Thermoregulation during exercise: Heat exchange
Thermoregulation during exercise is an important topic for every recreational and professional athlete. During intense exercise heat production is high and your internal or core temperature rises considerably. Therefore, your body has to respond in order to dissipate the heat and to prevent a dangerous increase in core temperature. The main goal of this article is to explain the process of heat exchange during exercise – the mechanism of heat production and all the forms of heat dissipation (transfer) during exercise, which function is to protect the body from overheating.
Thermoregulation at rest
The body produces internal heat due to the normal metabolic processes. At rest or during sleep, metabolic heat production is slow. In humans, normal body temperature is approximately 37°C (98.6°F). This value refers to the internal or core temperature, which is commonly measured orally and rectally. On the other hand, shell temperature, which represents the temperature or the skin and the tissues directly under it, varies considerably depending upon the surrounding environmental temperature. The body is able to maintain its core temperature by controlling the rate of heat production and the rate of heat loss.
Body temperature, therefore, reflects a careful balance between heat production and heat loss. When out of balance, the body either gains or loses heat. The temperature control center, which is located in the hypothalamus, works like a thermostat. It can initiate an increase in heat production when body temperature falls. Also, it can initiate an increase in heat loss when body temperature rises.
Types of heat production
Heat production can be either voluntary or involuntary.
Voluntary heat production
Voluntary heat production is brought about by exercise or physical activity.
Increased muscular activity during exercise causes an increase in heat production in the body owing to the inefficiency of the metabolic reactions that provide energy for contraction. The body is, at most, 20 to 30 percent efficient. Therefore, 70 to 80 percent of the energy expended during exercise appears as heat.
The total amount of heat produced in the body depends on the intensity and duration of the exercise. A more intense exercise will produce heat at a faster rate, while the longer the exercise lasts, the more the total heat is produced.
Involuntary heat production
Involuntary heat production results from shivering or the secretion of hormones, such as thyroxine and catecholamines.
This first type of heat production by shivering is the primary means of increasing heat production during exposure to cold. Maximal shivering can increase the body’s heat production by approximately five times the resting values. In addition, the release of thyroxine from the thyroid gland can also increase heat production. Thyroxine acts by increasing the metabolic rate of all cells in the body.
Finally, an increase in blood level of catecholamines (epinephrine and norepinephrine) can cause an increase in the rate of cellular metabolism.
Heat production during exercise
A normal body temperature at rest ranges from 36° to 38°C (97° to 100°F), as already stated earlier. However, it may rise to 38° to 40°C (100° to 104°F) during exercise. Further increases may result in heat exhaustion and subsequently heat-stroke.
So what exactly happens during the exercise? What causes heat production?
As you already know, your body produces internal heat due to normal metabolic processes that take place in your body. However, during exercise the total body metabolism may increase to 15-20 times the resting rate. Therefore, it’s an increase in metabolism during muscular activity that produces additional heat.
Under these conditions, metabolic heat production may increase to an extent that heat production is greater than heat dissipation; thus, the body will store heat and body temperature will increase, a condition known as hyperthermia. This heat must be dissipated to prevent a dangerous increase in core temperature.
Imbalance between the rates of heat production and heat dissipation
The elevated body temperature during exercise is not caused by resetting the hypothalamic set point. Rather, it is caused by the temporary imbalance between the rates of heat production and heat dissipation during early stages of exercise. A more rapid response of heat dissipation mechanisms can attenuate or delay the exercise-induced rise in body temperature. The process of thermoregulation during exercise is not perfect, but it justifies its task. This can then allow an athlete to compete at a relatively lower body temperature in a hot/warm environment. One of the heat acclimatization responses is that athletes will be able to respond to the heat more quickly on the commencement of exercise.
Body’s response to elevated body temperature during exercise
The heat from deep in the body (the core) is moved by the blood to the skin (the shell). Once heat nears the skin, it can be transferred to the environment by one of the four mechanisms: conduction, convection, radiation, and evaporation.
There are two main physiological mechanisms that allow the body to dissipate heat in an attempt to maintain thermal balance during exercise:
- vasodilation of the cutaneous vessels, and
- an increase in sweating rate
These mechanisms function to protect the body from overheating during strenuous exercise. The process of thermoregulation during exercise is performing a vital role for our body’s survival.
1. Vasodilation of the cutaneous vessels
Vasodilation (or the widening) in the cutaneous vessels brings the warmer blood close to the body’s surface so that heat can be dissipated to the environment via conduction, radiation, and convection, assuming that ambient temperature is cooler than the body.
Conduction is the transfer of heat from the body into the molecules of a cooler object in contact with its surface. In general, the body loses only small amounts of heat due to this process. For example, we can easily warm our hands by touching hot-water bottles. Conversely, we can cool our body by touching any object that has lower temperature relative to our current body temperature (for example sitting on a cold chair).
Radiation is heat loss in the form of infrared rays. This involves the transfer of heat from the surface of one object to the surface of another, with no physical contact between them. At rest, radiation is the primary means for dissipating the body’s excess heat. At normal room temperature, (i.e., 70 to 77°F or 21 to 25°C), the nude body loses about 60 percent of its excess heat by radiation.
This is possible because skin temperature is greater than the temperature of surrounding objects (e.g., walls, floors, furniture, etc.), and a net loss of body heat occurs due to the thermal gradient. If the temperature of the surrounding objects is greater than that of the skin’s surface, the body will experience a net heat gain via radiation.
Convection is a form of conductive heat loss in which heat is transmitted to either air or water molecules in contact with the body. In other words, it involves moving heat from one place to another by the motion of gas or liquid across the heated surface. Although we are not always aware of it, the air around us is in constant motion. As it circulates around us, passing over the skin, it sweeps away the air molecules that have been warmed by their contact with the skin.
The greater the movement of the air or liquid, such as water, the greater the rate of heat dissipation by convection. For example, cycling at high speeds would improve convective cooling when compared to cycling at slow speeds or running. Swimming in cool water also results in convective heat loss. In fact, the water’s effectiveness in cooling is about 25 times greater than that of air at the same temperature.
2. Increase in sweating rate (sweat evaporation)
Evaporation is our major defense against overheating during exercise. This is especially the case when the environmental temperature rises. For example, evaporation of sweat can account for as much as 70 percent of total heat loss when exercising in an ambient temperature of 30°C (86°F). During exercise, when body temperature rises above normal, the nervous system stimulates sweat glands to secrete sweat onto the surface of the skin.
As sweat evaporates, heat is lost to the environment, which in turn lowers skin temperature. Each liter of sweat that vaporizes transfers 2400 kJ (580 kcal) of heat energy from the body to the environment.
Sweat rate can vary considerably between individuals. Some of the most important non-hereditary factors that determine maximal sweat rates include age, sport, climate, acclimatization, hydration status, and perhaps body fat content.
Closing thoughts: Thermoregulation during exercise
During exercise at moderate to high intensity, heat loss always lags heat production. As intensity of exercise increases, the body will rely more on evaporative heat loss (sweating) in its effort to maintain a thermal balance. Therefore, the process of thermoregulation during exercise is not perfect, but it but it justifies its existence.