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# Reverse osmosis effect in narrow tubes concentrates urine in the kidney
Contrary to normal ("forward") osmosis, reverse osmosis moves water against the osmolarity gradient, from the salt water side to the fresh water side. It allows the production of fresh water from salt water, and is the most widely used desalination method worldwide. Reverse osmosis is normally pressure-based, but, a different type of reverse osmosis takes place in narrow tubes (micrometer scale) that also moves water in the direction opposite to normal osmosis, and therefore allows the production of fresh water. Such an effect is the basis of how the kidney concentrates urine, and it takes place in the thin segment of the nephron (it is why the nephron has evolved a narrow segment. )
In the kidney, the narrow tube effect is amplified by pressure, and the kidney adjusts how much it wants to concentrate the filtrate by adjusting the glomerular filtration pressure. To do this, it has evolved the tubuloglomerular feedback system in the juxtaglomerular apparatus. The role of tubuloglomerular feedback is simply to regulate the strength of the reverse osmosis effect in the thin segment, and it acts both locally via renin and systemically via angiotensin to achieve sufficient filtration pressure. This allows the body to prevent dehydration.
The kidney thus concentrates urine by pressure, although the pressure itself is not the basis of the effect, it only amplifies the effect that is inherent to narrow tubes.
# The role of adsorbed water in the mechanism of osmosis
It was discovered in 2009 that the driving force for osmosis is the thin layer of adsorbed water that forms at hydrophilic surfaces. This adsorbed phase of water has the unusual (and unpredicted) property of being charge polarized, it releases protons into the surrounding water that gets positively charged, and itself gains a negative charge from the surplus of hydroxide ions it is left with. The force behind this charge polarization effect is analogous to the force in semiconductor junctions: diffusion, from thermal energy. Water in the adsorbed phase (that is a semi-solid phase that is halfway between liquid and solid phase) auto-ionizes just like liquid water, but contrary to water in the liquid phase the hydroxide ions are locked into the semi-solid mass that is anchored to the hydrophilic surface. The more mobile hydrogen ion will diffuse outwards to a larger extent than the hydroxide ion, causing the charge separation effect.
Osmosis happens when there is an asymmetry between the thickness of the adsorbate on either side of a membrane. The asymmetry will cause protons to "even out their concentration" around the combined negative mass of the adsorbates on either side, so that the charge distribution is balanced out. The relative surplus and deficit of protons that results, will cause hydroxide ions on the exosmotic side (where water is moving from) to break down into dioxide, water and electrons, and the electrons will transfer over to the endosmotic side where there is a surplus of protons, and combine with the protons and dioxide to form dihydrogen monoxide, water. Water itself does not physically "move", it is broken down and built up on either side of the membrane. Only electrons and protons move during osmosis. The effect is analogous to what takes place in an acid-base battery.
Salt impairs the formation of the adsorbed phase in the same way it impairs the solid phase by freezing point depression (a colligative property), and an increase in osmolarity on one side will decrease the amount of adsorbate at the membrane on that side, causing the effect of proton equilibration and the subsequent breakdown of hydroxide ions on the hypoosmotic side of the membrane. Pressure, acts in an opposite way to increase the thickness of the adsorbate, and therefore generates an osmotic transfer of water in the reverse direction to osmolarity-based osmosis, away from the pressure.
In narrow tubes, the proximity of the adsorbate around the circumference of the tube and across the diameter causes an increase in the net positive charge of the lumen of the tube, and this effect will be yet another factor in how the protons even out their concentration (charge wise) across both sides of the membrane of the tube. This provides a method for reverse osmosis that can operate at very low pressures, such as that within the filtration system of the kidney. Since pressure increases the amount of adsorbate, it also increases the repulsion effect in the lumen of narrow tubes, and this allows the kidney to regulate the strength of the urine concentration mechanism by regulating the filtration pressure.
# The loop of Henle recirculates dioxide, the cathode in osmosis
The glomerular filtration tubular reabsorption pattern is physiologically complicated, and the peculiar and intricate loop design of the nephron has motivated a physiological model that explained why such as loop had been selected for, evolutionarily. This motivated the theory that the kidney operated by forward osmosis (osmolarity-based) and generated an osmolarity gradient using a counter-current multiplication mechanism, and that the loop of Henle had evolved to facilitate this process. The true reason for the loop of Henle is much more directly tied to the mechanism of osmosis: it recirculates the dioxide that is required for the cathode reaction in osmosis, so that the reverse osmosis process in the thin segment can reabsorb more water than would otherwise be possible with a single-pass of the dioxide.
The recirculation of dioxide is complicated and it explains the complicated patterns of reabsorbtion in the kidney. Dioxide is relased into the nephron in the thin segment (thin descending limb) when water is reabsorbed by reverse osmosis (and transported out of the kidney via the ascending vasa recta. ) This dioxide has to be returned into the medulla and the blood stream, or it will be extreted in the urine. To reabsorb the dioxide, the kidney uses osmosis all over again, and moves water back into the ascending thin limb of the loop of Henle. This seemingly counterproductive arrangement, where there is no net osmotic transfer of water, has evolved to faciliate the non-osmotic reabsorbtion of water from the collecting duct system. The reverse osmosis in the thin descending limb dehydrates the medulla, and favours the diffusion of water from the collecting duct tree and concentrates the urine before it leaves through the papilla. The water reabsorbed from the collecting duct, is in a 1:1 relationship with the water that is osmotically transferred into the thin ascending limb. The water that loops around from the collecting duct to the ascending thin limb and back again, faciliates the recirculation of dioxide via the vasa recta (that moves in the direction opposite to the loop of Henle. )
# Tubuloglomerular feedback is subordinated vasopressin, and operates reflexively to decisions by the hypothalamus
Tubuloglomerular feedback responds to vasopressin, and vasopressin is ultimately the control mechanism for urine concentration, thereby subordinating the kidney under the hypophysis and the brain. The role of vasopressin is to increase the resistance in the collecting duct tree, by contracting fibroblast-like cells in the kidney medulla, and also to increase water permeability of the collecting duct epithelium by aquaporin channels. The tubuloglomerular autoregulatory system responds to an increase in water reabsorbtion from the collecting duct, by sensing a decrease in filtrate osmolarity at the macula densa region of the thick ascending limb, and upregulates the reverse osmosis mechanism in the thin descending limb of the loop of Henle by increasing the glomerular filtration pressure through renin and angiotensin. Or, rather, it senses only osmolarity increase (concentrating urine more than what is required by vasopressins effect), and release of renin is the default and it gets downregulated whenever macula densa senses that the urine is concentrated more than it should be.