Imagine that you have just left your home for a mile jog on a cold, windy day in January. You did not dress warmly, so this exposure to exercise in cold air will provide an obvious challenge to the homeostatic balance of your body. After stretching for a few minutes, you begin to run. The cold air immediately decreases your skin temperature and your cutaneous vessels constrict strongly.
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Blood is diverted away from the skin to your central veins, and stroke volume increases due to increased venous return to the heart. Cardiac output and heart rate are thus maintained at levels below what they would be in a warmer environment. Vasoconstriction in skin and perhaps in muscle, coupled with a lower muscle temperature, may increase the rate of anaerobic metabolism, glycolysis and may reduce the clearance rate of metabolic by-products, such as lactate, from your muscles. This effect is dependent on exercise intensity, as witnessed by changes in plasma lactate. Verifying this increase of anaerobic metabolism, the rate of muscle glycogen utilization during your winter jog is greater than if you were running in the summer. Interestingly, research shows that you might utilize more fat as a fuel if you exercise in a cold environment, such as min of continuous cycling at an intensity of of maximal heart rate in an air temperature ofC F, or min of continuous cycling W in an air temperature ofF. However, it is fair to note than some studies disagree that this will occur and that other studies suggest that both your carbohydrate and fat utilization will increase, with the former serving as the predominant fuel source, as occurs during rest.
Increased shivering, which occurs concurrently with exercise, increases your VO above that experienced in a mild or warm environment. Because shivering does not produce external work, the energy utilized in muscle is converted almost entirely to heat. This warms internal organs and raises the core body temperature slightly, but the amount of shivering- induced heat production is quite small
Inadequate clothing compromises your body's ability to maintain homeostasis comparison to that produced by exercise. Shivering during exercise occurs only when your Tre falls below approximatelyF.
Your Tre will not be maintained in cold air if you jog at a slow pace. At higher exercise intensities, it will increase. This will send conflicting stimuli to your brain, which will decrease shivering and sympathetic vasoconstriction, encouraging heat loss while cold receptors in the skin simultaneously signal the hypothalamus to conserve heat. Although your overall heat loss will increase during intense running, the additional metabolic heat production will prevent a decline in rectal temperature. In contrast, you also may begin to sweat as your core body temperature rises, even though your skin is relatively cool; wearing sweat-soaked clothing on a windy day may result in hypothermia see section below titled Counteracting Cold and Windchill, pageThis explains why you should wear layers of quick-drying fabrics near your skin and have dry clothing on hand. This also illustrates why it is difficult to generalize about your body's responses to exercise in cold air: the physiological responses depend on the interactions of many factors.
In addition to maintaining rectal temperatureTre, it also is important to maintain skin temperature Tsk when the windchill is severe.
Increased blood flow, due to skin vasodilation and increased cardiac output, can warm peripheral tissues in your fingers, toes, ears, and nose, protecting them from frostbite. This response is related to exercise intensity, according to studies that involved mild to moderate exercise in aF environment. When light exercise was performed, Tsk cooled toF; subsequent moderate exercise caused a rapid rise in T to between and-F. This effect of increased exercise intensity on local blood flow can benefit the peripheral areas of your body that are prone to frostbite.
Heat loss through your lungs also modifies the thermal balance of your body's core, but only to a small degree of total heat production. The nasal sinus and pharynx region inside your skull warms air very rapidly and effectively. Large differences in the temperature of air inhaled during exercise, such asF versus F, cause virtually no change in rectal temperature.