Blood goes into the atria of the heart and the atria contract and force blood into the ventricles. The ventricles then contract and force the blood out of the heart.

There are valves in the heart which ensure that blood flows in unidirectionally in the right way.

Blood flows away from the heart to the organs through arteries and returns to the heart through veins.

In humans, there is a double circulation system, with 2 separate ones: one for the lungs and one for all other organs of the body.