IGCSE · Practice questions · The kidney

Filter, reabsorb, release.

Six original Cambridge-style questions on the structure of the kidney and the nephron: what is filtered, what is reabsorbed, what ends up in urine, with a concentration table and a water reabsorption calculation.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Filtered, then sorted.

Cortex outer, medulla inner.
Glomerulus filters water, glucose, urea and ions; blood cells and proteins stay in the blood.
Reabsorbed: all the glucose, some ions, most water.
Urine: urea, excess water, excess ions; no glucose, no protein.

Recall

[2 marks]

Extended. A kidney is cut in half lengthways. Name the outer region and the inner region that can be seen.

Outer region: cortex. ✓
Inner region: medulla. ✓

Explain

[3 marks]

Extended. Describe what happens to blood as it passes through the glomerulus.

Blood arrives at the glomerulus at high pressure. ✓
Water, glucose, urea and ions are filtered out of the blood into the nephron (Bowman’s capsule), forming the filtrate. ✓
Blood cells and proteins are too large to be filtered, so they remain in the blood. ✓

Data

[4 marks]

The table shows the concentrations of some substances in blood plasma, in the filtrate entering the nephron, and in urine.

Substance	Plasma / g per dm³	Filtrate / g per dm³	Urine / g per dm³
protein	75	0	0
glucose	1.0	1.0	0
urea	0.3	0.3	20

Extended. Explain the figures for (a) protein, (b) glucose, (c) urea.

(a) Protein is in plasma but not in the filtrate because protein molecules are too large to be filtered out of the glomerulus. ✓
(b) Glucose is filtered, so its concentration in the filtrate equals that in plasma. ✓
All of the glucose is then reabsorbed into the blood, so there is none in urine. ✓
(c) Urea is filtered and is not reabsorbed, while most of the water is reabsorbed, so urea becomes much more concentrated in the urine (about 67 times). ✓

Calculation

[2 marks]

Extended. The kidneys of an adult form about 180 dm³ of filtrate per day but produce only about 1.5 dm³ of urine. Assuming the filtrate is mostly water, calculate the percentage of the filtered water that is reabsorbed.

Water reabsorbed = 180 − 1.5 = 178.5 dm³. ✓
Percentage = (178.5 ÷ 180) × 100 = 99.2% (about 99%). ✓

Recall

[3 marks]

Extended. State the substances found in the urine of a healthy person, and name two substances that are filtered in the glomerulus but returned to the blood.

Urine contains urea, excess water and excess ions. ✓
Glucose is filtered and all of it is reabsorbed. ✓
Water (most of it) and ions (some of them) are also filtered and reabsorbed. ✓

Analysis

[3 marks]

Extended. A urine test shows glucose in a patient’s urine. Using your knowledge of the nephron, suggest what this tells a doctor.

In a healthy nephron all of the filtered glucose is reabsorbed, so normally urine contains no glucose. ✓
Glucose in urine means more glucose was filtered than the nephrons could reabsorb. ✓
This suggests the blood glucose concentration is abnormally high (for example in untreated diabetes), so further tests are needed. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.