(b) In healthy lungs, a gradient is maintained between the concentration of oxygen in the alveoli and the concentration of oxygen in the lung capillaries. (i) Describe how ventilation helps to maintain this difference in oxygen concentration.

Ventilation of the lungs is a mechanism designed to maintain a high concentration of oxygen present in the alveoli. Blood is delivered to the lungs from the heart deoxygenated, meaning the majority of haemoglobin in the blood at this stage is saturated with carbon dioxide, rather than oxygen. At the epithelium of the alveolus, gas is exchanged, where the high concentration of oxygen moves from inside the alveoli across to the haemoglobin inside the lung capillaries, with carbon dioxide travelling in the opposite direction. Ventilation then removes the remaining gas in the alveoli, which has a low concentration of oxygen and a comparatively high concentration of carbon dioxide, and replaces it with fresh atmospheric air, with a high concentration of oxygen and low carbon dioxide.