But the heartbeat is more like a squeezing or twisting than a thumping. It begins like this: Electricity from special cells (called pacemaker cells)-starting at the top of the heart and moving down-stimulates the heart muscle to squeeze the blood out through the aortic valve.
It's like wringing a wet towel to squeeze out the liquid. As the heart muscle becomes tightly wound, the blood is squeezed out until there is virtually no space between the individual muscle cells. The blood is then pushed through the valves. The wave of blood that has been squeezed out of the heart is ejected into the aorta itself, the body's largest artery, which carries oxygen-rich blood to the rest of the body.
Once that happens, the heart relaxes-as if your hands had just let go of the towel. As it does, the coronary vessels, which lie on the surface of the heart, also relax. Then the space between the tight muscle cells opens up, and the rich, oxygenated blood that was just ejected from the heart fills the arteries on the heart's surface and slips down between those cells and feeds them. Most of the ejected blood goes on to fuel the rest of the body-but not before the heart puts its own tax on it, taking its first cut of the life-sustaining fluid.
After the process of towel-wringing-squeezing blood into the aorta followed by muscle relaxation and the heart feeding itself-then, 60 or more times a minute, the pacemakers send out their next signal, beginning the process all over again.