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regarded as senility. The modern World Health Organization definitions of life's stages are no less arbitrary but provide a basis for modern work. They are given below. 45–59 years Middle age 60–74 years Elderly 75–89 Old 90+ Very old

Although the lifespan of some animals appears to be determined by a single gene (e.g. certain strains of mice), it is more likely that multiple factors are involved in determining the lifespan of humans. Nonetheless, aging is the result both of intrinsic factors, which are in part genetically determined, and of environmental factors, which may reflect the employment or lifestyle of an individual. In practice, it is often difficult to separate these two influences. There have been many theories of aging some cellular, some relating to whole body changes. Examples include: accumulation of toxic metabolites; exhaustion of irreplaceable substances (e.g. loss of elastic tissue); medical book shop endocrine changes; accumulation of errors in genetic material, caused for example by free-radical damage, the switching on of lethal genes, or a reduction in immunological surveillance. All are hypothetical and it is possible that some, or all, may play a part in the progressive alterations of function that are characteristic of old age. Old age is associated with disease but does not cause it When considering age-related changes to anatomy, biochemistry, and physiology, several immediate and obvious questions arise. First, do specific changes happen to everyone? If so, is the rate of change the same in everyone? Undoubtedly, things happen at different times and to different extents in different people. Some people considered elderly are fitter than many of those less than half their age. Related to this is a second question: clearly an 80-year-old does not exhibit the same anatomical and physiological characteristics that he or she did at the age of 20, but are the changes those of aging or pathology? It is always difficult to differentiate between a degenerative change that leads to dysfunction and a change that is to be expected as a normal consequence of aging, but it is important to realize that many of the changes that accompany aging are normal and not pathological. They are simply the expected consequences of changes in physiology and anatomy that occur as a result of progression through life. Old age itself does not cause disease. All organ systems are affected in some way by the aging process A small selection of the changes associated with the aging process will be considered briefly here but, for fuller discussion, a specialist text on geriatric medicine should be consulted. Perhaps the most obvious changes at the whole-body level are those that involve appearance and gait. The skin loses its elasticity over time, giving rise to wrinkling. The skin also thins, increasing the risk of pressure sores in immobile individuals. Loss of teeth is still common, despite improvements in dental Medical Library

care, and results in restructuring of the mandible and changes to the facial appearance. There is loss of muscle power and mass, stiffening of the joints, and loss of motor nerve fibers. These changes result in the characteristic changes in gait and balance seen in the elderly. With advancing age, the gait tends to be slow, often on a widened base, and shuffling is common. The standing posture is less erect due to loss of muscle power and the ability to maintain posture is reduced, particularly when the eyes are closed. These are the natural consequences of degenerative changes to the vestibular apparatus, cerebellum, skeletal musculature, and proprioceptors. The skeleton shows significant changes during life. While osteoporosis is associated mainly with postmenopausal women, all aging individuals show changes to bone tissue. Gradually bone mass is lost as bone erodes without equivalent periosteal deposition. Furthermore, there is an age-related decrease in circulating levels of hydroxylated vitamin D3 (25-hydroxychole-calciferol), leading to impaired intestinal absorption of calcium and an increasing reliance on the skeletal mineral reserves for maintaining adequate plasma calcium levels. There is enlargement of the Haversian canals and the developing spaces fill with adipose or fibrous tissue. As a result, the bone weakens and becomes liable to fracture even under relatively light loads. This helps to explain the high proportion of elderly orthopedic patients.