Physiological responses to exercise in the heat

Heat production is beneficial during exercise iin a cold environment because it helps maintain normal body temperature. However, even during exercise in a thermally neutral environment, such as 21 to 24°C( 70-75°F), the metabolic heat load places a considerable burden on the mechanisms that control body temperature. Heat stress means any environmental condition that increases body temperature and jeopardizes homeostasis.

Cardiovascular function

Exercise increases the demands of the cardiovascular system. When the need to regulate body temperature is added during exercise in the heat, the cardiovascular system can become burdened. During exercise in hot conditions, the circulatory system has to continue to transport blood not only to working muscle but also to the skin, where the heat generated by the muscles can be transferred to the environment. To meet this dual demand during exercise in the heat, two changes occur. First, cardiac output increases further(above that associated with a similar exercise intensity in cool conditions) by increasing both heart rate and contractility. Second, blood flow is shunted away from nonessential areas like the gut, liver, and kidneys and to the skin.

Consider what happens during running at a fast pace on a hot day. The exercise increases both metabolic heat production and the demand for blood flow and oxygen delivery to the muscles. This excess heat can be dissipated only if blood flow increases to the skin.

As the thermoregulatory center instructs the cardiovascular system to direct more blood flow to the skin, superficial blood vessels dilate to bring more of the warm blood to the body’s surface. Sympathetic nervous system signals also go to the heart to increase heart rate and allow the left ventricle to pump more forcefully. The gradual upward drift in heart rate helps compensate for the decrease in stroke volume as blood pools in the periphery. This phenomenon is known as cardiovascular drift. Because blood volume stays constant or even decreases some(as fluid is lost in sweat), a final phase of cardiovascular adjustment occurs. Sympathetic signals to the kidneys, liver, and intestines cause vasoconstriction of those regional circulations, which allows more of the available cardiac output to reach the skin without compromising muscle blood flow.

What limits exercise in the heat?

Seldom are records set in endurance events, such as distance running, when the environmental heat stress is great. The factors that cause early fatique when heat stress is superimposed on prolonged exercise have been the topic of some debate, and several theories have been proposed. None of these theories captures every situation, but taken together they demonstrate the multiple control systems at work during thermoregulation.

At some point, the cardiovascular system can no longer compensate for the increasing demands of continuing endurance exercise and efficiently regulating the body’s heat. Consequently, any factor that tends to overload the cardiovascular system or to interfere with heat dissipation can drastically impair performance and increase the risk of overheating. Exercise in the heat becomes limited when heart rate approaches maximum, especially in untrained or non-heat-acclimated exercisers, as illustrated in the figure below. Interestingly, working muscle blood flow is well maintained even at very high core temperatures unless significant dehydration occurs.

Another theory that helps explain limitations to exercise in the heat – especially in well-trained, acclimated athletes – is the critical temperature theory. This theory proposes that, regardless of the rate at which core temperature(and thus brain temperature) increases, the brain will send signals to stop exercise when some critical temperature is reached, usually between 40 and 41°C(104 and 105.8°F).

Body fluid balance

Under some conditions, the temperature of the environment approaches and can exceed both the skin and deep body temperatures. As mentioned earlier, this makes evaporation far more important for heat loss, because radiation, convection, and conduction become increasingly less effective is environmental temperature increases. In fact, these mechanism lead to heat gain when environmental temperature exceeds skin temperature. Increased dependence on evaporation means an increased demand for sweating.

The sweat glands are controlled by stimulation of the POAH. Elevated blood temperature causes this region of the hypothalamus to transmit impulses through the sympathetic nerve fibers to the millions of eccrine sweat glands distributed over the body’s surface. The sweat glands are fairly simple tubular structures extending through the dermis and epidermis, opening onto the skin, as illustrated in the picture below.

Sweat is formed in the coiled secretory portion of the sweat gland and at this stage has an electrolyte content similar to that of the blood, since plasma is the source of sweat formation. As this filtrate of plasma passes through the uncoiled duct of the gland, sodium and chloride are reabsorbed back into the surrounding tissues and then into the blood. As a result, the final sweat that is extruded onto the skin surface through the sweat gland pores is hypotonic to(has less electrolytes than) plasma. During light sweating, the filtrate sweat travels slowly enough through the duct that there is time for reabsorption of sodium and chloride. Thus, the sweat that forms during light sweating contains very little of these electrolytes by the time it reaches the skin. However, when the sweating rate increases during exercise, the filtrate moves more quickly through the tubules, allowing less time for reabsorption, and the sodium and chloride content of sweat can be considerably higher.

Example of sodium, chloride, potassium concentrations in the sweat of trained and untrained subjects during exercise
Subjects	Sweat Na+(mmol/L)	Sweat Cl-(mmol/L)	Sweat K+(mmol/L)
Untrained men	90	60	4
Trained men	35	30	4
Untrained women	105	98	4
Trained women	62	47	4

As seen in the table up, the electrolyte concentration of trained and untrained subjects sweat is significantly different. With training and repeated heat exposure(acclimation), more sodium is reabsorbed and the sweat is more dilute, in part because the sweat glands become more sensitive to the hormone aldosterone. Unfortunately, the sweat glands apparently do not have a similar mechanism for conserving other electrolytes. Potassium, calcium, and magnesium are not reabsorbed by the sweat glands and are therefore found in similar concentrations in both sweat and plasma. In addition to heat acclimation and aerobic training, genetics is a major determinant of both sweating rate and sweat sodium losses.

While performing heavy exercise in hot conditions, the body can lose more than 1L of sweat per hour per square meter of body surface. This means that during intense effort on a hot and humid day(high level of heat stress), an average-sized individual(50-75kg, or 110-165lb) might lose 1.6 to 2.0L of sweat, or about 2.5% to 3.2% of body weight, each hour. A person can lose a critical amount of body water in only a few hours of exercise in these conditions.

A high rate of sweating maintained for a prolonged time ultimately reduces blood volume. This limits the volume of blood returning to the heart, increasing heart rate and ultimately decreasing cardiac output, which in turn reduces performance potential, particularly for endurance activities. In long-distance runners, sweat losses can approach 6% to 10% of body weight. Such severe dehydration can limit subsequent sweating and make the individual susceptible to heat-related illnesses.

Loss of both electrolytes and water in the sweat triggers the release of both aldosterone and antidiuretic hormone(ADH), also know as vasopressin or arginine vasopressin. Aldosterone is responsible for maintaining appropriate sodium levels and ADH maintains fluid balance. Aldosterone is released from the adrenal cortex in response to stimuli such as decreased blood sodium content, reduced blood volume, or reduced blood pressure. During acute exercise in the heat, this hormone limits sodium excretion from the kidneys. More sodium is retained by the body, which in turn promotes water retention. Because of this, plasma and interstitial fluid volumes can increase 10% to 15%. This allows the body to retain water and sodium in preparation for additional exposures to the heat and subsequent sweat losses.

Similarly, exercise and body water loss stimulate the posterior pituitary gland to release ADH. This hormone stimulates water reabsorption from the kidneys, which further promotes fluid retention in the body. Thus, the body attempts to compensate for loss of minerals and water during periods of heat stress and heavy sweating by reducing their loss in urine. Also, blood flow to the kidneys is substantially reduced during exercise in the heat, which aids in fluid retention during acute bouts of exercise.