Physiology of Aging

Aging is accompanied by several gradual, expected, and progressive changes in all organs and systems. Every individual undergoes physiological changes at the molecular, organ, and functional levels over time. Peak function in most organs occurs in the third and fourth decades of life followed by a gradual decline. Age-related manifestations are subtle, largely asymptomatic, and seldom affect daily function. However, age-related physiological changes result in a decline in homeostatic reserves. During periods of increased demand or stress, typically induced by illness, the reserves in older people may be inadequate to meet the demands in a given situation. Physiological changes in most organs are expected, over time, and need to be distinguished from pathological changes that result from disease. In most situations, manifestations or symptoms indicate the presence of disease, occult or apparent, in older people. Chronological and biological aging do not necessarily match, as aging is highly variable and individualized. Life expectancy at birth and at age 65 years has increased worldwide, but dropped slightly in US females, who consistently have a longer life expectancy than males. The concepts of maximal life span and exceptional longevity are discussed. Successful aging is apparent in several areas of the world (Blue zones) and may be the result of several factors including genetics, healthy lifestyle, prevention of disease, and management of illness, besides environmental influences. Calorie restriction and physical activity have been consistently correlated with better health and longevity. Anorexia and frailty are common in the very old and are discussed. The chapter includes age-related physiological aspects followed by clinical relevance for every system.