Thermoregulation is the body’s ability to balance between heat gain and heat loss, thus maintaining a stable internal temperature of around 36 to 37.5° C or 96.8 to 99.5° F. This range is considered normal body temperature but it’s actually the average across the population. Normal body temperature can vary depending on the time of day, age of the individual, activity level, and many other factors. This normal body temperature is maintained by four mechanisms: neural and vascular control; heat production; heat loss; and behavioral control.
Okay, let’s discuss the physiology of these four mechanisms, starting with neural and vascular control. If we zoom into the brain, there's a region called hypothalamus. The hypothalamus acts like a thermometer. It has thermoreceptors that monitor the body’s internal, or core, temperature; and it also receives information about the peripheral temperature from the thermoreceptors in the skin.
Now, the hypothalamus is also the body’s thermostat, because it controls the internal temperature “set point” and when the core temperature deviates from the set point, the hypothalamus will signal the rest of the body to increase or decrease the body temperature and bring it back to the set point. So, on a hot summer day, the hypothalamus senses the core body temperature is getting too high and responds by initiating mechanisms like sweating and dilating skin blood vessels, resulting in body heat loss. On the flip side, on a cold winter day, the hypothalamus senses that the core body temperature is getting too low, and in turn, activates mechanisms like shivering which generates heat, and vasoconstriction in the skin that prevents heat loss.
Now normally the set point is 36 to 37.5° C or 96.8 to 99.5 °F; however, when the person has a fever, the hypothalamus can adjust the thermostat, raising the set point to a higher temperature. The body responds by increasing the core temperature to the new set point, which is helpful for fighting off infections.
The next mechanism is heat production, which is a result of internal metabolic processes that produce heat as a byproduct. All tissues in the body produce heat, but a major source of heat production comes from the muscles through voluntary movements like exercising, and involuntary movements like shivering. In neonates, heat production can happen without muscle activity, called non-shivering thermogenesis. This is because they have a higher percentage of “brown fat” than adults, which is more effective at generating heat.
Next up is heat loss, which is mostly a function of the skin that can happen in four ways: radiation, conduction, convection, and evaporation. Most body heat is lost through radiation which is the emission of body heat and the transfer of that heat to another object without touching. Heat generated deep inside the body is carried by blood to the peripheral vessels close to the skin where it can leave the body via radiation. So the body is constantly giving off heat like a radiator. When the peripheral vessels dilate and there’s more blood flow, more heat is lost. When these vessels constrict, there’s less blood flow and less heat is lost.
Conduction is the transfer of body heat to another object by touching it. For example, a person with a fever may use an ice pack which absorbs some of the body heat and cools their forehead. Convection is losing heat when air or water movement carries the heat away, so think of using a fan to cool down during a hot summer day. Last but not least is evaporation, which is losing heat by converting fluid into gas. This is most commonly seen in sweating where the sweat on your skin evaporates from body heat, taking that heat away. When it comes to behavioral control, this is as simple as moving to a colder or warmer environment or adding or removing clothing. However, this mechanism depends on factors like the availability of shelter and clothing, and the ability of the person to perform the necessary action.
Now, when the body’s thermoregulatory mechanisms can no longer keep the internal body temperature stable, it’s called hyperthermia if the body temperature is too high and hypothermia if the body temperature is too low. So, hyperthermia is when body temperature reaches above 36 to 37.5°C. It’s most commonly caused by extremely high environmental temperature, which decreases the body’s effectiveness in getting rid of body heat generated from metabolic processes. This is especially dangerous if combined with other factors that increase heat generation like exercise or increased metabolism from hyperthyroidism, infections, and serotonin syndrome. Additional causes of hyperthermia include anything that causes neurological damage such as trauma to the brain or spinal cord.