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 ventricle 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 heart through the pulmonary veins and enters the left atrium. From there, the oxygenated blood is pumped into the left ventricle, which then distributes it throughout the body via the systemic circulation.