Scheme of systemic and pulmonary circulation. (A) Normal physiology: The virtual network connections are present but are not permeable. (B) Collateral circulation: There is a shunt network between the systemic and the pulmonary circulation. These shunting connections are 1) the aortopulmonary collaterals (between the bronchial artery and the pulmonary artery), 2) the veno-venous collaterals (between the bronchial vein and the pulmonary vein) and 3) the arterio-venous shunts (direct connections between the bronchial artery and vein bypassing the capillary network). Adapted from Heimburg P, copyright notice 2011, with permission from BMJ Publishing Group Ltd.