The loop of Henle is also called the nephron loop. You have seen that it runs into the medulla (through its descending limb) and then back out (through its ascending limb). It turns out that the descending and ascending loop differ in their permeabilities. Unlike the PCT, the cells of the loop of Henle are not equipped with tons of protein channels to provide passage to all sorts of materials. Instead, they are quite specific.
You will see that their different permeabilities enables much more tubular reabsorption to occur.
The loop of Henle is mainly made up of simple squamous epithelium. The squamous cells are a little fatter than most, but much thinner than regular cuboidal epithelium like in the PCT and DCT.
The loop of Henle sticks into the medulla. So, the environment around the loop of Henle is the interstitial fluid of the medulla. However, the interstitial fluid in the medulla has an interesting arrangement. As you will see, lots of solute is removed from the filtrate in the medulla. When tons of solute is continually added to the deep medulla, the interstitial fluid in that area becomes extremely concentrated.
Because the solute is continually added to the deep medulla, there is no opportunity for diffusion to work fast enough to get rid of the higher concentration of solute in this area. Therefore, a high concentration of solute remains there. This is a much higher concentration of solute than was seen in the cortical interstitial fluid.
We can describe a solute gradient from the high solute concentration in the deep medulla to the much lower solute concentration in the cortex. That is shown in this picture. At least sort of!
When the filtrate left the PCT, it was isotonic with the interstitial fluid around the PCT, which is the cortical interstitial fluid. If the filtrate is isotonic with the cortical interstitial fluid, what is it to the medullary interstitial fluid? Is it hypertonic? hypotonic?
Think about this. Does the filtrate have a higher (hyper) or lower (hypo) concentration of solute than the medullary interstitial fluid.
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Therefore, when the filtrate runs through the loop of Henle, water will want to move across the tubule. You can probably figure that it will move out of the tubule because you know we are not done with reabsorption. But the reason that it moves out of the tubule is because the filtrate is hypotonic to the interstitial fluid, and WATER ALWAYS MOVES FROM HYPOTONIC TO HYPERTONIC.
What happens in the descending limb?
Because the descending limb is highly permeable to water, water moves by osmosis here. As described above, it moves out of the tubule and into the medullary interstitial fluid for reabsorption. It should be easy to imagine how come the descending limb epithelium doesn't allow the solutes across, right? The descending limb epithelial cells just do not have the protein channels on them.
What happens in the ascending limb?
In the ascending limb, the sodium-potassium pump is working like mad to keep shoving sodium out into the medullary interstitial fluid. This is a large part of the reason that the medullary interstitial fluid has its solute concentration gradient to begin with! You might think that if the cells shove sodium out, that potassium would be in low concentration in the interstitial fluid, but potassium ions are allowed to flow out passively through other channels. As the positive ions leave the ascending limb cells, the negative ions tend to follow, and that is why your book mentions that chloride ions also leave in the ascending limb.
Note that the ascending limb is NOT permeable to water. This is pretty unusual. Most cells allow some water to slip through at any time. These cells are specialized to prevent it. Even their tight junctions to one another are excessively tight so that water won't slip by between the cells.
Putting the descending and ascending limbs together...
The beauty of the loop of Henle is that each limb reinforces the other. By allowing sequential reabsorption, the actions within the loop are enhanced. Everytime fluid goes through the loop, the medulla becomes more concentrated. This continually provides an environment for the water to be drawn out of the tubule. This is enhanced even more by the vasa recta.
The vasa recta, the portion of the peritubular capillaries that overlies the loop of Henle, does not carry away much solute from the medullary interstitial fluid. In fact, it kicks out most of the sodium ions that it carries before ending in venules. So, the actions of the vasa recta also work to preserve the gradient set up in the medulla.
Keep this in mind...
In order to understand how the nephron works, you will need to understand osmosis and diffusion. I hope that you go back to look up and review osmosis... One of the hardest things to understand about osmosis is the fact that it is totally passive, yet we can force it to occur in the kidneys. How is that possible?
No matter what, water flows from hypotonic to hypertonic. That cannot change. So, if we want to force water to flow, we can force a change in solute concentration in one environment. So, if we shoved lots of solute into one compartment, water would flow into that (hypertonic) compartment. That is how the loop of Henle operates in the medulla.
© 2011 STCC Foundation Press