Why would a person need a heart bypass surgery?

There are blood vessels (coronary arteries) that feed the heart's cardiac muscle (myocardium) the oxygen it needs to continue pumping.

If these vessels have some kind of blockage (i.e. heart disease), then the heart wouldn't have oxygen it needs to function, leading to heart failure and possible death.

The heart bypass surgery is then done to fix this problem.

By taking a part of another blood vessel in the body (usually great saphenous vein is used), and is grafted to the vessel causing the problem for the heart.

And so, blood flow to the heart's important muscle is once again undisturbed.