information:classification, linguistic:voice_change, stylistic:style_transfer, structural:reformatting
A heart valve is a biological one-way valve that allows blood to flow in one direction through the chambers of the heart. Four valves are usually present in a mammalian heart and together they determine the pathway of blood flow through the heart. A heart valve opens or closes according to differential blood pressure on each side. The four valves in the mammalian heart are two atrioventricular valves separating the upper atria from the lower ventricles – the mitral valve in the left heart, and the tricuspid valve in the right heart. The other two valves are at the entrance to the arteries leaving the heart. These are the semilunar valves – the aortic valve at the aorta, and the pulmonary valve at the pulmonary artery. The heart also has a coronary sinus valve and an inferior vena cava valve, not discussed here.