Heat is an important byproduct of the many chemical activities constantly occurring in body tissues. At the same time, heat is always being lost through a variety of outlets. Under normal conditions, a number of regulatory devices keep body temperature constant within quite narrow limits. Maintenance of a constant temperature despite both internal and external influences is one phase of homeostasis, the tendency of all body processes to maintain a normal state despite forces that tend to alter them.
Heat is a byproduct of the cellular oxidations that generate energy. The amount of heat produced by a given organ varies with the kind of tissue and its activity. While at rest, muscles may produce as little as 25% of total body heat, but when muscles contract, heat production is greatly multiplied, owing to the increase in metabolic rate. Under basal conditions (at rest), the liver and other abdominal organs produce about 50% of total body heat. The brain produces only 15% of body heat at rest, and an increase in nervous tissue activity produces little increase in heat production. Although it would seem from this description that some parts of the body would tend to become much warmer than others, the circulating blood distributes the heat fairly evenly.
Factors Affecting Heat Production The rate at which heat is produced is affected by a number of factors, including exercise, hormone production, food intake, and age. Hormones, such as thyroxine from the thyroid gland and epinephrine (adrenaline) from the adrenal medulla, increase the rate of heat production. The intake of food is also accompanied by increased heat production. The nutrients that enter the blood after digestion are available for increased cellular metabolism. In addition, the glands and muscles of the digestive system generate heat as they set to work. These responses do not account for all the increase, however, nor do they account for the much greater increase in metabolism after a meal containing a large amount of protein. Although the reasons are not entirely clear, the intake of food definitely increases metabolism and thus adds to heat production.
More than 80% of heat loss occurs through the skin. The remaining 15% to 20% is dissipated by the respiratory system and with the urine and feces. Networks of blood vessels in the skin’s dermis (deeper part) can bring considerable quantities of blood near the surface, so that heat can be dissipated to the outside. This release can occur in several ways.
* Heat can be transferred directly to the surrounding air by means conduction.
* Heat also travels from its source as heat waves or rays, a process termed radiation.
* If the air is moving, so that the layer of heated air next to the body is constantly being carried away and replaced with cooler air (as by an electric fan), the process is known as convection.
* Finally, heat may be lost by evaporation, the process by which liquid changes to the vapor state.
To illustrate evaporation, rub some alcohol on your skin; it evaporates rapidly, using so much heat from the skin that your arm feels cold. Perspiration does the same thing, although not as quickly. The rate of heat loss through evaporation depends on the humidity of the surrounding air. When it exceeds 60% or so, perspiration
does not evaporate so readily, making one feel generally miserable unless some other means of heat loss is available, such as convection caused by a fan.
Prevention of Heat Loss Factors that play a part in heat loss through the skin include the volume of tissue compared with the amount of skin surface. A child loses heat more rapidly than does an adult. Such parts as fingers and toes are affected most by exposure to cold because they have a great amount of skin compared with total tissue volume. If the temperature of the surrounding air is lower than that of the body, excessive heat loss is prevented by both natural and artificial means. Clothing checks heat loss by trapping “dead air” in both its material and its layers. This noncirculating air is a good insulator. An effective natural insulation against cold is the layer of fat under the skin. Even when skin temperature is low, this fatty tissue prevents the deeper tissues from losing much heat. On the average, this layer is slightly thicker in females than in males. Naturally, there are individual variations, but as a rule, the degree of insulation depends on the thickness of this subcutaneous fat layer.
Given that body temperature remains almost constant despite wide variations in the rate of heat production or loss, there must be internal mechanisms for regulating temperature.
The Role of the Hypothalamus Many areas of the body take part in heat regulation, but the most important center is the hypothalamus, the area of the brain located just above the pituitary gland. Some of the cells in the hypothalamus control heat production in body tissues, whereas another group of cells controls heat loss. Regulation is based on the temperature of the blood circulating through the brain and also on input from temperature receptors in the skin. If these two factors indicate that too much heat is being lost, impulses are sent quickly from the hypothalamus to the autonomic (involuntary) nervous system, which in turn causes constriction of the skin blood vessels to reduce heat loss. Other impulses are sent to the muscles to cause shivering, a rhythmic contraction of many muscles, which results in increased heat production. Furthermore, the output of epinephrine may be increased if necessary. Epinephrine increases cell metabolism for a short period, and this in turn increases heat production.
If there is danger of overheating, the hypothalamus stimulates the sweat glands to increase their activity. Impulses from the hypothalamus also cause blood vessels in the skin to dilate, so that increased blood flow to the skin will result in greater heat loss. The hypothalamus may also promote muscle relaxation to minimize heat production. Muscles are especially important in temperature regulation because variations in the activity of these large tissue masses can readily increase or decrease heat generation. Because muscles form roughly one-third of the body, either an involuntary or an intentional increase in their activity can form enough heat to offset a considerable decrease in the temperature of the environment.
Age Factors Very young and very old people are limited in their ability to regulate body temperature when exposed to environmental extremes. A newborn infant’s body temperature decreases if the infant is exposed to a cool environment for a long period. Elderly people also are not able to produce enough heat to maintain body temperature in a cool environment. With regard to overheating in these age groups, heat loss mechanisms are not fully developed in the newborn. The elderly do not lose as much heat from their skin as do younger people. Both groups should be protected from extreme temperatures.
Normal Body Temperature The normal temperature range obtained by either a mercury or an electronic thermometer may extend from 36.2°C to 37.6°C (97°F to 100°F). Body temperature varies with the time of day. Usually, it is lowest in the early morning because the muscles have been relaxed and no food has been taken in for several hours. Temperature tends to be higher in the late afternoon and evening because of physical activity and consumption of food. Normal temperature also varies in different parts of the body. Skin temperature obtained in the axilla (armpit) is lower than mouth temperature, and mouth temperature is a degree or so lower than rectal temperature. It is believed that, if it were possible to place a thermometer inside the liver, it would register a degree or more higher than rectal temperature. The temperature within a muscle might be even higher during activity. Although the Fahrenheit scale is used in the United States, in most parts of the world, temperature is measured with the Celsius thermometer. On this scale, the ice point is at 0° and the normal boiling point of water is at 100°, the interval between these two points being divided into 100 equal units. The Celsius scale is also called the centigrade scale (think of 100 cents in a dollar).
Fever is a condition in which the body temperature is higher than normal. An individual with a fever is described as febrile. Usually, the presence of fever is due to an infection, but there can be many other causes, such as malignancies, brain injuries, toxic reactions, reactions to vaccines, and diseases involving the central nervous system (CNS). Sometimes, emotional upsets can bring on a fever. Whatever the cause, the effect is to reset the body’s thermostat in the hypothalamus. Curiously enough, fever usually is preceded by a chill-that is, a violent attack of shivering and a sensation of cold that blankets and heating pads seem unable to relieve. As a result of these reactions, heat is generated and stored, and when the chill subsides, the body temperature is elevated. The old adage that a fever should be starved is completely wrong. During a fever, there is an increase in metabolism that is usually proportional to the degree of fever. The body uses available sugars and fats, and there is an increase in the use of protein. During the first week or so of a fever, there is definite evidence of protein destruction, so a high-calorie diet with plenty of protein is recommended. When a fever ends, sometimes the drop in temperature to normal occurs very rapidly. This sudden fall in temperature is called the crisis, and it is usually accompanied by symptoms indicating rapid heat loss: profuse perspiration, muscular relaxation, and dilation of blood vessels in the skin. A gradual drop in temperature, in contrast, is known as lysis. A drug that reduces fever is described as antipyretic. The mechanism of fever production is not completely understood, but we might think of the hypothalamus as a thermostat that is set higher during fever than normally. This change in the heat-regulating mechanism often follows the injection of a foreign protein or the entrance into the bloodstream of bacteria or their toxins. Substances that produce fever are called pyrogens. Up to a point, fever may be beneficial because it steps up phagocytosis (the process by which white blood cells destroy bacteria and other foreign material), inhibits the growth of certain organisms, and increases cellular metabolism, which may help recovery from disease.