MYP Biology: The Excretory System & Kidneys FAQ

The excretory system is the group of organs — kidneys, skin, lungs and liver — that remove metabolic waste from the body. In MYP Biology the kidneys are the main focus: they filter blood to remove urea and excess water, producing urine, and help keep the internal environment stable (homeostasis).

The kidney's job is to filter blood, removing urea, excess salts and water while keeping useful substances — glucose especially — in the bloodstream. Each kidney contains around a million tiny filtering units called nephrons, and the fluid left over after filtration and reabsorption leaves the body as urine.

Excretion is removing waste made inside the body by metabolism — urea from the liver's breakdown of amino acids is the classic example. Egestion is just passing out undigested food as faeces, material that was never absorbed into the blood in the first place. Ask yourself: did this substance ever enter a cell?

A nephron is the microscopic filtering unit of the kidney — each kidney has roughly a million of them. Blood enters at the Bowman's capsule, gets filtered under pressure, and the fluid travels through a tubule where useful substances are reabsorbed and the rest becomes urine.

Blood arrives at the nephron under pressure and is squeezed through the Bowman's capsule wall — this is ultrafiltration. Small molecules (water, salts, urea, glucose) pass through; blood cells and proteins are too large and stay in the blood. The filtrate then moves through the tubule for selective reabsorption.

The kidneys filter urea, excess water and salts out of the blood and turn them into urine — that's the actual excretion step. The liver doesn't excrete urea itself; it makes urea by breaking down excess amino acids (deamination), then passes it to the kidneys. Students often credit the liver with excretion when it's really production.

ADH (antidiuretic hormone) is released by the pituitary gland when the blood is too concentrated — after exercise or not drinking enough water, for instance. It makes the collecting duct more permeable, so more water is reabsorbed into the blood and you produce a smaller, more concentrated volume of urine.

Excretory system content is mostly assessed under Criterion A (Knowing and Understanding), which tests recall and application of the nephron process and terminology. It can also appear under Criterion C (Processing and Evaluating) if you're interpreting data on urine concentration, and each criterion is marked out of 8 on the MYP 1-8 scale.

The most common command terms on this topic are define, describe, explain and label. A define question wants a precise statement; an explain question wants reasons and mechanisms, not just a description — that distinction alone is where a lot of marks get lost.

Structure your answer around cause and effect: state what happens, then say why, in that order. For a 4-mark explain question, examiners are looking for a linked chain of ideas, not four separate facts thrown together.

The biggest mark-loser I see is vague language — writing "the kidneys remove toxins" instead of naming urea specifically, or mixing up the ureter (tube from kidney to bladder) with the urethra (tube out of the body). Examiners marking Criterion A check for exact terminology, not general gestures at the idea.

Draw the nephron from memory, trace a molecule of urea from the liver to the toilet, then practise explain-style questions using cause-and-effect sentences. This topic rewards process knowledge over rote memorisation — you need to know why each step happens, not just its name.

It's moderately demanding rather than genuinely hard — the difficulty comes from unfamiliar vocabulary and a multi-step process (filtration, reabsorption, ADH control) rather than complex maths. Students who struggle usually haven't practised explaining the process in order; those who can trace the nephron step by step tend to do well.

Look for resources that combine a clear diagram-based explanation with practice questions using real MYP command terms — that combination matters more than volume of notes. The question bank, Revision Notes and Topical Worksheets on revisionprep.com are built around the Body Systems unit and the actual command-term wording your child will see in class assessments.