Question

Outline the physical and chemical factors controlling gas exchange between the alveoli and pulmonary capillaries. Diagram and describe the local signals and responses that help match ventilation and perfusion.

Answer 1

The gas exchange in the lungs occurs between the alveoli and pulmonary capillaries via the membrane shared between them, which allows the oxygen and carbon dioxide molecules to move freely. The haemoglobin in the blood flowing in the capillaries close to the alveoli binds to the oxygen present in the lungs because of the high O2 concentration in the air present in the lungs. The carboxyhaemoglobin is broken down resulting in the release of the carbon dioxide, which then moves from blood to the lungs (i.e. from high concentration to low concentration).

The partial pressure gradients of both oxygen and carbon dioxide allow for their exchange through the alveoli. The release of CO2 and intake of O2 occur at the same time. This maintains the equilibrium between the capillaries and the alveolar air.

It is critical to match ventilation and perfusion in order to maintain efficient external respiration. The perfusion adjusts according to the changes in the partial pressure of the O2. The constriction of the airways leads to a decreased partial pressure of oxygen as the airflow into the lungs is slowed down. In order to maintain the ventilation and perfusion match in such conditions, the arteries being supplied by the affected airways undergo vasoconstriction so that the blood flow is reduced and the perfusion is not increased when the ventilation has decreased.