<P> It is only as a result of accurately maintaining the composition of the 3 liters of 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 partial pressure of carbon dioxide 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". 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 causing a distressing respiratory alkalosis through the blowing off of too much CO from the blood into the outside air . </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 bicarbonate ions (HCO) 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 through 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 both directions depending on the prevailing partial pressure of CO . 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> <P> Ventilation of the lungs in mammals occurs via the respiratory centers in the medulla oblongata and the pons of the brainstem . These areas form a series of neural pathways which receive information about the partial pressures of oxygen and carbon dioxide in the arterial blood . This information determines the average rate of ventilation of the alveoli of the lungs, to keep these pressures constant . The respiratory center does so via motor nerves which activate the diaphragm and other muscles of respiration . </P>

What are the two main organs of the respiratory system