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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Temperature Regulation
Humans are homeotherms (warm-blooded animals) that need to maintain a constant internal body temperature of around 37°C regardless of the external environment. This process is called thermoregulation and is vital for our survival because enzymes that control metabolic reactions work optimally at this temperature.
Key Definitions:
If body temperature rises too high (hyperthermia), proteins can denature, enzymes stop working and cells die. If temperature falls too low (hypothermia), metabolic reactions slow down, potentially leading to organ failure. Even small deviations from 37°C can be dangerous!
Unlike cold-blooded animals (poikilotherms) that rely on their environment to regulate body temperature, humans actively generate, conserve, or lose heat to maintain a constant internal temperature.
The skin is our largest organ and plays a crucial role in temperature regulation. It contains various structures that help us respond to temperature changes:
Produce sweat that evaporates from the skin surface, removing heat from the body. A typical person has 2-4 million sweat glands!
Arterioles can dilate (vasodilation) or constrict (vasoconstriction) to control blood flow to the skin surface.
Connected to erector pili muscles that can raise or lower hair to trap or release heat.
The hypothalamus acts as the body's thermostat. It contains:
Temperature regulation is a perfect example of a negative feedback system. When body temperature changes, the hypothalamus initiates responses that reverse that change, bringing temperature back to normal.
1. Temperature deviates from 37°C
2. Hypothalamus detects change
3. Hypothalamus triggers appropriate responses
4. Responses bring temperature back to normal
5. Hypothalamus stops the responses
While the hypothalamus is the main control centre, we also have thermoreceptors in our skin that detect external temperature changes and send signals to the brain. This allows us to respond to environmental changes before they affect our core temperature.
When body temperature rises above 37°C, the hypothalamus triggers several mechanisms to increase heat loss:
Heat stroke occurs when the body's temperature regulation mechanisms are overwhelmed. During the 2003 European heatwave, over 70,000 excess deaths were recorded. Many victims were elderly people whose temperature regulation mechanisms were less efficient. Symptoms include a body temperature above 40°C, hot dry skin (sweating has stopped), confusion and unconsciousness. Heat stroke is a medical emergency requiring immediate cooling and fluid replacement.
When body temperature falls below 37°C, the hypothalamus triggers mechanisms to reduce heat loss and increase heat production:
When core body temperature falls below 35°C, hypothermia sets in. Symptoms progress from shivering and confusion to slurred speech, drowsiness and eventually unconsciousness and death if temperature continues to drop. Hypothermia is particularly dangerous because as it progresses, the victim becomes less able to help themselves.
Babies have a special type of fat called brown adipose tissue that can generate heat directly through a process called non-shivering thermogenesis. This helps compensate for their higher surface area to volume ratio, which makes them lose heat more quickly than adults.
Humans living in different climates have developed adaptations to help with temperature regulation:
People from hot regions often have more efficient sweating responses and higher sweat gland density. They may also have darker skin to protect from UV radiation.
People from cold regions tend to have more compact body shapes with lower surface area to volume ratios, reducing heat loss. They may also have more subcutaneous fat for insulation.
The body can adapt to new temperature conditions over time. For example, people who move to hot climates gradually develop more efficient sweating responses.
During infection, chemicals called pyrogens reset the hypothalamus's temperature set point to a higher level, causing a fever. This is actually a defence mechanism - the higher temperature makes the body less hospitable to pathogens and speeds up immune responses. Once the infection is controlled, the set point returns to normal and the body activates cooling mechanisms, which is why we often sweat as a fever "breaks".
Temperature regulation is a prime example of homeostasis - the maintenance of a stable internal environment. It demonstrates key principles that apply to other homeostatic systems:
Understanding temperature regulation helps us appreciate how our bodies maintain the precise conditions needed for optimal functioning, even in challenging environments.
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