Pulmonary circulation is the process by which blood moves from the heart to the lungs and back. It begins when deoxygenated blood is pumped from the right side of the heart through the pulmonary arteries to the lungs. In the lungs, carbon dioxide is exchanged for oxygen, allowing the blood to become oxygenated. Once the blood is oxygen-rich, it returns to the left side of the heart via the pulmonary veins. From there, the oxygenated blood is pumped out to the rest of the body through the systemic circulation, delivering essential oxygen to tissues and organs.