Schematic of the left heart (left ventricle [LV] and left atrium [LA]), right heart (right ventricle [RV] and right atrium [RA]), and pulmonary circulation (pulmonary arteries [PA] and pulmonary veins [PV]), in systole from a dog with mitral regurgitation (arrows) causing left heart failure (pulmonary edema). The pulmonary capillaries are the network of vessels between the PA and the PV. Intracardiac pressures (in millimeters of mercury) are shown as numbers (systolic/diastolic) on the left side of the circulation. Diastolic pressure in the LV (20 mm Hg) is elevated from normal (0–12 mm Hg). Diastolic pressures in the LV and LA are the same because the mitral valve is open (not shown). Systolic pressure in the LA is markedly elevated because of blood being expelled from the LV into the LA (through the hole in the mitral valve) in systole (mitral regurgitation). The pressures in the LA, PV, and pulmonary capillaries are all the same. The elevation in pulmonary capillary pressure causes pulmonary edema (pink material in the alveoli). AO, aorta; TR, tricuspid regurgitation.