Answer:
Oxygen enters the blood through the lungs, not directly through any part of the heart. The heart pumps deoxygenated blood to the lungs via the pulmonary artery, where the blood is oxygenated and carbon dioxide is removed through the process of respiration. The oxygenated blood then returns to the heart via the pulmonary veins and is pumped out to the rest of the body through the aorta.