Info

Hard exercise

Emotion or moderate exercise "A few normal adults Many active children

Usual range of normal

Early morning Cold weather, etc.

Rectal

Hard exercise

Emotion or moderate exercise "A few normal adults Many active children

Usual range of normal

Early morning Cold weather, etc.

Figure 73-1

Estimated range of body "core" temperature in normal people. (Redrawn from DuBois EF: Fever. Springfield, IL: Charles C Thomas, 1948.)

tissues to the skin, where it is lost to the air and other surroundings. Therefore, the rate at which heat is lost is determined almost entirely by two factors: (1) how rapidly heat can be conducted from where it is produced in the body core to the skin and (2) how rapidly heat can then be transferred from the skin to the surroundings. Let us begin by discussing the system that insulates the core from the skin surface.

Insulator System of the Body

The skin, the subcutaneous tissues, and especially the fat of the subcutaneous tissues act together as a heat insulator for the body. The fat is important because it conducts heat only one third as readily as other tissues. When no blood is flowing from the heated internal organs to the skin, the insulating properties of the normal male body are about equal to three quarters the insulating properties of a usual suit of clothes. In women, this insulation is even better.

The insulation beneath the skin is an effective means of maintaining normal internal core temperature, even though it allows the temperature of the skin to approach the temperature of the surroundings.

Blood Flow to the Skin from the Body Core Provides Heat Transfer

Blood vessels are distributed profusely beneath the skin. Especially important is a continuous venous plexus that is supplied by inflow of blood from the skin capillaries, shown in Figure 73-2. In the most exposed areas of the body—the hands, feet, and ears—blood is also supplied to the plexus directly from the small arteries through highly muscular arteriovenous anastomoses.

The rate of blood flow into the skin venous plexus can vary tremendously—from barely above zero to as great as 30 per cent of the total cardiac output. A high rate of skin flow causes heat to be conducted from the core of the body to the skin with great efficiency, whereas reduction in the rate of skin flow can decrease the heat conduction from the core to very little.

Figure 73-3 shows quantitatively the effect of environmental air temperature on conductance of heat

Estimated range of body "core" temperature in normal people. (Redrawn from DuBois EF: Fever. Springfield, IL: Charles C Thomas, 1948.)

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