ANATOMY

Detailed explanation-1: -Hemoglobin is the protein inside red blood cells. It carries oxygen.

Detailed explanation-2: -When we breathe in, the millions of air sacs in the lungs fill with fresh oxygenated air. The oxygen then moves into the blood by passing first through the very thin walls of the air sacs and then into the capillaries, which are tiny blood vessels in a network within the lungs.

Detailed explanation-3: -Red blood cells pick up oxygen in the lungs. Blood travels away from the heart and lungs through the arteries (ar-tuh-reez). Red blood cells drop off oxygen to the cells through tiny tubes called capillaries (cap-ill-air-ies). Blood then returns to the heart through the veins (vayns) and the cycle begins again.