2.38 understand the role of diffusion in gas exchange.

Diffusion is the movement of particles form an area of high density to an are of low density. In the circulatory 

system, oxygen enters the blood and carbon dioxide leave the blood through gaseous exchange. Gases move 

across the walls of the alveoli to an area of lower density than they are in.

Oxygen moves into the blood as there is a low density of oxygen in the blood. Carbon dioxide moves into the lungs as it is an area of lower density.