It simply represent  microscopic alveolus which absorbs oxygen and takes off carbon dioxide which is totally crucial in order the lungs can absorb and get rid unwanted gases.Oxygen from the inhaled air diffuses through the walls of each alveoli and adjacent capillaries into red blood cells waiting to its life giving gas. the oxygen is the carried by the blood to the body tissues which needed to support metabolism and to haCarbon dioxide produced by the body’s metabolism returns to the lung via the blood. It then diffuses across the capillary and alveolar walls into the air to be removed from the body with expiration. The alveoli have a structure specialized for efficient gaseous exchange,harness energy chemically from food and the carbon dioxide is then from body tissues gives off carbon dioxide that is carried in the blood and diffuse in the wall of the alveoli and exhale off.