A structure for gas exchange should be thin-walled and moist. Alveoli are exactly that in terms of morphology. Alveoli resemble clusters of grapes. They are located at the end of the broncioles, tiny tubes inside the lungs. When the lungs fill with air during inhalation, the alveoli expand and the oxygen inside can easily pass through the walls of the alveoli by diffusion. This is because a concentration gradient exists where there is a higher amount of oxygen inside the alveoli as compared to the capillaries outside the alveoli. Oxygen will diffuse from the alveoli into the capillaries where it is absorbed and transported by red blood cells in the circulating blood. Likewise, the higher amount of carbon dioxide in the blood can diffuse from the capillaries into the alveoli and when exhalation occurs, it is excreted from the body.