Describe how oxygen and carbon dioxide diffuse both between the 1) alveoli and the blood, and 2) blood and the body tissues. Include in your description the diffusion gradients present for both gases at both locations.

The change in partial pressure from the alveoli (high concentration) to the capillaries (low concentration) drives the oxygen into the tissue and the carbon dioxide into the blood (high concentration) from the tissues (low concentration), which is then returned to the lungs and exhaled.

Once in the blood of the capillaries, the O2 binds to the hemoglobin in red blood cells which carry it to the tissues where it dissociates to enter the cells of the tissues.

The lungs never fully deflate, so air that is inhaled mixes with the residual air left from the previous respiration, resulting in a lower partial pressure of oxygen within the alveoli.