Two separate selective pressures determine the structure of the blood-gas barrier in air breathing vertebrates. The first pressure which has been recognized for 100 years is to facilitate diffusive gas exchange. This requires the barrier to be extremely thin and have a large area. The second pressure has only recently been appreciated. This is to maintain the mechanical integrity of the barrier in the face of its extreme thinness. The most important tensile stress comes from the pressure within the pulmonary capillaries which results in a hoop stress. The strength of the barrier can be attributed to the type IV collagen in the extracellular matrix. In addition the stress is minimized in mammals and birds by complete separation of the pulmonary and systemic circulations. Remarkably the avian barrier is about 2.5 times thinner than that in mammals and it is also much more uniform in thickness. These advantages for gas exchange come about because the avian pulmonary capillaries are unique among air breathers in being mechanically supported externally in addition to the strength that comes from the structure of their walls. This external support comes from epithelial plates that are part of the air capillaries and the support is available because the terminal air spaces in the avian lung are extremely small owing to the flow-through nature of ventilation in contrast to the reciprocating pattern in mammals.