The internal anatomy of the heart is fascinating.  There are four chambers of the heart.  In order to go from one chamber to the next, the blood must travel across valves that keep the blood moving forward.

Here's the path that the blood travels:

The blood arrives into the heart through the superior and inferior vena cava and enters the right atrium.

Then the blood passes across the tricuspid valve to enter the right ventricle.

After leaving the right ventricle, the blood passes across the pulmonary valve and goes to the lung where the blood becomes oxygenated.

After leaving the lungs, the blood returns to the heart and enters the left atrium.

From the left atrium the blood crosses the mitral valve and enters the left ventricle.

Finally, the blood leaves the left ventricle and crosses the aortic valve to enter the aortic artery and then on to the rest of the body.