The interatrial septum is a thin wall that separates the left and right atria of the heart. This structure plays a crucial role in ensuring that oxygen-rich blood from the lungs does not mix with oxygen-poor blood returning from the body. During fetal development, there is a small opening in the interatrial septum called the foramen ovale, which allows blood to bypass the non-functioning lungs. After birth, this opening usually closes, forming a complete septum. Any defects in the interatrial septum can lead to conditions such as atrial septal defects (ASD), which may require medical attention.