Since the late s, numerous research studies have examined the responses of various ethnic groups that either inhabit cold climates or work in cold water. The goal of these studies has been to identify how natives of Peru, Australia, Korea, Tierra del Fuego, and the Arctic Circle maintain body temperature during daily cold exposures. Some of these natives live in nomadic bands or live in subsistence economies with limited access to modern technology. Thus, depending on their nutrition, shelters, and clothing, they encounter different levels of whole-body and extremity cold stress. For example, Inuits experience moderate whole-body cold stress because their clothing is so effectively insulated, with severe extremity chilling.
Australian Aborigines, in contrast, experience a prolonged moderate whole-body cold stress by sleeping in cool desert night air -C, -F. These varied environmental conditions result in different adaptive responses. Inuits exhibit a high metabolic heat production and high levels of blood flow to the extremities, resulting in warm hands and feet. The Aborigines oppose cold stress by constricting skin blood vessels increasing peripheral insulation and by tolerating mild hypothermia -C, -F without shivering-induced or nonshivering thermogenesis.
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Young’s Model of Acclimatization
More recent investigations have identified five strategies the human body employs to adapt to cold. These strategies differ considerably. Tabledifferentiates the various responses that occur in each strategy. It should be obvious to you that the responses to cold exposure are not as consistent as those that occur during heat acclimatization see tableon pageAnd, it is reasonable to wonder why so many different physiological strategies exist. One answer, for both cold-air and cold-water immersion, is that each adaptive pattern depends on the intensity of cold, the daily duration of the exposure, and the length of the acclimatization period. Another answer involves different scientific interpretations of data by investigators. As an example of this, physiologist Andrew Y oung has designed a theoretical scheme to explain the development of different patterns of cold adaptation that recognizes only three adaptive states: cold habituation, metabolic acclimatization, and insulative acclimatization. His paradigm appears in figure pageHabituation is a desensitization or dampening of the normal response to a stressor. A classic example of cold habituation was observed in fishermen who immersed their bare hands in cold water for many hours each day. Their skin vasoconstriction response was blunted locally by the central nervous system to allow increased cutaneous blood flow and a warmer skin temperature during their daily labor. You may remember experiencing cold habituation if you have immersed your hand or body in very cold water. After a few minutes, you probably noticed that the pain and discomfort subsided. This response involved habituation of feeling from your pain- sensitive neurons in or near the skin afferent information going to the brain. Metabolic acclimatization refers to changes in metabolism, either in the type of nutrient used for heat production or the metabolic pathway. Insulative acclimatization refers to changes in either skin blood flow or subcutaneous fat.
Physiological Responses Observed Subsequent to the Five Types of Human Cold Adaptation. The Titles of Each Column Arise from Various Studies and Authors,
Flowchart illustrating a theoretical scheme to determine which type of human cold acclimatization/acclimation occurs.
In fact, Bittel interpreted his data with physical characteristics in mind, and found that physical fitness and percent body fat both influenced the type of adaptation that occurred. However, this was true only in those subjects who exhibited extreme responses. Third, it is theoretically possible that the different patterns of adaptation to cold exposure metabolic, hypothermic, habituative, insulative are actually progressive stages in the development of complete cold acclimatization. This concept arose from observations of scuba divers in cold water, who initially responded by shivering but eventually lost this response and developed insulative adaptation.