This is when the ventricles contract and pump blood into the aorta and pulmonary artery. The first phase is called systole (SISS-tuh-lee).
One complete heartbeat is made up of two phases: From here, the electrical signal travels through the right and left ventricles, making them contract. Then the impulses travel down to the atrioventricular (or AV) node, which acts as a kind of relay station. These electrical impulses make the atria contract first.
This node is called the pacemaker of the heart because it sets the rate of the heartbeat and causes the rest of the heart to contract in its rhythm. It sends out an electrical signal to start the contracting (pumping) of the heart muscle. The sinus (or sinoatrial) node is a small area of tissue in the wall of the right atrium. How the heart beats is controlled by a system of electrical signals in the heart. But when we're exercising, the heart pumps faster so that our muscles get more oxygen and can work harder. For example, when we're sleeping, it pumps just enough to provide for the lower amounts of oxygen needed by our bodies at rest. The heart gets messages from the body that tell it when to pump more or less blood depending on a person's needs. Once the blood is back in the heart, it needs to re-enter the pulmonary circulation and go back to the lungs to drop off the carbon dioxide and pick up more oxygen. (The terms superior and inferior don't mean that one vein is better than the other, but that they're located above and below the heart.) Two large veins that lead into the heart are the superior vena cava and inferior vena cava. Valves in the veins keep blood flowing in the correct direction. Small veins lead to larger and larger veins as the blood approaches the heart. Waste products are brought into the capillaries.Ĭapillaries then lead into small veins. The capillaries have very thin walls, and through them, nutrients and oxygen are delivered to the cells. Like a tree, the branches gets smaller and smaller as they get farther from the aorta.Īt each body part, a network of tiny blood vessels called capillaries connects the very small artery branches to very small veins. Branches off of the aorta send blood to the muscles of the heart itself, as well as all other parts of the body. The aorta is a big artery that leaves the heart carrying this oxygenated blood.