The circulatory system moves blood throughout the body. It all starts in the lungs. When you inhale, your lungs get filled with oxygen. Red blood cells come from the heart and to the lungs, then release the carbon dioxide they picked up around the body. They pick up oxygen then travel back to the heart. It is then pumped all around the body. As the red blood cells travel, they drop off oxygen, nutrients, and water to your body cells. They also pick up carbon dioxide. When they are out of oxygen, the cells travel back to the heart, then into the lungs. The process happens over and over again.
There are other blood cells too. White blood cells help with fighting infections. Platelets can help stop bleeding when you get a cut.