Rectal temperature Tre is the most widely used measurement of deep body temperature in medical, industrial, and athletic settings. Skin temperature Tsk represents the thermal state of the body’s outer shell. Your brain controls these two areas separately, as it attempts to maintain thermal equilibrium throughout each day.
The combined effects of heat and humidity on deep body temperature during exercise are illustrated in figure This graph depicts the steady-state Tre of one hypothetical athlete who exercised on a treadmill in many combinations of air temperature and humidity, at the same constant treadmill speed, on separate occasions, until his body temperature reached a stable point heat production equaled heat loss. The three symbols each represent a different relative humidity either
Steady-state Tre of one hypothetical athlete who exercised in different controlled environments. Symbols represent three different humidities.
Steady-state Tre of one hypothetical athlete who exercised in nine different controlled environments, at three exercise intensities.
Adapted from Armstrong .
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Cardiovascular Responses to Heat and Humidity
Your cardiovascular system heart, blood vessels, and blood is essential for temperature regulation and exercise performance in a hot environment. As noted earlier, the central nervous system distributes the blood ejected from the heart cardiac output in liters per minute to both skin blood vessels and active muscles. This is accomplished by increasing the cardiac output from L/min at rest to L/min during strenuous exercise by increasing heart rate and the volume of blood ejected with each beat stroke volume. Further, the brain and nervous system divert blood temporarily away from inner organs such as the liver, intestines, and kidneys by constriction of the arteries that supply blood to them. The result of this increased cardiac output and – reduction of blood flow to inner organs is that the cardiac output of the heart can be utilized for two important tasks: heat dissipation increased skin blood flow and exercise in-
creased muscle blood flow. Figureillustrates this complex response as it is orchestrated by the brain and spinal cord. Constriction of arteries leading to inner organs is symbolized by C. Dilation of the arteries that supply skin and active muscle tissue is represented by the letter D.
Control of the distribution of cardiac output to organs during exercise in the heat. C, constriction of arteries; D, dilation of arteries.
From Human Circulation: Regulation During Physical Stress by Loring B. Rowell. Copyright © by Oxford University Press, Inc.
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