Thermoregulation and body temperature
Thermoregulation is the body’s ability to keep its core temperature stable enough, even when there’s a cold wind outside, a humid summer day, or your heart rate shoots up during a workout. The simple truth: the body produces heat all the time, but it also has to release it at the right moment.
Why is thermoregulation important?
Living organisms can be divided into two large groups. Poikilothermic, or cold-blooded, animals include invertebrates, fish, amphibians and reptiles. Their body temperature depends heavily on the external environment. When the environment warms up, so does their body temperature. When the temperature drops, their vital functions slow down too.
Homeothermic, or warm-blooded, animals are birds and mammals. Their bodies have mechanisms that help keep core temperature fairly constant. This is a big advantage, because the body’s functioning doesn’t swing so sharply with every small change in the weather.
In humans, temperature is not the same across the whole body. Skin temperature can vary a lot from area to area. It can be highest on the neck, at about 34, and lower on the feet, for example 24.4. The difference comes mainly from blood supply and how much heat a given area gives off.
Measured under the armpit, normal body temperature usually falls in the range of 36-37C. Over the course of a day it fluctuates by roughly 1C. Body temperature is usually lowest between 2 and 4 at night and highest between 4 and 7 in the evening. It is influenced by sleep, food, light and movement.
How does thermoregulation work?
You can think of the human body as having two layers. The outer part, especially the skin and limbs, partly behaves like a poikilothermic shell. This means skin temperature changes with the weather, wind, humidity and clothing. The inner part is a homeothermic core. There, the body tries to keep temperature stable.
In the cold, the blood vessels in the skin narrow and heat loss decreases. Wind carries heat away from the skin surface faster. That’s why damp, cold air feels especially biting. In hot, humid air the problem is the opposite: sweat doesn’t evaporate well, and cooling the body becomes harder.
During physical work, heat production increases. A small rise in body temperature, for example 1C, can raise the excitability of the central nervous system, support the work of the internal organs and help the body get ready for effort. That’s exactly why movement feels easier after a warm-up.
If the work is very intense, lasts a long time, and air temperature and humidity are high, body temperature can rise to 38.5-39.0 C. At that point it’s no longer simply a good warm-up. There is a risk of overheating and heat stroke.
The physical and chemical side
Physical thermoregulation mainly means changing how much heat is given off. A person gives off heat through the skin and the airways. A large share of it, 70-80%, happens through the skin. The main routes are conduction, radiation and evaporation of sweat.
As the temperature of the external environment approaches body temperature, the evaporation of sweat becomes more and more important. Evaporating 1ml of water gives off 0.58 kcal of heat. When the air is 100 % saturated with water vapor, sweat can’t evaporate from the skin normally. That’s why people often tolerate dry heat better than humid heat.
Swimming is a different situation. Water conducts heat better than air, so the body cools faster in water than in air of the same temperature. That’s why cool water can quickly become uncomfortable during a longer swim.
Chemical thermoregulation means changing heat production. When the environment cools, metabolic processes may intensify and the body produces more heat. A large share of heat is generated in the muscles. Even when lying still with tensed muscles, heat production can increase by 10 % or more compared with fully relaxed muscles.
Shivering also helps in the cold. It’s an involuntary but very practical reaction. The thermoregulatory center in the hypothalamus monitors blood temperature and receives information from receptors in the skin, mucous membranes and internal organs. Hormones, such as thyroxine and adrenaline, also play a role.
Thermoregulation in training and at different ages
In training, it’s worth watching for simple signs: unusual weakness, dizziness, chills in a hot room, or a feeling that the body is no longer cooling down. These are signals that you need to reduce the training load and your body needs rest.
Clothing matters a lot. In winter, it makes no sense to go outside in a way that leaves sweat trapped between your skin and a wet shirt. In summer, it’s important that the fabric lets moisture move through. The same effort can feel easy in a cool gym but much harder on a humid summer day.
Thermoregulation is special in newborns. In many mammals, thermoregulatory heat production only develops over a few weeks. In humans, thermoregulatory reactions kick in right after birth, but shivering is not the main defense. Non-shivering heat production is what matters.
A newborn’s body surface-to-volume ratio is 3 x larger than an adult’s. The skin and fat padding are thin, which makes holding on to heat harder. At the same time, heat production relative to body weight is 4 5 x higher. For this reason, a newborn at rest needs a higher external temperature than an adult.
FAQ: thermoregulation
Why does sweating cool the body?
Sweat cools you when it evaporates from the skin. When the air is very humid, evaporation is poorer and the cooling effect is smaller.
Why does a cold wind feel worse than just cold air?
Wind carries heat away from the skin surface faster. As a result, skin temperature drops faster and the body has to work harder to keep core temperature steady.
Does training always raise body temperature?
Yes, more heat is produced in the muscles during work. Usually the body can handle this, but heat, humidity and effort that goes on too long can make cooling difficult.
Author: EKFK
Source: WHO – physical activity.
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