My grandmother recently went through Bypass surgery. Her surgery was successful and she is currently in the recovery phase. I read more about Bypass surgery and here is what I found:
When an artery leading from the heart becomes clogged with plaque — material composed of cholesterol and fats that form and harden inside blood vessels — the heart can’t receive all of the blood that it needs. This can cause a heart attack, and in fact, my grandmother suffered a mild heart attack and was rushed to the hospital.
Bypass surgery involves taking a piece of a healthy blood vessel from elsewhere in the body and grafting, or attaching, it to the damaged vessel. In doing so, the blood flow is rerouted around the damaged part. (Source)