Heart 101 | National Geographic

• [Narrator] The heart pumps blood throughout the body, carrying oxygen and nutrients to every cell. It's this circulation of blood that is vital to sustaining life. The heart is an organ made up of several tough layers of muscle. The pericardium is the thin layer that covers the exterior, while the endocardium lines the inside walls.

The heart is divided into four chambers: two upper and two lower. The upper chambers, known as the atrium, receive blood coming into the heart. The lower chambers are the ventricles that pump blood out. Between each chamber are valves that open and close and help keep the blood moving. They are the tricuspid, mitral, pulmonary, and aortic valves.

A pumping cycle starts when oxygen-repleted blood returns to the heart after circulating throughout the body. The blood enters through the right atrium before flowing to the right ventricle. It's then pumped to the lungs via the pulmonary arteries. There, blood is rejuvenated by air that's breathed in. The oxygen-rich blood returns to the heart through the left atrium, where it goes to the left ventricle. Then, by way of the aorta artery, the fresh blood is pumped throughout the body before the process repeats itself.

That process happens with every heartbeat, and it's relentless. The heart beats 100,000 times a day, 40 million times a year, and up to three billion times over an average lifespan. But there are conditions that can disrupt a heartbeat and that normality. They can range from myocardial infarction or heart attack to heart disease and hypertension. In contrast, exercise and emotional excitement can also have an impact on a person's heartbeat.

The various blood vessels that comprise the circulatory system are a network of veins, arteries, and capillaries that span over 60,000 miles throughout the body. And the heart is the pump at the center of it. (heartbeat)