Editing Gas exchange (section)

===Pulmonary circulation===
{{Main |Pulmonary circulation}}
All the blood returning from the body tissues to the right side of the [[heart]] flows through the [[Pulmonary circulation|alveolar capillaries]] before being pumped around the body again. On its passage through the lungs the blood comes into close contact with the alveolar air, separated from it by a very thin diffusion membrane which is only, on average, about 2&nbsp;μm thick.<ref name=grays /> The gas pressures in the blood will therefore rapidly equilibrate with those in the [[Pulmonary alveolus|alveoli]], ensuring that the arterial blood that circulates to all the tissues throughout the body has an [[Blood gas tension|oxygen tension]] of 13−14&nbsp;kPa (100&nbsp;mmHg), and a [[Blood gas tension|carbon dioxide tension]] of 5.3&nbsp;kPa (40&nbsp;mmHg). These arterial partial pressures of oxygen and carbon dioxide are [[Homeostasis#Levels of blood gases|homeostatically controlled]]. A rise in the arterial <math>P_{{\mathrm{CO}}_2}</math>, and, to a lesser extent, a fall in the arterial <math>P_{{\mathrm{O}}_2}</math>, will reflexly cause deeper and faster breathing until the blood gas tensions return to normal. The converse happens when the carbon dioxide tension falls, or, again to a lesser extent, the oxygen tension rises: the rate and depth of breathing are reduced until blood gas normality is restored.

Since the blood arriving in the alveolar capillaries has a <math>P_{{\mathrm{O}}_2}</math> of, on average, 6&nbsp;kPa (45&nbsp;mmHg), while the pressure in the alveolar air is 13&nbsp;kPa (100&nbsp;mmHg), there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3&nbsp;liters of alveolar air slightly. Similarly, since the blood arriving in the alveolar capillaries has a <math>P_{{\mathrm{CO}}_2}</math> of also about 6&nbsp;kPa (45&nbsp;mmHg), whereas that of the alveolar air is 5.3&nbsp;kPa (40&nbsp;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 body|aortic bodies]], the [[Carotid body|carotid bodies]], and the [[Central chemoreceptors|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 circulation|pulmonary capillaries]], and are therefore responsible for directing the flow of air and blood to different parts of the lungs.

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 <math>P_{{\mathrm{CO}}_2}</math> has returned to 5.3&nbsp;kPa (40&nbsp;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&nbsp;mM (or 58&nbsp;ml per 100 ml),<ref name=ciba>{{cite book |last1=Diem |first1=K. | last2=Lentner |first2=C. | chapter= Blood – Inorganic substances| title= in: Scientific Tables | edition= Seventh |location=Basle, Switzerland |publisher=CIBA-GEIGY Ltd. |date=1970 |page=571}}</ref> compared to the concentration of oxygen in saturated arterial blood of about 9&nbsp;mM (or 20&nbsp;ml per 100 ml blood).<ref name=tortora1 /> This large concentration of carbon dioxide plays a pivotal role in the [[Acid-base homeostasis|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 [[Homeostasis#Levels of blood gases|carbon dioxide]] and  [[Homeostasis#Blood pH|pH homeostats]]

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<sup>+</sup> and HCO<sub>3</sub><sup>−</sup> 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<ref>{{cite journal|last=Shu|first=BC |author2=Chang, YY |author3=Lee, FY |author4=Tzeng, DS |author5=Lin, HY |author6=Lung, FW|title=Parental attachment, premorbid personality, and mental health in young males with hyperventilation syndrome.|journal=Psychiatry Research|date=2007-10-31|volume=153|issue=2|pages=163–70|pmid=17659783|doi=10.1016/j.psychres.2006.05.006|s2cid=3931401 }}</ref> thus blowing off too much CO<sub>2</sub> from the blood into the outside air, precipitating a set of distressing symptoms which result from an excessively high pH of the extracellular fluids.<ref name="Edward Newton">{{cite web |url=http://www.emedicine.com/emerg/topic270.htm |title=eMedicine - Hyperventilation Syndrome: Article by Edward Newton, MD |access-date=2007-12-20 }}</ref>

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 [[Iron(II) oxide|ferrous iron]]-containing [[heme]] groups per hemoglobin molecule. When all the heme groups carry one O<sub>2</sub> 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<sub>3</sub><sup>−</sup> ions in the plasma. However the conversion of dissolved CO<sub>2</sub> into HCO<sub>3</sub><sup>−</sup> (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 cell]]s.<ref name="Raymond&Swenson2000">{{Cite journal|vauthors=Raymond H, Swenson E | title=The distribution and physiological significance of carbonic anhydrase in vertebrate gas exchange organs| journal=[[Respiration Physiology]]| volume=121| issue=1| year=2000| pages=1–12| doi=10.1016/s0034-5687(00)00110-9| pmid=10854618}}</ref> 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<sub>2</sub>, and carbamino groups) in arterial blood (i.e. after it has equilibrated with the alveolar air) is about 26&nbsp;mM (or 58&nbsp;ml/100 ml),<ref name=ciba /> compared to the concentration of oxygen in saturated arterial blood of about 9&nbsp;mM (or 20&nbsp;ml/100 ml blood).<ref name=tortora1 />