Editing Blood vessel (section)

== Function ==
{{See also|Circulatory system}}
Blood vessels function to transport [[blood]] to an animal's body tissues. In general, arteries and arterioles transport oxygenated blood from the lungs to the body and its [[organ (anatomy)|organs]], and veins and venules transport deoxygenated blood from the body to the lungs. Blood vessels also circulate blood throughout the [[circulatory system]]. [[Oxygen]] (bound to [[hemoglobin]] in [[red blood cell]]s) is the most critical nutrient carried by the blood. In all arteries apart from the [[pulmonary artery]], hemoglobin is [[Oxygen saturation (medicine)|highly saturated]] (95–100%) with oxygen. In all veins, apart from the [[pulmonary vein]], the saturation of [[hemoglobin]] is about 75%.<ref>{{Cite web|title= Central Venous/Mixed Venous Oxygen Saturation |url=https://www.lhsc.on.ca/critical-care-trauma-centre/central-venous/mixed-venous-oxygen-saturation|access-date=2021-08-08 | location = London, Ontario, CA | work = London Health Sciences Centre }}</ref><ref>{{Cite web|title=Hypoxemia (low blood oxygen)|url=https://www.mayoclinic.org/symptoms/hypoxemia/basics/definition/sym-20050930|access-date=2021-08-08|website=Mayo Clinic|language=en}}</ref> (The values are reversed in the [[pulmonary circulation]].) In addition to carrying oxygen, blood also carries [[hormone]]s, and [[nutrient]]s  to the cells of a body and removes [[Metabolic waste|waste products]].<ref>{{Cite journal |last=Prisby |first=Rhonda D. |date=2017-08-16 |title=Mechanical, hormonal and metabolic influences on blood vessels, blood flow and bone |journal=The Journal of Endocrinology |language=en |volume=235 |issue=3 |pages=R77–R100 |doi=10.1530/JOE-16-0666 |pmc=5611884 |pmid=28814440}}</ref>

Blood vessels do not actively engage in the transport of blood (they have no appreciable [[peristalsis]]). Blood is propelled through arteries and arterioles through pressure generated by the [[Cardiac cycle|heartbeat]].<ref>{{cite encyclopedia | vauthors = Khan MG | title=Anatomy of the Heart and Circulation |encyclopedia=Encyclopedia of Heart Disease |year=2006 |publisher=Academic Press |location=Amsterdam | isbn = 978-0-08-045481-8 | pages = 13–22 | url = https://books.google.com/books?id=xco9aJ_Y9XIC&pg=PR13 }}</ref> Blood vessels also transport red blood cells. [[Hematocrit|Hematocrit tests]] can be performed to calculate the proportion of red blood cells in the blood. Higher proportions result in conditions such as dehydration or heart disease, while lower proportions could lead to [[anemia]] and long-term blood loss.<ref name="mayoclinic.org">{{cite web|url=https://www.mayoclinic.org/tests-procedures/hematocrit/about/pac-20384728|title=Hematocrit test – Mayo Clinic|website=www.mayoclinic.org}}</ref>

Permeability of the [[endothelium]] is pivotal in the release of nutrients to the tissue. It is also increased in [[inflammation]] in response to [[histamine]],<ref>{{Cite journal |last1=Ashina |first1=Kohei |last2=Tsubosaka |first2=Yoshiki |last3=Nakamura |first3=Tatsuro |last4=Omori |first4=Keisuke |last5=Kobayashi |first5=Koji |last6=Hori |first6=Masatoshi |last7=Ozaki |first7=Hiroshi |last8=Murata |first8=Takahisa |date=2015 |title=Histamine Induces Vascular Hyperpermeability by Increasing Blood Flow and Endothelial Barrier Disruption In Vivo |journal=PLOS ONE |volume=10 |issue=7 |pages=e0132367 |doi=10.1371/journal.pone.0132367 |issn=1932-6203 |pmc=4497677 |pmid=26158531 |doi-access=free}}</ref> [[prostaglandin]]s<ref>{{Cite journal |last1=Rittchen |first1=Sonja |last2=Rohrer |first2=Kathrin |last3=Platzer |first3=Wolfgang |last4=Knuplez |first4=Eva |last5=Bärnthaler |first5=Thomas |last6=Marsh |first6=Leigh M. |last7=Atallah |first7=Reham |last8=Sinn |first8=Katharina |last9=Klepetko |first9=Walter |last10=Sharma |first10=Neha |last11=Nagaraj |first11=Chandran |last12=Heinemann |first12=Akos |date=2020-12-01 |title=Prostaglandin D2 strengthens human endothelial barrier by activation of E-type receptor 4 |url=https://www.sciencedirect.com/science/article/pii/S000629522030513X |journal=Biochemical Pharmacology |volume=182 |pages=114277 |doi=10.1016/j.bcp.2020.114277 |pmid=33038299 |issn=0006-2952|doi-access=free }}</ref> and [[interleukin]]s,<ref>{{Cite journal |last1=Yu |first1=Hongchi |last2=Huang |first2=Xianliang |last3=Ma |first3=Yunlong |last4=Gao |first4=Min |last5=Wang |first5=Ou |last6=Gao |first6=Ting |last7=Shen |first7=Yang |last8=Liu |first8=Xiaoheng |date=2013 |title=Interleukin-8 regulates endothelial permeability by down-regulation of tight junction but not dependent on integrins induced focal adhesions |journal=International Journal of Biological Sciences |volume=9 |issue=9 |pages=966–979 |doi=10.7150/ijbs.6996 |issn=1449-2288 |pmc=3805902 |pmid=24155670}}</ref> which leads to most of the symptoms of inflammation (swelling, redness, warmth and pain).

=== Constriction ===
{{Main|Vasoconstriction}}
[[File:Microvessel.jpg|thumb|[[Transmission electron micrograph]] of a [[microvessel]] displaying an [[erythrocyte]] (E) within its [[lumen (anatomy)|lumen]] which is deformed due to vasoconstriction]]
Arteries—and veins to a degree—can regulate their inner diameter by contraction of the muscular layer. This changes the blood flow to downstream organs and is determined by the [[autonomic nervous system]]. Vasodilation and vasoconstriction are also used antagonistically as methods of [[thermoregulation]].<ref>{{cite journal | vauthors = Charkoudian N | title = Mechanisms and modifiers of reflex induced cutaneous vasodilation and vasoconstriction in humans | journal = Journal of Applied Physiology | volume = 109 | issue = 4 | pages = 1221–1228 | date = October 2010 | pmid = 20448028 | pmc = 2963327 | doi = 10.1152/japplphysiol.00298.2010 }}</ref>

The size of blood vessels is different for each of them. It ranges from a diameter of about 30–25 millimeters for the [[aorta]]<ref>{{Cite journal |last1=Erbel |first1=Raimund |last2=Eggebrecht |first2=Holger |date=2006 |title=Aortic dimensions and the risk of dissection |journal=Heart |language=en |volume=92 |issue=1 |pages=137–142 |doi=10.1136/hrt.2004.055111 |pmc=1861012 |pmid=16365370 |quote=The normal diameter of the abdominal aorta is regarded to be less than 3.0 cm.}}</ref> to only about 5 micrometers (0,005{{Nbsp}}mm) for the capillaries.<ref>{{Cite journal |last1=Potter |first1=R. F. |last2=Groom |first2=A. C. |date=1983 |title=Capillary diameter and geometry in cardiac and skeletal muscle studied by means of corrosion casts |url=https://pubmed.ncbi.nlm.nih.gov/6835100 |journal=Microvascular Research |volume=25 |issue=1 |pages=68–84 |doi=10.1016/0026-2862(83)90044-4 |issn=0026-2862 |pmid=6835100}}</ref> Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the [[vascular smooth muscle]] in the vessel walls. It is regulated by [[vasoconstrictor]]s (agents that cause vasoconstriction). These can include [[paracrine]] factors (e.g., [[prostaglandin]]s), a number of [[hormone]]s (e.g., [[Antidiuretic hormone|vasopressin]] and [[angiotensin]]<ref>{{Cite journal |last1=Kanaide |first1=Hideo |last2=Ichiki |first2=Toshihiro |last3=Nishimura |first3=Junji |last4=Hirano |first4=Katsuya |date=2003-11-28 |title=Cellular Mechanism of Vasoconstriction Induced by Angiotensin II |url=https://dx.doi.org/10.1161/01.res.0000105920.33926.60 |journal=Circulation Research |volume=93 |issue=11 |pages=1015–1017 |doi=10.1161/01.res.0000105920.33926.60 |pmid=14645130 |issn=0009-7330}}</ref>) and [[neurotransmitter]]s (e.g., [[epinephrine]]) from the nervous system.

[[Vasodilation]] is a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is [[nitric oxide]] (termed [[endothelium-derived relaxing factor]] for this reason).<ref>{{Cite journal |last=Cooke |first=John P |date=2000 |title=The endothelium: a new target for therapy |url=https://journals.sagepub.com/doi/10.1177/1358836X0000500108 |journal=Vascular Medicine |language=en |volume=5 |issue=1 |pages=49–53 |doi=10.1177/1358836X0000500108 |pmid=10737156 |issn=1358-863X}}</ref>

=== Flow ===
{{Main |Vascular resistance}}
The circulatory system uses the channel of blood vessels to deliver blood to all parts of the body. This is a result of the [[Cardiac cycle|left and right sides of the heart working together]] to allow blood to flow continuously to the lungs and other parts of the body. Oxygen-poor blood enters the right side of the heart through two large veins. Oxygen-rich blood from the lungs enters through the pulmonary veins on the left side of the heart into the aorta and then reaches the rest of the body. The capillaries are responsible for allowing the blood to receive oxygen through tiny air sacs in the lungs. This is also the site where carbon dioxide exits the blood. This all occurs in the lungs where blood is oxygenated.<ref>{{cite web | vauthors = Nazario B | date = 17 September 2021 |url=https://www.webmd.com/hypertension-high-blood-pressure/hypertension-working-heart|title=How Your Heart Works |website=WebMD}}</ref>

The blood pressure in blood vessels is traditionally expressed in [[torr|millimetres of mercury]] (1 mmHg = 133 [[Pascal (unit)|Pa]]). In the arterial system, this is usually around 120 mmHg [[Systole (medicine)|systolic]] (high pressure wave due to contraction of the heart) and 80 mmHg [[diastolic]] (low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg.<ref>{{Citation |last1=Yeo |first1=JuDong |title=Bioactive peptides in health and disease: an overview |date=2021 |work=Biologically Active Peptides |pages=1–26 |url=https://dx.doi.org/10.1016/b978-0-12-821389-6.00007-8 |access-date=2024-10-30 |publisher=Elsevier |isbn=978-0-12-821389-6 |last2=Shahidi |first2=Fereidoon|doi=10.1016/b978-0-12-821389-6.00007-8 }}</ref>

[[Vascular resistance]] occurs when the vessels away from the heart oppose the flow of blood. Resistance is an accumulation of three different factors: blood viscosity, blood vessel length and vessel radius.<ref name = "Saladin_2012">{{cite book | vauthors = Saladin KS |title=Anatomy & physiology : the unity of form and function |date=2012 |publisher=McGraw-Hill |location=New York, NY |isbn=978-0-07-131638-5 |edition=6th}}</ref> Blood viscosity is the thickness of the blood and its resistance to flow as a result of the different components of the blood. Blood is 92% water by weight and the rest of blood is composed of protein, nutrients, electrolytes, wastes, and dissolved gases. Depending on the health of an individual, the blood viscosity can vary (i.e., anemia causing relatively lower concentrations of protein, high blood pressure an increase in dissolved salts or lipids, etc.).<ref name="Saladin_2012" />

Vessel length is the total length of the vessel measured as the distance away from the heart. As the total length of the vessel increases, the total resistance as a result of friction will increase.<ref name="Saladin_2012"/> Vessel radius also affects the total resistance as a result of contact with the vessel wall. As the radius of the wall gets smaller, the proportion of the blood making contact with the wall will increase. The greater amount of contact with the wall will increase the total resistance against the blood flow.<ref name="Vessel Radius">{{cite news|title=Factors that Affect Blood Pressure|url=https://www.interactivephysiology.com/demo/misc/assignmentfiles/cardiovascular/Fact_Aff_Blood_Pressure.pdf|url-status=dead|access-date=21 October 2018|archive-url=https://web.archive.org/web/20170517024049/https://www.interactivephysiology.com/demo/misc/assignmentfiles/cardiovascular/Fact_Aff_Blood_Pressure.pdf|archive-date=17 May 2017}}</ref>