Hypothermia and Frostbite

Temperature Control in Mammals

Mammals are said to be homeothermic because they regulate their body temperature at a set point, although poikilothermic animals (so-called cold blooded animals) also attempt to regulate their body temperatures at a set point as well. The principle difference being that homeothermic organisms regulate their temperature by the generation of heat through metabolic processes (cooling is also accomplished through metabolic processes, but to a lesser extent since most homeotherms live in an evironment that is cooler than body temperature) while poikilotherms rely on behavior to regulate their body temperature (e.g. basking in the sun to warm up and seeking shade to cool down).

Human body temperature is 37C but this number is really just an average temperature for a healthy individual who is not sleeping. Normal temperature varies from 35.8C to 37.8 with activity and sleep. Body temperature is normally lowest in the early morning, before awakening; Napoleon's "4 o'clock in the morning courage" is aptly named, as mountaineers who have to make an "alpine start" are all too aware. During sustained and vigorous exercise, especially in hot, humid weather, body temperatures can easily rise to 40C, where heat stroke becomes likely.

One of the original Fermi problems actually looked at the power required to maintain a human at 37C. Fermi's reasoning went something like the following:

He was pretty well on the mark, too (as he usually was). An adult human consumes about 2000 kcal of food/day, corresponding to about 100 watts. Extreme activity might support about 600 watts for a short time, but only in a very fit individual. Room temperature is the temperature that a fully clothed person will not have to expend excess energy to maintain their body temperature. A nude human requires somewhat warmer temperatures, around 30C.

Most heat loss occurs through the skin, with perhaps 5% occuring through the lungs, except during aerobic exercise, where there is more heat loss from the lungs. Heat is moved around the body in the blood, and the subcutaneous blood flow can be regulated over a range of about 100 times, allowing heat loss to be controlled through vasoconstriction and vasodilation. Heat loss is also facilitated by evaporation of water from the surface of the skin (sweating). Heat production comes from the work done by muscles and nerves, and from chemical reactions in the liver.

Thermoregulation is accomplished in the hypothalamus, although there are also subcutaneous thermoregulatory components that work in denervated tissue. The posterior part of the hypothalamus controls heat retention and production when the organism is placed in a cold environment and the anterior region controls heat loss when in a hot environment. These systems operate independently of each other.

Humans have a limited capacity to adapt to extreme temperatures. Those who live at very cold temperatures are able to function more readily in the cold than those who are recently exposed. Similarly, people who live in very hot regions are able to function better in the heat than others. Cold or hot adaptation seems to impair function at the opposite extreme but the mechanisms are not well understood.


Hypothermia is the technical name for the condition in which the core temperature of a homeothermic organism falls below its setpoint. We have seen that there is variation in the core temperature with activity but hypothermia refers to a situation in which the temperature drops below the normal range.


Frostbite, or accidental cryosurgery, refers to the condition that occurs when ice crystals form inside the organism. This is almost always confined to the outer extremities, the feet and hands, as well as the face. Frostbite often occurs in conjunction with hypothermia, as the constriction of the peripheral blood vessels and thickening of the blood contribute to the inability to keep the extremities warm. Constrictive clothing can also play a role if it cuts down the circulation. Smokers, and those who have an impaired circulatory system, are more prone to frostbite than healthy people.

An under-appreciated mechanism of frostbite results from the low melting point of alcohol and its high thermal conductivity. Residents of cold climates often leave their favourite beverage outdoors to save space in the refrigerator. Unfortunately, a quick slash can instantly freeze the lips, tongue, and other parts of the mouth. Fatalities have resulted from freezing of the esophagus.

A Cryobiologist gets even Colder: A personal encounter with frostbite.

Trenchfoot, Paddyfoot, and the Chilblains

Trenchfoot primarily refers to a condition suffered extensively by soldiers in World War I, where they developed frostbite-like injury to their feet from being constantly immersed in cold water in the trenches. Freezing was not involved in the injury.

Paddyfoot refers to a condition endemic to more recent military conflicts that were fought in locations where low-level flooding was common (such as rice paddies). It is not a cold injury but simply due to constant immersion for 48-72 hours.

The chilblains is a charmingly named condition that seems to crop up in older literature. It is a drying and reddening of the skin caused by exposure to cold, wet, and windy conditions. Lee's Priceless Recipies, published in 1899 gives a treatment: "Flexible colodion 4 drams, castor oil 4 drams, spirits of turpentine 4 drams. Use 3 times daily with camel's hair brush." Just above this entry, a cure for cancer is provided (boiled red oak bark mixed with sheep fat).

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Document last updated Feb. 22, 1999.
Copyright © 1999, Ken Muldrew.