Heart surgery. Image 1 of 2. Artificial heart valve being lowered into place during keyhole surgery. The valve has been sown onto the heart by its soft fabric valve ring, and the threads are ready to be pulled tight to secure the valve in place. The four valves of the heart prevent blood from re-entering each of the heart chambers once it has been pumped out. If one of the valves becomes narrowed or leaky, due to damage or disease for example, it must be replaced with a prosthetic valve. See M560/472 for an image of the valve being lowered into place.