The blood-air barrier.
This is a colorized scanning electron micrograph showing the erythrocytes (red blood cells) within the capillary network of an alveola. The blood–air barrier (alveolar–capillary barrier or membrane) exists to prevent air bubbles from forming in the blood, and from blood entering the alveoli. It is permeable to molecular oxygen, carbon dioxide, carbon monoxide and many other gases and it is extremely thin (approximately 2μm) to allow sufficient oxygen diffusion, yet it is extremely strong. This strength comes from the type IV collagen in between the endothelial and epithelial cells.
Failure of the barrier may occur in a pulmonary barotrauma. This can be a result of several possible causes, including blast injury, and breathing gas entrapment or retention in the lung during depressurization, which can occur during ascent from underwater diving or loss of pressure from a pressurized vehicle, habitat or pressure suit. A possible consequence of rupture of the blood–air barrier is arterial gas embolism.
(Picture by The Cell Image Library).