Lung edema is defined as an accumulation of fluid within alveoli and small bronchi/bronchioles. Edema fluid enters the peripheral lung from the circulation via the interstitium into the alveoli. It can be induced by various causes. The most common form is due to congestion of the pulmonary circulation, most often caused by heart failure due to infarction, valvular diseases, and alike. In these cases, the venous flow into the left atrium is reduced, resistance in the venous part of the circulation increases, and leakage of the pulmonary veins increases. The gaps between the endothelial cells increase in size, and serum gets into the interstitium and causes interstitial edema.