This is how our body defends itself from extreme heat

With just one day left until the end of June 2025, Aemet says it's very likely to be the warmest on record in Spain . Reviewing data from the last 50 years, they've concluded that heat waves have come earlier and spread out over time, becoming more common this month.

Before we hit the fan button, turn on the air conditioner, or fan ourselves to cool down, our bodies have already set in motion to avoid the dangers of extreme heat .

Our body's ability to maintain constant levels of various parameters is called homeostasis, and within this, thermoregulation (our normal body temperature ranges between 36.5 and 37 ⁰C) is the most compromised by episodes of extreme heat, above 40 ⁰C. Thus, our body is forced to work harder to prevent an increase in heat that could compromise vital functions.

When faced with stress caused by excessive heat, the human body must cool itself. It does this through two mechanisms:

First, it redistributes blood flow to increase it in the skin through vasodilation, which improves heat transfer from the muscles to the skin and from the skin to the outside. Next, you sweat, because sweat evaporates and removes internal heat.

These physiological responses are necessary to limit the rise in internal temperature, but they can affect people differently depending on their age. Furthermore, they are aggravated by other illnesses or the use of certain medications, with consequent negative effects on the body.

Redistribution and increased blood flow to the skin, due to vasodilation, increases cardiac demand and decreases heart filling pressure.

This means our hearts must pump harder and faster, requiring more oxygen to the coronary tissue. In people with preexisting heart conditions, this extra demand could lead to cardiac ischemia (reduced blood flow), heart attack, and ultimately cardiovascular collapse.

Various studies have shown that cardiovascular disease is the leading cause of death during heat waves. And, given that nearly 500 million people worldwide are estimated to suffer from these conditions, any densely populated area affected by extreme heat will be at risk of increased mortality due to the combination of both factors.

The evaporation of sweat cools our bodies. However, it also means that, as water is lost, blood volume is reduced, compromising cardiovascular function. Furthermore, there is a risk of kidney damage and even acute kidney failure.

That's why it's so important to replenish our body's water deficit and avoid the effects of dehydration.

If our thermoregulation capacity fails, overheating can result in heat exhaustion, which can have long-term consequences due to damage to the central nervous system. It can even be fatal if not treated promptly.

Early symptoms of dizziness, disorientation, and seizures indicate a dysfunction that has been attributed to a possible combination of cerebral edema, cerebral ischemia, and metabolic disorders.

If, after blood redistribution, the lack of blood flow, or ischemia, persists, damage to cells, tissues, or organs can occur. Specifically, the brain, heart, kidneys, intestines, liver, and lungs are at greatest risk.

Heat-related lung damage is compounded by increased lung stress due to hyperventilation, which is directly related to the rise in temperature. Adding to this increased air pollution during heat waves is the second leading cause of mortality and morbidity during these episodes.

It turns out that heat also affects the functioning of the immune system. It's important to keep in mind that the innate immune system is activated by signs of tissue damage, that is, by the potential consequences of an infection with a pathogen we don't remember.

Furthermore, our immune system prioritizes, and its response to a threat (something it perceives as so dangerous that it could quickly end our life) forces it to ignore any other threat it considers less important, such as a cold, the flu, or even cancer. All of these responses are halted while the imminent danger is fought.

Many of the studies analyzing how the immune system reacts to extreme heat come from animal experiments—who cares about human volunteers to subject them to prolonged extreme heat?—but they can be extrapolated to humans.

The first concept that comes to mind when talking about heat and immunity is fever.

Fever is an increase in body temperature, not as a result of our immune system fighting a pathogen, but rather as a tool in that fight.

That is, the increase in body temperature is caused by a change in the internal temperature range considered optimal. It is triggered by immune cells, which secrete substances such as interleukins, interferon, and tumor necrosis factor. Under these conditions, the immune system achieves optimal function to fight a pathogen that potentially threatens to kill us.

As we have seen before, fever is very different from heat stroke, where body temperature increases due to external factors, but the internal temperature marked as optimal does not vary.

When external temperatures rise, proteins we share with yeast and flies, called "heat shock proteins" (HSPs), are released into the blood. Their function is to protect other, more sensitive proteins from potential conformational changes due to increased temperature that could affect their functionality. They also have pro-inflammatory activity: they act as sirens that warn the immune system of danger.

We have seen that, as a mechanism to lower body temperature, blood moves to the external capillaries to reduce temperature, and not enough blood reaches the rest of the body.

In the abdominal organs, changes occur in the intestinal mucosa, resulting in increased porosity. Bacterial debris is released into the bloodstream, which activates TLRs (Toll-Like Receptors) on immune cells.

At this point, our body reacts the same way as if we had an acute infection: everything is focused on fighting the supposed (nonexistent) pathogen. Intestinal tissue regeneration is inhibited, and what was already bad gets worse: lymphocytes respond and proliferate as if we had a bacterial infection in the blood (sepsis).

Given this situation, it's best to avoid extreme heat by taking the necessary measures. But not just the air conditioning control, the fan button, and the fan: it's essential to avoid prolonged exposure to high temperatures.

We must avoid outdoor work and exercise during the hottest hours and halt the global temperature rise that is plaguing us, causing these increasingly frequent episodes that put our health at risk.

*Article originally published in 'The Conversation'