Editing Bladder (section)

==Function==
{{Main|Urination}}[[File:Bladder.jpg|thumb|Female urinary bladder]][[Urine]] is excreted by the [[kidney]]s and flows into the bladder through the [[ureter]]s, where it is stored until [[urination]] (micturition).<ref name="Ganong2019">{{Cite book|title=Ganong's review of medical physiology| first1 = Kim E | last1 = Barrett | first2 = Susan M | last2 = Barman | first3 = Jason X-J | last3 = Yuan | first4 = Heddwen | last4 = Brooks |isbn=9781260122404 |edition=26th |location=New York |oclc=1076268769 |year=2019|chapter=37. Renal function & Micturition: The Bladder|pages=681–682}}</ref> Urination involves coordinated muscle changes involving a reflex based in the spine, with higher inputs from the brain.<ref name="Ganong2019" /> During urination, the detrusor muscle contracts, the external urinary sphincter and muscles of the [[perineum]] relax, and urine flows through the [[urethra]]<ref name="Ganong2019" /> and exits the [[penis]] or [[vulva]] through the [[urinary meatus]].<ref name="Grays2016">{{Cite book|url=https://www.worldcat.org/oclc/920806541|title=Gray's anatomy : the anatomical basis of clinical practice|year=2016|isbn=9780702052309|editor=Standring, Susan|edition=41st|location=Philadelphia|pages=1255–1261|section=Urinary bladder|oclc=920806541}}</ref>

The urge to pass urine stems from [[stretch receptor]]s that activate when between 300 - 400 mL urine is held within the bladder.<ref name="Ganong2019" /> As urine accumulates, the [[rugae]] flatten and the wall of the bladder thins as it stretches, allowing the bladder to store larger amounts of urine without a significant rise in internal pressure.<ref>{{cite book |author=Marieb, Mallatt |title=Human Anatomy |edition=5th |publisher=Pearson International |chapter=23 |page=700}}</ref> Urination is controlled by the [[pontine micturition center]] in the [[brainstem]].<ref name="Purves">{{cite book|last1=Purves|first1=Dale|title=Neuroscience|date=2011|publisher=Sinauer|location=Sunderland, Mass.|isbn=978-0-87893-695-3|page=471|edition=5.}}</ref>

[[Stretch receptor]]s in the bladder signal the [[parasympathetic]] nervous system to stimulate the [[muscarinic receptor]]s in the detrusor to contract the muscle when the bladder is distended.<ref name="Giglio">{{cite journal|last1= Giglio|first1= D|last2= Tobin|first2= G|title=Muscarinic receptor subtypes in the lower urinary tract |journal= Pharmacology|date= 2009|volume= 83|issue= 5|pages= 259–69|pmid= 19295256|doi=10.1159/000209255|doi-access= free}}</ref> This encourages the bladder to expel urine through the urethra. The main receptor activated is the [[Muscarinic acetylcholine receptor M3|M3 receptor]], although [[Muscarinic acetylcholine receptor M2|M2 receptors]] are also involved and whilst outnumbering the M3 receptors they are not so responsive.<ref name="Uchiyama">{{cite journal|last1= Uchiyama|first1= T|last2= Chess-Williams|first2= R|title= Muscarinic receptor subtypes of the bladder and gastrointestinal tract |journal= Journal of Smooth Muscle Research = Nihon Heikatsukin Gakkai Kikanshi|date= December 2004|volume= 40|issue= 6|pages= 237–47|pmid= 15725706|doi=10.1540/jsmr.40.237|doi-access= free}}</ref>

The main relaxant pathway is via the [[adenylyl cyclase]] [[Cyclic adenosine monophosphate|cAMP]] pathway, activated via the β3 adrenergic receptors. The [[Beta-2 adrenergic receptor|β2 adrenergic receptors]] are also present in the detrusor and even outnumber β3 receptors, but they do not have as important an effect in relaxing the detrusor smooth muscle.<ref name="Andersson" /><ref>{{Cite journal|last1=Moro|first1=Christian|last2=Tajouri|first2=Lotti|last3=Chess-Williams|first3=Russ|title=Adrenoceptor Function and Expression in Bladder Urothelium and Lamina Propria|journal=Urology|volume=81|issue=1|pages=211.e1–211.e7|doi=10.1016/j.urology.2012.09.011|pmid=23200975|year=2013}}</ref><ref name=":0">{{Cite journal|title=Neurophysiology of Stress Urinary Incontinence|journal=Rev. Urol.|volume=6|pages=S19–28|pmid=16985861|pmc=1472861|year=2004|last1=Chancellor|first1=M. B.|last2=Yoshimura|first2=N.|issue=Suppl 3 }}</ref>