Heat Balance

As humans are homoeothermic or have an internal body temperature that is kept nearly constant throughout life, thermal regulation (the body’s ability to maintain a constant deep-body temperature) is vital. If the core temperature is to remain in balance, heat production must match heat lost.

If heat loss is less than heat production, there is a net gain in body heat and therefore body temperature raises. If heat loss exceeds heat production, there is a net loss in body heat and body temperature decreases.

At rest internal body temperature (rectal) is approximately 37^oC. This can increase to approximately 40^oC during exercise.

Heat Production

Metabolic Heat

The body produces internal heat through the normal metabolic processes. At rest or during sleep, metabolic heat production is small; however, during intense exercise total metabolic rate can increase 20 to 25 times, therefore increasing heat production.

Powers and Howley place heat production into two classifications, being either voluntary or involuntary.

1. Voluntary heat production is referring to exercise heat production.
2. Involuntary heat production is referring to shivering and non-shivering thermogenesis.

Shivering is the primary means of increasing heat production during exposure to cold. Maximum shivering can increase the body’s heat production by up to five time the resting value.

Non-shivering thermogenesis (the generation of heat as a result of metabolic reactions) is the increased metabolic rate of all cells through the release of hormones such as thyroxine from the thyroid gland and through an increase in blood levels of adrenaline.

Environmental Heat

Heat can be gained by the body from the environment through radiation, convection and conduction. This occurs when both the air and the objects surrounding the body are warmer than the body.

Heat Loss

Radiation

Radiation is the primary means of heat lost at rest and would account for about 60 percent of the heat loss. Radiation is the transfer of heat between objects through electromagnetic waves. When we are seated in the classroom, for example, we are radiating heat to the walls of the room, while at the same time heat is being radiated from the walls to us. We gain heat through radiation when surrounding objects are warmer than our bodies; we loss heat when our body temperatures are warmer than the surrounding objects.

Conduction

Conduction is the transfer of heat between two objects of different temperatures that are in direct contact with each other. The direction of heat flow is always from the warmer to the colder object. For example, when touch a piece of ice the heat is conducted from the surface of the hand to the ice, whereas when we touch a hot stove the heat is conducted from the stove to the hand.

Convection

Convection is defined as the transfer of heat from one place to another by the motion of a heated substance. For example, a fan blowing over the surface of the skin removes air warmed by the body and replaces it with cooler air. Standing in the back of a moving truck produces the same effect as does the wind.

Evaporation

In evaporation, heat is transferred from the body to the water on the surface of the skin. When this water gains enough heat, it is converted to a vapour, taking the heat away from the body. It accounts for approximately 20 to 25 percent of heat loss at rest, but under most environmental conditions, it is the most important means of heat loss during exercise; approximately 80 percent of heat is lost through evaporation during exercise. As McArdle et al states, "Evaporation provides the major physiological defence against over heating."

Humidity and Heat Loss

When the relative humidity is high, the air holds closer to its maximum amount of water. Because of this, the air will accept less water in the form of a vapour from the skin. This leads to a problem, as deVreis and Housh (1994) state "liquid sweat must be converted to a gas by evaporation before any heat loss occurs."

Sweat must evaporate to provide cooling. Sweat that drips off the skin provides little or no cooling. When exercising in the heat, if you suddenly feel chilled and goose bumps form on your skin, stop exercising, get into a cool environment and drink plenty of cool fluids. The body’s thermoregulatory system has become confused and thinks that it has to increase body temperature even more! If left untreated, this condition could lead to heat stroke and death.

Heat Management Strategies

The most important heat management strategy is hydration, the regulation of bodily fluids that are necessary for internal heat regulation (temperature control) and the removal of waste products. Lack of these fluids is termed dehydration. Dehydration reduces endurance and increases the risk of heat illnesses (heat exhaustion and heat stroke). Contrary to popular belief, it does not have to be hot for you to dehydrate.

The basic symptoms of dehydration include the following:

• thirst
• strong thirst
• dry mouth, nausea
• light headiness, difficulty concentrating

Factors affecting dehydration include:

• Diuretics (e.g., alcohol and caffeine as they cause you to urinate)
• Weather and humidity - The more humid the weather, the less effective sweat becomes as a coolant.
• Clothing by not allowing sufficient heat loss. The colder it is, the more clothing we wear, and once we warm up, heat loss becomes difficult.
• Salt on food or strong doses of certain drinks like staminade

You can stay hydrated by doing the following:

• Drink 400 to 600 ml of water before exercise.
• Drink 100 to 200 ml of water every 10 to 15 minutes of exercise.
• Drink on schedule rather than relying on thirst.
• Drink cool water as this is more palatable, and by reducing the temperature of the stomach, fluid flow from the stomach to the small intestine is increased.
• Water consumption should continue well after the session concludes.

Cool water to date is still one the best hydrants and for your basic participant is all that is required. Those who train aerobically for over an hour may consider using a carbohydrate replacement drink. The most effective drinks contain six to eight percent carbohydrates. Beverages that exceed 10 percent carbohydrates eg sodas, fruit juices and other concentrated fructose drinks are associated with cramps, nausea and diarrhoea.