The pulmonary circuit involves blood flow from the heart to and from the:

Blood flow goes from the heart, to the lungs, and back to the heart again.

The blood in the human body goes from the heart, enters organs and muscles, then exits to go to the lungs to "drop CO2" and "pick up oxigen". It then returns in the heart to be pumped again in the blood vessels ...

More simply:

-Lungs - > - Heart - > - Blood vessels - > - Organs&Muscles - > - Lungs - > - Heart etc.