The human body must maintain a core temperature of approximately 37°C.
This is the OPTIMUM TEMPERATURE for human enzymes — the temperature at which they work most efficiently.
If temperature rises above ~40°C: enzymes denature → reactions stop → organ failure → death (hyperthermia).
If temperature falls below ~34°C: enzyme activity decreases dramatically → brain and heart function affected → loss of consciousness → death (hypothermia).
The body is constantly generating heat (from respiration in all cells) and losing heat (to the environment). Thermoregulation balances these.
The Hypothalamus — The Body's Thermostat
The HYPOTHALAMUS — a region in the brain — acts as the body's thermostat.
It has two types of thermoreceptors:
CENTRAL THERMORECEPTORS in the hypothalamus itself — monitor the temperature of blood flowing through the brain.
PERIPHERAL THERMORECEPTORS in the skin — detect changes in surface temperature (giving advance warning of environmental temperature changes).
When the hypothalamus detects deviation from 37°C, it sends signals via the nervous system and hormones to trigger corrective responses.
Responses When Too Hot
When body temperature rises ABOVE 37°C, the hypothalamus triggers responses to LOSE heat:
SWEATING:
Sweat glands in the skin produce sweat (mainly water + salts).
Sweat spreads over the skin surface.
As water EVAPORATES from the skin, it absorbs latent heat from the body → body cools.
More sweating occurs in hot conditions or during exercise.
VASODILATION:
Blood vessels (arterioles) near the skin surface WIDEN.
More blood flows close to the skin surface.
More heat is lost by radiation and convection from the skin.
This is why the skin looks flushed or red when hot.
HAIRS LIE FLAT:
Erector pili muscles RELAX → hairs lie flat.
No trapped air layer → less insulation → more heat can escape.
(More significant in other mammals than in humans due to less body hair.)
Responses When Too Cold
When body temperature falls BELOW 37°C, the hypothalamus triggers responses to GENERATE and RETAIN heat:
SHIVERING:
Skeletal muscles contract rapidly and repeatedly.
Muscle contractions require RESPIRATION → releases heat as a by-product.
Shivering can increase heat production up to 5× the resting rate.
VASOCONSTRICTION:
Blood vessels near the skin surface NARROW.
Less blood flows close to the skin surface.
Less heat lost by radiation from the skin.
Fingers and toes may become pale or blue in extreme cold.
HAIRS STAND ON END:
Erector pili muscles CONTRACT → hairs raise upright.
Traps a layer of warm air close to the skin → insulation layer.
(More effective in other mammals — in humans this causes 'goosebumps').
REDUCED SWEATING:
Sweat glands become less active.
Less evaporative cooling.
ADRENALINE RELEASED:
Hormone that increases metabolic rate → more respiration → more heat produced.
⚠️ Common Mistake
VASODILATION means blood vessels WIDEN — MORE blood near skin surface — MORE heat lost — occurs when TOO HOT. VASOCONSTRICTION means blood vessels NARROW — LESS blood near skin surface — LESS heat lost — occurs when TOO COLD. Students often get these mixed up. Remember: Dilation = Diameter increases = more heat lost.
📌 Key Note
Too hot: sweat (evaporation cools), vasodilation (more heat lost from skin), hairs flat. Too cold: shiver (respiration generates heat), vasoconstriction (less heat lost), hairs raised (insulation). Hypothalamus = the thermostat.
🎯 Matching Activity — Too Hot or Too Cold?
Sort each response — does it happen when the body is too hot or too cold? — drag the symbols on the right to match the component names on the left.
Too hot
Drop here
Too hot
Drop here
Too cold
Drop here
Too cold
Drop here
Too hot
Drop here
Too cold
Drop here
Sweating — evaporation removes heat from the body surface
Hairs stand on end — traps air for insulation (causes goosebumps)
Hairs lie flat — less insulation, more heat can escape
Vasoconstriction — blood vessels narrow, less heat lost from skin
Vasodilation — blood vessels widen, more heat lost from skin surface
Shivering — rapid muscle contractions generate heat via respiration
⭐ Higher Tier Only
The hypothalamus monitors core blood temperature and skin temperature (via peripheral thermoreceptors). Vasoconstriction: arterioles near skin narrow → less blood near surface → less heat lost. Vasodilation: arterioles widen → more blood near surface → more heat lost. Sweating: evaporation removes latent heat. Shivering: rapid muscle contraction generates metabolic heat. Hairs erect: trap air as insulation. Students should be able to explain these mechanisms in terms of negative feedback.
🎯 Test Yourself
Question 1 of 3
1. Why does sweating cool the body down?
2. What is vasodilation and when does it occur?
3. Why does shivering help when you are cold?
⭐ How Well Do You Understand This Topic?
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