11.1 Gas exchange in humans

How humans take in oxygen and remove carbon dioxide. The breathing system (lungs, trachea, bronchi, bronchioles, alveoli, ribs, intercostal muscles, diaphragm). Four features that make alveoli effective: thin (1-cell) walls, large surface area (~70 m²), good blood supply, and good ventilation. Inhaled vs exhaled air composition (O₂, CO₂, water vapour). Effect of physical activity on rate and depth of breathing. HL extends to the ventilation mechanism (volume + pressure changes), cartilage in the trachea, and the role of goblet cells, mucus, and ciliated cells in airway defence.