The heart is made of mainly cardiac muscle and connective tissue. It is the pump of the circulatory system and pushes blood to the lungs from the right side of the heart and to all parts of the body, from the left side of the heart. The heart has four chambers--two upper atria and two lower ventricles, with the left ventricle slightly larger than the right. Atria receive blood from the veins. They send blood through valves down to the ventricles. Ventricles contract and force blood out into the arteries, under pressure. The right atrium receives deoxygenated blood from the vena cava and sends this blood through the tricuspid valve, to the right ventricle. This ventricle pumps out blood towards the lungs in the pulmonary arteries. The blood in the pulmonary arteries is deoxygenated and once in the lungs, it becomes oxygenated. It returns to the left atrium via the pulmonary veins and is sent down to the left ventricle,after going through the mitral valve, where it is pushed through the aorta and all of its branches to transport oxygen to all parts of the body. The human heart beats approximately 72 times per minute and during a lifetime, it may beat more than 2.5 billion times. It weighs between 9 -12 ounces with male hearts slightly heavier than female hearts. The human heart is the size of a fist. It is behind the sternum or breastbone, which protects it. The sac around the heart is called the pericardium, which protects it and anchors it to surrounding structures.