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 CHAPTER 1 - Introduction

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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. In a natural population, conscious human intervention does  not alter individuals genetically or in other ways, and human intervention does not establish or control the 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.

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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 the myriad of body characteristics measured during medical check-ups 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 (Fig. 1.9).

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

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The third step in negative feedback is making the necessary adjustment to restore the condition to a normal level. Many body systems contribute to this process. For example, when the body becomes chilled, the blood vessels in the skin become narrow to reduce the loss of heat. The muscles may then cause shivering as they contract and relax quickly and repeatedly to produce more heat to warm the body. Using the muscles and bones, the person may move to a warmer location or turn on a heater. These and other activities can restore normal body temperature before any cells are significantly affected. Thus, negative feedback goes beyond preventing change by doing two things; it not only limits the amount of change, but it also causes a change in the opposite direction. Homeostasis is maintained, and the person stays alive and well.

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Fig. 1.9

A segment showing small frequent deviations from homeostasis will be added to Figure 1.9.

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The opposite of negative feedback is positive feedback. Positive feedback uses the same three steps found in negative feedback. However, in the third step, positive feedback causes the change to increase in amount and, often, to occur faster. Thus, positive feedback is usually harmful because it upsets homeostasis. Some people experience harmful positive feedback when they do some enjoyable activity to excess (e.g., overeating, over exercising, consuming too much alcoholic beverage or mind-altering drugs, driving too fast). The body does have positive feedback responses that are beneficial as long as some mechanism stops the increasing change before it becomes detrimental. Examples include (a) the inflammatory response to injury, (b) the immune response to a foreign substance or a cancer cell, (c) the movement of  impulses along a nerve cell (i.e., action potentials), (d) blood clotting, (e) ovulation, (f) aspects of human sexual response, and (g) uterine contractions during birth. Possible undesirable consequences when one of these positive feedback systems is not checked include, respectively, (a) disabling pain, swelling, and scar formation, (b) itching, swelling, and low blood pressure from allergies, (c) brain malfunction, (d) blocked blood vessels leading to heart attack or stroke, (e) ovarian cysts, (f) sexual misconduct, and (g) premature births.

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Second, other biological age changes reduce the functioning of negative feedback systems and of beneficial positive 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 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, and there is a decline in the ability to regulate the numerous activities in the immune response. Third, the structures that should restore conditions to an acceptable level by negative feedback, and some of the structures causing beneficial change through positive feedback are less able to do so. For examples, 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, and as aging reduces the immune response, there is increased susceptibility to infection and to cancer.

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In summary, most biological aging allows more of the conditions in the body to stray from the acceptable range farther and more frequently and to stay beyond the normal range longer. 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 a many cells are no longer able to perform adequately, the person becomes ill and dies.

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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. In a broad sense, a person has a disease if that person’s body has any one of three characteristics; (1) conditions are in the acceptable range but the body 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 infection and 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.) This definition of disease in a person would include local and widespread (systemic) diseases as well as short term (acute) and recurring or long term (chronic) diseases. 

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.

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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, immune responses against infection dwindle, and healing or restoration after injury take longer. 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 difficult 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.

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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.
 

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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; this is called primary prevention.

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. Slowing the progress of a disease is called secondary prevention.

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. Taking such actions is called tertiary prevention. Early detection is important because it greatly increases the success achieved by treatment.
 

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We have discussed several broad reasons for studying aging in general and biological aging in particular. We will now examine more specific reasons for studying biological aging. One of the most important is being able to distinguish true age changes from changes that occur by chance or are caused by abuse, misuse, disuse, or disease. Individuals with this ability will be able to recognize  changes in the body that represent the beginning of an abnormal condition. Then effective steps can be taken to prevent or combat undesirable changes that are not inevitable results of aging. Effort will not be wasted worrying about or attempting to alter conditions resulting from aging. Having knowledge about biological aging also makes it easier to select appropriate preventive or corrective measures. Furthermore, if one knows the course of age changes, the effects and effectiveness of new treatments can be better evaluated. There will be less chance of confusing the effects of a treatment with the effects of aging. Knowing the timing and nature of age changes also provides some predictability. Better estimates of a person=s future biological or medical status can be made, and it is easier to predict the life expectancy of an individual or a group of people. Finally, many recent theories and much research about how one may slow, prevent, and reverse aging are now being published.
 

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Go to http://www.biologyofhumanaging.com/activit.htm#Life Expectancy:Assignment  for activities related to determining life expectancy for an individual.
 

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The BLSA started in 1958. At first it had only a few hundred male subjects, most of whom were at or beyond middle age. It now includes more than 700 volunteer subjects, both female and male, ranging in age from the twenties through age 90. Subjects receive a thorough evaluation, including numerous biological and psychological characteristics, every two to three years (http://www.grc.nia.nih.gov/branches/blsa/blsa.htm). Other longitudinal studies include the NCHS Longitudinal Studies of Aging  (http://www.cdc.gov/nchs/lsoa.htm), the NHLBI Framingham Heart Study (http://www.nhlbi.nih.gov/about/framingham), the Canadian Longitudinal Study on Aging (http://www.cihr-irsc.gc.ca/e/22982.html), and the English Longitudinal Study of Ageing (http://www.natcen.ac.uk/elsa/).
 

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Information about anti-aging, including recent theories and research findings on possible methods that can prevent, slow, or reverse aging will be added here.
 

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While any of these techniques can produce seemingly meaningful results, there is a lack of consensus about the validity of the procedures. Disagreements arise over which approach should be used. There is also the question of whether all the functions tested are equally important. If they are not, attempts to select the useful ones or to rank those that are used result in more discord. For example, should the ability to feel vibrations be included? What about clarity of eyesight? If these are included, is either of them more important than the resting heart rate? Or is maximum heart rate a better indicator of biological age? Perhaps there is not such thing as an overall biological age but only separate biological ages for the various parts of the body or different biological ages under different environmental conditions and lifestyles. For an assignment and information on biomarkers of aging, go to http://www.biologyofhumanaging.com/readwrt.htm#Biomarkers%20of . Other sites with relevant information include http://www.niapublications.org/pubs/microscope/chapter4.pdf
http://sageke.sciencemag.org/cgi/content/abstract/2005/26/nf48 
http://sageke.sciencemag.org/cgi/content/abstract/2004/44/pe40
http://www.anti-aging-guide.com/62biomakers.php#11  
http://www.hbhealthonline.com/biomarkers.html 
http://ncof.com/AAeJournal/Issue11/aa_ejournal_biomarkers.htm
http://www.healthandage.com/html/min/afar/content/other8.htm .
 

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Recent research findings genes that affect aging will be added here.
 

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For recent data on life expectancies in the US, go to the NCHS Life Expectancy web page (http://www.cdc.gov/nchs/fastats/lifexpec.htm) .
 

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Mean longevity in the United States has continued to rise since 1980. It reached 75.4 years by 1990 and is expected to reach 77.3 by AD. 2000. It will probably rise slowly but steadily well beyond the year 2030, reaching as high as age 80 by the year 2050. Most of this increase is due more to decreased death rates for those above age 35 than to changes in death rates among younger people. The reason is that so much progress has been made in improving the extrinsic conditions that affect younger people that few advances in this area can be expected. Intrinsic factors and chronic diseases, which come into play in the later years of life, now have a more predominant influence on ML because they have become the main causes of death. This situation is expected to continue as long as human activity does not cause additional deterioration of the environment or become more self-destructive.

For graphs and statistics on life expectancies in the US, go to http://www.biologyofhumanaging.com/Graphs/plan22.htm#graphs .
 

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For a student activity, information, and discussions concerning potential benefits and drawbacks from altering life expectancies, go to http://www.biologyofhumanaging.com/Graphs/plan22.htm  .
For a relevant Reading/Writing assignment, go to http://www.biologyofhumanaging.com/readwrt.htm#The%20Future%20and%20Gerontology  
 

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age was 82.2, and was 91.1 for those age 85 (Fig. 1.6).
 

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Examples of different perspectives on factors determining quality of life may be found in the following web pages.
http://en.wikipedia.org/wiki/Quality_of_life
http://www.cdc.gov/hrqol/methods.htm
http://www.healthypeople.gov/Document/html/uih/uih_bw/uih_2.htm
http://www.geriatrictimes.com/g010719.html

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As a student activity and discussion, ask each student to write a list of factors that make his or her quality of life good this time in life. Have the students read their lists in class or tabulate their lists and distribute them. Discuss similarities and differences among the lists. Then, have each student write a list of  that he or she believes would make his or her quality of life good at age 65 or at age 85.