What Is Temperature Regulation And Excretion? A Visual Guide for Edexcel IGCSE Human Biology Students

Temperature regulation and excretion sit at the heart of homeostasis. Understanding how the body maintains its internal environment is essential for both Edexcel IGCSE Human Biology papers, and this topic is one of the most commonly examined.

Key facts

The body maintains a core temperature of approximately 37 degrees Celsius, which is the optimum for enzyme activity
The hypothalamus acts as the body's thermostat, detecting changes in blood temperature and triggering corrective responses
Excretion is the removal of metabolic waste: CO₂, water and urea
The kidneys filter the blood, remove urea, and regulate water content through the action of ADH
The liver produces bile, deaminates amino acids to form urea, regulates blood glucose and detoxifies harmful substances
Negative feedback is the mechanism by which all of these systems self-correct

Temperature regulation: the problem and the solution

Enzymes drive every metabolic reaction in the body, and they work best at around 37 degrees Celsius. If body temperature rises significantly above this, enzymes begin to denature: the active site changes shape, and reactions slow or stop. If temperature drops too far, molecular motion decreases, collisions between enzyme and substrate become less frequent, and reaction rates fall. The body must therefore keep its core temperature within a narrow range, regardless of external conditions.

How the body responds to overheating

The hypothalamus detects a rise in blood temperature (or receives signals from heat receptors in the skin) and triggers three main responses:

Increased sweating - sweat glands release more sweat onto the skin surface. As the sweat evaporates, it absorbs heat energy from the skin, cooling the body. The cooling effect depends on evaporation, not the sweat itself.
Vasodilation - arterioles supplying blood to the skin capillaries widen. More blood flows close to the skin surface, and heat is lost to the environment by radiation. The arterioles do not move; they simply increase in diameter.
Body hairs lie flat - the erector muscles in the skin relax, causing hairs to lie against the skin. This reduces the insulating layer of trapped air.

How the body responds to cooling

Reduced sweating - sweat production decreases, reducing evaporative heat loss.
Vasoconstriction - arterioles near the skin narrow, reducing blood flow to the surface and conserving heat in the core.
Shivering - skeletal muscles contract and relax rapidly and involuntarily. These contractions require energy from increased respiration, which releases heat as a by-product.
Body hairs stand erect - erector muscles contract, raising the hairs. The trapped air layer acts as insulation.

Worked example: A student runs 400 metres on a warm afternoon. Explain two ways the body prevents overheating during and after the run. (4 marks)

Model answer: Sweat glands produce more sweat (1), which evaporates from the skin surface, removing heat energy (1). Vasodilation occurs: arterioles near the skin widen (1), allowing more blood to flow near the surface so heat is lost by radiation (1).

Excretion: what it means and why it matters

Excretion is the removal of metabolic waste from the body. Metabolic waste is produced by chemical reactions inside cells. The three main excretory products are:

Waste product	Produced by	Removed by
Carbon dioxide (CO₂)	Aerobic respiration in all cells	Lungs (exhaled)
Water (H₂O)	Respiration and other metabolic reactions	Lungs, skin (sweat), kidneys (urine)
Urea	Deamination of excess amino acids in the liver	Kidneys (dissolved in urine)

Faeces are not excretory products. Egestion (the removal of undigested food from the gut) is a separate process. This distinction is tested directly on the Edexcel IGCSE exam and is one of the most commonly confused definitions in the specification.

The kidneys and the renal system

The renal system includes two kidneys, two ureters, the bladder and the urethra. Each kidney contains approximately one million nephrons, each a microscopic filtering and reabsorption unit. The processes within the nephron are:

1. Ultrafiltration

Blood arrives at the kidney through the renal artery and enters the glomerulus, a knot of capillaries inside the Bowman's capsule. High blood pressure forces small molecules (water, glucose, amino acids, urea, ions) out of the blood and into the capsule. Large molecules (proteins) and blood cells cannot pass through the capillary walls and remain in the blood. The liquid that enters the capsule is called the filtrate.

2. Selective reabsorption

As the filtrate passes along the nephron tubules, useful substances are reabsorbed back into the blood:

All glucose is reabsorbed by active transport in the proximal convoluted tubule
Most water is reabsorbed by osmosis (the exact amount depends on ADH levels)
Some ions are reabsorbed as needed

What remains (water, urea, excess ions) continues along the tubule, through the collecting duct, and into the ureter as urine.

Why urine composition varies

On a hot day or after intense exercise, the body loses water through sweat. The blood becomes more concentrated. More ADH is released, more water is reabsorbed, and urine becomes small in volume and dark in colour. After drinking a large volume of water, the blood becomes more dilute. Less ADH is released, less water is reabsorbed, and urine is large in volume and pale.

ADH and osmoregulation

ADH (antidiuretic hormone) is the key hormone in osmoregulation. The hypothalamus detects changes in the water potential of the blood. When the blood is too concentrated, the hypothalamus signals the pituitary gland to release more ADH. ADH travels in the blood to the kidneys, where it makes the walls of the collecting ducts more permeable to water. More water moves out of the collecting duct by osmosis and back into the blood. Less urine is produced, and it is more concentrated.

When the blood is too dilute, less ADH is released. The collecting duct walls become less permeable. Less water is reabsorbed, more urine is produced, and it is more dilute. This is negative feedback: the response (adjusting ADH) corrects the original deviation and returns the blood to its normal water potential.

Exam tip: Do not say "ADH controls the amount of water in the body." The precise mechanism is that ADH changes the permeability of the collecting duct walls. The water then moves by osmosis. State both: the hormone changes permeability, and water moves by osmosis. Two marks, two distinct points.

Blood glucose regulation

Blood glucose concentration is controlled by insulin and glucagon, produced by the islets of Langerhans in the pancreas. These hormones act on the liver as their target organ:

Blood glucose too high (after a meal) - pancreas releases insulin - liver converts glucose to glycogen for storage - blood glucose falls
Blood glucose too low (during exercise or fasting) - pancreas releases glucagon - liver converts glycogen back to glucose - blood glucose rises

This is a textbook example of negative feedback with antagonistic hormones. The correction in blood glucose removes the stimulus for further hormone release, completing the loop.

Worked example: blood glucose

Question: A person eats a meal rich in carbohydrates. Describe and explain the changes in blood glucose concentration over the next two hours. (4 marks)

Model answer: Blood glucose rises as carbohydrates are digested and glucose is absorbed into the blood (1). The pancreas detects the rise and releases insulin (1). Insulin causes the liver to convert excess glucose into glycogen (1). Blood glucose falls back towards the normal level as glucose is removed from the blood (1).

The liver

The liver performs four functions that the specification requires:

Bile production - bile is made in the liver, stored in the gall bladder, and released into the duodenum to emulsify fats and neutralise stomach acid
Blood sugar regulation - the liver converts glucose to glycogen (and vice versa) under the control of insulin and glucagon
Urea formation - excess amino acids cannot be stored. The liver removes the amino group (deamination), converting it first to ammonia and then to urea. Urea is carried in the blood to the kidneys
Detoxification - the liver breaks down toxic substances including alcohol. Long-term excessive alcohol consumption damages liver cells, potentially causing cirrhosis

Kidney failure: dialysis and transplant

When kidneys fail, two treatments are available:

Feature	Dialysis	Transplant
How it works	Blood passes through a machine with a partially permeable membrane. Urea diffuses from blood into dialysis fluid. Glucose and ions are at equal concentrations on both sides, so they are retained.	A healthy kidney from a donor is surgically placed in the patient. The donor kidney takes over filtration and reabsorption.
Advantages	Widely available; no donor needed; keeps patient alive indefinitely	Restores full function; patient can live normally; no regular sessions
Disadvantages	Time-consuming (several hours, 3+ times per week); restricts diet; risk of infection	Requires matching donor; risk of rejection; lifelong immunosuppressant drugs needed

Investigating diffusion with Visking tubing

The specification names Visking tubing as a model for a partially permeable membrane. A typical investigation: fill Visking tubing with a starch-glucose solution and place it in distilled water. After time, test the surrounding water for glucose (Benedict's reagent turns orange-red if positive) and starch (iodine stays brown-yellow if negative). Glucose passes through (small molecule); starch does not (large molecule). This models how the nephron and dialysis membrane work.

Negative feedback: the connecting thread

Every system described above, thermoregulation, osmoregulation and blood glucose regulation, operates by negative feedback. The pattern is always the same: a receptor detects a deviation from the normal value, a coordination centre processes the information and sends signals, an effector responds to correct the deviation, and the correction itself reduces the stimulus. When the exam asks about edexcel igcse temperature regulation and excretion explained in terms of negative feedback, your answer must close this loop. An answer that stops at the response stage without showing the return to normal is incomplete.

Self-check questions

Test yourself on what is temperature regulation and excretion igcse content at exam standard:

Define excretion. Why are faeces not considered an excretory product?
Name the structure in the brain that acts as the body's thermostat and describe how it detects temperature changes.
Explain the difference between vasodilation and vasoconstriction. Why is it incorrect to say that blood vessels "move"?
Describe the role of ADH when a person is dehydrated. Include the source of ADH, its target organ and its effect.
A person exercises vigorously. Predict the changes in their urine output and concentration. Explain your answer.
State the four functions of the liver covered in the Edexcel IGCSE specification.
Compare the advantages and disadvantages of kidney dialysis and kidney transplant.
Explain how insulin and glucagon work together as an antagonistic pair to regulate blood glucose.

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