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==Function== [[File:Arterial System en.svg|thumbnail|Arteries form part of the human [[circulatory system]]]] {{refimprove|section|date=May 2024}} {{main|Circulatory system}} Arteries form part of the [[circulatory system]]. They carry [[blood]] that is oxygenated after it has been pumped from the [[heart]]. [[Coronary circulation|Coronary arteries]] also aid the heart in pumping blood by sending oxygenated blood to the heart, allowing the muscles to function. Arteries carry oxygenated blood away from the heart to the tissues, except for [[pulmonary arteries]], which carry blood to the [[lung]]s for oxygenation (usually [[vein]]s carry deoxygenated blood to the heart but the [[pulmonary vein]]s carry oxygenated blood as well).<ref>{{cite book | last = Maton | first = Anthea | author2 = Jean Hopkins | author3 = Charles William McLaughlin | author4 = Susan Johnson | author5 = Maryanna Quon Warner | author6 = David LaHart | author7 = Jill D. Wright | title = Human Biology and Health | publisher = Prentice Hall | year = 1999 | location = Englewood Cliffs, New Jersey | url = https://archive.org/details/humanbiologyheal00scho | isbn = 0-13-981176-1 | url-access = registration }}</ref> There are two types of unique arteries. The [[pulmonary artery]] carries blood from the heart to the [[lung]]s, where it receives oxygen. It is unique because the blood in it is not "oxygenated", as it has not yet passed through the lungs. The other unique artery is the [[umbilical artery]], which carries deoxygenated blood from a fetus to its mother. Arteries have a [[blood pressure]] higher than other parts of the circulatory system. The pressure in arteries varies during the [[cardiac cycle]]. It is highest when the [[Systole (medicine)|heart contracts]] and lowest when [[diastole|heart relaxes]]. The variation in pressure produces a [[pulse]], which can be felt in different areas of the body, such as the [[radial pulse]]. [[Arterioles]] have the greatest collective influence on both local blood flow and on overall blood pressure. They are the primary "adjustable nozzles" in the blood system, across which the greatest pressure drop occurs. The combination of heart output ([[cardiac output]]) and [[systemic vascular resistance]], which refers to the collective resistance of all of the body's [[arteriole]]s, are the principal determinants of arterial blood pressure at any given moment. Arteries have the highest pressure and have narrow lumen diameter.{{what|date=May 2024}} [[Systemic circulation|Systemic arteries]] are the arteries (including the [[peripheral arteries]]), of the [[systemic circulation]], which is the part of the [[circulatory system|cardiovascular system]] that carries [[oxygen]]ated [[blood]] away from the heart, to the [[Human body|body]], and returns deoxygenated blood back to the heart. Systemic arteries can be subdivided into two types—muscular and elastic—according to the relative compositions of elastic and muscle tissue in their tunica media as well as their size and the makeup of the internal and external elastic lamina. The larger arteries (>10 mm diameter) are generally elastic and the smaller ones (0.1–10 mm) tend to be muscular. Systemic arteries deliver blood to the [[arterioles]], and then to the [[capillaries]], where nutrients and gasses are exchanged. After traveling from the [[aorta]], blood travels through peripheral arteries into smaller arteries called [[arterioles]], and eventually to [[capillaries]]. [[Arterioles]] help in regulating [[blood pressure]] by the variable contraction of the [[Smooth muscle tissue|smooth muscle]] of their walls, and deliver blood to the [[capillaries]]. This smooth muscle contraction is primarily influenced by activity of the sympathetic vasomotor nerves innervating the arterioles.<ref>{{Cite journal |last1=Aalkjær |first1=Christian |last2=Nilsson |first2=Holger |last3=De Mey |first3=Jo G. R. |date=2021-04-01 |title=Sympathetic and sensory-motor nerves in peripheral small arteries |url=https://journals.physiology.org/doi/10.1152/physrev.00007.2020 |journal=Physiological Reviews |language=en |volume=101 |issue=2 |pages=495–544 |doi=10.1152/physrev.00007.2020 |pmid=33270533 |s2cid=227282958 |issn=0031-9333|url-access=subscription }}</ref><ref>{{Cite journal |last1=Bruno |first1=Rosa Maria |last2=Ghiadoni |first2=Lorenzo |last3=Seravalle |first3=Gino |last4=Dell'Oro |first4=Raffaella |last5=Taddei |first5=Stefano |last6=Grassi |first6=Guido |date=2012 |title=Sympathetic regulation of vascular function in health and disease |journal=Frontiers in Physiology |volume=3 |page=284 |doi=10.3389/fphys.2012.00284 |pmid=22934037 |pmc=3429057 |issn=1664-042X |doi-access=free }}</ref> Enhanced sympathetic activation prompts vasoconstriction, reducing the lumen diameter. A reduced lumen diameter consequently elevates the blood pressure within the arterioles.<ref>{{Cite journal |last1=Renna |first1=Nicolás F. |last2=de las Heras |first2=Natalia |last3=Miatello |first3=Roberto M. |date=2013-07-22 |title=Pathophysiology of Vascular Remodeling in Hypertension |journal=International Journal of Hypertension |language=en |volume=2013 |pages=e808353 |doi=10.1155/2013/808353 |pmid=23970958 |pmc=3736482 |issn=2090-0384 |doi-access=free }}</ref> Conversely, decreased sympathetic activity within the vasomotor nerves causes vasodilation of the vessels thereby decreasing blood pressure.<ref>{{Citation |last1=Schwarzwald |first1=Colin C. |title=Chapter 3 - The Cardiovascular System |date=2009-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9781416023265000031 |work=Equine Anesthesia (Second Edition) |pages=37–100 |editor-last=Muir |editor-first=William W. |access-date=2023-11-17 |place=Saint Louis |publisher=W.B. Saunders |isbn=978-1-4160-2326-5 |last2=Bonagura |first2=John D. |last3=Muir |first3=William W. |editor2-last=Hubbell |editor2-first=John A. E.}}</ref> ===Aorta=== {{main|Aorta}} [[File:2121 Aorta.jpg|thumb|The [[aorta]] is the largest blood vessel in human body.]] The [[aorta]] is the root [[systemic circulation|systemic]] artery (i.e., main artery). In humans, it receives blood directly from the left [[Ventricle (heart)|ventricle]] of the heart via the [[aortic valve]]. As the aorta branches and these arteries branch, in turn, they become successively smaller in diameter, down to the [[arteriole]]s. The [[arteriole]]s supply [[capillaries]], which in turn empty into [[venule]]s. The first branches off of the aorta are the [[Coronary circulation|coronary arteries]], which supply blood to the heart muscle itself. These are followed by the branches of the aortic arch, namely the [[brachiocephalic artery]], the [[left common carotid]], and the [[subclavian artery|left subclavian]] arteries. === Capillaries === {{main|Capillaries}} The [[capillaries]] are the smallest of the blood vessels and are part of the [[microcirculation]]. The microvessels have a width of a single cell in diameter to aid in the fast and easy [[Diffusion#Diffusion vs. bulk flow|diffusion]] of gasses, sugars and nutrients to surrounding tissues. Capillaries have no [[smooth muscle]] surrounding them and have a diameter less than that of [[red blood cells]]; a red blood cell is typically 7 micrometers outside diameter, capillaries typically 5 micrometers inside diameter. The red blood cells must distort in order to pass through the capillaries. These small diameters of the capillaries provide a relatively large surface area for the exchange of gasses and nutrients.
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