Küttner showed in 1874 that simultaneous ligation of the pulmonary veins increased the frequency and severity of lung infarctions after pulmonary artery obstruction. The authors studied the possibility that a tidal pulmonary venous blood flow reflux from the left atrium could nourish the alveolar tissue. This could be driven by left atrial pressure transients and alternate expansion and compression of alveolar and extra alveolar vessels due to tidal lung volume changes. 5 anesthetized, closed chest goats were studied in the prone position after left pulmonary artery ligation and the obstruction of all bronchial blood flow to the left lung, checked by systemic microsphere injection. The inert, insoluble gas SF6 was infused into the left atrium and the exhaled gas from left and right lungs was collected separately. SF6 was found in the gas exhaled from the left lung, showing that left atrial blood had reached the alveolar tissues. The effective reflux blood flow was increased from control levels (no ventilation, normal left atrial pulses) by tidal volume changes, and by increased left atrial pressure transients (balloon induced mitral insufficiency). This venous reflux flow could explain why alveolar tissues do not suffer more severe injury when the pulmonary artery is obstructed.