An agent that is smaller than a virus causes scrapie. It is either a protein called a prion or it is another substance closely associated with the prion protein. Scientists still have a lot to learn about the scrapie agent and exactly how it works.
Tissue valves are made with tissues from porcine (pig) heart valves or bovine (cow) cardiac tissue because they function like human heart valves. Once the tissue is removed from the animal, it is chemically treated to preserve the tissue and prevent immulogic reactions once it is placed in a patient.
They are called the aortic, mitral, pulmonary, and tricuspid valves, and each is made of flaps of tissue called leaflets. Each time your heart bets, it pumps blood through these valves by contracting (squeezing) its chambers. These valves open in one direction, like one-way gates, allowing blood to flow forward.