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How the Body Cools Itself by Sweating
the Body Cools Itself by Sweating
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Sweating is one of the body’s main ways to keep internal temperature near a safe value. When muscles, warm air, sunlight, or fever add heat, the brain activates sweat glands in the skin. Sweat spreads onto the skin surface and removes heat as it evaporates. This cooling helps protect enzymes, organs, and the nervous system from overheating.
The process is controlled by the hypothalamus, a temperature control center in the brain. It sends nerve signals to eccrine sweat glands, which release mostly water with dissolved salts through pores. Nearby blood vessels widen so more warm blood flows close to the skin, where heat can leave the body. Sweating works best when air is dry enough for evaporation and when fluids are replaced by drinking water.
Key Facts
- Normal core body temperature is about 37°C, but it can vary slightly during the day.
- Evaporation cooling occurs because liquid sweat absorbs energy from the skin to become water vapor.
- Heat removed by evaporation can be estimated with Q = mL, where L for water is about 2.26 x 10^6 J/kg.
- Eccrine sweat glands are the main glands used for temperature regulation and are found over most of the body.
- Vasodilation increases skin blood flow, helping move heat from the body core to the skin surface.
- High humidity reduces sweating effectiveness because sweat evaporates more slowly.
Vocabulary
- Hypothalamus
- The part of the brain that helps regulate body temperature by controlling sweating, shivering, and blood vessel changes.
- Eccrine sweat gland
- A coiled gland in the skin that releases watery sweat through a duct to help cool the body.
- Evaporation
- The change of a liquid into a gas, which requires energy and can remove heat from a surface.
- Vasodilation
- The widening of blood vessels, which increases blood flow and heat transfer near the skin.
- Electrolyte
- A dissolved ion such as sodium or chloride that helps maintain fluid balance and nerve and muscle function.
Common Mistakes to Avoid
- Thinking sweat cools the body just by being on the skin. Sweat must evaporate to remove a large amount of heat, so dripping sweat that falls off provides much less cooling.
- Ignoring humidity when judging heat risk. Humid air slows evaporation, so the body may overheat even when it is producing plenty of sweat.
- Assuming all sweating means dehydration has already occurred. Sweating causes fluid loss, but dehydration depends on how much fluid is lost compared with how much is replaced.
- Forgetting that blood flow helps cooling. Sweat glands and skin blood vessels work together because vasodilation moves warm blood closer to the surface where heat can escape.
Practice Questions
- 1 A person evaporates 0.20 kg of sweat during exercise. Using Q = mL and L = 2.26 x 10^6 J/kg, how much heat is removed from the body?
- 2 During a hot practice, a student loses 1.2 L of sweat in 2 hours. If 1 L of sweat has a mass of about 1 kg, what is the average sweat loss rate in kg per hour?
- 3 Explain why a person can feel hotter on a humid 32°C day than on a dry 32°C day, even if the air temperature is the same.