Editing Collecting duct system (section)

====Papillary duct====
'''Papillary (collecting) ducts''' are anatomical structures of the [[kidneys]], previously known as the '''ducts of [[Lorenzo Bellini|Bellini]]'''. Papillary ducts represent the most [[distal]] portion of the [[collecting duct]]. They receive [[Renal filtration|renal filtrate]] (precursor to [[urine]]) from several [[medullary collecting duct]]s and empty into a [[minor calyx]]. Papillary ducts continue the work of water reabsorption and electrolyte balance initiated in the [[collecting tubules]].<ref>{{Cite book|title = Junqueira's Basic Histology|last = Mescher|first = Anthony|publisher = McGraw-Hill|year = 2013|isbn = 9780071807203|pages = 385–403}}</ref>

[[Medullary collecting duct]]s converge to form a central (papillary) duct near the apex of each [[Renal pyramids|renal pyramid]]. This "papillary duct" exits the renal pyramid at the [[renal papilla]]e. The [[Renal filtration|renal filtrate]] it carries drains into a [[Renal calyx|minor calyx]] as [[urine]].<ref name=":0">{{Cite book|title = Junqueira's Basic Histology|last = Mescher|first = Anthony|publisher = McGraw-Hill|year = 2013|isbn = 9780071807203|pages = 400}}</ref>

The cells that comprise the duct itself are similar to rest of the collecting system. The duct is lined by a layer of [[simple columnar epithelium]] resting on a thin [[basement membrane]]. The epithelium is composed primarily of [[principal cells]] and α-intercalated cells.<ref>{{Cite book|title = Color Atlas and Text of Histology|last1 = Gartner|first1 = Leslie|publisher = Lippincott & Wilkins|year = 2014|isbn = 9781451113433|location = Baltimore, MD 21201|pages = 383–399|last2 = Hiatt}}</ref> The [[simple columnar epithelium]] of the collecting duct system transitions into [[urothelium]] near the junction of a papillary duct and a minor calyx.<ref name=":0" />

These cells work in tandem to reabsorb water, sodium, and urea and secrete acid and potassium. The amount of reabsorption or secretion that occurs is related to needs of the body at any given time. These processes are mediated by [[hormones]] ([[aldosterone]], [[vasopressin]]) and the [[osmolarity]] (concentration of electrically charged chemicals) of the surrounding medulla. [[Hormones]] regulate how [[Permeability (earth sciences)|permeable]] the papillary duct is to water and electrolytes. In the medullary collecting duct specifically, [[vasopressin]] upregulates [[urea transporter|urea transporter A1]]. This increases the concentration of urea in the surrounding [[interstitium]] and increases the osmolarity.
[[Osmolarity]] influences the strength of the force that pulls (reabsorbs) water from the papillary duct into the medullary interstitium. This is especially important in the papillary ducts. '''Osmolarity increases''' from the base of the renal pyramid to the apex. It is highest at the renal apex (up to 1200 mOsm). Thus the force driving the reabsorption of water from the collecting system is the greatest in the papillary duct.<ref>{{Cite book|title = Physiology|last = Costanzo|first = Linda|publisher = Wolters Kluwer Health|year = 2011|isbn = 9781451187953|location = Baltimore, MD 21201|pages = 167–172}}</ref>