O-Level and SEC G3 Biology K325
B7: Excretion in Humans
Define excretion and trace urinary structures, nephron processes, urine production, and dialysis.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Excretion in Humans is a Pure Biology K325 topic. It follows metabolic wastes from their formation through transport and removal, with particular attention to urea, the urinary system, nephron processes, and the distinction between excretion, egestion, and secretion.
Metabolic wastes and the liver
Excretion removes toxic materials, products of metabolism, and substances in excess of requirements. Carbon dioxide is excreted by the lungs, while the kidneys remove urea, excess mineral ions, and excess water. Egestion removes undigested food from the gut and is not excretion.
Excess amino acids cannot be stored. In the liver, deamination removes the nitrogen-containing part and leads to urea formation. Urea travels dissolved in blood plasma to the kidneys. The liver also breaks down hormones, alcohol, and other toxins.
Urinary system and nephron processes
The urinary route is kidney to ureter to bladder to urethra. Renal arteries bring blood to the kidneys and renal veins carry blood away. Urine contains water, urea, and mineral ions, but a healthy person's urine normally contains little or no glucose and no blood cells.
At the glomerulus, high pressure filters small molecules into Bowman's capsule while cells and large plasma proteins remain in blood. Along the tubule, all glucose and amino acids and suitable amounts of water and mineral ions are selectively reabsorbed. The remaining fluid becomes urine.
Selective reabsorption and dialysis
Selective reabsorption returns all glucose and amino acids and suitable amounts of water and mineral ions from the tubule to blood. The process changes filtrate composition after ultrafiltration rather than preventing useful small molecules from entering the filtrate initially.
Dialysis uses a partially permeable membrane and dialysis fluid with controlled concentrations. Urea and excess ions diffuse out of blood while cells and large proteins remain. Useful substances are protected by matching their normal blood concentration in the dialysis fluid, and the fluid is renewed to maintain gradients.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: A patient's urine contains glucose repeatedly even though glucose is small enough to be filtered. Explain the normal nephron process that should prevent this.
- Glucose enters the filtrate during ultrafiltration.
- In a healthy nephron, glucose is selectively reabsorbed from the tubule into the blood.
- Persistent glucose in urine suggests that the normal handling or blood-glucose condition needs medical investigation.
Answer: Filtered glucose should be selectively reabsorbed into the blood, leaving little or none in urine.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Define excretion and explain why nitrogenous and excess substances must be removed.
- Trace the urinary route and distinguish ultrafiltration from selective reabsorption.
- Explain dialysis through membrane selectivity, gradients, and dialysis-fluid composition.
Official outcome coverage
K325 B7: 4 mapped outcomes, references B7(a), B7(b), B7(c), B7(d). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Interpret kidney, dialysis, or urine-composition data rather than attempting unsafe body-fluid tests.
Exam traps and retrieval check
Avoid these traps
- Calling egestion excretion because both remove material from the body.
- Saying the kidney makes urea instead of tracing urea formation from the liver.
- Claiming filtration removes only wastes even though useful small molecules also enter the filtrate.
Check from memory
Where is urea formed?
In the liver after deamination of excess amino acids.
What carries urine from kidney to bladder?
A ureter.
Why do blood cells remain during ultrafiltration?
They are too large to pass through the filtration barrier.
Pure versus Combined scope
This topic is part of Pure Biology K325 but is not a standalone Combined Biology topic in K327 or K328. Combined students should not assume it is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.