What divides the heart into right and left sides?

• A. The septum, a complete muscular wall
• B. The interatrial groove
• C. The myocardium
• D. The endocardium

Answer: (A)

The heart is split into right and left sides by a wall called the septum. There are two key septa: the interventricular septum, which is the thick muscular wall between the ventricles, and the interatrial septum, which separates the atria. Together, they form the two halves of the heart and help keep oxygen-poor blood on the right separate from oxygen-rich blood on the left. The ventricular portion is indeed primarily muscular, which is why a statement about a complete muscular wall fits for that part, though the atrial portion has a thinner, partly membranous region as well. The walls involved are part of the heart’s myocardium, and the inner lining is endocardium.