What kinds of valves are found in the human circulatory system and how do they differ?

The three main kinds of valves in the human circulatory system are atrioventricular valves, semi-lunar valves and pocket valves. Atrioventricular valves are found in the heart and separate the atria and ventricles. During ventricular systole (contraction), the pressure is higher in the ventricles compared to the atria, so AV valves prevent back-flow of blood into the atria. Semi-lunar valves are found in the aorta and pulmonary artery. During diastole, the ventricular walls relax and the elastic walls of the vessels recoil, so the pressure is higher in the vessels and semi-lunar valves prevent back-flow into the ventricles. Pocket valves are found in veins throughout the venous system. They ensure one-way flow of blood towards the heart.