What is the function of the semi-lunar valves?

The semi-lunar valves play a critical role in the circulatory system by ensuring unidirectional blood flow as the heart pumps. Specifically, these valves are located at the exit points of the ventricles, where blood leaves the heart to enter the aorta and the pulmonary artery.

The correct choice highlights that the semi-lunar valves prevent backflow of blood into the ventricles after contraction. When the ventricles contract during systole, they force blood into the aorta and pulmonary artery. Once the ventricles relax and the pressure within them decreases, the semi-lunar valves close to prevent blood that has just been ejected from flowing backwards into the ventricles. This function is vital for maintaining efficient circulation and ensuring that oxygenated and deoxygenated blood flow does not mix, which is crucial for effective oxygen delivery and waste removal in the body.

The other options misrepresent the function of these valves. For example, while the atrioventricular valves prevent backflow into the atria, the semi-lunar valves specifically address the scenario of blood flow out of the heart and back into the ventricles, which accurately relates to the correct function described.