Which structure separates the two atria in the heart?

The interatrial septum is the correct choice as it is the wall of tissue that divides the right and left atria of the heart. This septum plays a crucial role in maintaining the separation of oxygenated and deoxygenated blood within the heart, which is essential for efficient circulation.

During fetal development, there is a temporary opening in the interatrial septum known as the foramen ovale, allowing blood to bypass the non-functioning lungs. After birth, this opening typically closes and becomes the fossa ovalis, further solidifying the separation of the two atria.

Other structures listed serve different functions: the interventricular septum is located between the ventricles and primarily prevents mixing of the blood in the right and left sides of the heart, the coronary sulcus is a groove that encircles the heart and separates the atria from the ventricles, and the bicuspid valve (also known as the mitral valve) regulates blood flow from the left atrium to the left ventricle.