investigating life
How can we increase heat loss from the body to protect against heat stress?
In the opening story to this chapter you saw that body temperature is a critical factor in limiting physical performance. In Key Concept 39.4 you learned that body surface temperature is a critical factor in each path of heat loss. The relevant body surface temperature for furred mammals is mostly the surface of the fur. For us it is mostly the surface of our clothing. We can take off our clothes, but furred mammals cannot take off their fur. Mammals have evolved special blood vessel adaptations in their non-hairy skin that can accommodate large volumes of blood and therefore support high levels of heat loss. The experiment in Investigating Life: Can the Work Capacity of Muscle Be Increased by Extracting Heat from the Palms of the Hands? used a technology to extract large amounts of heat from the palm of one hand and showed a resulting improvement in ability to do muscular work. How does this heat-loss adaptation, and the technology designed to amplify it, work?
Blood flows from the heart through large arteries, then through small arteries, and then through the tiny capillaries that nourish the tissues and carry away waste products of metabolism, and then into veins that carry the blood back to the heart. The circulatory adaptations of the “heat portals” in non-hairy mammalian skin are an exception to this pattern. In the non-hairy skin, gated shunts deliver arterial blood directly to veins, bypassing the capillaries. These veins in non-hairy skin form networks that can accommodate a large volume of blood when the shunts are open. Based on this knowledge, biologists at Stanford University developed the “rapid-cooling” technology used in the experiment in Investigating Life: Can the Work Capacity of Muscle Be Increased by Extracting Heat from the Palms of the Hands? The palm of the hand (non-hairy skin) is placed in contact with a cooled surface and a mild vacuum is used to pull more blood into the large, heat-exchanging blood vessels. When this device is used, body temperature rises more slowly during exercise and cools more rapidly during rest after exercise.
As expected, this rapid cooling technology greatly increased endurance for exercise in the heat, but an unexpected discovery was enhanced athletic performance. Because muscle fatigue is partly due to increased muscle temperature, enhanced cooling reduces fatigue and increases exercise capacity, which in turn can lead to conditioning gains. In one study, first-year college students in a conditioning program doubled their rate of physical conditioning when they used the cooling technology. After 6 weeks of training, some men and women in the study achieved more than 900 push-ups or hundreds of pull-ups in a 45-minute workout session.
In this chapter you learned that virtually all biological processes are affected by temperature. Therefore, if we have a new technology that can rapidly and efficiently alter the heat content of the body, we should find many applications for it. What could some of those applications be besides improving athletic performance? Consider a few medical applications. Active thermoregulatory responses such as vasoconstriction, shivering, and sweating depend on control by the nervous system, but all of these mechanisms are inactivated by anesthesia and patients can become seriously hypothermic. Their body temperatures could be stabilized by heat exchange through their natural heat portals. Some medical conditions render patients terribly temperature-sensitive. An example is multiple sclerosis. Individuals with MS can suffer serious increases in their symptoms if their body temperature rises even a little bit. They may lose vision, balance, the ability to walk, or the ability to think clearly. Could they be protected from the effects of heat by cooling their natural heat portals? Evidence suggests that some cancer treatments could be augmented by the induction of hyperthermia and that the consequences of stroke or heart attack could be lessened by timely induction of hypothermia.