Editing Homeostasis (section)

===Blood oxygen content===
The [[kidneys]] measure the oxygen content rather than the [[partial pressure of oxygen]] in the arterial blood. When the [[Oxygen saturation (medicine)|oxygen content of the blood]] is chronically low, oxygen-sensitive cells secrete [[erythropoietin]] (EPO) into the blood.<ref name="Alberts">{{cite book|last1=Alberts|first1=Bruce|title=Molecular biology of the cell|date=2002|publisher=Garland|location=New York [u.a.]|isbn=978-0-8153-4072-0|pages=1292–1293|edition= 4th}}</ref> The effector tissue is the [[Bone marrow|red bone marrow]] which produces [[red blood cell]]s (RBCs, also called {{lang|la|erythrocytes}}). The increase in RBCs leads to an increased [[hematocrit]] in the blood, and a subsequent increase in [[hemoglobin]] that increases the oxygen carrying capacity. This is the mechanism whereby high altitude dwellers have higher hematocrits than sea-level residents, and also why persons with [[pulmonary insufficiency]] or [[right-to-left shunt]]s in the heart (through which venous blood by-passes the lungs and goes directly into the systemic circulation) have similarly high hematocrits.<ref name=tortora>{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |url= https://archive.org/details/principlesofanat05tort |url-access= registration |pages=[https://archive.org/details/principlesofanat05tort/page/444 444–445]|edition=  Fifth |location= New York |publisher= Harper & Row, Publishers|date= 1987 |isbn= 978-0-06-350729-6 }}</ref><ref name=Fisher1996>{{cite journal |vauthors=Fisher JW, Koury S, Ducey T, Mendel S |title=Erythropoietin production by interstitial cells of hypoxic monkey kidneys |journal=British Journal of Haematology |volume=95 |issue=1 |pages=27–32 |year=1996 |pmid=8857934 |doi=10.1046/j.1365-2141.1996.d01-1864.x |s2cid=38309595 }}</ref>

Regardless of the partial pressure of oxygen in the blood, the amount of oxygen that can be carried, depends on the hemoglobin content. The partial pressure of oxygen may be sufficient for example in [[anemia]], but the hemoglobin content will be insufficient and subsequently as will be the oxygen content. Given enough supply of iron, [[vitamin B12]] and [[folic acid]], EPO can stimulate RBC production, and hemoglobin and oxygen content restored to normal.<ref name=tortora /><ref name=pmid17253966>{{cite journal |vauthors=Jelkmann W |title=Erythropoietin after a century of research: younger than ever |journal=European Journal of Haematology |volume=78 |issue=3 |pages=183–205 |year=2007 |pmid=17253966 |doi=10.1111/j.1600-0609.2007.00818.x |s2cid=37331032 |doi-access= }}</ref>