The lungs perform a gaseous exchange between oxygen and carbon dioxide in the alveoli, which are small sac-like structures, surrounded by capillaries, the smallest blood vessels. When you breathe in, oxygen rich air enters your body through your nose and mouth. it travels down into the trachea, or wind pipe. The trachea branches off into the bronchial tubes, which lead into each of two lungs. The bronchi branch off into bronchioles, which eventually lead into the alveoli. The oxygen diffuses across the capillary walls directly into the blood stream where it is attached to the red blood cells by the heme molecule in hemoglobin. This oxygen is then transported to every cell in your body for the process of cellular respiration to take place.
The flip side of all this is that carbon dioxide is produced as a waste product in cellular respiration. The carbon dioxide is transported by the blood back to the lungs, where it diffuses across the capillary walls into the alveoli. When you exhale, the carbon dioxide starts the reverse trip to the outside world, first through the bronchioles, then the bfonchi, then the trachea, then the nose or mouth.
Blood carries oxygen, carbon dioxide, and hydrogen ions between tissues and the lungs. The majority of CO2 transported in the blood is dissolved in plasma (primarily as dissolved bicarbonate; 60%). A smaller fraction is transported in red blood cells combined with the globin portion of hemoglobin as carbaminohaemoglobin. This is the chemical portion of the red blood cell that aids in the transport of oxygen and nutrients around the body, but, this time, it is carbon dioxide that is transported back to the lung.
As CO2 diffuses into the blood stream, it is absorbed by red blood cells before the majority is converted into H2CO3 by carbonic anhydrase, an enzyme that is not present in the plasma. The H2CO3 dissociates into H+ and HCO−
3. The HCO−
3 moves out of the red blood cells in exchange for Cl− (chloride shift). The hydrogen ions are removed by buffers in the blood (Hb).