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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Sweating and Vasodilation
Your body is amazing at keeping its temperature just right - not too hot, not too cold. This process is called thermoregulation and it's super important for survival. Humans are endotherms (warm-blooded), which means we maintain a fairly constant internal temperature regardless of the environment. Our optimal body temperature is around 37°C and even small deviations can cause problems!
Key Definitions:
Your body has special temperature sensors called thermoreceptors located in your skin and hypothalamus (a region in your brain). These constantly monitor your internal and external temperature. When they detect a rise in temperature above the normal range, they send signals to the brain's temperature control centre - the hypothalamus.
The hypothalamus works like a thermostat in your home. When it receives signals that your body is getting too hot, it triggers responses to cool you down. The two main cooling mechanisms are sweating and vasodilation. These processes work together to help your body lose excess heat and return to its optimal temperature.
Sweating is one of the most effective ways your body cools itself. Let's explore how this clever system works!
When your body temperature rises, the hypothalamus sends signals to activate millions of tiny sweat glands in your skin. These glands produce sweat - a mixture of water, salt and other substances. The sweat travels through ducts to reach the skin surface, where it evaporates. This evaporation process requires heat energy, which is taken from your skin, cooling you down.
Sweat is about 99% water with small amounts of salt (sodium chloride), urea, lactic acid and minerals. This is why you need to replace both water and electrolytes when sweating heavily.
Humans have 2-4 million sweat glands! The two main types are eccrine glands (all over the body, produce watery sweat) and apocrine glands (in armpits and groin, produce thicker sweat).
When 1 litre of sweat evaporates from your skin, it removes about 580 kilocalories of heat energy from your body. On hot days, you can produce up to 12 litres of sweat!
The cooling effect of sweating only works when sweat can evaporate. In very humid conditions, sweat doesn't evaporate easily, which is why humid heat feels more uncomfortable than dry heat. This is also why fans help you feel cooler - they increase the rate of evaporation from your skin!
While sweating works to cool your body through evaporation, vasodilation helps by bringing more warm blood to the skin surface where heat can be lost to the environment.
When your body temperature rises, the hypothalamus sends signals that cause blood vessels near the skin surface (called arterioles) to widen or dilate. This process is called vasodilation. When these blood vessels expand, more blood can flow through them, bringing heat from your core to your skin where it can be lost to the environment.
During vasodilation, smooth muscles in the walls of arterioles relax, making the vessels wider. This allows more blood to flow through them. The increased blood flow to the skin makes it look redder (that's why you look flushed when hot). Heat from this blood transfers to the environment through radiation (direct heat transfer), conduction (transfer to objects in contact with skin) and convection (transfer to moving air).
Vasodilation can increase blood flow to the skin by up to 8 times the normal rate! This dramatically increases heat loss. However, there's a trade-off - blood diverted to the skin means less blood available for muscles and other organs. This is why intense exercise in hot weather is particularly challenging and can lead to heat exhaustion if you're not careful.
Sweating and vasodilation don't work in isolation - they're part of a coordinated response to help maintain your body temperature.
Thermoregulation works through a negative feedback loop. Here's how it happens when you get too hot:
During exercise, your muscles generate a lot of heat as they contract. This can raise your core temperature significantly. Your body responds by increasing both sweating and vasodilation to prevent overheating. A trained athlete's body becomes more efficient at thermoregulation - they start sweating earlier and produce more sweat, allowing for better cooling during intense exercise. This is one reason why regular exercise improves your heat tolerance!
Sometimes the body's cooling mechanisms can't cope with extreme conditions or may malfunction due to various factors.
Heat exhaustion occurs when the body can't cool itself efficiently, leading to symptoms like heavy sweating, weakness, dizziness and nausea. If not treated, it can progress to heat stroke, a life-threatening condition where body temperature rises above 40°C, sweating stops and confusion or unconsciousness may occur. Heat stroke requires immediate medical attention.
Hyperhidrosis is excessive sweating beyond what's needed for thermoregulation. Anhidrosis is the inability to sweat normally, which can be dangerous as it prevents proper cooling. Certain medications, nerve damage and skin conditions can affect sweating ability. People with these conditions need to take extra precautions in hot environments.
Understanding how sweating and vasodilation work has practical applications in everyday life and various fields.
Knowledge of thermoregulation can help you stay safe in hot weather:
Humans' exceptional sweating ability is actually an evolutionary advantage! While many mammals rely primarily on panting to cool down, humans can sweat over their entire body surface. This efficient cooling system allowed our ancestors to become persistence hunters - chasing prey until they overheated, even though the humans could keep going thanks to their superior cooling abilities.
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