Editing Collecting duct system (section)

===Segments===
[[File:Gray1133.png|thumbnail|Simple columnar epithelium and simple cuboidal epithelium in the collecting ducts of the [[pig]] [[kidney]]. The walls of the large and small [[connecting tubule]]s (a and b respectively), the circular structures, are formed by simple columnar epithelium (a) and simple cuboidal epithelium (b).]]
The segments of the system are as follows:

{| class="wikitable"
!Segment
!Description
|-
| ''[[connecting tubule]]'' ||Connects distal convoluted tubule to the cortical collecting duct 
|-
| ''initial collecting tubule'' || Before convergence of nephrons
|-
| ''cortical collecting ducts'' ||Receives filtrate from the initial collecting tubules, and descends into the [[renal medulla]], forming medullary collecting ducts
|-
| ''medullary collecting ducts'' ||
|-
| ''[[papillary ducts]]'' ||
|}

====Connecting tubule====
With respect to the [[renal corpuscle]], the '''connecting tubule''' ('''CNT''', or '''junctional tubule''', or '''arcuate renal tubule''') is the most proximal part of the collecting duct system. It is adjacent to the [[distal convoluted tubule]], the most distal segment of the [[renal tubule]]. Connecting tubules from several adjacent nephrons merge to form cortical collecting tubules, and these may join to form cortical collecting ducts (CCD).<ref>{{cite journal | author=Imai M | title=The connecting tubule: a functional subdivision of the rabbit distal nephron segments | journal=Kidney Int | year=1979 | pages=346–56 | volume=15 | issue=4 | pmid=513494 | doi=10.1038/ki.1979.46| doi-access=free }}</ref> Connecting tubules of some [[juxtamedullary nephron]]s may arch upward, forming an arcade. It is this "arcuate" feature which gives the tubule its alternate name.

The connecting tubule derives from the [[metanephric blastema]], but the rest of the system derives from the [[ureteric bud]].<ref>{{Cite book|last=Mitchell, B. S.|title=Embryology: an illustrated colour text|date=2009|publisher=Churchill Livingstone/Elsevier|others=Sharma, Ram, Britton, Robert.|isbn=978-0-7020-5081-7|edition=2nd|location=Edinburgh|pages=50–51|oclc=787843894}}</ref> Because of this, some sources group the connecting tubule as part of the [[nephron]], rather than grouping it with the collecting duct system.

{{anchor|Initial collecting tubule}}
The initial collecting tubule is a segment with a constitution similar as the collecting duct, but before the convergence with other tubules.

{{anchor|Cortical collecting duct}}
The "cortical collecting ducts" receive filtrate from multiple initial collecting tubules and descend into the [[renal medulla]] to form medullary collecting ducts.

It participates in the regulation of [[water]] and [[electrolyte]]s, including [[sodium]], and [[chloride]].<ref>{{cite book |author1=Eaton, Douglas C. |author2=Pooler, John P. | title=Vander's Renal Physiology | publisher=Lange Medical Books/McGraw-Hill | year=2004 | edition=6th | isbn=0-07-135728-9}}</ref> The CNT is sensitive to both [[isoprotenerol]] (more so than the cortical collecting ducts) and [[antidiuretic hormone]] (less so than the cortical collecting ducts), the latter largely determining its function in water reabsorption.

====Medullary collecting duct==== <!--Medullary collecting duct redirects here-->
"Medullary collecting ducts" are divided into outer and inner segments, the latter reaching more deeply into the medulla. The variable reabsorption of water and, depending on fluid balances and hormonal influences, the reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate ion continues here. Urea passively transports out of duct here and creates 500mOsm gradient.

{{anchor|Outer segment}}<!--Outer medullary collecting duct redirects here-->
The outer segment of the medullary collecting duct follows the cortical collecting duct. It reaches the level of the renal medulla where the [[thin descending limb of loop of Henle]] borders with the [[thick ascending limb of loop of Henle]]<ref>{{Cite book|last = Boron|first=Walter F.|title=Medical Physiology: A Cellular and Molecular Approach| edition = updated| publisher=Elsevier/Saunders|location=Philadelphia |year=2005 |isbn=1-4160-2328-3}}</ref>{{rp|837}}

{{anchor|Inner segment}} <!--Inner medullary collecting duct redirects here-->
The inner segment is the part of the collecting duct system between the outer segment and the papillary ducts.

====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>