Hypothesis of aging
Several lines of research indicates that aging has a genetic basis. Researchers working with simple organisms ,such as yeast and round worms , have identified a host of genes ,whose expression decreases the life span .What do these genes have in common ? Apparently ,when these genes are inactive ,mitochondria do not produce energy .The current mitochondria hypothesis of aging has been supported by engineering mice that have a defective DNA polymerase .These mice aged much faster than their peers ,why? Possibly because their defective mitochondria produced more free radicals than usual .Free radicals are unstable molecules that carry an extra electron .To become stable ,free radicals donate an electron to another molecule ,such as DNA or proteins {like enzymes } or lipids ,found in plasma membranes. Eventually these molecules are unable to function ,and the cell is destroyed .The well known observation that a low calorie diet can expand the life span is consistent with the mitochondrial hypotheses of aging .Caloric restriction also shuts down the genes that decrease the life span .A decline in the hormonal system can affect many different organs of the body ,for example , diabetes type 2 is common in older individuals .The pancreas makes insulin ,but the cells lack the receptors that enable them to respond . Menopause in women occurs for a similar reason. The blood stream contains adequate amounts of follicle-stimulating hormone ,but the ovaries do not respond .Perhaps aging results from the loss of hormonal activities and a decline in the functions they control .
The immune system ,such as thymus gland gradually decreases in size and it replaced by fat and connective tissues .
It is possible ,that aging is not due to the failure of a particular system that can affect the body as a whole ,but to a specific type of tissue change that affect all organs and even the genes , for example , collagen fibers present in many support tissues become increasingly cross-linked as people age .Undoubtedly ,this cross-linked contributes to the stiffening and loss of elasticity characteristic of aging tendons and ligaments .Some researchers have now found that glucose has the tendency to attach to any types of proteins ,which is the first step in a cross linking process.
Extrinsic factors
The current data about the effects of aging are often based on comparisons of the elderly to younger age groups. For example , osteoporosis ,this condition is associated with a progressive decline in bone density in both males and females so that fractures are more likely to occur after only minimal trauma .The occurrence of osteoporosis itself is associated with cigarette smoking ,heavy alcohol intake ,and inadequate calcium intake .A moderate exercise program has been found to slow down the progressive loss of bone mass. Persons 65 years of age and older can have well-functioning hearts and open coronary arteries if their health habits are good and they continue to exercise regularly .Effect of age on he body systems
*skin :1-As aging occurs, skin becomes thinner and less elastic because the number of elastic fibers decreases and the collagen fibers undergo cross-linking ,also ,there is adipose tissue in he subcutaneous layer , therefore ,older people are more likely to feel cold .
2-The loss of thickness partially accounts for sagging and wrinkling of the skin .
3- Homeostatic adjustment to heat is also limited because there are fewer sweat glands for sweating to occur .
4- The number of oil (sebaceous ) gland is reduced ,and the skin tends to cracks .
5- Decrease the number of melanocytes ,making their hair gray and their skin pale .
6- Some of the remaining pigment cells are larger ,and pigmented blotches appear on the skin.
*Processing and Transporting
The heart shrinks because of a reduction in cardiac muscle cell size ,this lead to loss of cardiac strength and reduced cardiac output .The arteries become more rigid with time because the middle layer of arteries contains elastic fibers ,which are more likely subject to cross-linking .Also their size is further reduced by plaque (a built up of fatty materials ) ,therefore ,blood pressure rereading gradually rise.
Blood flow to the liver is reduced ,and this organ does not metabolize drugs as efficiently as before .
Cardiovascular problems are often accompanied by respiratory disorders, and vice versa .
Blood supply to the kidneys is also reduced ,the kidneys become smaller and less efficient at filtering wastes .Difficulties involving urination include incontinence (lack of bladder control) .In men , the prostate gland may enlarge and reduce the diameter of the urethra .
The loss of teeth (in elderly) is more apt to be the result of long term neglect than aging .
The digestive tract loses tone ,and secretion of saliva and gastric juice is reduced ,there is no indication of reduction absorption ,therefore ,an adequate diet ,rather than vitamin and mineral supplements ,is recommended .
*Integration and Coordination :
Recent studies show that few neural cells of the cerebral cortex are loss during the normal aging process ,this means that cognitive skills remain unchanged even though a loss in short term memory characteristically occurs.
Neurons are extremely sensitive to oxygen deficiency ,and if neuron death does occur ,it may be due not to aging itself but to reduced blood flow in narrowed blood vessels .
After age 50 years ,the ability to hear tones at higher frequencies decreases gradually ,and this can make it difficult to identify individual voices and to understand conversation in a group .
The lens of the eye does not accommodate as well also may develop a cataract and glaucoma.
Loss of skeletal muscle mass is not uncommon but it can be controlled by following a regular exercise program .
Decrease in the strength of the respiratory muscles and inflexibility of the rib cage contribute to the inability of the lungs to expand as before.
Weight gain occurs because the basal metabolism decreases and inactivity increase .Muscles mass is replaced by stored fat and retained water.
*The reproductive system :
Females undergo menopause ,and thereafter the level of female of sex hormones in the blood falls markedly .The uterus and the cervix decrease in size ,and the walls of the oviducts and the vagina become thinner.
The external genitals become less pronounced .
In male the level of androgen falls gradually over the ages span of 50-90 years , but sperm production continuous enter death .
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