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insulation a Values are calculated for slabs 1 m2 in area and 1 cm thick, with a 1°C temperature difference between the two faces of the slab.

insulation a Values are calculated for slabs 1 m2 in area and 1 cm thick, with a 1°C temperature difference between the two faces of the slab.

which in the cold may include most of the limbs and the more superficial muscles of the neck and trunk—become cooler as they lose heat by conduction to cool overlying skin and, ultimately, to the environment. In this way, these underlying tissues, which in the heat were part of the body core, now become part of the shell. In addition to the organs in the trunk and head, the core includes a greater or lesser amount of more superficial tissue—mostly skeletal muscle— depending on the body's thermal state.

Because the shell lies between the core and the environment, all heat leaving the body core, except heat lost through the respiratory tract, must pass through the shell before being given up to the environment. Thus, the shell insulates the core from the environment. In a cool subject, the skin blood flow is low, so core-to-skin heat transfer is dominated by conduction,- the shell is also thicker, providing more insulation to the core, since heat flow by conduction varies inversely with the distance the heat must travel. Changes in skin blood flow, which directly affect core-to-skin heat transfer by convection, also indirectly affect core-to-skin heat transfer by conduction by changing the thickness of the shell. In a cool subject, the subcutaneous fat layer contributes to the insulation value of the shell because the fat layer increases the thickness of the shell and because fat has a conductivity about 0.4 times that of dermis or muscle (see Table 29.1). Thus, fat is a correspondingly better insulator. In a warm subject, however, the shell is relatively thin, and provides little insulation. Furthermore, a warm subject's skin blood flow is high, so heat flow from the core to the skin is dominated by convection. In these circumstances the subcutaneous fat layer, which affects conduction but not convection, has little effect on heat flow from the core to the skin.

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Essentials of Human Physiology

Essentials of Human Physiology

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