Lungs contain millions of alveoli which provide a surface for the exvhange of gases. As extensive network of blood vesseles is present in the wall of the alveoli. By lifting our ribs and flatten the diaphragm, the jchest cavity becomes spacious. Air is sucked into the lungs and alveoli. The oxygen from the breacth, diffuses into the blood and `CO_(2)` from the blood brought from the body, diffuses out into the air.
