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((Short description|Alternate blood flow around a blockage))
((refimprove|date=January 2016))
((Infobox anatomy
| Name            = Collateral circulation
| Latin           = 
| Image           = 
| Caption         = 
| Image2          = 
| Caption2        = 
| IsPartOf        =
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'''Collateral circulation''' is the alternate [[Circulatory system|circulation]] around a blocked [[blood vessel|artery or vein]] via another path, such as nearby minor vessels.<ref>[http://dictionary.reference.com/browse/collateral-circulation Dictionary.com collateral circulation]</ref> It may occur via preexisting vascular redundancy (analogous to [[redundancy (engineering)|engineered redundancy]]), as in the [[circle of Willis]] in the brain, or it may occur via new branches formed between adjacent blood vessels ([[neovascularization]]), as in the eye after a [[retina]]l embolism or in the brain when an instance of arterial constriction occurs due to [[Moyamoya disease]]. Its formation may be related by pathological conditions such as high [[vascular resistance]] or [[ischaemia]]. It is occasionally also known as '''accessory circulation''', '''auxiliary circulation''', or '''secondary circulation'''. It has [[surgery|surgically]] created analogues in which [[shunt (medical)|shunts]] or [[circulatory anastomosis|anastomoses]] are constructed to bypass circulatory problems.

An example of the usefulness of collateral circulation is a systemic thromboembolism in cats. This is when a thrombotic [[embolus]] lodges above the [[external iliac artery]] (common iliac artery), blocking the external and [[internal iliac arteries]] and effectively shutting off all blood supply to the hind leg. Even though the main vessels to the leg are blocked, enough blood can get to the tissues in the leg via the collateral circulation to keep them alive.

==Brain==
[[Cerebral circulation|Blood flow to the brain]] in humans and some other animals is maintained via a network of collateral arteries that [[anastomosis|anastomose]] (join) in the [[circle of Willis]], which lies at the base of the brain. In the circle of Willis so-called [[communicating artery (disambiguation)|communicating arteries]] exist between the front (anterior) and back (posterior) parts of the circle of Willis, as well as between the left and right side of the circle of Willis.

[[Leptomeningeal collateral circulation]] is another anastomosis in the brain.

==Heart==
Another example in humans and some other animals is after an acute [[myocardial infarction]] (heart attack). Collateral circulation in the heart tissue will sometimes bypass the blockage in the main artery and supply enough oxygenated blood to enable the cardiac tissue to survive and recover.

==Eye==
[[File:Collateral vein in central retinal vein occlusion.gif|thumb|Collateral tortuous vein (center, left) in central retinal vein occlusion, revealed by [[laser Doppler imaging]].]]
After [[central retinal vein occlusion]], [[neovascularization]] may restore some blood flow to the retina, but the new vessels' bulk also presents a risk of causing acute [[glaucoma]] by blocking the drainage of [[aqueous humour]]. Collateral circulation is created (within months) around the blocked central vein via a generally winding path, usually from a branch vein to the [[choroid]].

==Hand==
The circulatory system of the human hand features extensive collateral circulation in the form of the [[deep palmar arch|deep]] and [[superficial palmar arch|superficial]] palmar arches, and the [[proper palmar digital arteries]] of the digits, with the exception of the [[thumb]], which has its blood supplied by the [[princeps pollicis artery]]. As each finger has two proper digital arteries, blood can be continue to be supplied to specific digits if one of the two is injured.

==Truncal venous system==
[[Hepatic cirrhosis]] arising from  congestion in the [[hepatic portal vein]] may give rise to collateral circulation between branches of the portal and caval veins of the [[liver]], or between the two caval veins. Consequences of newly established venous collaterals arising from portal hypertension include [[esophageal varices]] and [[hemorrhoid]]s (portocaval collateral circulation).

==References==
((reflist))

((DEFAULTSORT:Collateral Circulation))
[[Category:Angiology]]
[[Category:Fault tolerance]]

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