The brain is an organ like all the other organs in the body. It requires a steady blood supply to deliver the oxygen and nutrients it needs to do its work. When the brain cells do not receive enough oxygen or nutrients, they die. Once the brain cells die, there is no chance for recovery. Cardiac death is different than brain death.
Cardiac death happens when the heart and lungs stop functioning.
People who die from cardiac death cannot donate their organs because of deterioration due to a lack of oxygen. Eye and tissue donation are, however, options. A person in a coma or vegetative state still has some brain activity and, therefore, has not died. The respirator or breathing machine provides oxygen to the organs keeping them healthy until they are removed for transplant. Once the organs have been removed, they are cooled and preserved.