Final answer:
Oxygen enters red blood cells in the pulmonary circulation, where gas exchange occurs in the alveoli of the lungs, allowing oxygenated blood to be carried back to the heart through the pulmonary veins.
Step-by-step explanation:
Oxygen enters red blood cells in the pulmonary circulation. The pulmonary arteries carry deoxygenated blood from the heart to the lungs. Here, in the alveoli, the exchange of gases takes place: oxygen is absorbed into the red blood cells and carbon dioxide is released. The now oxygenated blood is then transported back to the heart through the pulmonary veins, completing the path of the pulmonary circulation before it is pumped into the systemic circulation to provide oxygen to the rest of the body.