The heart's job is to pump blood to 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 stop blood flow in the wrong direction. The heart is made up of a special type of muscle, called the cardiac muscle, which never tires. The heart is about the same size as the person's closed fist. It sits almost in the middle of your chest and the lower end slopes towards the left side of the body. The average man has about 11 to 13 liters of blood; and the average woman has between 9 and 11 pints. Blood circulates throughout the body through tubes called arteries and veins. These branch into smaller tubes that reach every cell in the body. Blood circulates through 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 which give blood its red color. Red blood cells contain hemoglobin, which absorbs oxygen in the lungs. Blood that contains a lot of oxygen is bright red. Electrical impulses make your heart beat. These impulses are sent from special areas of tissue in the heart 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 several types of vessels connected to the heart: the first is the pulmonary trunk; which exports “used” blood to the left pulmonary artery. Next comes the aorta, which is the largest blood vessel that supplies blood to the heart muscle. Next comes the vena cava, which is divided into veins, one being the superior vena cava and the inferior vena cava. The superior vena cava is a large vein that channels oxygen-poor blood from the upper part of the body. The inferior vena cava carries “used” blood from the lower body to later return to the heart..