Heart valves are important because they guide the flow of blood through the heart in only one direction and prevent back-flow. In healthy heart valves, the pressure changes in the heart allow them to open their cusps (flap-like doors) and then close them tightly as needed. There are four valves in the heart with the aortic and mitral valves being the most commonly replaced. Cardiac surgery is needed to repair the valve or replace it with new synthetic (artificial) or biological ones. The tissue valve can come from an animal such as a pig or cow or a man-made valve can be made from metal or plastic. Regurgitation occurs when a heart valve does not close properly and blood backflows or leaks instead of moving forward. Surgery is able to tighten or replace the valve. This surgery can also be performed by minimally invasive methods (small incisions) depending on what the surgeon feels is safer for the patient.