Question

What structure regulates the direction of blood flow through the heart?

Answer 1

Valves.

Explanation:

The atrioventricular (AV) valves allow blood to flow from the atria to the ventricles, but not in the reverse direction. The right AV valve is called the tricuspid valve, the left is called the mitral valve.

Source: http://www.medicine.mcgill.ca/physio/vlab/cardio/heart_valves.htm