Final answer:
The interatrial septum separates the two atria of the heart and prevents the mixing of oxygenated and deoxygenated blood. It also played a role in fetal circulation by allowing blood to bypass the lungs. After birth, the opening in the septum closes and the septum becomes fully developed.
Step-by-step explanation:
The main function of the interatrial septum is to separate the two atria (upper chambers) of the heart. It prevents the mixing of oxygenated and deoxygenated blood, ensuring that oxygen-rich blood is pumped to the body while deoxygenated blood is sent to the lungs for oxygenation. In the fetal heart, the interatrial septum contains an opening called the foramen ovale, which allows blood to bypass the lungs. After birth, this opening closes and the septum becomes fully developed.