GLOSSARY

External respiration is gas exchange between alveolar air and pulmonary capillary blood, in which oxygen enters the blood and CO₂ leaves it; this process converts oxygen-poor venous blood into oxygen-rich arterial blood.

Process

• O₂: diffuses from the alveolus (PO₂ about 100) into venous blood (PO₂ about 40) and binds to haemoglobin.
• CO₂: diffuses from blood (PCO₂ about 46) into the alveolus (PCO₂ about 40) and is exhaled.
• Haldane effect: binding of oxygen to haemoglobin promotes the release of CO₂ from it.

Efficiency Factors

• Area and thickness of the respiratory membrane.
• Ventilation–perfusion (V/Q) matching: ideally about 1; about 0.8 for the whole lung at rest.
• Red cell transit time.

Clinical Application