Cambridge IGCSE Biology Notes 11: Gas Exchange in Humans
Cambridge IGCSE Biology 0610 and 0970 notes on gas-exchange surfaces, breathing anatomy, ventilation, air composition, exercise and airway defence.
Topic 11 of Cambridge IGCSE Biology 0610 and 0970 separates ventilation, which moves air, from gas exchange, which moves oxygen and carbon dioxide by diffusion. The official section covers exchange-surface features, breathing-system anatomy, inspired and expired air, exercise responses and airway protection. Smoking is not imported into this Topic 11 boundary.
The breathing-system route
Air passes through the larynx into the trachea. The trachea divides into two bronchi, one entering each lung. Bronchi branch into narrower bronchioles, which lead to alveoli. Alveoli are closely associated with blood capillaries.
Rings of cartilage in the trachea support the airway and prevent it collapsing during pressure changes while leaving the passage open for airflow. The syllabus names cartilage function in the trachea, so do not generalise it as the structure that performs gas exchange.
The lungs occupy the thorax and are ventilated through movements of the ribs, intercostal muscles and diaphragm. The lungs themselves do not contain muscles that actively pull them open in the required explanation.
Gas-exchange surface features
Alveoli collectively provide a large surface area. A large area permits many oxygen and carbon dioxide molecules to diffuse at the same time.
The alveolar surface and adjacent capillary wall are thin, creating a short diffusion distance. A good blood supply continually brings deoxygenated blood and carries oxygenated blood away. Good ventilation renews alveolar air.
Together, blood flow and ventilation maintain concentration gradients. Oxygen concentration is higher in alveolar air than in deoxygenated blood, so oxygen diffuses into blood. Carbon dioxide concentration is higher in blood arriving at the lungs than in alveolar air, so carbon dioxide diffuses into alveoli.
Gas exchange is diffusion down concentration gradients. Ventilation maintains those gradients but is not itself diffusion across the exchange surface.
Inspiration
During inspiration, external intercostal muscles contract. The ribs move upward and outward. The diaphragm muscles contract, making the diaphragm flatten and move downward.
These actions increase thoracic volume. As volume increases, pressure inside the thorax and lungs falls below atmospheric pressure. Air moves into the lungs down the pressure difference.
Use the complete causal order:
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