Human Physiological Adaptability Through the Life Sequence

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Abstract
Changes in organ function from birth to old age were investigated from published data. In essence, the observations of Shock and colleagues on maximal breathing capacity, vital capacity, glomerular filtration rate, renal plasma flow, basal metabolic rate (BMR), ulnar nerve conduction velocity, cardiac index, and intracellular water (ICW) for the age period 30 to 85 years were extended to the time of birth. There were two principal findings. First, with the exception of BMR and ICW, these functional indices increased from a low point at birth to a maximum between 3 and 20 years of age and then after 30 years declined progressively to 85 years; BMR declined from a maximum in infancy and ICW peaked at about 30 years and then declined. Second, the coefficient of variation increased regularly from birth to old age. Because the peak of organ function coincided with the period of minimal mortality, the curve of organ function across the life sequence was considered to represent changes in adaptive capacity. Supporting evidence from studies of physical fitness, thermoregulation, and seasonal change of mortality was considered. It was concluded that these shifts in adaptive capacity between birth and old age were fundamental baselines for investigations of the epidemiology of health.

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