How does the heart work?

The heart is a pump responsible for distributing blood throughout the body. A little larger than a fist, the heart is located behind the sternum and protected by the ribs.

A sac or membrane called the pericardium surrounds the heart. A layer of fluid in the pericardium lubricates the surface of the heart and allows it to move easily within the body.
 
As with any muscle, the heart is nourished by oxygenated blood, brought to the muscle by coronary arteries. Large arteries extend from the heart and connect its left side to the body  and right side to the lungs. Blood is pumped through the right side of the heart into the lungs and back to the left side of the heart, which pumps oxygenated blood to the body. 

One-way valves control the flow of the blood. Healthy valves form a tight seal, preventing blood from flowing back into the heart or lungs. 

Internally, the heart is divided into four chambers. The top two smaller chambers are called atria, while the bottom two larger chambers are known as ventricles. The heart is divided into a left and right side by a septum, which is essentially a wall. De-oxygenated blood flows from the body into the right atrium. The tricuspid valve acts as a trap door, which  controls the flow from the right atrium into the right ventricle. When the right ventricle beats or contracts, the tricuspid valve shuts and the pulmonary valve opens, allowing blood into the lungs where it becomes oxygenated. The oxygenated blood returns to the heart, pooling in the left atrium. The mitral valve opens and allows the blood to pass from the left atrium to the left ventricle. 

Once the left ventricle -- the most powerful of the four chambers -- contracts, the mitral valve closes and the aortic valve opens, allowing for blood to pass into the aorta where it is ultimately distributed throughout the body through an elaborate network of blood vessels.

Your heart acts as a pump to circulate blood throughout your body. Its pumping action is created by the division of the heart into four chambers, separated by valves. The chambers are the left and right atria (the upper chambers) and the left and right ventricles (the lower chambers). A thick wall called the septum separates the left side of the heart from the right. The walls of the heart and the valves keep the blood on a "one-way street" through the heart and out to the body and back.

Blood arrives through veins from the body, depleted of oxygen, and enters the right atrium of the heart. A valve called the tricuspid valve prevents the blood from flowing downward into the right ventricle until the appropriate time. Then the valve opens, the blood flows into the ventricle, and the ventricle contracts, forcing the blood through the open pulmonary valve and into the pulmonary artery. That artery then carries the blood to the lungs to release carbon dioxide and pick up oxygen.

From the lungs, blood returns to the heart through the pulmonary veins and enters the left atrium. The mitral valve keeps the blood in that chamber until it is time for it to move to the left ventricle. The ventricle contracts, and the blood is forced through the open aortic valve and into the aorta, the largest artery in the body. The aorta distributes blood to the smaller arteries and vessels that supply your entire body with oxygenated blood.

The heart’s pumping action is generated by a series of electrical impulses. The electrical impulse begins in the sinoatrial (SA) node in the top of the right atrium and travels through the muscles of both atria. The atria contract in response. The electrical impulse is then picked up by the atrioventricular (AV) node, which is between the atria and the ventricles, and passed to the fibers of the His-Purkinje system. The ventricles fill with blood from the atria before the electrical impulse causes them to contract. When the ventricles contract, they force the blood out of the heart and to the lungs and body.

The heart is a muscle. It has four chambers that pump blood. The upper two chambers are called the "atria" and the lower two chambers are called the "ventricles." The heart is really two pumps that work together to move blood through the body. The right-sided pump fills with blood returning from the body and pumps it to the lungs to receive oxygen. This right-sided pump is a thinner muscle than the left-sided pump because it only has to pump blood to the lungs. The left side of the heart receives blood from the lungs and pumps it out to all parts of the body. This left-sided pump is a thick, strong muscle. The heart has an electrical system that sends impulses to tell the heart muscle when to pump.