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How does the body control its temperature and the water content of the blood?

Explain thermoregulation by the skin including vasoconstriction, vasodilation and shivering, the structure and function of the kidney and nephron, and the role of ADH in osmoregulation.

A focused answer to Edexcel GCSE Biology 7.10B to 7.12B and 7.18B to 7.22B, covering thermoregulation by the skin, the structure and function of the kidney and nephron, the role of ADH in osmoregulation, and treatments for kidney failure.

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  1. What this dot point is asking
  2. Thermoregulation
  3. The kidney and the nephron
  4. ADH and osmoregulation
  5. Treating kidney failure
  6. Try this

What this dot point is asking

Edexcel statements 7.10B to 7.12B and 7.18B to 7.22B are Biology only. They want you to explain thermoregulation by the skin (including vasoconstriction, vasodilation and shivering), describe the structure and function of the kidney and nephron, explain the role of ADH in osmoregulation, and describe treatments for kidney failure.

Thermoregulation

A common exam trap: in vasodilation and vasoconstriction the blood vessels widen or narrow; they do not move up or down in the skin.

The kidney and the nephron

The kidneys remove waste and control the water and salt content of the blood. Each kidney contains thousands of tiny tubes called nephrons, where the blood is filtered and useful substances are reabsorbed:

  1. Filtration: in the glomerulus (a knot of capillaries) inside the Bowman's capsule, high pressure forces small molecules (water, glucose, ions and urea) out of the blood into the nephron. Large molecules and blood cells stay in the blood.
  2. Selective reabsorption: as the filtrate flows along the nephron, all the glucose, the useful mineral ions and much of the water are reabsorbed back into the blood.
  3. Urine formation: what remains, urea (made in the liver from excess amino acids), excess water and excess salts, continues as urine, which passes to the bladder.

ADH and osmoregulation

This is a negative-feedback system:

  • If the blood is too concentrated (dehydrated), the brain detects it and more ADH is released. ADH makes the collecting duct more permeable, so more water is reabsorbed, and a small volume of concentrated urine is made. Blood water content rises.
  • If the blood is too dilute, less ADH is released, less water is reabsorbed, and a large volume of dilute urine is made. Blood water content falls.

Treating kidney failure

If the kidneys fail, urea and excess water build up in the blood. Two treatments:

  • Dialysis: a machine filters the patient's blood, removing urea and excess substances across a partially permeable membrane, restoring the correct balance. It must be done regularly (often several times a week).
  • Kidney transplant (organ donation): a healthy kidney from a donor replaces the failed ones. It is a long-term solution, but donor organs are limited and the kidney may be rejected by the immune system, so immunosuppressant drugs are needed.

Try this

Q1. State what happens to the skin blood vessels when the body is too cold, and name the process. [2 marks]

  • Cue. They narrow (constrict), reducing blood flow near the surface so less heat is lost; this is vasoconstriction.

Q2. Name the waste substance removed from the blood by the kidney and where it is made. [2 marks]

  • Cue. Urea, made in the liver from the breakdown of excess amino acids.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20194 marksExplain how the body reduces its core temperature when a person becomes too hot.
Show worked answer →

A 4-mark explain question rewards the cooling responses linked to losing heat.

  1. Sweat glands release more sweat onto the skin; as the sweat evaporates it transfers heat away from the body, cooling it.
  2. The blood vessels supplying the skin capillaries widen (vasodilation), so more blood flows near the surface and more heat is lost by radiation.
  3. These changes are coordinated to bring the core temperature back down to normal.

Markers reward sweating and the cooling effect of evaporation, and vasodilation increasing heat loss at the surface. Saying the blood vessels move to the surface is wrong; they widen, they do not move.

Edexcel 20214 marksExplain how ADH controls the water content of the blood when a person is dehydrated.
Show worked answer →

A 4-mark explain question (Biology only) rewards the ADH negative-feedback loop.

  1. When the blood is too concentrated (dehydrated, low water content), this is detected by the brain (hypothalamus).
  2. The pituitary gland releases more ADH into the blood.
  3. ADH makes the collecting ducts of the kidney nephrons more permeable to water, so more water is reabsorbed back into the blood.
  4. As a result, a smaller volume of more concentrated urine is produced, and the water content of the blood rises back to normal.

Markers reward detection of low water content, more ADH released, increased permeability of the collecting duct, more water reabsorbed, and concentrated urine. Saying ADH adds water to the blood directly is wrong; it increases reabsorption.

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