Skin and Temperature Control
The normal oral temperature is 37°C (98.6°F), which is .5 degrees (32.9°F) lower than the rectal tempera-ture representative of core body temperature. Temperatures vary at different parts of the body. In gen-eral, the extremities are cooler than the rest of the body. Body temperature must be maintained within a narrow range despite wide temperature fluctuations in the environment. The rate of chemical reactions varies with temperature and enzymes function only properly within a narrow temperature range.
The major processes by which heat is lost from the body are conduction and radiation (70%), sweat va-porization (27%), respiration (2%), and urination and defecation (1%). Conduction is the heat ex-change between two objects in contact with each other. The amount of heat lost in this way depends on the temperature difference between the objects. Con-duction is helped by convection. Convection is the movement of molecules away from the area of con-tact. For example, if the air is cool and it comes in contact with warm skin, the air around the body is warmed; this warm air rises and fresh cool air reaches the skin. Heat can be lost by convection whether the object moves through the medium (e.g., swimming in cold water) or the medium moves over object (e.g., a cool breeze moving over the skin). Ra-diation is transfer of heat by high frequency wavesfrom one object of a higher temperature to another.
It is because of radiation that a person can feel cold in a warm room with cold walls.
Because heat is conducted from an object’s surface to the surrounding environment, the amount of body heat lost is largely determined by skin temperature. The temperature of the skin, in turn, depends on the amount of blood that reaches the skin from the skin’s deeper layers. Body temperature can be controlled by altering the amount of warm blood reaching the skin. Hair traps some of the heat lost from the skin to the air. When the outside environment is cold, the smooth muscles attached to the individual hairs con-tract and make the hairs stand on end, trapping a layer of air between the hairs. This layer slows down the loss of heat. In man, clothes supplement the layer of hair. Therefore, the amount of heat lost across the clothing depends on the texture and thickness of the clothing. Dark clothing absorbs radiated heat, while light clothing reflects heat.
Transfer of heat causes another mechanism—the evaporation of sweat. Vaporization of 1 gram of wa-ter removes approximately 0.6 kcal of heat. During heavy exercise in a hot environment, sweat secretion may be as high as 1,600 mL/hour. Heat loss by va-porization can then be as high as 900 kcal/hour. The rate of vaporization depends on the humidity of the environment and the movement of air around the body.
The body’s adjustment to the changing environ-mental temperature is largely controlled by the hypo-thalamus and is a result of autonomic, somatic, en-docrine, and behavioral changes. Local reflex responses also contribute. For example, when cuta-neous blood vessels are cooled, they become more sensitive to circulating catecholamines (e.g., epineph-rine) and the arterioles and venules constrict. Other adjustments include shivering, hunger, increased vol-untary activity, increased secretion of norepinephrine and epinephrine, and hair “standing on end.” When hot, cutaneous vasodilation, sweating, increased res-piration, anorexia, apathy, and inertia (to decrease heat production), are some of the adjustments.
The signals that activate the hypothalamus come from temperature-sensitive cells in the hypothalamus and cutaneous temperature receptors.