Blood entering the right atrium has just finished delivering oxygen to the tissues and cells in your body. So, the blood in this atrium is oxygen-poor blood.
Oxygen-poor blood passes from the right atrium into the right ventricle. This ventricle pumps blood to vessels in the lungs, where the blood “resupplies” with oxygen. This ventricle pumps blood only to the lungs.
After the right side of the heart does its job and pumps blood through the lungs to pick up oxygen, the blood flows from the lungs and re-enters the heart through the left atrium.
Then, the left ventricle pumps the oxygen-rich blood to the cells and tissues throughout the entire body. This cycle occurs continuously.