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.
The pulmonary and the aortic valves make up the semilunar valves. The pulmonary valve, made up of two leaflets, guards the opening between the pulmonary artery and the right ventricle, while the aortic valve, made up of three leaflets, guards the opening between the left ventricle and the aorta.