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A2 · Practice questions · The kidney: filtering and reclaiming

Pressure out, pumps back.

Six original Cambridge-style questions on the kidney and nephron, from naming structures to explaining how ultrafiltration and selective reabsorption make urine.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Liver, filter, reclaim.

01
Recall
[3 marks]

Describe how urea is formed in the body and how it reaches the kidney.

  • Excess amino acids cannot be stored; in the liver they are deaminated: the amine group is removed, forming ammonia (and a keto acid that can be respired). ✓
  • Ammonia is combined with carbon dioxide to form urea, which is less toxic. ✓
  • Urea dissolves in the blood plasma and is carried to the kidneys in the renal artery. ✓
02
Recall
[3 marks]

Name the region of the kidney in which each of these is found: (a) glomeruli, (b) loops of Henle, (c) the place where urine collects before entering the ureter.

  • (a) cortex ✓
  • (b) medulla ✓
  • (c) renal pelvis ✓
03
Explain
[4 marks]

Explain how the structure of the glomerulus and the Bowman's capsule is related to the process of ultrafiltration.

  • The afferent arteriole is wider than the efferent arteriole, so hydrostatic pressure in the glomerular capillaries is high. ✓
  • The capillary endothelium has pores (fenestrations), so fluid can leave easily. ✓
  • The basement membrane acts as a filter, preventing large proteins (above about 69 000 relative molecular mass) and cells from passing. ✓
  • Podocytes have processes with slits between them, so filtrate passes into the capsule space easily. ✓
04
Explain
[5 marks]

Explain how glucose is reabsorbed from the filtrate in the proximal convoluted tubule, and how the cells of the PCT are adapted for this.

  • Na+/K+ pumps in the basal membrane actively transport Na+ out of the cell into the blood, using ATP. ✓
  • This keeps the Na+ concentration in the cell low, so Na+ diffuses in from the lumen through cotransporter proteins. ✓
  • Each cotransporter carries glucose in with Na+ (secondary active transport), so glucose is taken up against its concentration gradient. ✓
  • Glucose then moves into the blood by facilitated diffusion through the basal membrane. ✓
  • Adaptations: microvilli give a large surface area; many mitochondria supply ATP; close contact with capillaries; folded basal membrane. ✓
05
Data
[3 marks]

The concentrations of three substances, in g dm−3, are: blood plasma: protein 80, glucose 1.0, urea 0.3; glomerular filtrate: protein 0.05, glucose 1.0, urea 0.3; urine: protein 0, glucose 0, urea 20. Explain the differences shown.

  • Protein is almost absent from the filtrate because protein molecules are too large to pass through the basement membrane. ✓
  • Glucose is at the same concentration in plasma and filtrate (freely filtered) but absent from urine because all of it is reabsorbed in the PCT. ✓
  • Urea is about 67 times more concentrated in urine than in filtrate because most of the water is reabsorbed while most urea is not. ✓
06
Analysis
[3 marks]

A section of kidney tissue seen with a light microscope shows round knots of capillaries and tubules lined with cells that have a fuzzy border facing the lumen. Identify the region of the kidney and the two structures, giving a reason for each.

  • Region: cortex, because it contains glomeruli (and convoluted tubules). ✓
  • Round knots of capillaries are glomeruli (inside Bowman's capsules). ✓
  • Tubules with a fuzzy border are proximal convoluted tubules; the border is the brush border of microvilli. ✓

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.