Thermoregulation and the Kidney

In the last lesson you met homeostasis and saw how blood glucose is held steady by negative feedback. This lesson applies exactly the same principle to two more vital conditions: body temperature and the water content of the body. First you will study thermoregulation — how the body keeps its core temperature near $37\,°\text{C}$37°C whether you are exercising in the heat or shivering in the cold. Then you will study the kidney, the organ that removes waste urea and balances the body's water and ions. Both are controlled by negative feedback, so the framework from the previous lesson carries straight over. This is part of Topic B3 of OCR Gateway Science A.

Higher / separate-science note: The detail of ADH and the control of water content, and the comparison of dialysis versus transplant, go beyond the Foundation core. These sections are flagged Higher tier where appropriate.

By the end of this lesson you should be able to explain how the body responds to overheating and cooling, describe the role of the kidney in removing urea and balancing water, explain (Higher) how ADH controls water content by negative feedback, and compare dialysis with a kidney transplant.

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Thermoregulation: Controlling Body Temperature

The human body works best at a core temperature of about $37\,°\text{C}$37°C — the temperature at which its enzymes are most active. If the body gets too hot, enzymes may begin to denature; if it gets too cold, reactions slow down too much. Keeping the temperature steady is therefore essential, and this is called thermoregulation.

Body temperature is monitored and controlled by the thermoregulatory centre in the brain. This centre contains receptors that detect the temperature of the blood flowing through it. There are also temperature receptors in the skin that send impulses to the centre about the temperature of the surroundings. When the centre detects that the body is too hot or too cold, it triggers responses through effectors in the skin and muscles — a textbook example of negative feedback.

The skin and its role in temperature control

The skin has three structures that matter for temperature control: sweat glands, blood vessels near the surface, and hairs controlled by tiny erector muscles.

When the body is too hot

The thermoregulatory centre detects that the blood is too warm and triggers responses to lose heat:

• Vasodilation: the blood vessels supplying the skin capillaries widen (dilate), so more blood flows near the surface and more heat is lost to the surroundings by radiation. (This is why a hot person looks flushed.)
• Sweating: the sweat glands release more sweat onto the skin. As the sweat evaporates, it takes heat energy from the body, cooling it down.
• The hairs lie flat (the erector muscles relax), so less air is trapped and heat escapes more easily.

When the body is too cold

The thermoregulatory centre detects that the blood is too cool and triggers responses to conserve and generate heat:

• Vasoconstriction: the blood vessels supplying the skin capillaries narrow (constrict), so less blood flows near the surface and less heat is lost.
• Sweating stops, so no cooling by evaporation occurs.
• Shivering: the muscles contract rapidly and repeatedly; this muscle contraction requires respiration, which releases heat and warms the body.
• The hairs stand up (the erector muscles contract), trapping a layer of insulating air next to the skin to reduce heat loss.

Condition	Blood vessels	Sweat	Hairs	Shivering
Too hot	Vasodilation (widen)	More sweat (evaporation cools)	Lie flat	No
Too cold	Vasoconstriction (narrow)	Less/no sweat	Stand up	Yes (releases heat)

Exam Tip: "Vasodilation" does not mean the blood vessels move closer to the surface — they cannot move. It means the vessels widen, so more warm blood flows through the capillaries already near the surface and more heat is lost. Use the words vasodilation (cooling) and vasoconstriction (warming) precisely.

Exam Tip: Be careful with sweating and shivering. Sweating cools you down (evaporation removes heat). Shivering warms you up (muscle contraction releases heat from respiration). Mixing these up loses easy marks.

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The Kidney

The kidneys carry out two homeostatic jobs at once: they remove the waste product urea from the blood, and they balance the water and ions in the body. There are two kidneys, each connected to the bladder by a tube called the ureter.

Where urea comes from

When you eat more protein than you need, the body cannot store the excess amino acids. Instead they are broken down in the liver in a process called deamination, which produces ammonia. Ammonia is toxic, so the liver immediately converts it into the less harmful waste product urea. The urea is carried in the blood to the kidneys, which remove it and excrete it in the urine.

How the kidney works: filtration and selective reabsorption

The kidney cleans the blood in two stages:

1. Filtration. Blood at high pressure is filtered in the kidney. Small molecules — water, glucose, ions and urea — are forced out of the blood and into the kidney tubules. Large molecules such as proteins and blood cells are too big to be filtered out and stay in the blood.
2. Selective reabsorption. The filtered liquid still contains useful substances, so the kidney reabsorbs the ones the body needs back into the blood: all of the glucose, as much water as the body needs, and the right amount of ions. The urea, excess water and excess ions are not reabsorbed and pass on to form urine.

flowchart TD
  A["Blood enters the kidney"] --> B["FILTRATION:<br/>water, glucose, ions and urea<br/>filtered out of the blood<br/>(proteins and cells stay in)"]
  B --> C["SELECTIVE REABSORPTION:<br/>all glucose, needed water<br/>and ions returned to the blood"]
  C --> D["Remaining urea, excess water<br/>and excess ions form URINE"]
  D --> E["Urine passes to the bladder"]

The clever part is selective reabsorption: glucose is far too valuable to lose, so all of it is taken back; the amount of water and ions reabsorbed is adjusted to keep the body balanced; and the waste urea is left behind to be excreted.

Exam Tip: If healthy, there should be no glucose and no protein in the urine. Glucose is all reabsorbed; protein is never filtered out in the first place (its molecules are too big). Glucose appearing in urine is a classic sign of diabetes.

Controlling water content with ADH (Higher tier only)

Higher tier only: The amount of water reabsorbed by the kidney is controlled by a hormone called ADH (antidiuretic hormone), released by the pituitary gland. This is another negative-feedback system.