Editing Homeostasis (section)

===Levels of blood gases===
{{main |Respiratory center |Gas exchange}}
{{Further |Blood gas tension}}
[[File: 2327 Respiratory Centers of the Brain.jpg|thumb|The respiratory center]]
Changes in the levels of oxygen, carbon dioxide, and plasma pH are sent to the [[respiratory center]], in the [[brainstem]] where they are regulated.
The [[partial pressure]] of [[oxygen]] and [[carbon dioxide]] in the [[arterial blood]] is monitored by the [[peripheral chemoreceptors]] ([[Peripheral nervous system|PNS]]) in the [[common carotid artery|carotid artery]] and [[aortic arch]]. A change in the [[PCO2|partial pressure of carbon dioxide]] is detected as altered pH in the [[cerebrospinal fluid]] by [[central chemoreceptors]] ([[Central nervous system|CNS]]) in the [[medulla oblongata]] of the [[brainstem]]. Information from these sets of sensors is sent to the respiratory center which activates the effector organs – the [[Thoracic diaphragm|diaphragm]] and other [[muscles of respiration]]. An increased level of carbon dioxide in the blood, or a decreased level of oxygen, will result in a deeper breathing pattern and increased [[respiratory rate]] to bring the blood gases back to equilibrium.

Too little carbon dioxide, and, to a lesser extent, too much oxygen in the blood can temporarily halt breathing, a condition known as [[apnea]], which [[freediving|freedivers]] use to prolong the time they can stay underwater.

The [[PCO2|partial pressure of carbon dioxide]] is more of a deciding factor in the monitoring of pH.<ref name="Spyer">{{cite journal|last1=Spyer|first1=KM|last2=Gourine|first2=AV|title=Chemosensory pathways in the brainstem controlling cardiorespiratory activity.|journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences|date=12 September 2009|volume=364|issue=1529|pages=2603–10|doi=10.1098/rstb.2009.0082|pmid=19651660|pmc=2865116}}</ref> However, at high altitude (above 2500&nbsp;m) the monitoring of the partial pressure of oxygen takes priority, and [[hyperventilation]] keeps the oxygen level constant. With the lower level of carbon dioxide, to keep the pH at 7.4 the kidneys secrete hydrogen ions into the blood and excrete bicarbonate into the urine.<ref name=pubmed>{{cite journal|title=Oxygen at high altitude|journal=British Medical Journal|first=Andrew J|last=Peacock|date=17 October 1998|volume=317|pages=1063–1066|pmid=9774298|issue=7165|pmc=1114067|doi=10.1136/bmj.317.7165.1063}}</ref><ref name=BordenHuman>{{cite book |last1=Young|first1=Andrew J|last2=Reeves|first2=John T. |chapter=Human Adaptation to High Terrestrial Altitude|title=Medical Aspects of Harsh Environments |volume=2 |location=Borden Institute, Washington, DC |year=2002 |citeseerx=10.1.1.175.3270|chapter-url=https://www.usariem.army.mil/Pages/download/harshenvironmentsvol2.pdf|archive-url=https://web.archive.org/web/20120916195023/https://www.usariem.army.mil/Pages/download/harshenvironmentsvol2.pdf |publisher=Office of The Surgeon General Department of the Army, United States of America |access-date=5 January 2009|archive-date=16 September 2012}}</ref> This is important in [[Effects of high altitude on humans#Acclimatization|acclimatization to high altitude]].<ref name=harris>{{cite journal|url=https://emedicine.medscape.com/article/768478-overview|journal=EMedicine Specialties &gt; Emergency Medicine &gt; Environmental|title=Altitude Illness – Cerebral Syndromes|first1=N Stuart|last1=Harris|first2=Sara W|last2=Nelson|date=16 April 2008|access-date=30 June 2016|archive-date=12 June 2016|archive-url=https://web.archive.org/web/20160612041149/https://emedicine.medscape.com/article/768478-overview|url-status=live}}</ref>