The process of acclimatization allows for humans to physiologically adapt to a wide range of environmental stressors over the course of their lifetime. These adaptive responses maintain the body's normal biological function (homeostasis) in the face of ecological challenges. Acclimatization to cold stress involves increasing the efficiency of metabolic heat production and more effectively delivering warm blood to the extremities of the body. Indigenous Arctic populations show enhanced metabolic responses to cold. These populations have elevated basal metabolic rates (BMR) and maintain higher peripheral temperatures in response to cold challenges. In contrast, acclimatization to tropical environments involves enhancing the capacity to dissipate metabolic heat. Indigenous tropical populations maintain thermal homeostasis with much lower sweat rates than their nonindigenous counterparts. Their lower body weights and more linear physiques promote greater heat loss through passive, nonsweating mechanisms. Tropical populations also show reduced BMRs. Acclimatization to high-altitude hypoxia involves increasing the uptake and delivery of oxygen to the body. Native high-altitude populations have enlarged chests and lungs, and greater ability to transfer oxygen from the lungs into the bloodstream. Additionally, they have increased hemoglobin levels, allowing for greater oxygen-carrying capacity in the bloodstream.