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Chapter: Medical Physiology: Sports Physiology

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Body Heat in Exercise

Almost all the energy released by the body’s metabolism of nutrients is eventually converted into body heat.

Body Heat in Exercise

Almost all the energy released by the body’s metabolism of nutrients is eventually converted into body heat. This applies even to the energy that causes muscle con-traction for the following reasons: First, the maximal efficiency for conversion of nutrient energy into muscle work, even under the best of conditions, is only 20 to 25 per cent; the remainder of the nutrient energy is con-verted into heat during the course of the intracellular chemical reactions. Second, almost all the energy that does go into creating muscle work still becomes body heat because all but a small portion of this energy is used for (1) overcoming viscous resistance to the move-ment of the muscles and joints, (2) overcoming the friction of the blood flowing through the blood vessels, and (3) other, similar effects—all of which convert the muscle contractile energy into heat.

Now, recognizing that the oxygen consumption by the body can increase as much as 20-fold in the well-trained athlete and that the amount of heat liberated in the body is almost exactly proportional to the oxygen con-sumption, one quickly real-izes that tremendous amounts of heat are injected into the internal body tissues when performing endurance athletic events. Next, with a vast rate of heat flow into the body, on a very hot and humid day so that the sweat-ing mechanism cannot eliminate the heat, an intolera-ble and even lethal condition called heatstroke can easily develop in the athlete.

Heatstroke. During endurance athletics, even undernormal environmental conditions, the body tempera-ture often rises from its normal level of 98.6° to 102° or 103°F (37° to 40°C). With very hot and humid condi-tions or excess clothing, the body temperature can easily rise to 106° to 108°F (41° to 42°C). At this level, the ele-vated temperature itself becomes destructive to tissue cells, especially the brain cells. When this happens, mul-tiple symptoms begin to appear, including extreme weakness, exhaustion, headache, dizziness, nausea, profuse sweating, confusion, staggering gait, collapse, and unconsciousness.

This whole complex is called heatstroke, and failure to treat it immediately can lead to death. In fact, even though the person has stopped exercising, the tempera-ture does not easily decrease by itself. One of the reasons for this is that at these high temperatures, the temperature-regulating mechanism itself often fails. A second reason is that in heatstroke, the very high body temperature itself approximately doubles the rates of all intracellular chemical reactions, thus liberating still more heat.

The treatment of heatstroke is to reduce the body temperature as rapidly as possible. The most practical way to do this is to remove all clothing, maintain a spray of cool water on all surfaces of the body or continually sponge the body, and blow air over the body with a fan. Experiments have shown that this treatment can reduce the temperature either as rapidly or almost as rapidly as any other procedure, although some physicians prefer total immersion of the body in water containing a mush of crushed ice if available.


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