Human thermoregulation in a warm environment

Thermoregulation is the ability for an organism to control body temperature within a specified physiological range even when environmental temperatures vary.

A recent article published in Physiological Reviews explored thermoregulation in humans and how various factors such as age, body shape, adaptation, and biological sex impact a person’s ability to maintain body temperature. They also explored how various diseases such as multiple sclerosis, diabetes, cardiovascular diseases as well as injuries (e.g. skin grafts, burns) affected thermoregulation during heat stress.  

Humans use a variety of behavioral mechanisms to regulate body temperature in a hot environment including seeking shade, water sources, going inside, wearing cool clothing, drinking something cold, etc. There are also two main physiological ways humans get rid of excess body heat in a hot environment – evaporative heat loss (i.e. sweating) and blood vessel dilation, which brings the warm blood close to the skin where heat can dissipate to the environment. If the amount of heat loss is the same as the amount of heat produced through metabolism or exercise, then body temperature does not change. But if heat production is greater than heat loss, body temperature increases.

For humans, normal internal body temperature ranges from about 36-38°C. Heat stress can happen at body temperatures ranging from 38-40°C, whereas heat injury and stroke can happen at temperatures above 40°C. Temperatures that high can damage and even kill cells. Although temperatures in this zone do not always cause cell damage, however. For example, the body temperature of an elite athlete can reach 41.9°C after a race without causing heat illness.

Thermoregulation and heat stress in animals

Endothermic species, like humans and mammals, produce their own body heat. Ectothermic (or poikilothermic) species, on the other hand, obtain their body heat from the environment and can regulate body temperature by moving from one location to another.

Like humans, endothermic animals use behavioral and physiological mechanisms to regulate body temperature. The mechanisms used vary between animals that have feathers, fur, or glabrous (i.e. hairless) skin as reviewed recently by Mota-Rojas and colleagues (2021):

Glabrous skin – Behavioral mechanisms used by hairless animals to deal with heat include reducing movements and energy intake as well as seeking cooler environments. Physiological mechanisms include sweating, panting, and dilation of blood vessels in the skin. They can also lose heat from the skin through radiation, conduction, convection, and evaporation. They may also lose heat through respiratory evaporation.

Hair – Like hairless animals, behavioral mechanisms used by furry animals include reducing movements and energy intake as well as seeking cooler environments. They are also known to stretch out their bodies to promote heat loss. Although these animals do not sweat, physiological mechanisms include panting and dilation of blood vessels in the skin to promote heat loss via radiation, conduction, and convection.

Feathers – Like mammals, birds can cool off using behavioral methods such as reducing movements and energy intake as well as seeking cooler environments. They are also known to extend limbs to increase the surface area and promote heat loss. Additional methods include splashing water on feather-free areas. Physiological mechanisms include panting and dilation of blood vessels in their extremities. They can also lose heat from the skin through radiation, conduction, convection, and evaporation and they may lose heat through respiratory as well as cloacal evaporation. Because feathers are very good insulators, birds are at risk of heat stress.

How do you stay cool?

Sources

MN Cramer, D Gagnon, O Laitano, CG Crandall. Human temperature regulation under heat stress in health, disease, and injury. Physiological Reviews. 102(4): 1907-1989, 2022.

JA Akin. Homeostatic processes for thermoregulation. Nature Education Knowledge. 3(10), 2011.

D Mota-Rojas, CG Titto, A de Mira Geraldo, J Martinez-Burnes, J Gomez, A Hernandez-Avalos, A Casas, A Dominguez, N Jose, A Bertoni, B Reyes, AMF Pereira. Efficacy and function of feathers, hair, and glabrous skin in the thermoregulation strategies of domestic animals. Animals. 11(12): 3472, 2021.

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Categories: Climate Change, Environment, Nature's Solutions

Tags: American Physiological Society, body temperature, Climate Change, Exercise, heat, heat stress, Physiological Reviews, thermoregulation