Editing Countercurrent exchange (section)

==== In the kidney ====
[[File:Kidney nephron molar transport diagram.svg|right|400px|Nephron Ion flow diagram]]
[[File:Gray1128.png|thumb|right|400px|Loop of Henle (''[[Gray's Anatomy]]'' book)]]
A circuit of fluid in the [[loop of Henle]]—an important part of the kidneys—allows for gradual buildup of the concentration of urine in the kidneys, by using [[active transport]] on the exiting [[nephron]]s (tubules carrying liquid in the process of gradually concentrating the urea). The active transport pumps need only to overcome a constant and low gradient of concentration, because of the countercurrent multiplier mechanism.<ref>See the [http://www.colorado.edu/intphys/Class/IPHY3430-200/countercurrent_ct.swf countercurrent multiplier animation] {{Webarchive|url=https://web.archive.org/web/20110606095648/http://www.colorado.edu/intphys/Class/IPHY3430-200/countercurrent_ct.swf |date=2011-06-06 }} at the [[University of Colorado]] website.</ref>

Various substances are passed from the liquid entering the nephrons until exiting the loop (See the nephron flow diagram). The sequence of flow is as follows:
* [[Renal corpuscle]]: Liquid enters the nephron system at the [[Bowman's capsule]].<ref>Beginning with the [[afferent arteriole]], a [[blood vessel]] leading to the [[Glomerulus]], filtered blood is passed to the nephrons in the Bowman's capsule which surrounds the Glomerulus. (The blood leaves the Glomerulus in the [[efferent arteriole]]).</ref>
* [[Proximal convoluted tubule]]: It then may reabsorb urea in the thick [[descending limb of loop of henle|descending limb]].<ref>The liquid from the Bowman's capsule reaches the thick descending limb. [[Urea]] may be reabsorbed into the low (300 [[mOsm]]) osmotic concentration in the limb nephrons. The urea absorption in the thick descending limb is inhibited by [[Sartan]]s and catalyzed by [[Lactic acid|lactates]] and [[ketone]]s.</ref> Water is removed from the nephrons by [[osmosis]] (and glucose and other ions are pumped out with [[active transport]]), gradually raising the concentration in the nephrons.<ref>[[Glucose]], [[amino acid]]s, various [[ions]] and organic material leave the limb, gradually raising the concentration in the nephrons. [[Dopamin]] inhibits the secretion from the thick descending limb, and [[Angiotensin II]] catalyzes it</ref>
* Loop of Henle Descending: The liquid passes from the thin descending limb to the thick ascending limb. Water is constantly released via osmosis.<ref>The semipermeable membrane of the thin descending limb does not permit passage of ions or large dissolved molecules</ref>{{citation needed|date=August 2015}} Gradually there is a buildup of osmotic concentration, until 1200&nbsp;mOsm is reached at the loop tip, but the difference across the membrane is kept small and constant.
:For example, the liquid at one section inside the thin descending limb is at 400&nbsp;mOsm while outside it is 401. Further down the descending limb, the inside concentration is 500 while outside it is 501, so a constant difference of 1 mOsm is kept all across the membrane, although the concentration inside and outside are gradually increasing.{{citation needed|date=August 2015}}
* Loop of Henle Ascending: after the tip (or 'bend') of the loop, the liquid flows in the ''thin'' [[Thin ascending limb of loop of Henle|ascending limb]].<ref>The thin ascending limb's membrane does not permit free passage of any substance including water.</ref>{{citation needed|date=August 2015}} Salt–[[sodium]] Na<sup>+</sup> and [[chloride]] Cl<sup>−</sup> ions are pumped out of the liquid<ref>[[Furosemide]] inhibits salt secretion from the thin ascending limb, while [[aldosterone]] catalyzes the secretion.</ref>{{citation needed|date=August 2015}} gradually lowering the concentration in the exiting liquid, but, using the [[#Countercurrent multiplier|countercurrent multiplier]] mechanism, always pumping against a constant and small osmotic difference.
:For example, the pumps at a section close to the bend, pump out from 1000&nbsp;mOsm inside the ascending limb to 1200&nbsp;mOsm outside it, with a 200&nbsp;mOsm across. Pumps further up the thin ascending limb, pump out from 400&nbsp;mOsm into liquid at 600&nbsp;mOsm, so again the difference is retained at 200&nbsp;mOsm from the inside to the outside, while the concentration both inside and outside are gradually decreasing as the liquid flow advances.

:The liquid finally reaches a low concentration of 100 mOsm when leaving the ''thin'' ascending limb and passing through the ''thick'' one<ref>Water or liquid with very low osmotic concentration leaving the nephrons is reabsorbed in the [[Peritubular capillaries]] and returned to the blood.</ref>
* [[Distal convoluted tubule]]: Once leaving the loop of Henle the thick ascending limb can optionally reabsorb and re increase the concentration in the nephrons.<ref>Reabsorbing and increasing the concentration is done by optionally absorbing [[potassium]] (K<sup>+</sup>) and [[hydrogen]] (H<sup>+</sup>) cations, while releasing water and the continued pumping out of calcium (Ca<sup>+</sup>) and salt (Na<sup>+</sup> and Cl<sup>−</sup> ions). The repeated concentration by secretion of calcium and salt ions is inhibited by [[thiazides]] and catalyzed by [[Aantidiuretic hormone]] and [[aldosterone]]</ref>
* [[Collecting duct]]: The collecting duct receives liquid between 100&nbsp;mOsm if no re-absorption is done, to 300 or above if re-absorption was used. The collecting duct may continue raising the concentration if required, by gradually pumping out the same ions as the Distal convoluted tubule, using the same gradient as the ascending limbs in the loop of Henle, and reaching the same concentration.<ref>[[Atrial natriuretic peptide]] and [[urodilatin]] inhibit water salt and calcium secretion from the collecting duct, while antidiuretic hormone and aldosterone catalyze it.</ref>
* Ureter: The liquid urine leaves to the [[ureter]].
* Same principle is used in hemodialysis within artificial kidney machines.