A、The machine keeps the blood warm enough. B、The machine pushes the blood back into the vessels. C、The machine works very gently

The heart is really just a very strong pump. It has four valves made of very thin, strong tissues. If the damage to the heart is not too bad, (21) we put in pieces of natural heart-valve, which are taken from other people who have just died. (22) 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? (23) 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. (24) The machine keeps the blood warm all the time. Then it pumps the blood back into a large artery. It does all this in a very gentle way; the blood cells are not damaged. So we can work on a heart which is empty of blood.
    (25)" If you aren’ t breathing, and if your heart isn’ t beating, you are dead," people used to say. But that is not true now. Although the heart is not pumping, and the patient is not breathing, his blood is still going around his body with the help of the machine. He is very much alive. The heart-lung machine saves thousands of lives every year. It’ s really a wonderful invention.
21．When the heart is not too badly damaged，what can we do to it?
22．Where did the piece of vein used to replace a damaged artery come from?
23．When operating on a patient’s heart，how can the engineers help out?
24．When the blood passes through the heart—lung machine，why are the blood cells not damaged?
25．Previously，when was a person considered dead?