Body temperature and osmotic balance
Thermoregulation
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Purpose: to hold core temperature near a set point of about 37 °C in humans, so enzymes operate near their optimum. Too cold and reaction rates fall; too hot and enzymes are denatured as their tertiary structure is disrupted.
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Components:
- Receptors — peripheral thermoreceptors in the skin detect external temperature; central thermoreceptors in the hypothalamus detect the temperature of the blood, which reflects core temperature.
- Control centre — the hypothalamus.
- Effectors — skin arterioles, sweat glands, skeletal muscle, and endocrine glands adjusting metabolic rate.
Responses to overheating
- Vasodilation. The arterioles supplying the skin capillaries dilate, so more blood flows near the surface. Heat is then lost by radiation and conduction to the surroundings. The skin appears flushed.
- Note the common error: it is the arterioles that dilate, not the capillaries — capillaries have no muscle in their walls and cannot change diameter.
- Sweating. Sweat glands secrete water onto the skin surface. As it evaporates, it takes latent heat of vaporisation from the body, cooling it.
- Evaporation is what cools, not the sweat itself. In high humidity the water potential gradient between skin and air is small, so evaporation is slow and sweating is much less effective — which is why humid heat is more dangerous than dry heat.
- Reduced metabolic rate and reduced muscular activity, generating less heat internally.
- Behavioural responses — moving to shade, reducing activity.
Responses to cooling
- Vasoconstriction. Skin arterioles constrict, so less blood flows near the surface and less heat is lost by radiation.
- Shivering. Rapid involuntary contraction of skeletal muscle. Muscle contraction requires respiration, which is not fully efficient, so much of the energy is released as heat.
- Increased metabolic rate, raised by thyroxine and adrenaline, generating more heat from respiration.
- Piloerection — hair-raising muscles contract, trapping an insulating layer of still air. Effective in furred mammals, of little practical value in humans.
- Behavioural responses — seeking shelter, huddling, increasing activity.
Osmotic balance (osmoregulation)
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Purpose: to maintain the water potential of blood and tissue fluid within narrow limits.
- If blood water potential falls too low, water leaves cells by osmosis, so they shrink and their contents become too concentrated for reactions to proceed normally.
- If it rises too high, water enters cells, which swell and may lyse.
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Components:
- Receptors — osmoreceptors in the hypothalamus, which detect the water potential of the blood.
- Control centre — the hypothalamus, which triggers release of antidiuretic hormone (ADH) from the posterior pituitary.
- Effector — the kidney, specifically the collecting duct and distal convoluted tubule.
How ADH works
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This is the biophysical mechanism the Excellence criterion refers to, and it is worth knowing precisely.
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When blood water potential falls (dehydration, salt intake, sweating):
- Osmoreceptors in the hypothalamus detect the fall.
- ADH is released from the posterior pituitary into the blood.
- ADH binds to receptors on the collecting duct cells, causing vesicles containing aquaporins — water channel proteins — to fuse with the membrane facing the tubule.
- This increases the permeability of the collecting duct to water.
- More water is therefore reabsorbed by osmosis, down the water potential gradient into the concentrated tissue fluid of the medulla.
- The result is a small volume of concentrated urine, and blood water potential rises toward the set point.
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When blood water potential rises (drinking a large volume):
- Osmoreceptors detect the rise, and ADH secretion is inhibited.
- Aquaporins are removed from the membrane, so the collecting duct becomes less permeable to water.
- Less water is reabsorbed, producing a large volume of dilute urine, and blood water potential falls toward the set point.
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The key biophysical point: ADH does not pump water. It changes membrane permeability, and water then moves passively by osmosis down an existing water potential gradient. The gradient itself is maintained by the loop of Henle.
Selective advantage
- Thermoregulation allows an animal to remain active across a wide range of external temperatures, with enzymes near their optimum continuously — so it can forage, escape predators and reproduce when non-regulating competitors are sluggish.
- Osmoregulation allows an animal to maintain cell function despite variable water and salt intake, and is what allows occupation of environments where water is scarce or intermittent. The ability to produce concentrated urine specifically permits survival on limited water, which is why desert mammals have exceptionally long loops of Henle.
Worked Example
Worked Example
A person exercises hard in hot, dry conditions for two hours without drinking.
- Core temperature rises from 37.0 °C to 38.1 °C, then stabilises.
- Sweat rate is high throughout.
- Urine volume falls sharply, and the urine produced is dark and concentrated.
- Blood ADH concentration is measured as elevated.
Explain the responses shown, identifying the control systems involved and how each restores stability.
Answer:
Two control systems are operating at once, and they interact — which is what makes this scenario worth analysing.
System 1: thermoregulation.
Exercise increases the rate of respiration in muscle, and because respiration is not fully efficient, much of the energy is released as heat. Core temperature therefore rises.
- Receptors: central thermoreceptors in the hypothalamus detect the raised blood temperature; peripheral thermoreceptors detect the hot environment.
- Control centre: the hypothalamus.
- Effectors and responses:
- Vasodilation of skin arterioles increases blood flow near the surface, increasing heat loss by radiation. However, in a hot environment this is limited, because heat loss by radiation depends on a temperature gradient between skin and air, and that gradient is small.
- Sweating at a high rate. Water evaporating from the skin takes latent heat of vaporisation from the body. Because the air is dry, the water potential gradient between skin and air is large, so evaporation is rapid and this is the principal cooling mechanism here.
Why temperature stabilised above the set point. Core temperature settled at 38.1 °C rather than returning to 37.0 °C because heat is still being produced faster than at rest. The system reached a new balance where heat loss equalled the raised rate of heat production — so it is limiting the rise rather than fully correcting it. This shows negative feedback opposing a change without necessarily eliminating it when the disturbance is continuous.
System 2: osmoregulation.
Sweating loses a large volume of water, so the water potential of the blood falls (becomes more negative).
- Receptors: osmoreceptors in the hypothalamus detect the fall in blood water potential.
- Control centre: the hypothalamus, which triggers release of ADH from the posterior pituitary — which is why measured ADH is elevated.
- Effector: the collecting duct of the kidney.
- Mechanism: ADH causes vesicles containing aquaporins to fuse with the collecting duct membrane, increasing its permeability to water. Water is then reabsorbed passively by osmosis, down the water potential gradient into the concentrated tissue fluid of the medulla.
- Result: a small volume of concentrated urine — exactly what was observed — so water is conserved and blood water potential rises back toward the set point.
How the two systems interact — and conflict.
This is the important part. The two systems make competing demands on the same resource:
- Thermoregulation requires losing water as sweat, to lose heat by evaporation.
- Osmoregulation requires conserving water, to maintain blood water potential.
The body prioritises temperature, continuing to sweat even as dehydration develops, because a rise in core temperature of a few degrees would denature enzymes and is rapidly fatal, whereas moderate dehydration is survivable for longer. The kidney response is therefore compensating for a loss the thermoregulatory system is deliberately continuing to make.
Why this cannot continue indefinitely. If water is not replaced, blood volume falls, which reduces the blood available for delivery to the skin, so vasodilation becomes less effective and sweat production itself eventually declines. At that point the cooling mechanism fails and core temperature rises steeply — which is how heat illness develops. The two systems are not independent, and failure of one propagates to the other.