Editing Loop of Henle (section)

==Physiology==
The descending loop of Henle receives [[Tonicity#Isotonicity |isotonic]] (300 [[milliosmole|mOsm]]/L) fluid from the proximal convoluted tubule (PCT). The fluid is isotonic because as ions are reabsorbed by the gradient time system, water is also reabsorbed maintaining the [[osmotic concentration|osmolarity]] of the fluid in the PCT. Substances reabsorbed in the PCT include urea, water, potassium, sodium, chloride, glucose, amino acids, lactate, phosphate, and bicarbonate. Since water is also reabsorbed the volume of fluid in the loop of Henle is less than the PCT, approximately one-third of the original volume.

The interstitium of the kidney increases in osmolarity outside as the loop of Henle descends from 600 mOsm/L in the outer medulla of the kidney to 1200 mOsm/L in the inner medulla. The descending portion of the loop of Henle is extremely permeable to water and is less permeable to ions, therefore water is easily reabsorbed here and solutes are not readily reabsorbed.  The 300 mOsm/L fluid from the loop loses water to the higher concentration outside the loop and increases in tonicity until it reaches its maximum at the bottom of the loop. This area represents the highest concentration in the nephron, but the collecting duct can reach this same tonicity with maximum ADH{{clarify|date=July 2023}} effect.<ref name=Kaplan.Physio>{{cite book|title=Kaplan USMLE Step 1 Physiology Lecture Notes|year=2011|author=Dunn R. B.|author2=Kudrath W. |author3=Passo S.S. |author4=Wilson L.B. |pages=209–223|chapter=8}}</ref>

The ascending limb of the loop of Henle receives an even lower volume of fluid and has different characteristics compared to the descending limb. In the ascending portion, the loop becomes impermeable to water and the cells of the loop actively reabsorb solutes from the luminal fluid; therefore water is not reabsorbed and ions are readily reabsorbed.  As ions leave the lumen via the Na–K–Cl symporter and the Na–H antiporter, the concentration becomes more and more hypotonic until it reaches approximately 100–150 mOsm/L.  The ascending limb is also called the diluting segment of the nephron because of its ability to dilute the fluid in the loop from 1200 mOsm/L to 100 mOsm/L.<ref name=Kaplan.Physio />

Flow of the fluid through the entire loop of Henle is considered slow.  As flow increases, the ability of the loop to maintain its osmolar gradient is reduced. The vasa recta (capillary loops) also have a slow flow as well. Increases in vasa recta flow wash away metabolites and cause the medulla to lose osmolarity as well. Increases in flow will disrupt the kidney's ability to form concentrated urine.<ref name=Kaplan.Physio />

Overall the loop of Henle reabsorbs around 25% of filtered ions and 20% of the filtered water in a normal kidney. These ions are mostly Na<sup>+</sup>, Cl<sup>−</sup>, K<sup>+</sup>, Ca<sup>2+</sup> and HCO<sub>3</sub><sup>−</sup>. The powering force is the [[Na/K-ATPase]] on the basolateral membrane which maintains the ion concentrations inside the cells. On the luminal membrane, Na enters the cells passively; using the Na–K–Cl symporter. Then the Na/K-ATPase will pump 3 Na out into the peritubular fluid and 2 K into the cell on the non-lumen side of the cell. This gives the lumen of the fluid in the loop a positive charge in comparison and creates a Na concentration gradient, which both push more Na into the cell via the [[Na–H antiporter]]. The hydrogen ion for the antiporter comes from the enzyme [[carbonic anhydrase]], which takes water and carbon dioxide and forms bicarbonate and a hydrogen ion. The hydrogen ion is exchanged for the Na in the tubular fluid of the loop of Henle.<ref name=Kaplan.Physio />