Physiological Adaptations to Cold Stress in Humans
There are several different ways that humans adapt physiologically to acute and chronic cold and stress. Maintaining internal body temperature is essential for basic metabolic processes so these adaptations are necessary for survival (Molnar, 2006:246). Physiological responses to chronic cold stress include Bergman’s rule, Allen’s rule, and the hunter’s response. Hypothermia, frostbite, and Raynaud’s disease are responses to acute cold stress.
Chronic Cold Stress
In Arctic environments, sub-zero temperatures pose a significant challenge to inhabitants. Cold climates, lack of food, and difficult terrain all require inhabitants to develop unique biological adaptations if they want to survive. These individuals are exposed to chronic cold stress which is cold stress that is moderate and experienced for longer periods of time (Frisancho, 1993:81). Therefore, of these different adaptations, one of the most important is the conversation of heat. Bergmann’s rule states that latitude is related to body mass in animals. Allen’s rule states that endotherms from colder climates have shorter limbs. Both of these rules demonstrate that squat, short body shapes retain heat better (Molnar, 2006:249). This is evident in short, squat body shape of Arctic inhabitants like the Inuit. Another physiological response to chronic cold is the hunter’s response. This has also been observed among the Inuit and other Arctic fishermen. Hunter’s response is the tendency for capillaries in the extremities to open periodically when they would otherwise be constricted. This allows blood to flow which preserves the body’s core temperature (Frisancho, 1993:84).
Acute Cold Stress
There are also several physiological responses to acute cold stress. Acute cold stress is severe cold stress that is experience over short periods of time (Frisancho, 1993:81). Raynaud’s disease occurs when the blood vessels that supply blood to the skin have limited circulation. This is caused by a narrowing of the arteries. Fingers, toes, the tip of the nose, and ears will all feel numb due to the lack of blood flow. When a normal body temperature is regained, these areas will burn and tingle until normal blood flow is reached. Another response to acute cold stress is hypothermia. Moderate hypothermia occurs at body temperatures between 82 and 90 degrees Fahrenheit. This will lead to shivering that becomes more violent, apparent lack of muscle coordination, slow labored movements, and mild confusion. Blood vessels on the surface of the skin contract so the body can focus its remaining resources on the warmth of the vital organs. This causes the individual to become pale, and the lips, ears, fingers, and toes may start to become blue. As body temperature drops further, the individual will experience difficulty speaking and sluggish thinking that will eventually result in amnesia. Exposed skin will become blue and puffy, and muscle coordination will become even more difficult until the individual is no longer able to walk. At this point cellular metabolic processes shut down. Finally, frostbite is another response to acute cold stress. Before frostbite sets in, blood vessels dilate to keep the skin warm (Frisancho, 1993:87). However, this is only effective for a certain period of time. Frostbite occurs when soft tissue is damaged and skin freezes. Frozen tissues will usually blister. Untreated, the skin will become red and then purple. Blisters begin to fill with fluid within 36 hours. Over the next 10 days, the affected tissue will start to blacken.
All of these physiological responses allow humans to respond to acute and chronic cold stress. It is important to note that these different mechanisms work to conserve heat and also produce and dissipate heat. Conserving body heat will always be more economical than producing heat, but depending on the degree of cold stress experienced the body will do whatever it needs to maintain internal body temperature (Frisancho, 1993:87).
Frisancho AR. 1993. Human adaptation and accommodation. Ann Arbor, MI: University of Michigan Press.
Molnar S. 2006. Human variation: races, types, and ethnic groups. Upper Saddle River, NJ: Pearson Prentice Hall.