Explanation: Oxygen and carbon dioxide travels to and from tiny air sacs in the lungs, through the walls of the capillaries, into the blood. Blood leaves the heart through the pulmonic valve, into the pulmonary artery, and to the lungs.
In the lungs, the blood refills its oxygen supply and gets rid of carbon dioxide. The oxygenated blood returns from the lungs to the left atrium through the pulmonary veins to the heart. The left ventricle then pumps this oxygen-rich blood throughout the body. Blood first enters the heart's right atrium. A muscle contraction forces the blood through the tricuspid valve into the right ventricle. When the right ventricle contracts, blood is forced through the pulmonary semilunar valve into the pulmonary artery. Then it travels to the lungs.
