How does the structure of the heart relate to its function?

The easiest way to understand the way that the heart works is to imagine a double pump with high pressure and low pressure tubing. One part of the pump is low pressure all the way, and that brings the blood from the right side of the heart, to the lungs to be oxygenated, and back to the left side of the heart. The second part of the pump takes this newly oxygen rich blood from the left side of the heart, and pumps it out under high pressure to the rest of the body. Once the blood has reached its destination, it no longer needs to be under such high pressure, and it is returned to the right side of the heart to start the cycle again. Generally, the high pressure tubes are arteries, which carry blood away from the heart (artery = away), whilst the low pressure tubes are veins, which return blood to the heart. Naturally, though, there are exceptions to this rule!