This figure demonstrates the heart and the two cycles of blood through the body. It is crucial for the heart to keep the two cycles separate, so heart is divided into the right and the left halves, which have no direct connections whatsoever. The right half is responsible for the pulmonary cycle and the the left - for the systemic.
Blood comes into the heart through the atria and leaves through the ventricles.
The heart pumps blood by contracting and relaxing the muscles which line the heart walls, squeezing blood out and then letting new blood in. When the heart contracts, it exerts pressure on the blood which is in it, forcing it to move out. This difference in pressure is what drives the blood flow through the body. The exact equation which describes this relationship is on page 4. One very important parameter to consider is the heart rate, which is the number of times it contracts in a minute.