Editing Red blood cell (section)

===Vertebrates===
[[Image:Erythrocytes in vertebrates.jpg|thumb|200px|There is an immense size variation in vertebrate red blood cells, as well as a correlation between cell and nucleus size. Mammalian red blood cells, which do not contain nuclei, are considerably smaller than those of most other vertebrates.<ref name="Gulliver1875">{{Cite journal | volume = 1875 | pages = 474–495 | vauthors = Gulliver G | title = On the size and shape of red corpuscles of the blood of vertebrates, with drawings of them to a uniform scale, and extended and revised tables of measurements | journal = Proceedings of the Zoological Society of London | year = 1875 }}</ref>]] [[File:Cytological abnormalities in peripheral blood erythrocytes of penguins Pygoscelis papua 5.jpg|thumb|Mature red blood cells of birds have a nucleus, however in the blood of adult females of penguin ''[[Gentoo penguin|Pygoscelis papua]]'' enucleated red blood cells ('''B''') have been observed, but with very low frequency. |222x222px]] The vast majority of vertebrates, including [[mammal]]s and humans, have red blood cells. These erythrocytes are cells present in blood to transport oxygen. The only known vertebrates without red blood cells are the crocodile icefish (family [[Channichthyidae]]); they live in very oxygen-rich cold water and transport oxygen freely dissolved in their blood.<ref>{{cite journal | vauthors = Ruud JT | title = Vertebrates without erythrocytes and blood pigment | journal = Nature | volume = 173 | issue = 4410 | pages = 848–850 | date = May 1954 | pmid = 13165664 | doi = 10.1038/173848a0 | bibcode = 1954Natur.173..848R | s2cid = 3261779 }}</ref> While they no longer use hemoglobin, remnants of hemoglobin genes can be found in their [[genome]].<ref>{{cite book|title=The Making of the Fittest|publisher=W.W. Norton|year=2006|isbn=978-0-393-06163-5| vauthors = Carroll S |author-link=Sean B. Carroll}}</ref>

Vertebrate red blood cells consist mainly of [[hemoglobin]], a complex [[metalloprotein]] containing [[heme]] groups whose iron atoms temporarily bind to oxygen molecules (O<sub>2</sub>) in the lungs or gills and release them throughout the body. Oxygen can easily [[diffusion|diffuse]] through the erythrocyte's [[cell membrane]]. Hemoglobin in the red blood cells also carries some of the waste product [[carbon dioxide]] back from the tissues; most waste carbon dioxide, however, is transported back to the [[alveolar-capillary barrier|pulmonary capillaries]] of the [[lung]]s as [[bicarbonate]] (HCO<sub>3</sub><sup>−</sup>) dissolved in the [[blood plasma]]. [[Myoglobin]], a compound related to hemoglobin, acts to store oxygen in [[muscle]] cells.<ref>{{cite book  | vauthors = Maton A, Hopkins J, McLaughlin CW, Johnson S, Warner MQ, LaHart D, Wright JD | title = Human Biology and Health  | publisher = Prentice Hall  | year = 1993  | location = Englewood Cliffs, New Jersey| isbn = 978-0-13-981176-0  | url-access = registration  | url = https://archive.org/details/humanbiologyheal00scho  }}</ref>

The color of red blood cells is due to the heme group of hemoglobin. The [[blood plasma]] alone is straw-colored, but the red blood cells change color depending on the state of the hemoglobin: when combined with oxygen the resulting oxyhemoglobin is scarlet, and when oxygen has been released the resulting deoxyhemoglobin is of a dark red burgundy color. However, blood can appear bluish when seen through the vessel wall and skin.<ref>{{Cite web|title = Why Are Veins Blue?|url = http://scienceblogs.com/scientificactivist/2008/04/17/why-are-veins-blue/|access-date = 23 April 2015|date = 17 April 2008|website = Scienceblogs| vauthors = Anthis N }}</ref> [[Pulse oximetry]] takes advantage of the hemoglobin color change to directly measure the [[artery|arterial]] blood [[oxygen saturation]] using [[colorimetric]] techniques. Hemoglobin also has a very high affinity for [[carbon monoxide]], forming carboxyhemoglobin which is a very bright red in color. Flushed, confused patients with a saturation reading of 100% on pulse oximetry are sometimes found to be suffering from carbon monoxide poisoning.{{citation needed|date=December 2021}}

Having oxygen-carrying proteins inside specialized cells (as opposed to oxygen carriers being dissolved in body fluid) was an important step in the evolution of vertebrates as it allows for less [[viscosity|viscous]] blood, higher concentrations of oxygen, and better diffusion of oxygen from the blood to the tissues. The size of red blood cells varies widely among vertebrate species; red blood cell width is on average about 25% larger than [[capillary]] diameter, and it has been hypothesized that this improves the oxygen transfer from red blood cells to tissues.<ref name=snyder>{{Cite journal |doi=10.1093/icb/39.2.189 |title=Red Blood Cells: Centerpiece in the Evolution of the Vertebrate Circulatory System |year=1999 | vauthors = Snyder GK, Sheafor BA |journal=Integrative and Comparative Biology |volume=39 |pages=189–198 |issue=2|doi-access=free }}</ref>