DAVID LEAH / SCIENCE PHOTO LIBRARY. DAVID LEAH / SCIENCE PHOTO LIBRARY.
Open heart surgery, showing the implantation of an artificial heart valve made from a pig's dura mater, the tough outer membrane of the brain. The valve is tricuspid, meaning it has three "leaflets": blood flows through the valve from the front, but will not be allowed to return when back pressure forces the leaflets shut. The artificial valve (left) is about to be implanted.
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