This guide covers IGCSE Biology topic 11: gas exchange in humans. You will revise the respiratory system (trachea, bronchi, bronchioles, alveoli), breathing (inhalation, exhalation, role of diaphragm and ribs), and how alveoli are adapted for gas exchange.

Key concepts to revise

• Path of air – Nose/mouth → trachea → bronchi → bronchioles → alveoli. Know the order.
• Alveoli – Large surface area, thin wall (one cell thick), moist lining, good blood supply; diffusion of O₂ in, CO₂ out.
• Breathing – Inhalation: diaphragm contracts and flattens, ribs move up and out, volume increases, air in. Exhalation: opposite.
• Gas exchange – Oxygen diffuses from alveolus into blood; carbon dioxide diffuses from blood into alveolus (concentration gradient).

Exam tips and command words

• Label the respiratory system – Trachea, bronchi, lungs, diaphragm, alveolus.
• State the function – Of alveoli (gas exchange); of diaphragm (changes volume of thorax).
• Describe – The process of inhalation and exhalation (muscles, volume, pressure).
• Explain – How alveoli are adapted for efficient gas exchange (surface area, thin wall, blood supply).

Common mistakes

• Confusing breathing (ventilation; moving air in and out) with gas exchange (diffusion of O₂ and CO₂ in alveoli).
• Saying the diaphragm “expands” (it contracts and flattens on inhalation).
• Not stating that gas exchange is by diffusion down a concentration gradient.

Revision checklist

• Label the respiratory system and state the path of air from nose to alveoli.
• Describe inhalation and exhalation in terms of diaphragm, ribs and volume.
• List four adaptations of alveoli for gas exchange and explain each.
• Explain why smoking damages gas exchange (e.g. tar, reduced surface area).

Next steps

Book a free trial with an IGCSE Biology tutor to practise respiratory system and gas exchange questions, or explore Tutopiya’s learning portal for more revision resources.