concentration gradient

Diffusion is the movement of molecules from a region of high concentration into a region of lower concentration.

Change in the concentration of a substance from one area to another. Particles in a fluid, such as sugar molecules, gradually move so that they become evenly distributed throughout the fluid. In particular, they move from an area of high concentration to an area of low concentration; that is, they diffuse along the concentration gradient (see diffusion).

This explains why oxygen in the lungs will diffuse into the blood supply. The oxygen molecules are more concentrated in the lungs than they are in the capillaries surrounding the alveoli (air sacs). As it diffuses along the concentration gradient, oxygen will tend to pass into the blood. Gas exchange therefore depends on the maintenance of a concentration gradient, so that oxygen will continue to diffuse across the respiratory surface.