Heart Chambers and Heart ValvesThe heart has four chambers. The two ventricles (right and left) are muscular chambers that propel the blood out of the heart (the right ventricle to the lungs, and the left ventricle to all other organs). The two atria (right and left) hold the blood returning to the heart, and at just the right moment empty into the right and left ventricles. The four heart valves (tricuspid, pulmonic, mitral and aortic) keep the blood moving in the right direction through the heart.
How The Heart Pumps BloodIt is helpful to visualize the heart as two separate pumps, working in series; the right heart pump, and the left heart pump.
The Right Heart PumpFigure 1: The right heart pump consists of the right atrium (RA), tricuspid valve (TV), right ventricle (RV), pulmonic valve (PV), and pulmonary artery (PA). Poorly oxygenated, "used" blood returning to the heart from the body's organs enters the right atrium, and is stored there until the right atrium contracts.
Figure 2: When the right atrium contracts, the tricuspid valve opens, allowing the blood to enter the right ventricle.
Figure 3: Then, when the right ventricle contracts, the pulmonic valve opens, and the blood is propelled into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up oxygen.