The heart is a hollow muscle located in the pericardial cavities within the thorax. The heart consists of four chambers: the right and left atrium and the left and right ventricle. Blood returning to the heart from the peripheral organs arrives via the vena cava to the right atrium first. From here most of the blood flows directly through the tricuspid valve into the right ventricle. When the heart is contracting the tricuspid valve prevents that blood regurgitating in to the right atrium. Instead blood is squeezed through the pulmonary valve and into the pulmonary trunk to the lungs where the blood takes up oxygen and releases carbon dioxide. The blood returns to the heart via four pulmonary veins to the left atrium. Blood flows through the mitral valve into the left ventricle, the heart chamber with the thickest muscle wall. When the heart contracts blood is pushed through the aortic valve into the aorta. From here the blood is delivered to all the body’s organs, including the heart muscle itself through the coronary circulation.