The heart is essentially two pumps in parallel on its left and right side, both work together. The right side forces deoxegenated blood from the body through the lungs. The left side sends oxygenated blood from the lungs to the body. The left side having more to do, is larger than the right. Each side has two chambers connected. The smaller chamber is called an atrium (or auricle) and the larger chamber the ventricle. There are valves at the exit and between the chambers which only permit blood to flow into the atrium, to flow from the atrium to the ventricle, and to leave the ventricle, and prevent flow in the other direction. The atria contract, forcing their blood into the ventricles. In a second big contraction, the ventricles shut the connecting valve and force the blood into the exit vessel. EDIT: Science Geek, you're right - veins have valves in them anyway. I have updated this answer. Please update your rating.

The heart is composed of four chambers, two atria and two ventricles. The atria are on the top and the ventricles are on the bottom. There are also four major vessels entering the heart, all from the top. The whole system is organized in series. The blood flows through a vein into the right atrium then into the right ventrical and then it is pumped into the pulmonary artery(arteries go away from the heart), which pumps blood to the lungs to recieve Oxygen. Then the blood flows through a vein into the left atrium then into the left ventricle. From the left ventricle, the blood is pumped to the rest of the body through the Aorta(An artery). There are valves between the atria and ventricles that are designed in order to prevent blood from flowing back into the atrium when the ventricles contract. There are also valves dividing the arteries from the ventricles to keep blood from flowing back into the ventricles from the arteries. Contraction begins in an area of the heart called the SA node. The SA node is concentrated with cells called pacemaker cells that can cause contractions without external stimulation. This is accomplished by a type of channel called "funny channels". These funny channels allow Sodium ions to flow into the pacemaker cell when the cell is polarized and causes it to move toward its depolarization threshold. As the cell becomes depolarized, it causes Calcium channels to open and allow Calcium to flood into the cell. This eventually leads the cell to threshold and causes the cell to depolarize. The depolarization spreads to the neighboring cells through gap junctions and causes the atrium to contract. The depolarization also spreads to the AV node in the floor of the Atrium. It then moves to the "Bundle of His", which is between the two ventricles, and then to the Perkinje fibers where it causes the cell to contract from bottom to top. So, now a further description of the flow of blood. The blood begins by flowing into the right atrium and left atrium, with the AV valves closed, and causes the two atria to fill. At the same time, the ventricles are contracting in unison, pushing blood into the arteries that come out of the top of the heart to the lungs and to the body. When the ventricles are done contracting and begins to relax, the valves between the arteries and ventricles close. The AV valves open and blood flows from the atria into the ventricles without contraction. The atria then contract to push the remaining blood(about 20%) into the ventricles. Then relaxation of the atria occur and the process begins again. The channels are called funny channels because, unlike normal Sodium channels, they allow Sodium into the cell when the cell is polarized. Because of this, the heart can beat without any stimulation from the nervous system. This phenomenon has been observed in documentation of ancient Aztec human sacrifices. They would remove the heart from the body and it would continue to beat for a little while.