The loop of Henle

https://youtu.be/3U4AhlOX3Tw

Here’s a quick summary of how the loop of Henle functions. I have separated out the two processes that, in combination, set up a salt concentration gradient in the kidney. I start by showing that active transport of salt from the ascending limb of the loop of Henle leads to a high salt concentration in the surrounding tissue. The salt concentration gradient is set up by the fact that the descending limb is permeable to water. This means that as fresh filtrate, with a lower concentration, continuously enters the descending limb, water diffuses out of the tubule to balance the salt concentrations inside and outside the tubule. This lowers the salt concentration in the cortex, but at the same time concentrates the filtrate as it moves down the descending limb into the medulla.

The low salt concentration in the cortex makes possible for the cells in the ascending limb to keep pumping out salt. They are pumping salt against a relatively low salt concentration gradient because the salt is continuously being cleared away (diluted) by water from fresh filtrate as it diffuses from the descending limb. If the tubule was not set up in a loop, the amount of salt that could be pumped out of the tubule would be limited by the high concentration of salt accumulating outside the ascending limb, with no means of clearing it away.

The high salt concentration in the medulla is important because, after the loop of henle, the tubule changes direction again and loops back down into the medulla as the collecting duct. In the medulla, water again diffuses out of the tubule to balance the salt concentration in the medulla. This concentrates the urine and conserves water. This time, however, the permeability of the collecting duct walls to water is dependent on water permeability channels (aquaporins) whose abundance in the cell membranes of the cells lining the collecting duct is controlled (increased) by antidiuretic hormone (ADH).

My video was inspired by this excellent video:

https://youtu.be/cYyJF_aSC6o

which goes into the mechanism in more detail and gives a really clear numerical explanation.

There are errors in the A level biology textbooks that make this all seem a lot more complicated than it really is. You can see these on twitter here

https://twitter.com/biologypictures/status/1320316042506342402?s=21

and here

https://twitter.com/biologypictures/status/1320279380862636038?s=21