The heart is a four-chambered organ with four main vessels, which either bring blood to or carry blood away from the heart. The four chambers of the heart are the right atrium, the right ventricle, the left atrium, and the left ventricle.
The great vessels of the heart include the superior and inferior vena cava, which bring blood from the body to the right atrium, the pulmonary artery, which transports blood from the right ventricle to the lungs. The last of the great vessels is the aorta, the body's largest artery, which transports oxygen-rich blood from the left ventricle to the rest of the body.
If we remove some of the tough fibrous coating of the heart and great vessels, you can get a better look at the heart beating. If you look carefully, you can see a series of one-way valves that keep the blood flowing in one direction. If we inject dye into the superior vena cava, you can watch it pass through the heart as it goes through the cardiac cycle.
The blood first enters the heart into the right atrium. Blood passes from the right atrium through the tricuspid valve and into the right ventricle. When the right ventricle contracts, the muscular force pushes blood through the pulmonary semilunar valve into the pulmonary artery.
The blood then travels to the lungs, where it receives oxygen. Next, it drains out of the lungs via the pulmonary veins, and travels to the left atrium. From the left atrium, the blood is forced through the mitral valve into the critically important left ventricle. The left ventricle is the major muscular pump that sends the blood out to the body systems. When the left ventricle contracts, it forces the blood through the aortic semilunar valves and into the aorta. From here, the aorta and its branches carry blood to all the tissues of the body.