SEC G3 Combined Science Biology component K327/K328
B6: Respiration in Humans
Connect gas-exchange structures, tobacco-smoke effects, respiration, and oxygen debt.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Respiration releases energy in cells, while the gas-exchange system supplies oxygen and removes carbon dioxide to support that process.
Air passages and alveoli
The larynx provides an air passage into the trachea. The trachea carries air to two bronchi, bronchi divide into bronchioles through the lungs, and bronchioles deliver air to alveoli. Capillaries beside alveoli carry blood for oxygen uptake and carbon dioxide removal.
Alveoli provide a large moist surface, a wall one cell thick, and close capillary supply. Ventilation and blood flow maintain steep gas concentration differences.
Tobacco smoke and health
Nicotine is addictive and raises heart rate and blood pressure. Tar contains carcinogens and damages cilia and lung tissue, increasing the risks of bronchitis, emphysema, and lung cancer. Carbon monoxide reduces oxygen transport by binding strongly to haemoglobin.
Connect each component to its effect rather than assigning every smoking harm to tar alone.
Aerobic, anaerobic, and oxygen debt
Aerobic respiration releases energy from glucose in oxygen: . Human anaerobic respiration releases less energy without oxygen: .
During vigorous exercise, oxygen delivery may not meet demand, so some cells respire anaerobically and lactic acid accumulates. Rapid deep breathing afterwards supplies the extra oxygen needed to remove lactic acid and restore the body to its resting condition, thereby removing the oxygen debt.
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: Why does breathing remain rapid after a sprint?
- Working muscles used some anaerobic respiration.
- Lactic acid accumulated and an oxygen debt developed.
- Continued rapid breathing supplies oxygen needed during recovery.
Answer: Breathing remains elevated to remove the oxygen debt after anaerobic respiration.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Air passages and alveoli with the named terms, evidence, and causal links kept distinct.
- Use Tobacco smoke and health to interpret the evidence given and justify each conclusion.
- Apply Aerobic, anaerobic, and oxygen debt to a new example, then check the conclusion against the information given.
Official outcome coverage
K327 B6: 6 mapped outcomes, references B6(a), B6(b), B6(c), B6(d), B6(e), B6(f). Check the official K327 syllabus.
K328 B6: 6 mapped outcomes, references B6(a), B6(b), B6(c), B6(d), B6(e), B6(f). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Compare breathing or respiration data before and after a controlled activity, or interpret gas-exchange specimens. Keep activity duration and recovery intervals consistent, repeat measurements, and distinguish ventilation rate from cellular respiration.
Exam traps and retrieval check
Avoid these traps
- Confusing breathing with cellular respiration.
- Writing ethanol as the human anaerobic product.
- Saying carbon monoxide damages lungs only by coating them.
Check from memory
Where does gaseous exchange occur?
At the alveoli.
What human anaerobic product forms?
Lactic acid.
How does carbon monoxide affect transport?
It binds haemoglobin and reduces oxygen carriage.
Official Combined Science scope
This shared Combined Biology owner serves both K327 and K328. Gas-exchange structures, alveolar adaptation, tobacco smoke, aerobic and anaerobic respiration, and oxygen debt.