The heart is made of cardiac muscle and is split into 4 chambers known as atria and ventricles.Low pressure, deoxygenated blood from the tissue enters the right atrium via the superior and inferior vena cava. This blood passes through the right atrio ventricular or tricuspid valve into the right ventricle. Valves prevent the back flow of blood. From here the blood passes through the right semi lunar or pulmonary valve, deoxygenated blood is directed to the lungs via the pulmonary arteries to become oxygenated.From the lungs the now oxygenated blood passes into the left atrium via the pulmonary veins, oxygenated blood passes from the left atrium into the left ventricle through the left bicuspid or mitral (also can be called left atrio ventricular) valve. From the left ventricle oxygenated blood passes through the semi lunar or aortic valve into the aorta which pumps oxygenated blood around the body.