Page Links:
This is often seen as one of the most complicated systems in the kidney! You are forewarned! But it is actually not so complicated (if explained properly …).
One of the functions of the kidney is to keep the body fluids at a reasonable level of osmolality (= amount of solutes in water).
The kidney regulates this osmolality by losing or retaining water, depending on what is necessary.
If, however your water concentration becomes too low (such as when you are sweating too much in a hot environment) then you want to reduce the loss of water in the urine.
The kidney does that by either producing diluted urine (light colour; more water than solute) or by producing concentrated urine (dark colour; little water).
Henle: Friedrich Gustav Henle (1809-1885) was a German physician, anatomist and pathologist (link: https://en.wikipedia.org/wiki/Friedrich_Gustav_Jakob_Henle)
Remember, the loop of Henle has a descending and an ascending limb.
The descending limb is permeable to water but not to salts (both thick and thin part). Therefore, the water can go out of the limb as it flows down the limb while the salts remain in the pre-urine.
Therefore, the pre-urine inside the descending loop becomes more and more concentrated; its osmolality increases.
In the thick ascending limb however, the wall of the loop of Henle is NOT permeable to water and the cells activelypump salt out of the pre-urine (Na+, K+, Cl–). Therefore, the osmolality of the pre-urine decreases while flowing upwards in the ascending limb.
SO, because salt is being transported out of the ascending loop, the interstitial fluid becomes more osmotic, and that will attract water from the descending limb!
But, if the osmolality of the pre-urine at the end of the ascending limb is equal to or even lower than that at the beginning of the loop, what then is the POINT of all this?
The whole point is that with this counter-current system, an osmotic gradient is created in the adjacent interstitium.
As you go down the descending limb, the pre-urine and the neighbouring interstitial fluid gets more concentrated.
In other words, because of the workings of the loop, the interstitium is getting more concentrated deeper into the medulla and less concentrated closer to the cortex.
14.
So, this is purpose of the loop of Henle; to create an osmotic gradient in the interstitium of the medulla, from the cortex down to the papilla. Why? Watch what happens at the collecting ducts! (link).
Some teachers then confuse things with the vasa recta. Indeed, its role is important but NOT in the creation of the medullary osmotic gradient!
All these capillaries together run along the loop of Henle, called the vasa recta (which is maybe a strange name as this is a loop and not a straight vessel (=recta!). Well, ok, two recta’s, one up and one down ;-))
Now, if these capillaries had been normal, then the blood would immediately take up all the excess salt in the medulla, take it away from the kidney and destroy the osmotic gradient!
What is so special about these vasa recta vessels is that the blood flow is very slow (= sluggish).
Because of this slow blood flow, NaCl will easily flow from the interstitium into the descending blood but will also easily leave the blood in the ascending blood. There is therefore a constant equilibrium between the blood and the local interstitium.
In other words, the osmolality of the blood at the end of the vasa recta is the same as at the beginning of the descending limb.
Search:
This website does not want to use copyrights or cookies etc
(see info) and … does not use AI, ChatGPD, etc. !! 😆
