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1.2Cells, Organ Systems and Ecosystems

Respiration and the Respiratory System in Humans — WJEC GCSE Biology Revision Notes & Topical Questions

5 min read 4 parts Lesson 2 of 6

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Section 1 of 4Aerobic vs Anaerobic respiration

1Aerobic respiration
Breakdown of glucose in the presence of oxygen releasing large amounts of energy:

Glucose + Oxygen → Carbon Dioxide + Water (+ ATP)

C6H12O6 + 6O2 → 6CO2 + 6H2O

2Anaerobic respiration in human muscle
Occurs during vigorous exercise when oxygen cannot be supplied fast enough:

Glucose → Lactic Acid (+ small amount of ATP)

  • Builds up an oxygen debt; lactic acid causes muscle fatigue and cramps.
  • Oxygen debt is repaid during recovery to oxidise lactic acid in the liver back to carbon dioxide and water or glucose.

Section 2 of 4Ventilation and alveolar gas exchange

ActionInspiration (Breathing in)Expiration (Breathing out)
External Intercostal MusclesContract, pulling ribs up and outwardsRelax, ribs move downwards and inwards
DiaphragmContracts and flattens downwardsRelaxes and domes upwards
Thorax Volume & PressureVolume increases; pressure drops below atmospheric; air rushes inVolume decreases; pressure increases above atmospheric; air forced out
  • Adaptations of alveoli for gas exchange:
    • Millions of alveoli provide an immense surface area.
    • Thin walls composed of a single layer of flattened epithelial cells (short diffusion path).
    • Dense surrounding network of blood capillaries maintaining steep concentration gradients.
    • Moist inner lining allowing respiratory gases to dissolve before diffusing.

Section 3 of 4Key terms

TermMeaning
Aerobic respirationThe cellular enzymatic breakdown of glucose using oxygen to produce CO2, water, and ATP.
Oxygen debtThe extra volume of oxygen required after strenuous exercise to metabolise accumulated lactic acid.
AlveolusA microscopic thin-walled air sac in the lungs where gas exchange occurs with capillary blood.

Section 4 of 4Exam technique

Avoid these mistakes
Confusing breathing (ventilation) with respiration — respiration is a chemical cellular reaction releasing energy; ventilation is the mechanical movement of air in and out of the lungs.
Saying diaphragm moves down during expiration — the diaphragm relaxes and domes upwards during breathing out.

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