# Medullary Osmolarity Gradient --- The **medullary osmolarity gradient** is the steep [[concentration gradient]] maintained in the [[kidneys#Anatomy|renal medulla]] in order to effectively and powerfully concentrate [[urine]]. Due to this high gradient, as filtrate moves towards **minor calyx**, water is strongly [[osmosis|pulled out]] of it due to high osmolarity. The cortex of the kidney is at about 300 milliosmoles, and this increases and increases until deep in the medulla, near the renal papillae it reaches 1200 (or even 1400) milliosmoles. ![[nephron concentration gradient.png]] ## Countercurrent Mechanisms The **countercurrent mechanisms** are important to the function of the [[nephron]], because they help build up and maintain the steep [[osmosis|osmolarity gradient]] of the [[kidneys|renal medulla]]. They are made possible because the flow of fluid through two different tubes are opposite to each other. They are the *countercurrent multiplier* and the *countercurrent exchanger*. > [!tip] The nephron loop >Without the nephron loop, none of this would be possible. This is why we have them, and not just a straight line tubeule. ![[countercurrent flow.png]] ### Countercurrent Multiplier The **countercurrent multiplier** used the opposite flow between the two limbs of the [[nephron#Renal Tubule|nephron loop]] to allow the [[active transport]] of solutes up such a steep [[concentration gradient]]. Without this multiplier, even ATP-powered active transport would only be able to transport molecules up a concentration gradient of around 200 millimoles difference. In this case the osmolarity gradient would only be able to go from 300 mmols in the capillaries to 500 in the kidney medulla—but with the countercurrent multiplier it can reach up to 1200-1400 mmols. The juxtamedullary nephrons, with their long nephron loops, do the work of maintaining this steep gradient, thanks to the countercurrent multiplier. ### Countercurrent Exchanger The **countercurrent exchanger** uses the opposite flow **between the [[nephron#Renal Tubule|nephron loop]] and the vasa recta to maintain the medullary osmolarity gradient** by making sure reabsorbed water does not accumulate in the interstitial space, and solutes are not simply washed away by the [[nephron|peritubular capillaries]]. Water leaving (i.e. being reabsorbed by) the descending loop of Henle is picked right up by the vasa recta; likewise, salt ions leaving (i.e. being reabsorbed by) the ascending loop of Henle are also picked right up by the vasa recta. Solutes are in constant transport, so even though it gets picked back up pretty much right away, they're still in the interstitial *en route*, so the concentration gradient is maintained. ## Urea Recycling **Urea recycling** is a method of maintaining the osmolarity gradient by using the waste product [[urea]] as a medullary solute before letting it be sent down the collecting duct. Although ultimately urea is [[tubular reabsorption|secreted into the urine]], this also lets urea be concentrated in the urine to a really high level—essentially using a countercurrent multiplier. If it could do this it would not be able to concentrate itself too high, and we would need to produce a *lot* more urine to get rid of it all, because we produce a ton of urea. And as the point of the kidneys is not to get rid of water, so we don't want that. ___