Physiological Adaptations to Environmental Stressors

Human biologists have long recognized the important role that environmental stressors play in shaping variation in human physiology and morphology. Among human populations around the world, body size and proportions are strongly influenced by differences in environmental temperature. Human populations living in cold, arctic climates are relatively heavy with large trunks and shortened limb lengths. Conversely, tropical populations are lighter and typically have more linear physiques. These patterns of variation are consistent with the classic "ecological rules" of Bergmann and Allen. Physiological adaptations to temperature stress involve strategies that regulate heat loss and production. Acclimation to cold stress involves increasing the efficiency of metabolic heat production and of heat delivery to peripheral tissues through earlier vasodilation. In contrast, acclimation to heat stress involves enhancing the capacity to dissipate heat production through greater vasodilation of peripheral blood vessels and increased sweat rates. Adaptations to high altitude hypoxia involve increasing the uptake and delivery of oxygen to peripheral tissues. Sea-level visitors to altitude respond by increasing the rate and depth of breathing and markedly increasing the oxygen carrying capacity of the blood. In contrast, native high altitude populations show a different set of responses including enlarged lung volumes and a modest increase in oxygen carrying capacity of the blood. The adaptations acquired during growth and development at altitude are critical for promoting complete functional adaptation to hypoxic stress.