<P> The word diffusion derives from the Latin word, diffundere, which means "to spread out". A substance that "spreads out" is moving from an area of high concentration to an area of low concentration . </P> <P> A distinguishing feature of diffusion is that it dependends on particle random walk, and results in mixing or mass transport without requiring directed bulk motion . Bulk motion, or bulk flow, is the characteristic of advection . The term convection is used to describe the combination of both transport phenomena . </P> <P> An example of a situation in which bulk motion and diffusion can be differentiated is the mechanism by which oxygen enters the body during external respiration known as breathing . The lungs are located in the thoracic cavity, which expands as the first step in external respiration . This expansion leads to an increase in volume of the alveoli in the lungs, which causes a decrease in pressure in the alveoli . This creates a pressure gradient between the air outside the body at relatively high pressure and the alveoli at relatively low pressure . The air moves down the pressure gradient through the airways of the lungs and into the alveoli until the pressure of the air and that in the alveoli are equal i.e. the movement of air by bulk flow stops once there is no longer a pressure gradient . </P> <P> The air arriving in the alveoli has a higher concentration of oxygen than the "stale" air in the alveoli . The increase in oxygen concentration creates a concentration gradient for oxygen between the air in the alveoli and the blood in the capillaries that surround the alveoli . Oxygen then moves by diffusion, down the concentration gradient, into the blood . The other consequence of the air arriving in alveoli is that the concentration of carbon dioxide in the alveoli decreases . This creates a concentration gradient for carbon dioxide to diffuse from the blood into the alveoli, as fresh air has a very low concentration of carbon dioxide compared to the blood in the body . </P>

Where does diffusion take place in our body