During exercise, the increased muscular activity tends to raise temperature, instead of drop it. However, humans have a number of protective mechanisms to maintain a constant body temperature. These include dilating the arteries near the skin, sweating and behavioral changes. Because the numerous chemical reactions in the body occur within a very narrow temperature range, these mechanisms are important to keep the body functioning during an intense workout.
Source of Body Heat
Exercising muscle requires large amounts of energy to function. This energy comes from the breaking down of high energy bonds from sugars and fatty acids. This process is known as cellular respiration and results in the formation of adenine triphosphate, or ATP, a high-energy compound that fuels muscle activity. While cellular respiration is efficient — about 40 percent of the energy is captured, compared with about a 25 percent fuel efficiency of most cars — the remaining energy is released as heat. As a result, the body temperature has a tendency to rise during intense muscular exertion.
Dilating Arteries
The skin is considered the largest human organ. The surface area of the skin in an adult measures approximately 2 square meters. Small blood vessels, called arterioles, supply the inner surface of the skin. During exercise, these blood vessels dilate. This dilation is due to neural impulses that arrive from the brain. By dilating the arteries near the skin, blood reaches closer to the atmosphere, which allows it to lose extra heat. This effect accounts for the red complexion that is so common in athletes after a workout.
Sweating
Sweating is another important mechanism that promotes heat-loss during exercise. The amount of energy to evaporate a single g of water is about 0.58 kilocalories — a considerable amount of heat. During exercise, nervous impulses to the sweat glands are activated. Eccrine glands, the type of sweat gland involved in heat regulation, are found in the skin throughout the body. As heat is evaporated off of the body's surface, the temperature of the body is subsequently reduced.
Other Mechanisms of Heat Loss
Radiation refers to the loss of heat via infrared waves, conduction refers to the loss of heat to surrounding objects and convection refers to the loss of heat due to surrounding air currents. All of these occur irrespective of exercise. Psychological changes that occur while the body is heating are important and are responsible for the desire to sit in the shade and have a cool glass of lemonade after a workout. Some mammals lose heat by panting, but this does not occur in humans.
References
- Ventura College: Harvesting Energy — Glycolysis and Cellular Respiration
- The Physics Factbook: Surface Area of Human Skin
- "Textbook of Medical Physiology"; John Hall; 2010
- American Academy of Dermatology: Eccrine and Apocrine Sweat Glands
