Thermoregulation and Fluid Balance

Core temperature is regulated by balancing heat production against heat loss, with the hypothalamus sensing the temperature of the blood and skin and driving the responses that correct it. At rest the body produces heat through metabolism, and during exercise the working muscles add a great deal more. Heat is lost by four routes: radiation, conduction, convection and evaporation. During exercise, and especially in the heat, evaporation of sweat from the skin is by far the most important, helped by widening of the skin blood vessels, which brings warm blood to the surface. This is why high humidity is so dangerous, since it reduces how well sweat can evaporate, and convection can even add heat to the body once the surrounding air is warmer than the skin, while radiation adds heat when the surrounding surfaces are hotter than the skin.

The balance between the heat being produced and the heat being lost determines whether core temperature stays steady, rises or falls, and in hard exercise in the heat that margin can be narrow. The body defends this set-point actively: when it is too hot it sweats and widens the skin vessels, and when it is too cold it shivers and narrows them, so exercise in the heat pushes hard against the heat-losing side of the system.

Sweating links temperature control to fluid balance. Sweat is water with sodium and other electrolytes, so heavy sweating depletes both. Losing fluid reduces the volume of circulating blood, which impairs both performance and the ability to lose heat, so dehydration and overheating tend to go together. Drinking too much hypotonic fluid, such as plain water or a dilute sports drink, during prolonged exercise can dilute the blood sodium, especially when water excretion is impaired because antidiuretic hormone is not fully switched off. Sweat rates vary widely between individuals and rise with fitness and acclimatisation, so fluid and salt losses over a long, hot event can be substantial and differ greatly from one athlete to the next. Because both under-drinking and over-drinking carry risks, drinking guided by thirst and avoiding weight gain from overdrinking is safer than following a fixed schedule. This can happen alongside heat illness rather than being its mirror image.

Heat illness develops when the heat load outpaces the body's ability to lose it and core temperature climbs. Milder heat exhaustion leaves thinking clear, whereas heat stroke is defined mainly by disturbance of the central nervous system (CNS) alongside marked overheating, and it is a medical emergency: the core temperature is usually above 40 degrees Celsius, though it may read lower if measured late or after some cooling, and an athlete with an altered mental state during or after exertion, with marked overheating or suspected heat illness, should be treated as heat stroke while other causes are rapidly considered. Exercise-associated hyponatraemia (EAH) is a problem of too much fluid rather than too much heat, and it can be equally serious.

When exercise starts in the heat, the body responds quickly, widening the skin blood vessels and beginning to sweat to shed the extra heat, at the cost of diverting some blood flow to the skin and away from the muscles. Repeated exposure to exercise in the heat then produces acclimatisation over one to two weeks. The body starts to sweat earlier and more, the sweat becomes more dilute so less sodium is lost, the plasma volume expands, and the heart rate and core temperature for a given effort fall. An acclimatised athlete tolerates heat far better, which is why deliberate heat preparation is used before competing in hot conditions. The gains of acclimatisation fade over a similar timescale once heat exposure stops, so the preparation has to be timed to the event.

At events and pitchside this physiology becomes clinical. Heat illness ranges from heat exhaustion, with fatigue, dizziness and nausea but clear thinking, to heat stroke, which is defined mainly by disturbance of the central nervous system alongside marked overheating and is a medical emergency, so an altered mental state during or after exertion, with marked overheating or suspected heat illness, should be treated as heat stroke while other causes are rapidly considered. Exercise-associated hyponatraemia, from drinking too much during a long event, can look similar, with nausea, headache and later confusion, and it can occur alongside heat illness rather than being its opposite, so recognising which is present matters. Exercise-associated muscle cramps are separate again, with a multifactorial cause, and do not necessarily signal overheating or salt loss.

• •The hypothalamus regulates core temperature by balancing heat production and loss.
• •Heat is lost by radiation, conduction, convection and evaporation.
• •Evaporation of sweat is the main route in exercise and its efficiency falls in humidity.
• •Sweating links temperature control to fluid and electrolyte balance.
• •Heat stroke is defined by brain dysfunction with marked overheating; heat exhaustion leaves thinking clear.
• •Exercise-associated hyponatraemia comes from overdrinking, not from dehydration.