ANTONIA REEVE / SCIENCE PHOTO LIBRARY ANTONIA REEVE / SCIENCE PHOTO LIBRARY
Heart surgery. Surgeon's hands stitching the soft fabric valve ring of an artificial heart valve onto the heart. The four heart valves prevent blood from re-entering each of the heart chambers once it has been pumped out. If one of these valves becomes narrowed or leaky, due to damage or disease for example, it must be replaced with a prosthetic valve.
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