What is the primary function of the semi-lunar valves?

The primary function of the semi-lunar valves is to prevent backflow of blood into the heart. Located at the exit points of the heart, these valves are positioned at the aorta and the pulmonary artery, playing a crucial role in maintaining unidirectional blood flow during the cardiac cycle. When the heart contracts and pumps blood out, the semi-lunar valves open to allow blood to flow into the arteries. However, when the heart relaxes, these valves close tightly to prevent the blood that has just been ejected from flowing back into the heart's ventricles. This mechanism is vital for maintaining efficient circulation and ensuring that the heart can effectively pump blood to the lungs and the rest of the body without any backflow that could compromise the circulatory system.

The other options do not accurately describe the primary function of the semi-lunar valves. Allowing blood flow between the atria and ventricles is the role of the atrioventricular valves, not the semi-lunar valves. Facilitating blood flow from the heart to the body is a general function of the entire heart but not specific to the semi-lunar valves alone. Regulating oxygen levels in the blood involves the lungs and respiratory system rather than a direct function of any cardiac valve.