Editing Collecting duct system (section)

==Function==
[[File:Kidney nephron molar transport diagram.svg|thumbnail|Diagram outlining movement of ions in nephron, with the collecting ducts on the right.]]
The collecting duct system is the final component of the kidney to influence the body's [[electrolyte]] and fluid balance. In humans, the system accounts for 4–5% of the kidney's reabsorption of [[sodium]] and 5% of the kidney's reabsorption of water. At times of extreme dehydration, over 24% of the filtered water may be reabsorbed in the collecting duct system.

The wide variation in water reabsorption levels for the collecting duct system reflects its dependence on hormonal activation. The collecting ducts, in particular, the outer medullary and cortical collecting ducts, are largely impermeable to water without the presence of [[antidiuretic hormone]] (ADH, or vasopressin).
* In the ''absence'' of ADH, water in the renal filtrate is left alone to enter the urine, promoting [[:wikt:diuresis|diuresis]].
* When ADH is ''present'', [[aquaporin]]s allow for the reabsorption of this water, thereby inhibiting diuresis.

The collecting duct system participates in the regulation of other [[electrolyte]]s, including [[chloride]], [[potassium]], [[hydrogen ion]]s, and [[bicarbonate]].

An extracellular protein called [[hensin (protein)]] mediates the regulation of secretion of acid by alpha cells in acidosis, and secretion of bicarbonate by beta cells in alkalosis.<ref>{{Cite book|title=Harrison's principles of internal medicine|others=Jameson, J. Larry,, Kasper, Dennis L.,, Longo, Dan L. (Dan Louis), 1949-, Fauci, Anthony S., 1940-, Hauser, Stephen L.,, Loscalzo, Joseph|date=13 August 2018|isbn=978-1-259-64403-0|edition=20th|location=New York|pages=2097|oclc=1029074059}}</ref><ref name="Takito">{{cite journal |last1=Takito |first1=J |last2=Hikita |first2=C |last3=Al-Awqati |first3=Q |title=Hensin, a new collecting duct protein involved in the in vitro plasticity of intercalated cell polarity. |journal=The Journal of Clinical Investigation |date=15 November 1996 |volume=98 |issue=10 |pages=2324–31 |doi=10.1172/JCI119044 |pmid=8941650|pmc=507683 }}</ref>