Editing Kidney (section)

===Formation of urine===
[[File:Physiology of Nephron.png|thumb|260px|Four main processes are involved in the creation of [[urine]].]]

====Filtration====
Filtration, which takes place at the [[renal corpuscle]], is the process by which cells and large proteins are retained while materials of smaller molecular weights are<ref>{{cite book | vauthors = Hall JE |title=Guyton and Hall textbook of medical physiology |date=2016 |publisher=Elsevier Health Sciences |location=Philadelphia, PA |isbn=978-0-323-38930-3 |edition=13th | page = 1129 }}</ref> filtered from the blood to make an [[Ultrafiltration (kidney)|ultrafiltrate]] that eventually becomes urine. The adult human kidney generates approximately 180 liters of filtrate a day, most of which is reabsorbed.<ref>{{Cite book |last1=Alpern |first1=Robert J. |url=https://books.google.com/books?id=w5nEg7VLEQ4C&pg=1405 |title=Seldin and Giebisch's The Kidney: Physiology and Pathophysiology |last2=Caplan |first2=Michael |last3=Moe |first3=Orson W. |date=2012-12-31 |publisher=Academic Press |isbn=978-0-12-381463-0 |pages=1405 |language=en |access-date=2022-07-28 |archive-date=2023-07-22 |archive-url=https://web.archive.org/web/20230722105802/https://books.google.com/books?id=w5nEg7VLEQ4C&pg=1405 |url-status=live }}</ref> The normal range for a twenty four hour urine volume collection is 800 to 2,000 milliliters per day.<ref>{{cite web|url=https://www.mountsinai.org/health-library/tests/urine-24-hour-volume|title=Urine 24-hour volume|website=mountsinai|access-date=21 November 2022|archive-date=21 November 2022|archive-url=https://web.archive.org/web/20221121180728/https://www.mountsinai.org/health-library/tests/urine-24-hour-volume|url-status=live}}</ref> The process is also known as hydrostatic filtration due to the hydrostatic pressure exerted on the capillary walls.

====Reabsorption====
[[File:2618 Nephron Secretion Reabsorption.jpg|thumb|Secretion and reabsorption of various substances throughout the nephron]]
Reabsorption is the transport of molecules from this ultrafiltrate and into the peritubular capillary network that surrounds the nephron tubules.<ref name="2024-Kumaran">{{cite journal |vauthors=Kumaran GK, Hanukoglu I |title=Mapping the cytoskeletal architecture of renal tubules and surrounding peritubular capillaries in the kidney |journal=Cytoskeleton (Hoboken) |volume=81 |issue=4–5 |pages=227–237 |date=2024 |pmid=37937511 |doi=10.1002/cm.21809 |url=}}</ref> It is accomplished via selective [[Cell surface receptor|receptor]]s on the luminal cell membrane. Water is 55% reabsorbed in the proximal tubule. Glucose at normal plasma levels is completely reabsorbed in the proximal tubule. The mechanism for this is the Na<sup>+</sup>/glucose cotransporter. A plasma level of 350&nbsp;mg/dL will fully saturate the transporters and glucose will be lost in the urine. A plasma glucose level of approximately 160 is sufficient to allow glucosuria, which is an important clinical clue to diabetes mellitus.

Amino acids are reabsorbed by sodium dependent transporters in the proximal tubule. [[Hartnup disease]] is a deficiency of the tryptophan amino acid transporter, which results in [[pellagra]].<ref name="Tao p 1">Le, Tao. ''First Aid for the USMLE Step 1'' 2013. New York: McGraw-Hill Medical, 2013. Print.</ref>

{| class="wikitable"
|-
! Location of Reabsorption !! Reabsorbed nutrient !! Notes
|-
| Early proximal tubule || Glucose (100%), amino acids (100%), bicarbonate (90%), Na<sup>+</sup> (65%), Cl<sup>−</sup> (65%), phosphate (65%) and H<sub>2</sub>O (65%) || 
* [[Parathyroid hormone|PTH]] will inhibit phosphate reabsorption.
* [[Angiotensin II|AT II]] stimulates Na<sup>+</sup>, H<sub>2</sub>O and HCO<sub>3</sub><sup>−</sup> reabsorption.
|-
| Thin descending loop of Henle || H<sub>2</sub>O || 
* Reabsorbs via medullary hypertonicity and makes urine hypertonic.
|-
| Thick ascending loop of Henle || Na<sup>+</sup> (10–20%), K<sup>+</sup>, Cl<sup>−</sup>; indirectly induces para cellular reabsorption of Mg<sup>2+</sup>, Ca<sup>2+</sup> || 
* This region is impermeable to H<sub>2</sub>O and the urine becomes less concentrated as it ascends.
|-
| Early distal convoluted tubule || Na<sup>+</sup>, Cl<sup>−</sup> || 
* PTH causes Ca<sup>2+</sup> reabsorption.
|-
| Collecting tubules || Na<sup>+</sup>(3–5%), H<sub>2</sub>O || 
* Na<sup>+</sup> is reabsorbed in exchange for K<sup>+</sup>, and H<sup>+</sup>, which is regulated by aldosterone.
* ADH acts on the V2 receptor and inserts [[aquaporins]] on the luminal side
|-
! colspan="3" |Examples of substances that are reabsorbed in the kidneys, and the hormones that influence those processes.<ref name="Tao p 1"/>
|}

====Secretion====
Secretion is the reverse of reabsorption: molecules are transported from the peritubular capillary through the interstitial fluid, then through the renal tubular cell and into the ultrafiltrate.

==== Excretion ====
The last step in the processing of the ultrafiltrate is ''excretion'': the ultrafiltrate passes out of the nephron and travels through a tube called the ''collecting duct'', which is part of the [[collecting duct system]], and then to the ureters where it is renamed ''urine''. In addition to transporting the ultrafiltrate, the collecting duct also takes part in reabsorption.