Editing Red blood cell (section)

== Structure ==

===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>

===Mammals===
[[Image:Gray453.png|thumb|300px|Typical mammalian red blood cells: (a) seen from surface; (b) in profile, forming rouleaux; (c) rendered spherical by water; (d) rendered crenate (shrunken and spiky) by salt. (c) and (d) do not normally occur in the body. The last two shapes are due to water being transported into, and out of, the cells, by [[osmosis]].]]

The red blood cells of [[mammal]]s are typically shaped as biconcave disks: flattened and depressed in the center, with a [[dumbbell]]-shaped cross section, and a [[torus]]-shaped rim on the edge of the disk. This shape allows for a high surface-area-to-volume (SA/V) ratio to facilitate diffusion of gases.<ref>{{Cite web|url=https://www.bbc.co.uk/education/guides/ztp9q6f/revision/2|title=BBC Bitesize – GCSE Biology – Blood – Revision 2|website=www.bbc.co.uk|language=en-GB|access-date=26 November 2017}}</ref> However, there are some exceptions concerning shape in the [[artiodactyl]] order (even-toed [[ungulates]] including cattle, deer, and their relatives), which displays a wide variety of bizarre red blood cell morphologies: small and highly ovaloid cells in [[llama]]s and camels (family [[Camelidae]]), tiny spherical cells in mouse deer (family [[Tragulidae]]), and cells which assume fusiform, lanceolate, crescentic, and irregularly polygonal and other angular forms in red deer and wapiti (family [[Cervidae]]). Members of this order have clearly evolved a mode of red blood cell development substantially different from the mammalian norm.<ref name="Gulliver1875" /><ref name="The bigger the C-value, the larger">{{cite journal | vauthors = Gregory TR | title = The bigger the C-value, the larger the cell: genome size and red blood cell size in vertebrates | journal = Blood Cells, Molecules & Diseases | volume = 27 | issue = 5 | pages = 830–843 | year = 2001 | pmid = 11783946 | doi = 10.1006/bcmd.2001.0457 | citeseerx = 10.1.1.22.9555 }}</ref> Overall, mammalian red blood cells are remarkably flexible and deformable so as to squeeze through tiny [[capillary|capillaries]], as well as to maximize their apposing surface by assuming a cigar shape, where they efficiently release their oxygen load.<ref>{{cite journal | vauthors = Goodman SR, Kurdia A, Ammann L, Kakhniashvili D, Daescu O | title = The human red blood cell proteome and interactome | journal = Experimental Biology and Medicine | volume = 232 | issue = 11 | pages = 1391–1408 | date = December 2007 | pmid = 18040063 | doi = 10.3181/0706-MR-156 | s2cid = 32326166 }}</ref>

Red blood cells in mammals are unique amongst vertebrates as they do not have nuclei when mature. They do have nuclei during early phases of [[erythropoiesis]], but extrude them during development as they mature; this provides more space for hemoglobin.  The red blood cells without nuclei, called [[reticulocytes]], subsequently lose all other cellular [[organelle]]s such as their [[mitochondrion|mitochondria]], [[Golgi apparatus]] and [[endoplasmic reticulum]].

The [[spleen]] acts as a reservoir of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells, which are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.

[[Image:Red White Blood cells.jpg|thumb|200px|Scanning electron micrograph of blood cells. From left to right: human red blood cell, [[Platelet|thrombocyte]] (platelet), [[leukocyte]].]]

===Human===
[[Image:NIK 3232-Drops of blood medium.JPG|thumb|200px|Two drops of blood are shown with a bright red oxygenated drop on the left and a darker red deoxygenated drop on the right.]]
[[Image:Erytrocyte deoxy to oxy v0.7.gif|thumb|200px|Animation of a typical human red blood cell cycle in the circulatory system. This animation occurs at a faster rate (~20 seconds of the average 60-second cycle) and shows the red blood cell deforming as it enters capillaries, as well as the bars changing color as the cell alternates in states of oxygenation along the circulatory system.]]

A typical human red blood cell has a disk diameter of approximately [[Orders of magnitude (length)#Cellular to human scale|6.2–8.2 μm]]<ref>{{cite book|title=Clinical Hematology: Theory and Procedures| vauthors = Turgeon ML |publisher=Lippincott Williams & Wilkins|year=2004|page=100|url=https://books.google.com/books?id=cHAjsUgegpQC&q=erythrocyte%20size&pg=PA100|isbn=9780781750073}}</ref> and a maximum thickness of 2–2.5&nbsp;μm and a minimum thickness in the centre of 0.8–1&nbsp;μm, being much smaller than most other [[List of distinct cell types in the adult human body|human cells]]. These cells have an average volume of about [[Femto-|90 fL]]<ref>{{cite journal | vauthors = McLaren CE, Brittenham GM, Hasselblad V | title = Statistical and graphical evaluation of erythrocyte volume distributions | journal = The American Journal of Physiology | volume = 252 | issue = 4 Pt 2 | pages = H857–H866 | date = April 1987 | pmid = 3565597 | doi = 10.1152/ajpheart.1987.252.4.H857 | citeseerx = 10.1.1.1000.348 }}</ref> with a surface area of about 136 μm<sup>2</sup>, and can swell up to a sphere shape containing 150 fL, without membrane distension.

Adult humans have roughly 20–30&nbsp;trillion red blood cells at any given time, constituting approximately 70% of all cells by number.<ref>{{cite journal | vauthors = Bianconi E, Piovesan A, Facchin F, Beraudi A, Casadei R, Frabetti F, Vitale L, Pelleri MC, Tassani S, Piva F, Perez-Amodio S, Strippoli P, Canaider S | display-authors = 6 | title = An estimation of the number of cells in the human body | journal = Annals of Human Biology | volume = 40 | issue = 6 | pages = 463–471 | date = 1 November 2013 | pmid = 23829164 | doi = 10.3109/03014460.2013.807878 | s2cid = 16247166 | doi-access = free | hdl = 11585/152451 }}</ref> Women have about 4–5&nbsp;million red blood cells per microliter (cubic millimeter) of blood and men about 5–6&nbsp;million; [[Effects of high altitude on humans#Acclimatization|people living at high altitudes]] with low oxygen tension will have more. Red blood cells are thus much more common than the other blood particles: there are about 4,000–11,000 [[white blood cells]] and about 150,000–400,000 [[platelet]]s per microliter.

Human red blood cells take on average 60 seconds to complete one cycle of circulation.<ref name="Blom20032" /><ref name=pierige/><ref>{{Cite book  | vauthors = Hillman RS, Ault KA, Rinder HM | year = 2005  | title = Hematology in Clinical Practice: A Guide to Diagnosis and Management  | edition = 4th | publisher = McGraw-Hill Professional  | page = 1  | isbn = 978-0-07-144035-6  }}</ref>

The blood's red color is due to the spectral properties of the [[heme|hemic]] iron [[ion]]s in [[hemoglobin]]. Each hemoglobin molecule carries four heme groups; hemoglobin constitutes about a third of the total cell volume. Hemoglobin is responsible for the transport of more than 98% of the oxygen in the body (the remaining oxygen is carried dissolved in the [[blood plasma]]). The red blood cells of an average adult human male store collectively about 2.5 grams of iron, representing about 65% of the total iron contained in the body.<ref>[http://www.med-ed.virginia.edu/courses/path/innes/nh/iron.cfm Iron Metabolism], University of Virginia Pathology. Accessed 22 September 2007.</ref><ref>{{Cite web|title=Transferrin and Iron Transport Physiology |url= https://sickle.bwh.harvard.edu/iron_transport.html |access-date= 26 March 2023 | vauthors = Bridges KR | work = Information Center for Sickle Cell and Thalassemic Disorders}}</ref>