Excretion is how the body gets rid of the waste its own cells make. Homeostasis is how the body keeps its internal conditions steady while the outside world changes. The two sit in one topic because both depend on the blood carrying substances to organs that adjust or remove them.
Check the current Cambridge IGCSE Biology 0610 syllabus page (linked under Sources) for the wording in your syllabus year. This page gives you a study route through our lessons. The wider subject is mapped in the Biology learning guide.
What should I know before starting?
You need the idea of metabolism and respiration from gas exchange and respiration, because carbon dioxide is a waste product of respiration. You also need the blood and its vessels from circulation and blood, and diffusion and osmosis from movement across membranes. Enzymes from biological molecules and enzymes explain why a steady temperature matters.
One orienting example
Here is the whole topic in one question. On a hot afternoon a student’s skin turns red and she starts to sweat. Why, and what happens as she cools? The answer uses the feedback idea from several lessons.
Her core temperature rises above its set point. Temperature receptors detect the change and the brain sends a signal to the skin. The skin responds: more sweat is produced and its evaporation removes heat, and blood vessels near the surface widen so more heat is lost.
As her temperature falls back towards the set point, the signal weakens and the responses reduce. The response opposes the change, which is why the pattern is called negative feedback. The same steps explain glucose control, only with different parts.
In what order should I study the lessons?
- Distinguish excretion from egestion: it fixes the key words before anything else uses them.
- Explain filtration and selective reabsorption conceptually: it shows the kidney as a filter that then takes back what the body needs.
- Interpret a negative-feedback loop: it gives you the general pattern, and the mechanism and feedback builder lets you practise ordering it.
- Explain temperature regulation using competing effects: it applies the loop to the skin, where two responses pull in opposite directions.
- Read a glucose-regulation diagram without diagnostic claims: it applies the loop to blood glucose and keeps the language academic.
- Excretion and homeostasis practice set: it tests the whole route with original questions.
What are the common traps?
- Calling egestion excretion. Faeces are mostly undigested food that never entered the cells, so they are not metabolic waste.
- Saying the kidney “makes” urea. The liver makes urea from excess amino acids. The kidney filters the blood and removes it.
- Writing “the capillaries move closer to the skin”. Vessels do not move. Arterioles widen or narrow to change blood flow.
- Describing a change without the return. Negative feedback ends with conditions coming back towards the set point.
How do I use the practice set?
Work through the practice set after two or three lessons, then again after all five. Write each answer before opening the worked one. Sort mistakes by type and go back to the lesson the set names.
Students who want someone to check their written explanations can find out what online one-to-one Biology tuition looks like.