text: how human lungs absorbs oxygen only from inhaled air ?   the tiniest part of our lung is called alveolus (plural is alveoli). it is like an inflated balloon . the air in side this balloon is the fresh air we have inhaled.  now imagine your ten fingers covering an inflated balloon from all the sides. that is how the blood capillaries cover the alveoli . these blood capillaries have extremely thin wall through which the oxygen, which is at a higher concentration in the fresh air of the alveoli, diffuses in the blood capillary. it is immediately picked up by the hemoglobin in the red blood cells forming oxyhemoglobin  . it is the source of oxygen to our body. this is then taken to different parts of the body and given to them. at the same time , co2 collected from the body parts and brought to the alveoli by those very capillaries diffuses in to the air of the alveoli because its concentration in the capillaries is greater.  the air in the alveoli is now stale air rich in co2 and poor in o2. it is exhaled out. thus , the gaseous exchange can occur only on the surface of the alveoli . their walls are as thin as the walls of a soap bubble, only that they are not fragile like soap bubble.