If you have ever had a phone or music player stop working after you have been using it for a while, you know that things can wear down with time. The same thing happens to the human body, except in this case it's a much more serious problem than trying to figure out how you're going to get your iTunes fix. Heart valves can become problematic over time. When this happens it can be extremely harmful. A heart valve can have stenosis, which occurs when the valve no longer open as widely as it would normally. It can also become leaky, which means that blood can backflow into chambers that it should have already been pumped from. If you are trying to get ketchup out of a bottle when the tip is plugged, you know that you have to work extra hard to get the desired effect. This is exactly what the heart has to do to get all of the oxygen and nutrients required to the rest of the body. If it has to work too hard for too long, the heart can fail.
When valves begin to become problematic, they must be replaced. Many advances have been made in life saving valve replacement technology. Pig heart valves were discovered to be very similar to those of human hearts. The first successful valve xenograft (meaning a tissue implantation from one species to another) occurred in 1965. Since then, heart valves from pigs, horses, and cows have been utilized extensively to replace ailing heart valves throughout the world.
The major setback with using heart valves from other species is that the human body's immune system rejects foreign objects. This makes the replacement valves useless after a period of time. Recent attempts that have shown some success in alleviating these problems include using xenograft valves that have had all of the cells removed, leaving behind just the matrix. No, not that matrix. This allows the valve recipient's cells to replace the missing cells and avoid rejection of the valve. It's a win-win—maybe not for the animal that donated the valve.