Body Temperatures And Heat Transfer In The Body

The body is divided into a warm internal core and a cooler outer shell (Fig. 29.2). Because the temperature of the shell is strongly influenced by the environment, its temperature is not regulated within narrow limits as the internal body temperature is, even though thermoregulatory responses strongly affect the temperature of the shell, especially its outermost layer, the skin. The thickness of the shell depends on the environment and the body's need to conserve heat. In a warm environment, the shell may be less than 1 cm thick, but in a subject conserving heat in a cold environment, it may extend several centimeters below the skin.

Temperature Body Shell

k Distribution of temperatures in the body's core and shell. A, During exposure to cold. B, In a warm environment. Since the temperatures of the surface and the thickness of the shell depend on environmental temperature, the shell is thicker in the cold and thinner in the heat.

The internal body temperature that is regulated is the temperature of the vital organs inside the head and trunk, which, together with a variable amount of other tissue, comprise the warm internal core.

Heat is produced in all tissues of the body but is lost to the environment only from tissues in contact with the en-vironment—predominantly from the skin and, to a lesser degree, from the respiratory tract. We, therefore, need to consider heat transfer within the body, especially heat transfer (1) from major sites of heat production to the rest of the body, and (2) from the core to the skin. Heat is transported within the body by two means: conduction through the tissues and convection by the blood, a process in which flowing blood carries heat from warmer tissues to cooler tissues.

Heat flow by conduction varies directly with the thermal conductivity of the tissues, the change in temperature over the distance the heat travels, and the area (perpendicular to the direction of heat flow) through which the heat flows. It varies inversely with the distance the heat must travel. As Table 29.1 shows, the tissues are rather poor heat conductors.

Heat flow by convection depends on the rate of blood flow and the temperature difference between the tissue and the blood supplying the tissue. Because the vessels of the mi-crovasculature have thin walls and, collectively, a large total surface area, the blood comes to the temperature of the surrounding tissue before it reaches the capillaries. Changes in skin blood flow in a cool environment change the thickness of the shell. When skin blood flow is reduced in the cold, the affected skin becomes cooler, and the underlying tissues—

Material

Conductivity kcal/(s-m-°C)

Rate of Heat Flow* kcal/hr Watts

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Responses

  • walter
    Is the skin the tissue that transfers heat the easiest?
    3 years ago
  • flavio
    What is core body temperature?
    4 months ago

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