Gas exchange
Gas exchange is the process by which oxygen enters an organism and carbon dioxide is removed from an organism. Diffusion (when molecules move from an area of high concentration to an area of low concentration) occurs during the gaseous exchange as the blood in the capillaries surrounding the alveoli has a lower oxygen concentration of oxygen than the air in the alveoli which has just been inhaled. Both alveoli and capillaries have walls which are only one cell thick and allow gases to diffuse across them. The same happens with carbon dioxide (CO2). The blood in the surrounding capillaries has a higher concentration of CO2 than the inspired air due to it being a waste product of energy production. Therefore CO2 diffuses the other way, from the capillaries, into the alveoli where it can then be exhaled.
