Editing Blood vessel (section)

== Structure ==
The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins:<ref>{{Citation |last1=Taylor |first1=Anthony M. |title=Histology, Blood Vascular System |date=2024 |work=StatPearls |url=https://www.ncbi.nlm.nih.gov/books/NBK553217/ |access-date=2024-04-07 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31985998 |last2=Bordoni |first2=Bruno}}</ref>
* The inner layer, ''[[tunica intima]]'', is the thinnest layer. It is a single layer of flat cells ([[simple squamous epithelium]]) glued by a [[polysaccharide]] intercellular matrix, surrounded by a thin layer of subendothelial [[connective tissue]] interlaced with a number of circularly arranged elastic bands called the ''[[internal elastic lamina]]''. A thin membrane of elastic fibers in the tunica intima run parallel to the vessel.
* The middle layer of ''[[tunica media]]'' is the thickest layer in arteries. It consists of circularly arranged elastic fiber, connective tissue and polysaccharide substances; the second and third layer are separated by another thick elastic band called ''external elastic lamina''.<ref>{{Cite journal |last1=Qin |first1=Guiping |last2=Wang |first2=Leiming |last3=Hua |first3=Yulan |last4=Hou |first4=Haina |last5=Zou |first5=Qiuju |last6=Wang |first6=Daye |last7=Hu |first7=Zijing |last8=Lu |first8=Dehong |date=2020-04-01 |title=Comparative morphology of the internal elastic lamina of cerebral and peripheral arteries |journal=International Journal of Clinical and Experimental Pathology |language=en |volume=13 |issue=4 |pages=764–770 |pmc=7191140 |pmid=32355525}}</ref> The tunica media may (especially in arteries) be rich in [[vascular smooth muscle]], which controls the caliber of the vessel. Veins do not have the external elastic lamina, but only an internal one. The tunica media is thicker in the arteries rather than the veins.
* The outer layer is the ''[[tunica adventitia]]'' and the thickest layer in veins. It is entirely made of connective tissue. It also contains [[nerve]]s that supply the vessel as well as nutrient capillaries ([[vasa vasorum]]) in the larger blood vessels.

[[Capillaries]] consist of a single layer of [[endothelial cell]]s with a supporting subendothelium consisting of a [[basement membrane]] and [[connective tissue]]. When blood vessels connect to form a region of diffuse vascular supply, it is called an [[circulatory anastomosis|anastomosis]]. Anastomoses provide alternative routes for blood to flow through in case of blockages. Veins can have [[Vein#Venous valves|valves]] that prevent the [[Regurgitation (circulation)|backflow of the blood]] that was being pumped against gravity by the surrounding muscles.<ref>{{Cite web |last=D. Douketis |first=James |date=2023 |title=Overview of the Venous System |url=https://www.msdmanuals.com/home/heart-and-blood-vessel-disorders/venous-disorders/overview-of-the-venous-system |website=MSD Manual}}</ref> In humans, arteries do not have valves except for the two 'arteries' that originate from the heart's ventricles.<ref>The exception is the [[pulmonary artery]] and the [[aorta]].</ref>

Early estimates by Danish physiologist [[August Krogh]] suggested that the total length of capillaries in human muscles could reach approximately {{convert|100000|km|mi}} (assuming a high muscle mass human body, like that of a [[bodybuilder]]).<ref>{{Cite book |last=Krogh |first=August |url=https://archive.org/details/anatomyphysiolog00kroguoft/page/10/mode/2up?q=length |title=The anatomy and physiology of capillaries |date=1922 |publisher=New Haven, Yale Univ. Press |others=Gerstein - University of Toronto}}</ref> However, later studies suggest a more conservative figure of {{convert|9000|-|19000|km|mi}} taking into account updated capillary density and average muscle mass in adults.<ref>{{Cite journal |last=Poole |first=David C. |last2=Kano |first2=Yutaka |last3=Koga |first3=Shunsaku |last4=Musch |first4=Timothy I. |date=2021|title=August Krogh: Muscle capillary function and oxygen delivery |journal=Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology |volume=253 |pages=110852 |doi=10.1016/j.cbpa.2020.110852 |issn=1531-4332 |pmc=7867635 |pmid=33242636}}</ref>

=== Types ===
There are various kinds of blood vessels:<ref>{{Citation |last1=Tucker |first1=William D. |title=Anatomy, Blood Vessels |date=2024 |work=StatPearls |url=https://www.ncbi.nlm.nih.gov/books/NBK470401/ |access-date=2024-04-17 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=29262226 |last2=Arora |first2=Yingyot |last3=Mahajan |first3=Kunal}}</ref>
* [[Artery|Arteries]]
** [[Elastic artery|Elastic arteries]]
** [[Distributing artery|Distributing arteries]]
** [[Arteriole]]s
* [[Capillary|Capillaries]] (smallest type of blood vessels)
* [[Venule]]s
* [[Vein]]s
** Large collecting vessels, such as the [[subclavian vein]], the [[jugular vein]], the [[renal vein]] and the [[iliac vein]].
** [[Venae cavae]] (the two largest veins, carry blood into the heart).
* [[Capillary#Sinusoidal|Sinusoids]]
** Extremely small vessels located within bone marrow, the spleen and the liver.

They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing ''away from'' (arterial) or ''toward'' (venous) the [[heart]]. The term "arterial blood" is nevertheless used to indicate blood high in [[oxygen]], although the [[pulmonary artery]] carries "venous blood" and blood flowing in the [[pulmonary vein]] is rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.{{cn|date=February 2024}}