<P> Since the blood arriving in the alveolar capillaries has a P O 2 (\ displaystyle P_ ((\ mathrm (O)) _ (2))) of, on average, 6 kPa (45 mmHg), while the pressure in the alveolar air is 13 kPa (100 mmHg), there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3 liters of alveolar air slightly . Similarly, since the blood arriving in the alveolar capillaries has a P C O 2 (\ displaystyle P_ ((\ mathrm (CO)) _ (2))) of also about 6 kPa (45 mmHg), whereas that of the alveolar air is 5.3 kPa (40 mmHg), there is a net movement of carbon dioxide out of the capillaries into the alveoli . The changes brought about by these net flows of individual gases into and out of the functional residual capacity necessitate the replacement of about 15% of the alveolar air with ambient air every 5 seconds or so . This is very tightly controlled by the continuous monitoring of the arterial blood gas tensions (which accurately reflect partial pressures of the respiratory gases in the alveolar air) by the aortic bodies, the carotid bodies, and the blood gas and pH sensor on the anterior surface of the medulla oblongata in the brain . There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the bronchioles and pulmonary capillaries, and are therefore responsible for directing the flow of air and blood to different parts of the lungs . </P> <P> It is only as a result of accurately maintaining the composition of the 3 liters alveolar air that with each breath some carbon dioxide is discharged into the atmosphere and some oxygen is taken up from the outside air . If more carbon dioxide than usual has been lost by a short period of hyperventilation, respiration will be slowed down or halted until the alveolar P C O 2 (\ displaystyle P_ ((\ mathrm (CO)) _ (2))) has returned to 5.3 kPa (40 mmHg). It is therefore strictly speaking untrue that the primary function of the respiratory system is to rid the body of carbon dioxide "waste". In fact the total concentration of carbon dioxide in arterial blood is about 26 mM (or 58 ml per 100 ml), compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml per 100 ml blood). This large concentration of carbon dioxide plays a pivotal role in the determination and maintenance of the pH of the extracellular fluids . The carbon dioxide that is breathed out with each breath could probably be more correctly be seen as a byproduct of the body's extracellular fluid carbon dioxide and pH homeostats </P> <P> If these homeostats are compromised, then a respiratory acidosis, or a respiratory alkalosis will occur . In the long run these can be compensated by renal adjustments to the H and HCO concentrations in the plasma; but since this takes time, the hyperventilation syndrome can, for instance, occur when agitation or anxiety cause a person to breathe fast and deeply thus blowing off too much CO from the blood into the outside air, precipitating a set of distressing symptoms which result from an excessively high pH of the extracellular fluids . </P> <P> Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with hemoglobin . The oxygen is held on the hemoglobin by four ferrous iron - containing heme groups per hemoglobin molecule . When all the heme groups carry one O molecule each the blood is said to be "saturated" with oxygen, and no further increase in the partial pressure of oxygen will meaningfully increase the oxygen concentration of the blood . Most of the carbon dioxide in the blood is carried as HCO ions in the plasma . However the conversion of dissolved CO into HCO (through the addition of water) is too slow for the rate at which the blood circulates through the tissues on the one hand, and alveolar capillaries on the other . The reaction is therefore catalyzed by carbonic anhydrase, an enzyme inside the red blood cells . The reaction can go in either direction depending on the prevailing partial pressure of carbon dioxide . A small amount of carbon dioxide is carried on the protein portion of the hemoglobin molecules as carbamino groups . The total concentration of carbon dioxide (in the form of bicarbonate ions, dissolved CO, and carbamino groups) in arterial blood (i.e. after it has equilibrated with the alveolar air) is about 26 mM (or 58 ml / 100 ml), compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml / 100 ml blood). </P>

Where does gas exhange happen in the lungs