Editing Nephron (section)

=== Collecting duct system ===
{{main|Collecting duct system}}
[[File:Gray1133.png|thumbnail|Fig.6) Cross-sectional histologic preparation showing (b)small connecting tubules with simple columnar epithelium and (a) large connecting tubules with simple cuboidal epithelium.]]

Each distal convoluted tubule delivers its filtrate to a [[collecting duct system|system of collecting ducts]], the first segment of which is the [[connecting tubule]]. The collecting duct system begins in the renal cortex and extends deep into the medulla. As the urine travels down the collecting duct system, it passes by the medullary interstitium which has a high sodium concentration as a result of the loop of Henle's [[Countercurrent exchange#In the kidney|countercurrent multiplier system]].<ref name="lote" />{{rp|67}}

Because it has a different origin during the [[development of the urinary and reproductive organs]] than the rest of the nephron, the collecting duct is sometimes not considered a part of the nephron. Instead of originating from the metanephrogenic blastema, the collecting duct originates from the [[ureteric bud]].<ref name="mitchell">{{cite book | vauthors = Mitchell B, Sharma R |title= Embriology | edition = 2nd |year=2009|publisher=Churchill Livingstone Elsevier}}</ref>{{rp|50–51}}

Though the collecting duct is normally impermeable to water, it becomes permeable in the presence of [[antidiuretic hormone]] (ADH). ADH affects the function of [[aquaporins]], resulting in the reabsorption of water molecules as it passes through the collecting duct. Aquaporins are membrane proteins that selectively conduct water molecules while preventing the passage of ions and other solutes. As much as three-quarters of the water from urine can be reabsorbed as it leaves the collecting duct by osmosis. Thus the levels of ADH determine whether urine will be concentrated or diluted. An increase in ADH is an indication of [[dehydration]], while water sufficiency results in a decrease in ADH allowing for diluted urine.<ref name="junqueiras" />{{rp|406}}
[[Image:Kidney Nephron Cells.png|thumb|250px|Fig.7)
Cross-sectional diagram of the juxtaglomerular apparatus and adjacent structures: 1) top, yellow - distal convoluted tubule; 2) top, brown -macula densa cuboidal cells surrounding arterioles; 3) small blue cells - juxtaglomerular cells; 4) large blue cells - mesangial cells; 5) tan - podocytes lining [[Bowman's capsule]] adjacent to capillaries, and parietal layer of capsule, 6)center - five glomerular capillaries, and the 6)bottom, purple - exiting tubule. Structures (2), (3), and (4) constitute the juxtaglomerular apparatus.]]
Lower portions of the collecting organ are also permeable to [[urea]], allowing some of it to enter the medulla, thus maintaining its high concentration (which is very important for the nephron).<ref name="lote" />{{rp|73–74}}

Urine leaves the medullary collecting ducts through the [[renal papilla]]e, emptying into the [[Minor calyx|renal calyces]], the [[renal pelvis]], and finally into the [[urinary bladder]] via the [[ureter]].<ref name="junqueiras" />{{rp|406–407}}