why does oxygen moves onto red blood cells when they are near the lungs but moves off of red blood cells when they are near other body cells.

Oxygen moves onto red blood cells when they are close to the lungs because oxygen needed by cells of other parts of the body is carried by the red blood cells and the only the lungs makes this oxygen available to the body and also ensures to remove other harmful gases such as CO2. Thus, red blood cells must be able to absorb oxygen in the lungs, pass through narrow blood vessels, and release needed oxygen to respiring body cells.