Editing Blood (section)

==Physiology==
===Circulatory system===
[[File:Diagram of the human heart (cropped).svg|thumb|Circulation of blood through the human heart]]

{{main|Circulatory system}}
Blood is circulated around the body through [[blood vessels]] by the pumping action of the [[heart]]. In humans, blood is pumped from the strong [[left ventricle]] of the heart through [[arteries]] to peripheral [[Tissue (biology)|tissues]] and returns to the right [[Atrium (heart)|atrium]] of the heart through [[veins]]. It then enters the right [[Ventricle (heart)|ventricle]] and is pumped through the [[pulmonary artery]] to the [[lungs]] and returns to the left atrium through the [[pulmonary veins]]. Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, and [[venous blood]] carries carbon dioxide, a waste product of [[metabolism]] by [[Cell (biology)|cells]], to the lungs to be exhaled. However, one exception includes pulmonary arteries, which contain the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.

Additional return flow may be generated by the movement of [[skeletal muscles]], which can compress veins and push blood through the valves in veins toward the [[right atrium]].

The blood circulation was described by [[William Harvey]] in 1628.<ref>{{cite web |title=Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus |url=http://www.rarebookroom.org/Control/hvyexc/index.html |first=William |last=Harvey | name-list-style = vanc |author-link=William Harvey |language=la |year=1628 |url-status=live |archive-url=https://web.archive.org/web/20101127002138/http://rarebookroom.org/Control/hvyexc/index.html |archive-date=27 November 2010 }}</ref>

===Cell production and degradation===
In vertebrates, the various cells of blood are made in the [[bone marrow]] in a process called [[hematopoiesis]], which includes [[erythropoiesis]], the production of red blood cells; and [[myelopoiesis]], the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the [[thymus]] gland, found in the [[mediastinum]], is an important source of [[T lymphocytes]].<ref>{{cite book |last1=Williams |first1=Peter W. |url=https://archive.org/details/graysanatomy0000gray_m7a1 |title=Gray's anatomy |last2=Gray |first2=Henry David |publisher=C. Livingstone |year=1989 |isbn=978-0-443-02588-4 |edition=37th |location=New York |url-access=registration |name-list-style=vanc}}</ref>
The proteinaceous component of blood (including clotting proteins) is produced predominantly by the [[liver]], while hormones are produced by the [[endocrine gland]]s and the watery fraction is regulated by the [[hypothalamus]] and maintained by the [[kidney]].

Healthy [[erythrocytes]] have a plasma life of about 120 days before they are degraded by the [[spleen]], and the [[Kupffer cells]] in the liver. The liver also clears some proteins, lipids, and amino acids. The kidney actively secretes waste products into the [[urine]].

==={{anchor|Oxygen transport}}Oxygen transport===
{{Further|Oxygen saturation (medicine)}}
[[File:Oxyhaemoglobin dissociation curve.png|thumb|Basic hemoglobin saturation curve. It is moved to the right in higher acidity (more dissolved carbon dioxide) and to the left in lower acidity (less dissolved carbon dioxide)|left]]
About 98.5%<ref>{{Cite book|title=Fundamentals of anatomy & physiology|last=Frederic|first=Martini|date=2009|publisher=Pearson/Benjamin Cummings|others=Nath, Judi Lindsley|isbn=978-0321539106|edition= 8th |location=San Francisco|pages=657|oclc=173683666}}</ref> of the [[oxygen]] in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the [[hemoglobin]]. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in [[mammal]]s and many other species. Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O<sub>2</sub> per gram hemoglobin,<ref>{{cite journal | vauthors = Dominguez de Villota ED, Ruiz Carmona MT, Rubio JJ, de Andrés S | s2cid = 10029560 | title = Equality of the in vivo and in vitro oxygen-binding capacity of haemoglobin in patients with severe respiratory disease | journal = British Journal of Anaesthesia | volume = 53 | issue = 12 | pages = 1325–8 | date = December 1981 | pmid = 7317251 | doi = 10.1093/bja/53.12.1325 | doi-access = free }}</ref> which increases the total [[blood oxygen capacity]] seventyfold,<ref name=brsphys>{{cite book |last=Costanzo |first=Linda S. |name-list-style=vanc |title=Physiology |publisher=Lippincott Williams & Wilkins |location=Hagerstown, Maryland |year=2007 |isbn=978-0-7817-7311-9 |url-access=registration |url=https://archive.org/details/physiology00cost_0 }}</ref> compared to if oxygen solely were carried by its solubility of 0.03&nbsp;ml O<sub>2</sub> per liter blood per mm&nbsp;Hg partial pressure of oxygen (about 100&nbsp;mm&nbsp;Hg in arteries).<ref name=brsphys/>

With the exception of pulmonary and [[umbilical arteries]] and their corresponding veins, arteries carry '''oxygenated blood''' away from the heart and deliver it to the body via [[arterioles]] and [[capillaries]], where the oxygen is consumed; afterwards, [[venule]]s and veins carry '''deoxygenated blood''' back to the heart.

Under normal conditions in adult humans at rest, hemoglobin in blood leaving the lungs is about 98–99% [[Oxygen saturation (medicine)|saturated with oxygen]], achieving an oxygen delivery between 950 and 1150 ml/min<ref name=edwards>[http://www.edwards.com/SiteCollectionImages/edwards/products/presep/ar04313hemodynpocketcard.pdf Edwards Lifesciences LLC – Normal Hemodynamic Parameters – Adult] {{webarchive|url=https://web.archive.org/web/20101110081655/http://www.edwards.com/SiteCollectionImages/edwards/products/presep/ar04313hemodynpocketcard.pdf |date=10 November 2010 }} 2009</ref> to the body. In a healthy adult at rest, oxygen consumption is approximately 200–250 ml/min,<ref name=edwards/> and deoxygenated blood returning to the lungs is still roughly 75%<ref>{{cite web |url=http://home.hia.no/~stephens/ventphys.htm|archive-url=https://web.archive.org/web/20100323054138/http://home.hia.no/~stephens/ventphys.htm|url-status=dead|archive-date=23 March 2010 |title=Ventilatory Physiology and Endurance |date=23 March 2010 |access-date=4 March 2017}}</ref><ref>[https://web.archive.org/web/20040224085741/http://groups.msn.com/TransplantSupportLungHeartLungHeart/oxygen2.msnw Transplant Support- Lung, Heart/Lung, Heart] MSN groups</ref> (70 to 78%)<ref name=edwards/> saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions.<ref>{{cite journal | vauthors = Mortensen SP, Dawson EA, Yoshiga CC, Dalsgaard MK, Damsgaard R, Secher NH, González-Alonso J | title = Limitations to systemic and locomotor limb muscle oxygen delivery and uptake during maximal exercise in humans | journal = The Journal of Physiology | volume = 566 | issue = Pt 1 | pages = 273–85 | date = July 2005 | pmid = 15860533 | pmc = 1464731 | doi = 10.1113/jphysiol.2005.086025 | display-authors = etal }}</ref> Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia). Sustained hypoxia (oxygenation less than 90%), is dangerous to health, and severe hypoxia (saturations less than 30%) may be rapidly fatal.<ref>{{cite web |url=http://manbit.com/PAC/chapters/P30.cfm|archive-url=https://web.archive.org/web/20100925053056/http://manbit.com/PAC/chapters/P30.cfm|url-status=dead|archive-date=25 September 2010 |title=Blood gas and Saturation measurements |date=25 September 2010 |access-date=4 March 2017}}</ref>

A [[fetus]], receiving oxygen via the [[placenta]], is exposed to much lower oxygen pressures (about 21% of the level found in an adult's lungs), so fetuses produce another form of hemoglobin with a much higher affinity for oxygen ([[Fetal hemoglobin|hemoglobin F]]) to function under these conditions.<ref>{{cite web |url=http://members.aol.com/Bio50/LecNotes/lecnot20.html|archive-url=https://web.archive.org/web/19990502195422/http://members.aol.com/Bio50/LecNotes/lecnot20.html|url-status=dead|archive-date=2 May 1999 |title=Lecture Notes-20 |date=2 May 1999 |access-date=4 March 2017}}</ref>

===Carbon dioxide transport===
CO<sub>2</sub> is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood). Most of it (about 70%) is converted to bicarbonate ions {{chem2|HCO3−}} by the enzyme [[carbonic anhydrase]] in the red blood cells by the reaction {{chem2|CO2 + H2O -> H2CO3 -> H+ + HCO3−}}; about 7% is dissolved in the plasma; and about 23% is bound to hemoglobin as [[carbamino]] compounds.<ref>{{cite book | last = Martini | first = Frederic | name-list-style = vanc |year=2007 |title=Anatomy and Physiology |url=https://books.google.com/books?id=joJb82gVsLoC&pg=PA643 |publisher=Rex Bookstore, Inc. |page=643 |isbn=9789712348075 |display-authors=etal |url-status=live |archive-url=https://web.archive.org/web/20160501022224/https://books.google.com/books?id=joJb82gVsLoC&pg=PA643 |archive-date=1 May 2016 }}</ref><ref>''Vander's Human Physiology'' reported similar numbers: 60% carried as bicarbonate, 30% bound to hemoglobin as [[carbaminohemoglobin]], and 10% physically dissolved. {{cite book |last1=Widmaier |first1=Eric P. |url=https://archive.org/details/humanphysiologym0000widm/page/493/mode/2up |title=Vander's Human Physiology |last2=Raff |first2=Hershel |last3=Strang |first3=Kevin T. |date=2003 |publisher=[[McGraw-Hill Education]] |isbn=978-0-07-288074-8 |edition=9th |at=p. 493 (ch. Respiratory physiology § Transport of carbon dioxide in blood) |url-access=registration |name-list-style=vanc}}</ref>

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO<sub>2</sub> bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of [[allosteric regulation|allosteric]] effects on the hemoglobin molecule, the binding of CO<sub>2</sub> decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the [[Haldane effect]], and is important in the transport of carbon dioxide from the tissues to the lungs. A rise in the partial pressure of CO<sub>2</sub> or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the [[Bohr effect]].

===Transport of hydrogen ions===
Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin binds most of the hydrogen ions as it has a much greater affinity for more hydrogen than does oxyhemoglobin.

===Lymphatic system===
{{main|Lymphatic system}}
In mammals, blood is in equilibrium with [[lymph]], which is continuously formed in tissues from blood by capillary ultrafiltration. Lymph is collected by a system of small lymphatic vessels and directed to the [[thoracic duct]], which drains into the left [[subclavian vein]], where lymph rejoins the systemic blood circulation.

===Thermoregulation===
Blood circulation transports heat throughout the body, and adjustments to this flow are an important part of [[thermoregulation]]. Increasing blood flow to the surface (e.g., during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss. In contrast, when the external temperature is low, blood flow to the extremities and surface of the skin is reduced and to prevent heat loss and is circulated to the important organs of the body, preferentially.

===Rate of flow===
Rate of blood flow varies greatly between different organs. Liver has the most abundant blood supply with an approximate flow of 1350 ml/min. Kidney and brain are the second and the third most supplied organs, with 1100 ml/min and ~700 ml/min, respectively.<ref name=":0">{{Cite book |url=https://archive.org/details/guytonhalltextbo0000hall/page/204/mode/2up |title=Guyton and Hall Textbook of Medical Physiology |publisher=Saunders |year=2015 |isbn=978-1455770052 |page=204 |url-access=registration}}</ref>

Relative rates of blood flow per 100 g of tissue are different, with kidney, adrenal gland and thyroid being the first, second and third most supplied tissues, respectively.<ref name=":0" />

===Hydraulic functions===
The restriction of blood flow can also be used in specialized tissues to cause engorgement, resulting in an [[erection]] of that tissue; examples are the [[erectile tissue]] in the [[penis]] and [[clitoris]].

Another example of a hydraulic function is the [[jumping spider]], in which blood forced into the legs under pressure causes them to straighten for a powerful jump, without the need for bulky muscular legs.<ref>{{cite encyclopedia |url=https://www.britannica.com/eb/topic-559817/spider |encyclopedia=Encyclopædia Britannica online |title=Spiders: circulatory system |access-date=25 November 2007 |url-status=live |archive-url=https://web.archive.org/web/20071112190558/https://www.britannica.com/eb/topic-559817/spider |archive-date=12 November 2007 }}</ref>