Editing Respiratory system (section)

===Gas exchange===
{{Main|Gas exchange}}
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| caption1          = '''Fig. 11''' A highly diagrammatic illustration of the process of gas exchange in the mammalian lungs, emphasizing the differences between the gas compositions of the ambient air, the alveolar air (light blue) with which the pulmonary capillary blood equilibrates, and the blood gas tensions in the pulmonary arterial (blue blood entering the lung on the left) and venous blood (red blood leaving the lung on the right). All the gas tensions are in kPa. To convert to mm Hg, multiply by 7.5.
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| caption2          = '''Fig. 12''' A diagrammatic histological cross-section through a portion of lung tissue showing a normally inflated [[Pulmonary alveolus|alveolus]] (at the end of a normal exhalation), and its walls containing the [[Pulmonary circulation|pulmonary capillaries]] (shown in cross-section). This illustrates how the pulmonary capillary blood is completely surrounded by alveolar air. In a normal human lung, all the alveoli together contain about 3 liters of alveolar air. All the pulmonary capillaries contain about 100{{nbsp}}ml of blood.
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[[File:Alveolar Wall.svg|thumb|300 px|left|'''Fig. 10''' A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary [[endothelial]] and alveolar type&nbsp;I&nbsp;[[epithelial]] cells (or type&nbsp;1&nbsp;[[pneumocyte]]s). The two red objects labeled "RBC" are [[red blood cell]]s in the pulmonary capillary blood.]]

The primary purpose of the respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood (Fig. 11). This process occurs by simple [[Diffusion#Diffusion vs. bulk flow|diffusion]],<ref>{{cite book|last1=Maton|first1=Anthea|first2=Jean Susan|last2= Hopkins|first3=Charles William|last3=Johnson|first4=Maryanna Quon|last4= McLaughlin|first5=David|last5=Warner|first6=Jill|last6= LaHart Wright|title=Human Biology and Health|publisher=Prentice Hall|year=2010 |location=Englewood Cliffs|pages= 108–118|isbn=978-0134234359}}</ref> across a very thin membrane (known as the [[blood–air barrier]]), which forms the walls of the [[pulmonary alveoli]] (Fig.&nbsp;10). It consists of the [[Pneumocytes|alveolar epithelial cells]], their [[basement membrane]]s and the [[Endothelium|endothelial cells]] of the alveolar capillaries (Fig.&nbsp;10).<ref name=grays>{{cite book |last1=Williams |first1=Peter L. |last2=Warwick |first2=Roger |last3=Dyson|first3=Mary |last4=Bannister |first4=Lawrence H. |title=Gray's Anatomy| pages=1278–1282 |location=Edinburgh|publisher=Churchill Livingstone | edition=Thirty-seventh |date=1989|isbn= 0443-041776 }}</ref> This blood gas barrier is extremely thin (in humans, on average, 2.2&nbsp;μm thick). It is folded into about 300 million small air sacs called [[Pulmonary alveolus|alveoli]]<ref name=grays /> (each between 75 and 300&nbsp;μm in diameter) branching off from the respiratory [[bronchiole]]s in the [[lung]]s, thus providing an extremely large surface area (approximately 145&nbsp;m<sup>2</sup>) for gas exchange to occur.<ref name=grays />

The air contained within the alveoli has a semi-permanent volume of about 2.5–3.0&nbsp;liters which completely surrounds the alveolar capillary blood (Fig.&nbsp;12). This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly. The blood leaving the alveolar capillaries and is eventually distributed throughout the body therefore has a [[partial pressure]] of oxygen of 13–14&nbsp;kPa (100&nbsp;mmHg), and a [[PCO2|partial pressure of carbon dioxide]] of 5.3&nbsp;kPa (40&nbsp;mmHg) (i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli).<ref name=tortora1 /> As mentioned in [[#Mechanics of breathing|the section above]], the corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21&nbsp;kPa (160&nbsp;mmHg) and 0.04&nbsp;kPa (0.3&nbsp;mmHg) respectively.<ref name=tortora1 />

This marked difference between the composition of the alveolar air and that of the ambient air can be maintained because the [[functional residual capacity]] is contained in dead-end sacs connected to the outside air by fairly narrow and relatively long tubes (the airways: [[nose]], [[pharynx]], [[larynx]], [[trachea]], [[bronchi]] and their branches down to the [[bronchioles]]), through which the air has to be breathed both in and out (i.e. there is no unidirectional through-flow as there is in the [[Bird anatomy#Respiratory system|bird lung]]). This typical mammalian anatomy combined with the fact that the lungs are not emptied and re-inflated with each breath (leaving a substantial volume of air, of about 2.5–3.0&nbsp;liters, in the alveoli after exhalation), ensures that the composition of the alveolar air is only minimally disturbed when the 350&nbsp;ml of fresh air is mixed into it with each inhalation. Thus the animal is provided with a very special "portable atmosphere", whose composition differs significantly from the [[Great Oxygenation Event|present-day ambient air]].<ref>{{cite book |last1=Lovelock |first1=James | title=Healing Gaia: Practical medicine for the Planet|url=https://archive.org/details/healinggaiaprac00love |url-access=registration |pages=21–34, 73–88|location=New York |publisher=Harmony Books |date=1991|isbn= 0-517-57848-4}}</ref> It is this portable atmosphere (the [[functional residual capacity]]) to which the blood and therefore the body tissues are exposed – not to the outside air.

The resulting arterial partial pressures of oxygen and carbon dioxide are [[Homeostasis#Levels of blood gases|homeostatically controlled]]. A rise in the arterial partial pressure of CO<sub>2</sub> and, to a lesser extent, a fall in the arterial partial pressure of O<sub>2</sub>, will reflexly cause deeper and faster breathing until the [[blood gas tension]]s in the lungs, and therefore the arterial blood, 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 partial pressure of O<sub>2</sub> of, on average, 6&nbsp;kPa (45&nbsp;mmHg), while the pressure in the alveolar air is 13–14&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 partial pressure of CO<sub>2</sub> 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 alveolar air necessitate the replacement of about 15% of the alveolar air with ambient air every  5&nbsp;seconds or so. This is very tightly controlled by the monitoring of the arterial blood gases (which accurately reflect composition of the alveolar air) by the [[Aortic body|aortic]] and [[Carotid body|carotid bodies]], as well as by 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&nbsp;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&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". 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 [[Acid-base homeostasis|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 thus causing a distressing [[respiratory alkalosis]] through the blowing off of too much CO<sub>2</sub> from the blood into the outside air.<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>

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 bicarbonate ions (HCO<sub>3</sub><sup>−</sup>) 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 through alveolar capillaries on the other. The reaction is therefore catalyzed by [[carbonic anhydrase]], an [[enzyme]] inside the [[red blood cell]]s.<ref name="pmid10854618">{{cite journal | vauthors = Henry RP, Swenson ER | title = The distribution and physiological significance of carbonic anhydrase in vertebrate gas exchange organs | journal = Respiration Physiology | volume = 121 | issue = 1 | pages = 1–12 | date = June 2000 | pmid = 10854618 | doi = 10.1016/S0034-5687(00)00110-9}}</ref> The reaction can go in both directions depending on the prevailing partial pressure of CO<sub>2</sub>.<ref name=tortora1 /> 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>{{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/100 ml blood).<ref name=tortora1 />