How are oxygen carbon dioxide and other gases exchanged in the lungs and body tissues?
In the lungs, the respiratory zones end in sacs called alveoli, which are the site of gas exchange. The alveoli have walls only one cell thick, and are surounded by pulmonary capillaries which again have walls only once cell thick; gases diffuse, along their concentration gradients (i.e. from the area of high concentration to the area of low concentration), through these two thicknesses of cells.
