How (the typical) body tries to keep cool: Thermoregulation 101

Illustration of a brain, showing the hypothalamus highlighted with the basal ganglia around it, surrounded by the neocortex. Below the basal ganglia is the amygdala, and below that the hippocampus

Among the hypothalamus’ roles is maintaining thermal homeostasis. Homeostasis comes from the Greek words homo, meaning “same” and stasis, meaning “steady”. Minimal deviation to core temperature is essential to survival.

Typically a relay of messages takes place inside the body. Sensors in the skin tell the hypothalamus that it is hot and it responds by sending a message to the sympathetic nervous system to open up blood vessels (vasodilation), drop blood pressure, elevate the heart rate, and tell adrenal glands to start producing sweat. As the sweat evaporates there is a transfer of heat that occurs when water changes from a liquid to a gas (vaporization) and this cools the body. This is why humidity plays a crucial role in human health in hot weather. If the air is humid and sweat doesn’t evaporate the body is unable to cool.

Diabetes mellitus, multiple sclerosis, spinal cord injury, and anhidrotic ectodermal dysplasia are among the conditions associated with a significant reduction in sweating.

Sweating results in loss of fluid and electrolytes. When these losses cannot be replaced with intake, dehydration and electrolyte disturbances can result. Dehydration can cause reduced blood flow to the kidneys which can either lead to a temporary drop in kidney function or an injury to the kidneys themselves. . But the endocrine system, the liver, pancreas, gastrointestinal system and even the skeletal muscles are also impacted by heat and/or the process of thermoregulation so any condition could play a role in how your body and mind respond to heat.

Once the brain is alerted that the body is in a hot environment it typically begins the process of trying to cool that body. But it’s important to be aware that in addition to how different conditions might alter that response, age is also a factor on its own.

There are two kinds of sweat glands (eccrine and apocrine) in our skin. Both are present at birth but only one (eccrine) is active in infants and young children. The second type (apocrine) becomes active during puberty in most people. This means that infants and children do sweat but they sweat less than adults. This is important to keep in mind as, again, sweating is the main mechanism the body has for cooling itself.

a cubic cross section of skin, labelled with “thin epidermis / results in slower repair functions, decreased vitamin d production and reduced number of langerhans cells” , “dry epidermis / sebaceous and sweat glands’ activity reduced”, “thin dermis / wrinkling and sagging from collagen fibre loss”, “fewer active follicles / hair is thinner and sparser”, “changes in distribution of fat & hair / due to sex hormone level reduction”, “reduced blood supply / leads to slower healing and lessened ability to reduce heat”, “reduced sweat gland activity / leads to tendency to overheat”, “fewer melanocytes / results in paler skin & reduced tolerance to sun exposure”.

Skin (as shown in the diagram) consists of layers - the epidermis (the outer most layer), the dermis (the thickest layer) and the hypodermis (the innermost subcutaneous tissue). In hot weather, the sympathetic nervous system stimulates the dermal blood vessels to dilate (vasodilation) allowing sweat glands to increase the amount of sweat from the usual 500 ml/day up to as much as 12L a day. The evaporation of sweat cools the body and prevents it from overheating.

If it is too humid the sweat is unable to evaporate and the body is unable to cool itself.

As well, there are intrinsic (time) and extrinsic causes of ageing of skin. Extrinsic causes include exposure to UV light, trauma to the skin, smoking and certain chemicals. These will change the skin’s ability to play its role in thermoregulation.

The dermis becomes thinner and the blood flow is reduced as is the amount of sweat that is produced.

And, again, disability can occur at any age.

A graphic showing 3 humans. The text on the top left reads “the body’s system for thermoregulating (maintaining a consistent core temperature) changes over a lifetime and can be different for some disabled people of any age”. The first illustration is of a baby, labelled “babies and toddlers”, with a thin red line around them labelled “skin blood flow”; there are 3 thin, wavy arrows pointing away from them, labelled “heat release.” The second illustration is of an adult, labelled “typical, non-disabled young adult”, with a thick red line around them labelled “skin blood flow”; there are 5 thick, wavy arrows pointing away from them labelled “heat release”. The third illustration is of an adult, labelled “older or disabled adult”, with a thin red line around them labelled “skin blood flow”; there are 3 thin wavy arrows pointing away from them labelled “heat release”.

power chair user and bedbound person_v2-02 (1).png

Cholesterol-narrowed arteries can limit blood flow to the skin. People with heart failure are at particular risk. The extra work for the heart, along with the loss of sodium and potassium and the release of stress hormones, can lead to trouble. In addition to the stress on the heart the increased blood flow to the skin combined with dehydration may cause blood pressure drops that result in dizziness or falls.

People with mental illness have a two to three-fold greater risk of mortality during extreme heat. There is some evidence that certain mental illnesses may affect the way the brain is able to alert someone that they are unwell. And, in addition to the conditions themselves, some psychiatric medications have the potential to impair the body’s heat regulatory functioning; sweating and dehydration can increase lithium levels, putting patients at greater risk for dangerous toxicity during heat waves. Source

Pregnancy: Multiple studies found that preterm births increased by 16 percent during heat waves. Four U.S. studies found, compared to white women, Black or Hispanic women with heat exposure had double or more the risk of preterm birth or stillbirth. Pregnancy elevates the body’s temperature and as a result pregnant people who work outdoors in high temperatures have a higher risk for heat exhaustion and possible effects on the fetus resulting in preterm birth, low birth weight, heart birth defects, and infant mortality. Pregnant people who work outside are at particular risk during hot weather.

Note: Babies in disposable diapers and adults wearing disposable incontinence briefs can face additional risk of heat rash in those areas during hot weather.

People with dysautonomia (which occurs in many chronic conditions) may experience worsening symptoms in the heat.

There are a number of studies documenting the heat impacts on

cognitive function and mood, mental health and well-being and people with mental illness can be particularly impacted by heat both as a result of their conditions, the medications, and/or the impacts of ableism.

In addition to wheelchair users and those in bed regardless of condition, experiencing diminished heat loss because of the body being in direct contact with a surface and thus no evaporation of sweat occurring, research suggests a significant risk of pressure sores developing as a result of the heat.

Additional Reading

Anatomy and Physiology of Ageing: Skin
Global Warming, Heat-related Illnesses and the Dermatologist
High Ambient Temperature and the Risk of Preterm Delivery

An illustration of a tan skin pregnant person with a red gradient emanating from their body’s core, next to a very hot thermometer. Underneath is the label “pregnant person”. Text on the left reads “Pregnancy naturally elevates the body’s core temperature, making pregnant people more susceptible to harm caused by heat. Additionally, fetuses exposed to extreme heat are more likely to suffer from fetal distress and breathing problems at birth.”