Explain the way in which blood moves throughout the heart, including nodes and valves.

First, the blood enters through the right and left atria. The atrioventricular valves are closed, so the blood pools up inside them. Then, the sinoatrial node (SAN) sends a pulse for the atria to contract, forcing the blood past the valves and into the ventricles. The force closes the valves, preventing backflow. The pulse from the SAN travels down to the atrioventricular node (AVN) and then down the bundle of His and Purkinje fibers, delaying them so that the ventricles contract after the atria. When the pulse reaches the ventricles, they contract, forcing the blood into the arteries. The blood from the right ventricle goes to the lungs and then to the left atrium, while the blood from the left ventricle goes on to the body then the right atrium.