It is covered by a protective membrane, the pericardium . Deoxygenated blood from the body enters the right atrium . It flows through the tricuspid valve into the right ventricle . The term tricuspid refers to the three flaps of tissue that make up the valve. Contraction of the ventricle then closes the tricuspid valve and forces open the pulmonary valve.
Blood flows into the pulmonary artery .
This branches immediately, carrying blood to the right and left lungs . Here the blood gives up carbon dioxide and takes on a fresh supply of oxygen. The capillary beds of the lungs are drained by venules that are the tributaries of the pulmonary veins . Note that the blood draining the stomach, spleen, and intestines passes through the liver before it is returned to the heart. Here surplus or harmful materials picked up from those organs can be removed before the blood returns to the general circulation.