtaining proper medical treatment or purchasing adequate food. Loss of contact with business colleagues and lowered self-esteem can have social and psychological effects.

    Spiritual Aging

    Spiritual aging consists of age changes in a person’s spirituality. This topic seems to be the most recent aspect of aging to be studied and discussed. Consequently, there is not yet a consensus on what happens during spiritual aging. Moreover, there is no consensus on the meaning and definition spirituality. To some people, spirituality is a person’s religion, religious thinking and the outward expression thereof. To others, spirituality has nothing to do with religion. Still others view spirituality as a combination of religious and non-religious aspects of a person. Thus, spiritual aging includes age changes in a combination of beliefs, understandings, experiences, and awareness included in spirituality. (Table 1.1)

    Table 1.1 Possible components of spirituality
    Beliefs and understandings regarding: Experiences of: Awareness or sense of:
    • the existence of one’s soul (Soul), its importance, and its status
    • a sense of well-being
    • death, including one’s own and that of others
    • essential motivations
    • fundamental values and what are of essential importance
    • hardship and suffering
    • one’s interior mental thoughtful life
    • one’s true essential nature – what constitutes one’s existence
    • the essential purposes of living
    • the fundamental principles that motivate and direct all other activities
    • the value of survival
    • what constitutes quality of life
    • what is true happiness
    • the essential purposes of living
    • the fundamental principles that motivate and direct all other activities
    • the value of survival
    • core values
    • guiding principles
    • fundamental philosophy of existence
    • non-material aspect of existence
    • the existence and nature of the supernatural
    • perspectives on the entire human race; past, present, and future
    • relationships with other people and relationships among people
    • a greater power that is outside of the one’s self
    • one’s self, defined with regard to relationships to others
    • faith
    • hope
    • inner peace and calmness, or inner conflict and turmoil
    • awe and transcendence
    • internal inward-looking, which may include self-evaluation
    • values and thinking that guides one’s conduct
    • relationships with a Creator, God (by any name), gods, and supernatural beings
    • overall success and accomplishment in life
    • purpose for existing and one’s life as a whole
    • interconnectedness with all that exists

    Interactions among Types of Aging

    As is seen from the above examples, most types of aging can result from any of the other types. Also, each type can influence the others, and complex series of interactions can develop. The one exception is that chronological aging cannot be altered by the other types of aging (Figure 1.11).

    clipboard_e240658183c9073416fcac94a425f432a.png
    Figure 1.11 Interactions among types of aging.(Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission. )

    What Aging Is Not

    It is important to note that many changes in the elderly are thought to be age changes but are abnormal rather than true age changes. Some abnormal changes result from abuse or misuse of parts of the body. Examples include skin wrinkling caused by sunlight (farmers), hearing loss caused by loud noise (factory workers), and joint stiffness caused by repeated traumatic injury (athletes). Other abnormal changes that are often thought to be age changes result from disuse. Examples include reductions in the pumping capacity of the heart, muscle power, and bone strength caused by inadequate exercise. Changes from extrinsic factors such as abuse, misuse, and disuse frequently accompany or amplify true age changes. For example, aging does cause some skin wrinkling, hearing loss, joint stiffness, muscle weakening, and bone weakening. Since many extrinsically caused abnormal changes in aging individuals are not severe enough to be considered disease, they are called “usual” age changes by some authors. People who avoid these abnormal changes while undergoing normal age changes are said to have achieved “successful” aging.

    In addition to abnormal changes mentioned above, many abnormal changes that accompany aging are not part of true aging but are aspects of a disease. A person has a disease if that person’s body has any one of three characteristics; (1) it has homeostasis but it cannot maintain homeostasis when it encounters a mild adverse condition that would not destroy homeostasis in most people (e.g., having diabetes where ingesting a small amount of sugar causes the blood sugar level to rise excessively, having AIDS where the immune system cannot kill certain types of cancer cells); (2) conditions in at least a part of the body are severe enough to be causing injury or death to cells there (e.g., having an infected finger, having a broken thigh bone); (3) conditions in many or all parts of the body are out of the acceptable range (e.g., having high blood pressure, having kidney failure.) A disease may be short term (acute), long term or recurring (chronic), in a small area (local), or widespread (systemic).

    However, aging is not a disease, does not mean disease, and does not automatically include disease. The elderly are more susceptible than the young to certain diseases, but no diseases occur only in the elderly or occur in every elderly person.

    Why then is aging often equated with disease? This probably stems from the much higher incidence of diseases among the elderly. One reason for this increase in disease is that most age changes reduce the ability of the body to keep conditions within the normal range. As examples, timing mechanisms may only delay diseases under genetic control, the sensory function of the nervous system declines, reflexes become slower and weaker, and immune responses against infection dwindle. However, there are compensating mechanisms that make up for many of these detrimental changes. Something as simple as wearing warmer clothing can compensate for the reduced ability to maintain an adequate body temperature. The use of eyeglasses and brighter lighting can restore much of the decline in vision. Allowing more time for tasks can make up for slower reactions and slower learning or remembering. Practicing and using experience can make accomplishing a task quick, easy and efficient. Avoiding exposure to infectious agents places less demand on defense mechanisms. If one creatively develops and uses compensating strategies, many undesirable consequences of aging that increase the likelihood of disease can be reduced or eliminated.

    A second reason for the increase in disease with advancing age is that years must pass before some diseases become serious enough to be noticed. Sometimes this is because of the reserve capacity found in many parts of the body. Having reserve capacity means that under normal resting conditions, only a fraction of the full functional capacity of certain organs is needed to maintain homeostasis. For example, up to 50 percent of the functional capacity of the kidneys may be lost before a person notices that something is wrong. When body structures have little reserve capacity, a disease is not noticed because it progresses very slowly. For example, osteoarthritis, the most common type of arthritis, seems to require the cumulative effect of years of abuse of the joints before it becomes a problem. Also, atherosclerosis, which is a type of hardening of the arteries (arteriosclerosis), frequently begins before a heart attack because deterioration of the arteries occurs slowly, the heart attacks and strokes it causes usually do not occur until several decades later.

    A third reason for the age-related increase in disease is that as time passes, there is a greater chance that a person will be subjected to factors that promote disease and that these exposures will occur many times and for longer periods. Examples include physical trauma, infectious organisms, air pollution, harmful radiation, and bad nutrition.

    These facts indicate a very important point: Many abnormal changes associated with aging can be prevented, and the progress of many other diseases can be slowed enough so that their detrimental effects may be delayed for many years. It is even possible that their effects will not become apparent before death from other causes occurs. Of course, not all cases of every disease are preventable. Diseases such as Alzheimer’s disease and rheumatoid arthritis cannot be prevented at all. However, for many age-related diseases, the use of disease prevention strategies before the disease begins often reduces the seriousness of its effects. For example, avoiding cigarette smoking reduces the effects of emphysema caused by other types of air pollution or by genetic factors.

    Usually, all a person needs to do is avoid the factors that increase the risk of developing abnormal changes and the diseases that cause many of them. For risk factors, such as air pollution, that cannot be completely avoided, reducing their intensity or the frequency of exposure can help. This can reduce or nearly eliminate the chances of developing certain abnormal changes.

    To be most effective, the avoidance of risk factors must begin early in life, but changing bad habits will probably help at any age. Even when a person begins to develop an abnormal change or disease, reducing risk factors can slow its progress so much that the change or disease may never become a significant problem. Some of the most important risk factors are smoking, stress, poor nutrition, inadequate exercise, and excessive exposure to harmful chemicals and sunlight. Others can be identified only by a medical checkup, including high blood pressure and high levels of cholesterol in the blood.

    Finally, there is good news for those who develop a disease. Many diseases, including serious ones such as certain types of cancer and dementia, can be cured. Many others, such as arthritis, can be treated so that they have a minimal impact on a person’s lifestyle. Early detection is important because it greatly increases the success achieved by treatment.


    This page titled 1.2: What is Aging? is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Augustine G. DiGiovanna via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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