Figure 1.

The pulmonary capillary filter and right-to-left shunts. Cartoon of the circulations, indicating the pulmonary capillary bed and the mechanical filter through which venous blood should pass before returning to the systemic arterial tree. Red arrows represent right-to-left shunts which allow venous blood to return to systemic arteries having bypassed filtration, gas exchange, and other pulmonary capillary functions. Right-to-left shunting is continuous through pulmonary AVMs, but only intermittent through intracardiac shunts such as patent foramen ovale, when flow is predominantly left-to-right (blue arrow, see text). Note that the lungs have a second arterial supply from the systemic circulation (bronchial vessels indicated) that can preserve arterial supply to lung tissue if the pulmonary circulation is compromised.