The heart is simply a bag of muscle that - about once every second-squeezes itself to send blood flowing through the body's network of blood vessels. The heart has two pumps, working side by side. The left one sends blood out along arteries to the whole body, from head to toes. This blood gives up oxygen and returns along veins to the right pump. From here it is sent on a much shorter trip to the lungs to gather more oxygen. Two special features make the heart incredibly hard-working and versatile. The first is the muscle in the heart's walls. It is called cardiac muscle and, unlike the normal muscle, it never tires. The second is the heart's ability to change its pumping speed and force to match the body's needs.