temperature regulation
The human body is a heat-generating object. Even at complete rest at a comfortable temperature, the vital functions of the body generate heat. When it is at a minimum this is called basal metabolic heat. Heat production rises with activity: the more vigorous the activity the greater the quantity of heat produced. Excess heat has to be lost. Considering this variation in internal heat production, together with the wide variety of thermal environments inhabited by humans, it is remarkable that, except during exercise or fever, the body temperature can be kept within such narrow limits. This is achieved partly by the physiological mechanisms that control the rate of heat loss and heat production to achieve a balance. If the rate of heat loss exceeds the rate of heat production, the body temperature will fall, leading ultimately to hypothermia. Conversely, if the rate of heat loss is less than the rate of heat production, or is defected by external heating the body temperature will rise, leading eventually to heatstroke. Death may be the end result of either.
Body temperature is controlled through a central mechanism in the brain which, while it acts in a manner similar to a thermostat, is not a simple on/off device but is more akin to a ‘black box’ with a complex system of neurons cross-linking the sensory input and the effector output. The thermostat is activated by impulses from central receptors, which detect changes in the temperature of the blood, and from peripheral receptors mainly in the skin. The thermostat regulates the temperature of the body by adjusting heat production and heat loss, but the setting of the thermostat itself may be altered. During sleep the cerebral thermostat is reset to a new low level, skin blood vessels relax, causing an immediate rise in skin temperature and heat loss, and the metabolic rate is reduced.
There are racial variations in the response to heat and cold, and at the extremes of age there is an increased risk of hypothermia and heatstroke. Many medical disorders and a range of drugs predispose to temperature disorders.
Heat is lost, and gained, through convection, radiation, and conduction, while evaporation contributes only to heat loss. Even for humans, the standard thermodynamic laws of physics apply. The rate at which the body loses heat depends on the temperature difference between the skin and the environment. In the cold, therefore, the aim is to reduce the skin temperature as much as possible, whereas in the heat the skin temperature should be as high as possible.
Conductive heat transfer depends on the surface area in contact with a substance, the temperature of the substance, and its thermal conductivity. Air is a poor conductor of heat, and therefore most insulating systems, including clothing, act by trapping air. Conductive transfer is high in water. Wet clothing not only has decreased insulating properties, but it rapidly conducts heat from the body to the colder surface of the clothing. In an emergency in the cold, conductive heat loss may be forgotten, with the casualty being well covered with blankets but continuing to lie with nothing between him and the ground.
Convective heat loss is increased by wind, hence the ‘wind chill’ index, which gives the total heat loss under any particular combination of cold and wind speed, and relates it to the still air temperature which would produce the same rate of heat loss. The rate of heat loss at -10°C in still air is almost exactly the same as that at +10°C with a 10 mph wind. Convective heat loss is also increased by movement, not only by the person physically moving forward but also through the ‘pendulum’ movement of the limbs, and through the clothing producing a bellows effect and blowing warm air out to be replaced by cold air, or water, at the next movement. Increasing convective loss is dangerous in the cold but beneficial in the heat.
Radiant heat exchange is often overlooked. Standing near a glacier causes a feeling of chill even though there is no contact and no wind. Similarly, people may gain heat radiating from hot walls, concrete, or sand in a hot environment, as well as from fires or central heating radiators in the cold.
Evaporation is an effective way of losing heat because of the high latent heat of evaporation. However, the rate of evaporation depends on the surface area of fluid and its vapour pressure immediately above the surface. Evaporative heat loss is greatest when the air is dry and the moisture vapour is constantly being removed from the body surface, and least when there is high humidity and no air movement. Any factor which increases convective heat loss also increases evaporative heat loss. In man evaporative heat loss occurs through sweating and when clothing is wet. Wet clothing is used to reduce the temperature in heatstroke.
In animals with lungs, heat is also lost by warming and humidifying the inspired air during breathing, with the greater proportion being through humidification. Even in a mist in winter, with an ambient relative humidity of 100%, when the inspired air is warmed to core body temperature the relative humidity falls and the body expends heat in raising the relative humidity to 100% before it reaches the alveolar surfaces of the lung. Although some of the heat and moisture is retrieved from the air on its way out again, in temperate conditions 10-15% of the total heat loss from the body is from the respiratory tract, and the proportion increases in colder or drier air.
In the cold
The body responds by constriction of the superficial blood vessels, mainly via the sympathetic nervous system but also through direct action of cold on the blood vessels. Vasoconstriction is very effective in reducing heat loss by limiting blood flow to the periphery, thus increasing the depth of ‘shell’ insulation and reducing the temperature differential between the skin and the environment (see Figure). In fact vasoconstriction can result in the outermost inch of the body having a thermal conductivity equivalent to that of cork. To reduce the surface area exposed to cold the person may adopt a ball-like fetal position. There is also a counter-current exchange of heat between the arteries and veins in the distal half of the limbs. Below a limb temperature of 10-12°C the peripheral vasoconstriction fails and alternating vasodilatation and vasoconstriction occurs, though there may actually be very little increase in the volume of blood circulating in the skin during the vasodilatation, which would preserve the insulating effect of the vasoconstriction. Though there is some vasoconstriction in the face, it is minimal in the scalp, and this plus the lack of tissue depth makes the regulation of heat loss from the head inefficient; the rate of heat loss by this route increases in a linear manner between ambient temperatures of +32°C and -20°C, and indeed may be equal to half the total body heat production at rest at -4°C. Mental stress even of as mild a degree as mental arithmetic increases this heat loss, as also do nausea, vomiting, fainting, trauma, and haemorrhage.
Heat production rises with muscle activity, either deliberate activity (10-15-fold during hard physical exercise) or shivering. In severe cold stress catecholamine secretion increases and stimulates increased heat production. The less fit a person is, the less the degree of cold stress at which the catecholamine ‘overdrive’ occurs. Any increase in heat production is always accompanied by a rise in oxygen consumption: in shivering it may double or treble. Activity and shivering are not economical in thermoregulation, because they are accompanied by an increased blood supply to the muscles and this in turn raises the surface temperature and increases heat loss. In fact only 48% of the extra heat generated is retained in the body.
The greatest rate of heat production which an individual can achieve depends on his maximal ability for muscular work, linked to his maximal rate of oxygen usage. This in turn depends on the supply of oxygen and therefore on the greatest rate at which the heart can pump blood around the body, and on the efficiency with which the muscles can utilize the oxygen. These all increase with improving fitness. For any level of exercise the oxygen consumption and cardiac output need to be higher in a cold environment than in a warm one; this explains, for example, why angina may develop during a particular level of activity in the cold but not at normal temperatures. Also, during sleep in the cold, unfit people are repeatedly awakened by shivering, whereas the greater heat generating ability of ‘fit’ muscles allows sleep undisturbed by shivering.
In conditions of very severe cold, if the person has to undertake very vigorous exercise, the maximal oxygen uptake may be insufficient to provide for the high demand of both the exercise and the severe cold stress, and a person can develop unexpected and unsuspected hypothermia despite vigorous muscular activity. If hypoxia is also present, as at high altitude, this decreases the total possible oxygen uptake, and therefore heat production and shivering may be inhibited. Finally, if the person is exhausted or suffering from malnutrition he cannot increase heat production because of the lack of substrate (fuel) for metabolism.
Alcohol produces a number of effects which increase the risk of hypothermia; the greatest danger is decreasing the awareness of cold and increasing bravado, while impairing the ability to assess risks.
Left: In the cold: conservation of body heat by 'shell' insulation. Right: In the heat: increasing heat loss by warming the skin
In the heat
The skin and superficial blood vessels open up and the skin temperature rises, thus increasing the possibility of heat loss. Sweating increases and, in a dry, hot environment, evaporation can become the major means of losing heat. Unfortunately in a hot and humid environment there is less scope for evaporation, and sweating becomes less effective. The person tends to adopt an ‘open’ posture e.g. lying spread-eagled to maximize the surface area available. This is usually done for suntanning but also works for heat loss. By contrast with the cold, there is little scope for adjusting the production of heat except by reducing activity.
In fever the thermostat in the brain is set at a higher level. Therefore the person shivers to raise the temperature to the new, higher setting, and, when the fever breaks and the thermostat setting drops back to normal, the person sweats to dispose of the (now) excess heat.
Clothing and shelter
Clothes are the obvious protection from the cold, especially those that trap air. It is more efficient to have several thin layers than one thick. Cold weather clothing effectively reduces the temperature of the outermost surface and therefore reduces heat loss. A windproof layer stops or reduces convective loss. In the heat, clothing should be thin and loose to cause the least barrier to heat loss by conduction and convection. White or light-coloured clothing reduces the heat load by reflecting radiant heat.
Shelter may take many forms, from simply sheltering behind a boulder or digging a snow hole to the high technology of modern housing. In cold climates houses tend to have wooden floors with carpets, fires or central heating, attic insulation, and double glazing, and everything is done to reduce draughts. In a hot climate many houses have stone floors, the windows and doors are placed opposite each other to encourage through draughts, and the buildings are often painted white to reflect radiant heat from the sun.
Comfort
Humans are very sensitive to differences and changes in temperature, especially changes downwards. The sensation of cold is related to the lowered average skin temperature, and the sensation of local cold becomes more intense with increasing duration of exposure. A hot or cold stimulus is considered pleasant when it tends to restore temperature towards normal, and unpleasant if it has the reverse tendency. Thus cold applied to the skin is pleasant if the core temperature is raised but unpleasant if the core temperature is lowered. A person may feel warm on moving from the cold into the warmth, even if the body temperature is still low; this suggests that the discomfort caused by cold may be the result of the superficial vasoconstriction. Subjective discomfort is greater if certain areas, such as the forehead or feet, are cold, and the discomfort is increased by shivering. Because cold is a common human experience, the fact that it can cause injury, illness, and death is often overlooked.
Symptomless cooling
The central thermostat responds to the rate of firing of temperature-sensitive neurons, and the rate of firing is partially dependent on the rate of change of the stimulus. If the peripheral and central temperatures change slowly enough, the body may fail to activate the mechanisms for reducing heat loss and increasing heat production. During experimental cooling (in a water bath) it is possible to abolish shivering and the sensation of cold without interrupting the progressive reduction in body temperature, by slightly raising the temperature of the water, and therefore of the skin, at the onset of shivering; the perceived warmth is increased although the water temperature remains cold enough to cause continued cooling.
Perceived warmth has practical and clinical implications:
(i) Deep divers have hot water flowing over the surface of the body, which gives a sensation of warmth whilst it may not be enough to balance the large quantities of heat they lose through breathing cold oxyhelium. They may therefore become hypothermic while feeling warm.
(ii) Coming out of a refrigerated room at -30°C into a cold room at 0°C gives a feeling of warmth and comfort but the risk of hypothermia is still present, especially if the person is moving frequently between the two chambers.
(iii) For the elderly a single bar fire in a large cold room gives a sensation of warmth which prevents vasoconstriction and therefore allows an increased heat loss for which the heat from the small fire cannot compensate; so they can become hypothermic without being aware of discomfort. Inadequate heating may therefore be more dangerous than no heating.
It is salutary to realize that humans can live in and explore very hostile areas of the world, not because of the physiology of thermoregulation, but because of man's technological ability to manipulate his thermal environment by the use of clothing, heating, and shelter. The proof of this is the fact that the temperature of the skin of the abdominal wall is the same in the Arctic as in the tropics.
— Evan L. Lloyd
Bibliography
- Lloyd, E. L. (1986). Hypothermia and cold stress. Croom Helm, London.
- Maclean, D. and Emslie-Smith, D. (1977). Accidental hypothermia. Blackwell, Oxford
See also cold exposure; heat exposure; hypothermia; sweating.
