The human body operates within a remarkably narrow temperature range. When the surrounding environment becomes extremely hot, the body does not simply “feel uncomfortable” it begins a complex physiological battle to keep its core temperature stable.
And as heatwaves become more frequent and intense, understanding that battle is becoming increasingly important.
Your first line of defence is sweating
When your body detects rising temperature, the brain activates several cooling mechanisms. Blood vessels near the skin widen, increasing blood flow to the surface so heat can escape. Sweat glands release fluid, which cools the skin as it evaporates.
But there is a limit.
If the air is extremely humid, sweat evaporates less efficiently. This means the body can produce plenty of sweat while gaining relatively little cooling benefit. Heat, therefore, is not determined by temperature alone the combination of temperature, humidity, radiation, wind and activity matters.
The heart starts working harder
Sending more blood toward the skin requires the cardiovascular system to compensate. At the same time, sweating reduces body water and can decrease circulating blood volume.
The heart may therefore have to beat faster to maintain circulation.
A 2025 meta-analysis combining more than 400 laboratory heat-exposure studies and over 6,800 participant exposures found substantial increases in core temperature and heart rate during heat exposure, demonstrating the cardiovascular workload imposed by rising temperatures.
For people with cardiovascular disease, this additional strain can become particularly dangerous. Extreme heat has been associated with increased risks of heart failure, arrhythmias, heart attacks and stroke.
Your kidneys also pay a price
Sweating means losing water and electrolytes. The kidneys respond by conserving fluid, but prolonged heat exposure can place considerable stress on the renal system.
Repeated heat exposure particularly combined with strenuous physical work and dehydration has been associated with acute kidney injury and may contribute to chronic kidney disease in some heavily exposed populations.
Eventually, the brain is affected
As core temperature continues rising, the nervous system becomes increasingly vulnerable. Heatstroke can cause confusion, altered consciousness, seizures and, in severe cases, coma and multi-organ failure.
Recent research also indicates that heat stress can alter cerebral blood flow, potentially reducing the brain’s ability to dissipate heat.
When cooling fails
The progression can move from heat stress → heat exhaustion → heatstroke.
Heatstroke is a medical emergency because the body’s temperature-regulation system is no longer able to keep up with the environmental and metabolic heat load. Cellular damage, inflammation, impaired blood flow and organ dysfunction can follow rapidly.
The frightening part is that the danger does not require an extraordinary temperature in isolation. Duration, humidity, physical exertion, nighttime temperatures, acclimatization and individual health all influence how much heat the body can tolerate.
Earth getting hotter therefore means more than hotter afternoons.
It means that one of the most fundamental systems keeping us alive the ability to regulate our own temperature is being placed under increasing physiological stress.
Sources:
https://www.who.int/news-room/fact-sheets/detail/climate-change-heat-and-health?
