The heart's job is to pump blood into the lungs and then throughout the body. The right side of the heart takes blood from the body and pumps it to the lungs. The left side of the heart takes oxygen-filled blood from the lungs and pumps it to the rest of the body. Valves inside the heart prevent blood from flowing the wrong way. The heart is made up of a special type of muscle, called the cardiac cardiac muscle, that never tires. The heart is about the same size as a person's closed fist. It is located almost in the center of the chest and the lower end slopes towards the left side of the body. An average man has approximately 11-13 liters of blood; and the average woman has between 9 and 11 pints. Blood travels around the body through tubes called arteries and veins. These branch into smaller tubes that reach every cell in the body. Blood circulates around the body about once a minute or 1,500 times a day. Each small drop of blood contains up to 5 million red blood cells that give blood its red color. Red blood cells contain hemoglobin, which absorbs oxygen in the lungs. Oxygen-rich blood is bright red. Electrical impulses make the heart beat. These impulses are produced by special areas of the heart's tissues called nodes. The nodes are filled with nerves and send signals that help heart muscle cells contract. The heart contains four chambers; a right and left atrium and a right and left ventricle. The septum that separates the left and right sides of the heart. There are different types of vessels connected to the heart: the first is the pulmonary trunk; which exports the “used” blood to reach the left pulmonary artery. Then there is the aorta, which is the largest blood vessel that supplies blood to the heart muscle. Next comes the vena cava, which divides into two veins, one of which is the superior vena cava and the inferior vena cava. The superior vena cava is a large vein that carries oxygen-poor blood from the upper body. The inferior vena cava carries “used” blood from the lower part of the body and then returns to the heart.