How your body keeps cool when you are running

As runners, we are always looking for a challenge. In the heat however, our performances are markedly reduced. If we want to know what we can do about this, we first have to understand what is happening.

Our bodies are divided into an inner core (brain, vital organs such as heart and liver) and an external shell (skin and underlying structures).  The temperature of the core is regulated within narrow limits while that of the shell depends on the environment and on our need to lose or conserve warmth. The thickness of the shell can therefore vary from one centimetre in warm conditions to several centimetres in the cold.

The amount of heat your body generates is roughly proportional to your body mass and size. Women produce slightly less heat than men, probably because their bodies contain a higher proportion of fat.

The core loses heat by conduction (transfer between tissues that are in contact with each other) and convection (transfer of heat into air or liquids, in this case blood which carries it away to the shell).

Most of the heat exchanges between the body and the environment happen at the skin surface. The skin can dissipate heat into the air by convection, especially if the air is cooler than the skin and if there is wind, but sweating is more efficient. To cool you down, sweat has to evaporate. If it drips off from your body, it is useless.

Exercising in the heat

When you exercise, your muscles generate heat. An increased temperature in your muscles allows them to produce more power (that’s why you warm up!), but your core needs to stay cooler than 40C. Moreover, exercise and thermoregulation impose competing demands on your body. Your muscles need a large increase in blood to perform well, but to cool your core down you need to direct blood to your skin.

As the blood flow through the skin increases, large superficial vessels dilate and blood pools in them. This reduces the blood available to fill your heart chambers. The amount your heart pumps around at each beat therefore decreases, and to maintain the same intensity of exercise your heartbeat has to increase. Moreover, if the fluid lost through sweat is not replaced, your plasma volume will decrease, making the situation even more difficult. To help out, your body reduces the blood flow through your bowels and kidneys. Nevertheless, if you continue to generate more heat than you can dissipate, your core temperature will go up.

As the brain realises that the core temperature is rising, it will slow you down by preventing some muscle fibres to contract and making you feel tired. Fatigue is therefore triggered by the rate of core temperature increase, and not by its absolute value.

This is a safety brake to keep your core temperature below 40C. It is clearly not an “all or nothing” mechanism, but a gradual phenomenon. If it does not work and your core temperature continues to increase, you will suffer a heatstroke.

 Acclimatisation

To be able to go harder and for longer in warm conditions, you need to become better at dissipating heat.

A full acclimatisation will take up to 14 days, but after a few days of training in warm conditions your plasma volume will have expanded, your heart rate reduced, and your blood flow more efficiently redirected to your skin and working muscles. Your sweat rate will increase, and sweat production will start earlier and become more important on your limbs than on your torso. Your sweat becomes dilute, allowing you to save salt.

Your core temperature at rest will have decreased by 0.2 0r 0.3C, which means that it will now take longer to reach 40C.

You can only acclimatise correctly by exercising in the heat: just resting is not enough. You will not be able to train hard during that period and this can be a problem, as you might lose fitness.

Fitness

As you can see, physically fit people have already many of the necessary adaptations, and training in the heat will further enhance them. You will therefore have an advantage over your sedentary counterparts, and you will also acclimatise quicker than them.

However, as you are fitter you are able to exercise harder and therefore generate more heat. Even though fitter people are better at dissipating heat, you might produce more than you can dissipate. In shorter races (e.g.: 10 Km), faster runners are therefore more at risk of heatstroke than slower ones. On the other hand, if a group of individuals are working out at a set intensity or pace (e.g. team sport or work activities), the aerobically unfit ones will suffer most.

What you can do

Scientists have shown that dehydration makes it more difficult for your body to keep the core temperature down, but drinking more than you need is useless and dangerous as it puts you at a higher risk of hyponatremia (too little salt in your plasma). Make sure that you are well hydrated when you start and simply drink to your thirst.

Other risk factors for heatstroke include sleep deprivation, recent infections, alcohol use, some drugs and a lack of physical fitness.

Do not forget to sponge yourself on a regular basis when exercising. Sponging your limbs will make the dilated veins contract and therefore reduce blood pooling.

Interval training will be harder than continuous work, as your heat dissipating mechanisms take some time to respond while the sudden bouts of intense exercise produce a large amount of heat. In non-acclimatised and untrained people the heat dissipating will start even slower.

What you cannot change

The heat your body produces depends on its size. Smaller people will thus generate less and, as they have relatively more skin surface, are better at dissipating it.

Some people are more vulnerable to heat illness than others.

Disclaimer: This article is for information only, and cannot be used as a guide for treatment or diagnosis. If you have any questions or concerns, talk to your health care provider.

References

T Kuwahara, Y Inoue, M Abe et al. Effects of menstrual cycle and physical training on heat loss responses during dynamic exercise at moderate intensity in a temperate environment. AJP-Regu Physiol 2005 (288); 5: R1347-R1353

L Nybo. Hyperthermia and fatigue. Journal of Applied Physiology 2008 (104); 3:871-878

R  Mora-Rodriguez. Influence of aerobic fitness on thermoregulation during exercise in the heat. Exercise & Sport Sciences Reviews 2012(40); 2:79-87.

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2 thoughts on “How your body keeps cool when you are running

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