Body temperature regulation (thermoregulation)

Definitions & Key takeaways

Thermoregulation is the process by which an organism maintains its internal body temperature within a certain range, despite changes in external conditions. For the human body, it ranges between 36.5 �C to 37.5 �C. The main purpose of thermoregulation is to keep the enzyme systems of the body working properly. The part of the brain responsible for thermoregulation is called the hypothalamus. It receives information about the temperature status from some specialized nerve cells called thermoreceptors.

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Introduction0:00–0:38

Body temperature regulation, also known as thermoregulation, is how an organism keeps its body temperature within certain limits.
For humans, the normal body temperature ranges between 36.1°C, or 97 °F, and 37°C, or 98.6°F. When body temperature increases above 38.5° C, or 101.3°F, that’s called hyperthermia.
The opposite condition, when body temperature decreases below 35 °C, or 95 °F, is known as hypothermia. Thermoregulation is needed in response to internal and external temperature variations.

Hypothalamus and thermoreceptors0:38–1:42

Internal temperature variations are sensed by specialized nerve cells, called thermoreceptors, located in the anterior hypothalamus.
Now, the hypothalamus works as a thermostat. The front part or the anterior hypothalamus responds to increased environmental temperatures and it also controls the core temperature of the body.
The back part or the posterior hypothalamus, on the other hand, responds to decreased environmental temperatures. Changes in the external temperature are sensed by the skin thermoreceptors, which are specialized nerve cells located in the skin.
For example, during winter, when the environmental temperature is less than the body temperature, the skin receptors sense these variations and send the information to the anterior hypothalamus which will then inform the posterior hypothalamus that the body has to generate heat.
Now, besides the behavioral habits, such as putting more clothes on or drinking hot tea, there are several other physiologic mechanisms through which heat production is increased.

Thyroid hormones1:42–2:34

First, thyroid hormone action is stimulated. Thyroid hormones are thyroxine or T4 and triiodothyronine or T3, which is the active form.
One of their roles is to increase heat production and they do that by stimulating conversion of T4 to T3. T3 then increases the production of the energy molecule adenosine triphosphate or ATP in the body.
ATP is basically the energy currency in the cell and the more we have, the more of it can be used to generate heat. Because these hormones are thermogenic hormones, any excess or deficit of these hormones will disturb the thermoregulation.
For example, with hypothyroidism, when there are low levels of thyroid hormones, there is an extreme sensitivity to cold.
With hyperthyroidism, on the other hand, there are high levels of thyroid hormones, and this leads to heat intolerance. Now, in order to produce maximal body heat, the posterior hypothalamus also sends signals that activate the sympathetic nervous system.

Sympathetic nervous system2:34–3:20

The sympathetic nervous system is normally activated by fight-or-flight situations, and it leads to increased catecholamine production, like epinephrine or norepinephrine.
Catecholamines bind to β receptors in brown fat, which is also referred to as “good fat” because brown adipose cells can burn calories and generate heat - instead of energy, like the other kind of fat would.
The activation of the sympathetic nervous system also stimulates the α1 receptors in vascular smooth muscle of skin blood vessels, causing vasoconstriction, or narrowing of the arterioles.
This decreases blood flow to the surface of the skin, and, in turn, reduces heat loss. Finally, the posterior hypothalamus signals the skeletal muscles, causing rhythmic contractions known as shivering.

ATP3:20–4:10

When we shiver and these muscles contract, ATP breaks into adenosine diphosphate or ADP, which means that one of 3 phosphoryl groups from ATP gets popped off and turned into a phosphate molecule.
Now, normally, these phosphoryl groups are bonded with the adenosine and these bonds have a lot of energy, meaning that the electrons in this bond are in a high energy state.
Well, when a bond is broken by a chemical reaction, such as hydrolysis, the electrons go into a lower energy state. As they do that, they release that high energy they previously had and we get what is called an exothermic reaction.
Exo means exit and thermo refers to heat or energy. And so, with more energy released, more heat is generated.

Response to high temperatures4:10–5:52

Now, let’s switch gears and see how the body responds when the environmental temperature increases, such as during torrid summer days.
The first thing that comes to our mind is to wear less clothes or use fanning. However, when this is not enough, the body has to find ways to increase heat loss and it is doing that by dissipating the heat.
When the skin thermoreceptors sense that the body heat has become too high, they send signals to the anterior hypothalamus to activate the heat dissipating mechanisms.
First, the anterior hypothalamus increases the activity of the sympathetic cholinergic fibers which innervate thermoregulatory sweat glands leading to increased sweating.
The sweat glands spew out perspiration which then evaporates from the skin. Evaporation is a heat losing mechanism through which a liquid changes to a gaseous state due to an increase in temperature.
The anterior hypothalamus also decreases the activity of the sympathetic nervous system in skin blood vessels which means the arterioles dilate and the blood flow increases.
More blood flow leads to more warm blood from the body core taken to the body surface, from where heat is lost by radiation, conduction or convection.
Now, radiation occurs between any two objects when their temperature differs. For example, on a sunny day the radiation from the sun warms the skin.
Conduction is the transfer of heat between objects that are in direct contact with each other. For instance, when holding an ice pack on the forehead, the internally generated heat is transferred to the ice pack.
And convection refers to the transfer of heat by the movement of air or liquid moving past the body. That explains why a breeze across the skin may cool one down.

Effects of uncontrolled body temperature5:52–6:28

Normally, the body response to variations in environmental temperature is well controlled. However, there are some situations when this control is disturbed.
For instance, heat exhaustion happens when one is exposed to high temperatures, and this condition can lead to dizziness, rapid heartbeat and fainting.
Heat stroke occurs when the core body temperature is higher than 40°C (104 °F) leading to confusion, seizures or even coma.
Malignant hyperthermia happens when heat dissipating mechanisms are unable to keep pace with an excessive heat production, possibly leading to life-threatening consequences.

Infections6:28–7:26

Finally, a particular situation describing an abnormal increase of body temperature is fever, which most frequently occurs in response to infections.
Now, during an infection, the microbes produce pyrogens, which are molecules that increase the production of interleukin-1 in phagocytic cells, like neutrophils.
Interleukin-1 then signals the anterior hypothalamus to increase local prostaglandin production, which are lipid molecules that increase the set point temperature, basically, telling the anterior hypothalamus that the right body temperature is waaay above 37 degrees celsius.
The result of this is that the new set point will make the normal core temperature appear too low and so the anterior hypothalamus activates heat generating mechanisms, such as shivering, to increase body temperature up to the value of the new set point.
Due to the fact that prostaglandins are so important in the fever appearing mechanism, medications like aspirin, which blocks their synthesis pathway, can be used to reduce fever.
Alright, as a quick recap: Thermoregulation is the ability of the organism to keep its temperature at a normal value of 37°C, or 98.6°F.

Review7:26–7:53

The part of the body responsible for that is the brain in a place called hypothalamus which receives information about the temperature status from some specialized nerve cells called thermoreceptors.
Disturbances in the control of temperature can lead to several conditions, such as heat stroke or heat exhaustion.