What type of valves are the pulmonary and aortic valves classified as?

The pulmonary and aortic valves are classified as semilunar valves due to their structure and function. These valves are located at the exit of the heart's ventricles; specifically, the pulmonary valve is positioned between the right ventricle and the pulmonary artery, while the aortic valve is found between the left ventricle and the aorta.

The term "semilunar" derives from the valve's half-moon shape. Each of these valves consists of three cusps or leaflets that open and close in response to pressure changes during the cardiac cycle, facilitating unidirectional blood flow from the ventricles to the arteries while preventing backflow. This function is crucial in maintaining proper circulation and pressure in the pulmonary and systemic circuits.

In contrast, atrioventricular valves, such as the tricuspid and bicuspid (mitral) valves, are situated between the atria and ventricles and have a different structure and function, allowing blood to flow from the atria into the ventricles. The tricuspid valve has three leaflets, while the bicuspid valve has two, but neither of these categories pertains to the pulmonary and aortic valves.