A、An animal’s body. B、Another person who has just died. C、A person who has more veins than he really needs. D、The patient’s own

The heart is really just a very strong pump. It has four valves made of very thin, strong tissue. If the damage to the heart is not too bad, we put in pieces of natural heart-valve, which are taken from other people who have just died. If a damaged artery is not too big, we replace it with a piece of vein that it really needs. But how can we operate on the heart? How does the blood continue to flow around the body? Here engineers really have helped us. We use the heart lung machine. It does the work of both the heart and the lungs. It pumps the patient’ s blood out of a large vein near the heart and passes the blood through a " bath", oxygen. The blood is filled with oxygen and other gases are taken away from it, as in our lungs.
    Q16: When the heart is not too badly damaged, what can we do to it?
    Q17: Where did the piece of vein used to replace a damaged artery come from?
    Q18: When operating on a patient’s heart, how can the engineers help out?