"The gradual changes in the structure and function of humans and animals that occur with the passage of time, that do not result from disease or other gross accidents, and that eventually lead to the increased probability of death as the person or animal grows older (Biology-Online, 2006)." The following paper will describe the aging process in humans from a biological systems perspective. In addition, changes that occur within the various systems of the body will be discussed in detail.
Integument comes from the word integumentum, meaning, "cover," or "enclosure." The human integumentary system is essential to life. Composed primarily of the skin, it protects, nourishes, insulates, and cushions. Accessory structures within this system include hair, nails, and certain exocrine glands. Accounting for approximately 7 percent of the body, the skin is the largest and heaviest organ of the body (Cohen & Taylor, 2005). Without it, bacteria would attack an individual immediately, or death from heat and dehydration would occur.
Chronological aging affects biochemical changes to skin. The supportive fibers of collagen and elastin break down. Skin does not retain as much moisture as it once did. The skin's ability to fight infection, feel sensations, and regulate body temperature also diminishes. Fine lines around the eyes, deepened expression lines at the corners of the mouth and across the forehead, and sagging skin are some visible signs of aging skin. In addition, hair may become coarser, gradually lose color, or begin to thin, and nails become more brittle with age.
Photoaging of the skin is far more damaging than age alone. Over the years, sun exposure causes fine and coarse wrinkles; baggy skin with a yellow, leathery appearance; and dry, scaly skin. Because sun exposure diminishes collagen, which supports a network of blood vessels, photoaging can cause skin to bruise more easily. Many medications for arthritis and other conditions common to aging can cause photosensitivity, and increase the aging of the skin.
Mucosal membranes in the body that assist in the prevention of infection by trapping organisms in secreted mucus and removing them by a process called ciliary transport. This transport system works like a tiny escalator conveying the trapped material toward a body opening such as the mouth.
Aging may compromise this barrier function, which commonly occurs in the mouth, urethra, and vagina. Additionally the cough mechanism decreases with age, further reducing the ability to eliminate organisms. Changes in the lung, such as the collapse of small airways and the overall loss of lung elasticity, also increase the risks of infection.
It is normal for taste buds that perceive sweetness and saltiness to diminish with aging. Tooth enamel thins, saliva production decreases, and incidences of periodontal disease increase as part of normal aging. In addition, esophageal peristalsis slows and sphincters in the digestive system are less effective, causing a delay of the entry of food into the stomach, increasing the likelihood of heartburn. Gastric emptying slows, causing food to remain in the stomach longer, compounded by peristalsis in the large intestine, subsequently incidence of constipation increase with aging.
Liver size decreases naturally with age, resulting in the decreased production of liver enzymes, causing a slowing of the metabolism and making it more difficult to detoxify the body. Aging also effects efficient functioning of the gallbladder, increasing the potential for gallstones. Detoxifying the body also occurs by way of the urinary tract. Changes occur within the urinary tract; increasing the risk of infection due to lower levels of prostatic fluid.
One of the primary functions of this system is to maintain homeostasis, or balancing mechanisms within the body. The adult-onset of diabetes mellitus is the result of decreased insulin secretion by the pancreas. Additional changes associated with aging linked to the endocrine system are loss of muscle and bone tissue. Another natural process of aging is the decrease of sex hormones for both men and women. Changes in the thyroid gland can decrease metabolic rate, resulting in unawareness to temperature, increasing the risk for heat stroke. Hormonal changes occur as well. Women experience menopause, the cessation of menstruation.
The uterus and ovaries decrease in size, as well as external genitalia. Vaginal dryness and loss of elasticity are common aging issues. Men experience lower levels of testosterone, shrinking of the testes, and erections are slower to develop. Sexual activity may decrease, however the reasons may be related to fatigue, weakness, reduced mobility, or pain rather than disease (Cohen & Taylor, 2005).
This system is comprised of 206 bones and more than 500 muscles that provide the framework for the body and allow for movement (Sorrentino, 2001). A membrane called the periosteum, which contains blood vessels that supply food and oxygen to bone cell, covers bones. Cartilage connects and cushions the joints, where two bones meet. Synovial fluid produced by the lining of the joints provides lubrication for smooth movement.
The process of bone formation and resorption, the breakdown of bone tissue, continues throughout life. As aging occurs, the bone renewal process slows, resulting in weaker more fragile bones (Cohen & Taylor, 2005). This results in slower healing from fractures and a gradual loss of height, strength, and mobility. Inactivity leads to loss of muscle tone, subsequently, aging results in fragile bone structure supported by weaker muscles.
This system has two main structural divisions, the central nervous system (CNS), and the peripheral nervous system. It is divided functionally to depict voluntary and involuntary controls within the body as well. The autonomic nervous system (ANS) refers to automatic functions of the body, e.g., heartbeat, blood pressure, intestinal secretions, and glandular secretions. The sense organs are vital to the communication of information to the nervous system. The five major senses are sight, hearing, taste, smell, and touch.
Aging affects changes in vision; the most commonly the lens of the eye loses elasticity, causing difficulty focusing on close objects. This is a condition called presbyopia, which literally means "old eye", which is easily corrected with eyeglasses. Other common age related changes in eyesight are loss of peripheral vision, decreased ability to judge depth, and decreased clarity of colors, such as pastels and blues.
Hearing and equilibrium are the functions of the ear. Essentially, the inner ear consists of semicircular canals and the cochlea. The canals contain fluid that transmits sound and contain receptors for equilibrium. The cochlea contains the receptors for hearing (Cohen & Taylor, 2005). Extended exposure to loud noises can damage the receptors in the ear. There is a slight loss of hearing acuity and a decreasing ability to distinguish sounds when there is background noise because of the aging process.
As one of the first systems to develop within the embryo, the nervous system undergoes changes beginning with maturity. The brain decreases in size and weight due to cell loss. Processing information slows due to the decrease of synapse and neurotransmitters. The loss of nerve cells in the brain and nervous system cause slower reactions, however memory loss is not part of the normal aging process (Carter, 2006). This progressive loss of brain cells however does affect mental function, increasing forgetfulness and confusion.
Several age related changes occur in the cardiovascular system. First, the heart muscle requires more time to relax between contractions. Second, the wall of the aorta is less flexible, creating more resistance during the contraction of the left ventricle. Third, the heart muscle is less responsive the stimulation of the pacemaker cells. Fourth, the amounts of elastin, cologne, and fat in the walls of the heart increase while the amount of muscle decreases (Schaie & Willis, 2002).
The alveoli in the lungs thicken, causing less effective oxygen exchange. Subsequently, reduced oxygenation of the blood occurs. Absent disease, there is a gradual change in the efficiency of the respiratory system due to aging combined with accumulated damage to the lungs from air pollution, respiratory infections, and smoking.
One of the major components of the immune system are T cells, a form of white blood cell. These cells are programmed to look for certain kinds of disease-causing pathogens, then destroy them and the cells infected by them. As we age, our T cell population becomes less effective at recognizing and fighting off infection, resulting in a higher level of susceptibility to disease. The body's innate response to infection is mounting a fever to kill cells causing illness. Often adults over age 65 who have serious bacterial infections do not have fevers.
The body at this age probably still has the ability to generate fevers ; however the central nervous system is simply less sensitive to immune signals and doesn't react as quickly or efficiently to infection (Sorrentino, 2001). As a result of aging the probability of disease increases.
To summarize aging, the cumulative changes overtime create deterioration to such a degree that one or more of the body's systems no longer functions. Death is a natural outcome of these system failures. Although longevity can be influenced by lifestyle, genetics, and science to varying degrees, the body with all its resilience cannot withstand the rigors of eternity. "First we ripen, and then we rot!" (Alexander Pope, 1733)
Biology Online (2006). Retrieved 08/05/2006, from http://www.biology-online.org/dictionary/Aging.
Carter, A. (2006, January). Health Changes With Aging. McKesson Health Solutions LLC, 2006 i1, NA. Retrieved 07/20/2006, from Clinical Reference Systems database (A146486678).
Cohen, B. C., & Taylor, J. T. (2005). The Integumentary System. In J. Goucher & D. Knighten (Eds.), Memmler's the Structure and Function of the Human Body (8th ed., pp. 72-80). Baltimore, MD: Lippincott Williams & Wilkins.
Schaie, K. W., & Willis, S. L. (2002). Biological Development. In (Ed.), Adult Development and Aging (5th ed.). Prentice-Hall. Retrieved July 14, 2006, from University of Phoenix Web Site: https://mycampus.phoenix.edu/esource
Sorrentino, S. A. (2001). Mosby's Essentials for Nursing Assistants (2nd ed.). St. Louis, MO: Mosby, Inc.