The<unk>  (or membrane, or blood-air barrier) exists in the gas exchanging region of the lungs.  It exists to prevent air bubbles from forming in the blood, and from blood entering the alveoli. It is formed by the type 1 pneumocytes of the<unk>  wall, the endothelial cells of the<unk> es and the basement membrane between the two cells.  The barrier is permeable to molecular oxygen, carbon dioxide, carbon monoxide and many other gases.
This blood gas barrier is extremely thin (600-800 nm; some places merely 200 nm) so as to allow sufficient oxygen diffusion, yet it is extremely strong. This strength comes from the type 4 collagen in between the endothelial and epithelial cells. Damage can occur to this barrier at pressures of around 40mmhg. Failure of the barrier is often seen in racehorses and other domesticated horses due to exercise induced blood pressure rising above normal.
Alveolar-capillary barrier
Pulmonary surfactant
Blood-brain barrier
Fluid compartments
Systemic circulation
Microcirculation
Pulmonary stretch receptors
Reticuloendothelial system