During diastole, the relaxation phase of the cardiac cycle, the pressure in the right ventricle drops to near zero. The pressure gradient formed between the right atrium and ventricle, plus slight contraction by the atrium, causes blood to flow into the ventricle. As the ventricle fills, the blood passes through the tricuspid valve, pushing its leaflets aside. As systole, the pumping phase, starts, the ventricle starts to contract, increasing intraventricular pressure. This causes the leaflets of the tricuspid valve to snap shut, and the cusps of the pulmonary valve to open.
Blood then flows out of the ventricle through the pulmonary artery and on to the lungs.
As the ventricle relaxes, intraventricular pressure drops below the pressure in the pulmonary artery, causing the pulmonary valve to close. In this way, blood returns from the body to the right side of the heart and is pumped to the lungs for gas exchange. The story on the left side of the heart is practically identical to the right, but the names change. Oxygenated blood from the lungs arrives at the left atrium from the pulmonary vein. During diastole, blood flows through the mitral valve into the left ventricle.