Blood flows through the heart in a specific pattern in order to pump deoxygenated, or blue blood to the lungs and oxygenated, or red blood to the body. Blue blood returns from the body and collects in the right atrium. Red blood returns from the lungs and collects in the left atrium. Blood from the atria flow into their respective ventricles which then contract to pump the blue blood to the lungs and the red blood to the body.
The heart has four valves that keep the blood flowing forward through the heart and prevent it from going backward. The mitral valve and the tricuspid valve (or atrioventricular valves) are located between the atria and ventricles. The aortic valve and pulmonary valve are known as the semilunar valves and are located between the ventricles and two major arteries: the aorta which brings blood to the body and the pulmonary artery which brings blood to the lungs.
Visit Houston Cardiovascular Institute Website for more information.