Water resorption

For "Water resorption, hyper- and hypotonic urine, Basic mechanisms of water reabsorption in tubules" see See Tubular Processes

Countercurrent multiplication system
To maintain the physiological level of osmolarity of body fluids, it is essential that the kidneys are able to produce hypertonic or hypotonic urine, depending on the body's current needs - if it is necessary to eliminate excess solutes or excess water. To do this, it is necessary to consistently separate the process of water and solute resorption, which takes place in the Henle loop.

For the ability of the kidneys to adjust the osmolarity of urine (in the osmolarity range of definitive urine 50-1200 mosm / l ) it is necessary to create a hypertonic medulla with an osmolarity gradient increasing towards the depth of the medulla. The process used to create the hypertonic marrow is called the countercurrent multiplication system. This, by repeated transfer of chloride and sodium ions along the course of the Henle loop, creates an osmotic gradient in the medullary interstitium, which is necessary for the concentration of urine as it flows through the collection channels.

The fluid leaving the proximal tubule still represents about 20% of the primary ultrafiltrate, from which the substances needed by the body have already been absorbed, the fluid is still isoosmotic with blood plasma. Elimination of such an amount of fluid could impair the stability of the body's internal environment. It is therefore necessary to reduce the volume of fluid excreted and increase the concentration of substances dissolved in it according to the current needs of the body. In the case of excess water, on the other hand, the osmolarity of the urine must decrease and the volume of excreted fluid must increase. The renal concentration apparatus therefore controls the volume and osmolarity of the excreted fluid and the concentration of the substances contained in it (the amount of definitive urine is called diuresis ). The renal concentration apparatus consists mainly of Henle's loop ,distal tubules, collecting ducts , vasa recta and interstitium of the renal marrow.

Vasa recta
In the vasa recta there is a countercurrent exchange of water, which is based on its permeability to water and the already mentioned hypertonicity of the medullary interstitium. The descending arms of the vasa recta descend deeper into the hypertonic marrow, where solutes enter and exit water. At the top, therefore, the blood has the same osmolarity as the interstitium and the tubular fluid in the bend of the Henle loop. The ascending arms point back to the environment with a lower concentration, so in this section they surrender NaCl from the blood and receive water. Part of the water changes from the descending to the ascending arm of the vasa recta and flows away, which increases the osmolarity of the blood in the vasa recta towards the papilla, in parallel with the increasing concentration of the osmolarity of the marrow. The blood flowing out of the marrow thus has almost the same osmolarity as the blood flowing into it. If this "bypass" of water did not occur, the water would pass out of the vessel at the top of the vasa recta (where the surrounding interstitium is highly hypertonic), reducing the osmolality of the marrow, thereby impairing its concentration. This arrangement thus minimizes the leaching of osmotically active substances from the interstitium.

In the deep layers of the marrow, osmolarity reaches values ​​that may be critical for erythrocytes in the vasa recta (especially the area around the apex of the loop, where the osmolarity of the blood equals the high osmolarity of the interstitium). Therefore, part of the erythrocytes are diverted in the proximal floors of the descending arm by a direct junction to the ascending arm.

The descending arm of Henle's loop
The thin part, where the epithelial cells are relatively flat, shows almost no transport activity. It is very permeable to water, which moves in the direction of the osmotic gradient into the interstitium of the marrow and further into the vasa recta , which flows away. As a result, the tubular fluid is increasingly concentrated and at the apex of the loop has an osmolarity almost identical to the surrounding hypertonic marrow. The ions hardly pass through here, so the tubular fluid contains more NaCl and less urea (the osmolarity of the interstitium and tubular fluid is the same, it is only the distribution of osmotically active substances). For urea, the descending arm is relatively permeable, but due to already balanced osmolarities and impermeability to ions, most of the urea remains in the interstitium.

A thin segment of the ascending arm of Henle's loop
This part is impermeable to water. The ions Na +, Cl - and urea pass very well here, so the concentrations of Na + , Cl - and urea , which diffuse along their concentration gradients, equalize. Thus, the osmolarity and volume of the tubular fluid still remain the same, only its composition changes to match the composition of the interstitial marrow.

The thick segment of the ascending arm of Henle's loop
Epithelial cells here show signs of high transport activity. This part is completely impermeable to water, but there is an active pumping of a number of solutes. Importantly, it is an active transport by Na + / K + -ATPase, not a free diffusion, which is not possible in this region, and the ions are not followed by water. In the tubular fluid, the osmolarity decreases to hypoosmolarity values ​​(200 mosm / l).

Distal tubule and collecting ducts
If osmoreceptors stored in the hypothalamus capture the increased osmolarity of the internal environment, ADH is released and the permeability of the apical membrane of the distal tubule and especially the water collection channel increases rapidly, thereby immediately increasing its resorption. The hypotonic fluid at the beginning of the distal tubule thus becomes isotonic during its course and then hypertonic in the collecting ducts in proportion to the osmolarity of the marrow (water progresses along an osmolar gradient). At low values ​​of systemic osmolarity, ADH output is low and the walls of the collecting ducts remain impermeable to water, water is not resorbed and thus a larger amount of hypotonic fluid is eliminated.