In the lungs, oxygen diffuses from the alveoli (air sacs) into the blood in the surrounding capillaries. This happens because the concentration of oxygen is higher in the alveoli compared to the blood. The oxygen then binds to hemoglobin in red blood cells, which transport it to various tissues throughout the body.
In the tissues, the direction of oxygen diffusion reverses. Oxygen diffuses from the blood into the tissue cells. This occurs because the concentration of oxygen is higher in the blood than in the tissue cells. The oxygen is then used by the cells for various metabolic processes, including cellular respiration.
In summary, oxygen diffuses from the alveoli into the blood in the lungs and from the blood into the tissues in the body. This process ensures that oxygen is delivered to where it is needed most.