Expansion
Filtration, metabolism, immune defence, acid-base balance and a blood reservoir
Expansion
- Filtration - traps emboli, clots and small air bubbles before they reach the systemic circulation
- Metabolic - angiotensin converting enzyme on the pulmonary endothelium converts angiotensin I to II and inactivates bradykinin; the lung also inactivates serotonin, noradrenaline and prostaglandins, while leaving adrenaline, dopamine and histamine largely untouched
- Immune defence - mucociliary escalator, alveolar macrophages, secretory IgA, lysozyme and defensins
- Acid-base regulation - by controlling carbon dioxide excretion, the fastest of all compensatory mechanisms
- Blood reservoir - holds about 500 ml, which can be mobilised
- Synthesis - surfactant, and heparin from mast cells
- Heat and water loss
The metabolic role explains two familiar drug effects: ACE inhibitors act largely on pulmonary endothelium, and their cough is attributed to accumulated bradykinin and substance P.
The filtration role explains why a small venous air embolism is usually tolerated, and why a right to left shunt removes that protection and permits paradoxical embolism.