Perhaps the most important observation regarding normal aging is how much does not change. Most hormone levels, liver enzymes, electrolytes, body temperature, and basal glucose remain constant throughout the lifespan. Long-term studies have indicated there is no age-related anemia, although the hematocrit declines slightly in men presumably because of the age-related decline in testosterone.

Physiology also is affected by the passage of time. More prolonged exposure results in the emergence of abnormalities and illnesses, such as skin cancer owing to cumulative sun exposure. Much of the lifetime exposure to sun is received before age 20 years, yet the resulting cancers follow decades later. Another example is polycystic kidney disease, which is an inheritable condition that does not appear until well into adulthood; the passage of time, not aging, is the key factor.

Some conditions become less likely with advancing age. Immune disorders, such as systemic lupus erythematosus and multiple sclerosis, rarely appear in late life, presumably because changes in the immune system lead to less aggressive autoimmune disease. Similarly, although many cancers are more common in the elderly, many of the most aggressive tumors occur at a young age, suggesting that immune tolerance develops with advancing age.

Physiologic aging can change the ways certain illnesses appear. The elderly individual with hyperthyroidism often does not present with the systemic findings of agitation, irritability, hyperactivity, hyperphagia, and increased bowel movements, but rather presents with apathy, anorexia, and atrial fibrillation. The underlying pathophysiology may be no different, but age-related changes in physiology influence the clinical presentation.

The most important physiologic change of aging is the predisposition to more disease and more severe disease. The lung function of a healthy 70-year-old is about 50% that of a 30-year-old. Renal function commonly declines by 50% or more by age 70. This lack of physiologic reserve capacity does not affect day-to-day function but can affect the ability to recover from an extreme illness.

Geriatric syndromes emerge from these age-related findings. The many systems that support the ability to maintain an upright posture are compromised by age, leading to postural sway; as a result, an older individual is much more likely to fall after a slip or a push. The consequences of that fall are more serious owing to the age-related loss of bone mass, which can predispose to fall-related fractures.

Some physiologic changes imitate illness when they may be a normal part of aging. Diabetes mellitus may "appear" and "disappear" in the elderly. The ability of insulin to stimulate glucose uptake declines with age and usually is manifested as postprandial hyperglycemia with a maintenance of normal fasting insulin and glucose levels. Under stress situations, older individuals can appear to be diabetic, but when the stressful situation is relieved, they no longer have chemical evidence of diabetes. Nevertheless, this loss of physiologic reserve understandably contributes to the increasing prevalence of diabetes with advancing age.

The age-related changes that make the elderly more vulnerable in daily life are more subtle. Older individuals are more likely to develop hypothermia or hyperthermia during extreme environmental exposure because of a variety of neurologic and thermoregulatory changes. The loss of brainstem neurotransmitters in older persons not only may cause a senile gait pattern, but also predisposes to genetically determined conditions such as Parkinson's disease.

Some age-related changes cause specific medical sequelae. Menopause is a normal aging process that produces symptoms and predisposes to future bone loss and atherosclerosis. Another example is senile cataracts, which are caused by post-translational modifications in lens proteins combined with the inability of the lens to dispose of breakdown products; as a result, the lens becomes stiffer, thicker, and more opaque with age.