STEVE ALLEN / SCIENCE PHOTO LIBRARY STEVE ALLEN / SCIENCE PHOTO LIBRARY
Artificial heart valves. Two prosthetic heart valves of the ball-and-cage type, showing the closed (left) and open (right) positions. A heart valve regulates the flow of blood from the heart to the main arteries which distribute blood to the lungs or around the body. Malfunctions of a heart valve due to disease or malformation may result in it narrowing with a consequent reduction in blood flow or backflow. An incompetent heart valve can be replaced with a prosthetic valve through surgery.
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