Loop of henle

What you have so far seems to make sense....from what I can remember about the loop of henle.

You should add that SODIUM chloride is pumped out by active transport into the surrounding tissue fluid, not just chloride ions! Sodium is pumped out first before the chloride ions, therefore sodium chloride is actually pumped into the surrounding tissue...this would significantly lower the water potential gradient.

The vasa recta are specialised capillaries that absorb the water, I really don't think you need to know more than that for A level syllabus. The fluid will have a lower concentration of sodium chloride because it has been pumped by active transport out of the filtrate.

It will have a higher concentration BY PERCENTAGE of urea because a lot of water and sodium chloride has been removed. The urea has always been there it just now has a higher percentage concentration, due to the loss of other substances.

Hope that helps I can remember the grief that the kidney and loop of henle gave me at A level.

hypertonic = more concentrated than plasma
hypotonic = less concentrated than plasma.
osmolarity/osmotic pressure/Osm/pressure = simply a measure of 'concentration' - higher it is, higher the 'concentration'.

The loop of henle's function is to create a high medullary interstitial (tissue fluid) osmotic pressure. It achieves this by uncoupling water and salt reabsorption (it absorbs about 25% of the soulte and 10% of the water of the original glomerular filtrate (GF)). Consequently there is always a hypotonic tubular fluid produced at the end of the loop but a hypertonic medulla.

Start with the ascending limb:

It has a Na+/2Cl-/K+ transporter on the luminal surface which is driven by the concentration gradients of Na set-up by the Na/K ATPase. There are channels which allow the relevant ions (Cl- and Na+) to exit into the interstitium via the basolateral surface of the cell. Both Cl- and Na+ are transported actively in this way, but Na+ is also transported by a resulting transepithelial gradient (set-up by the active processes).

Cells in the ascending limb have tight junctions between them which prevent water from leaking through - so water permeability is minimal. Therefore, this part of the loop of henle has almost single handedly achieved what is required, increased salt reabsorption with very little water reabsorption. However, this mechanism can only generate a relatively small osmotic gradient of around 200 between medulla and tubule ie. Imagine that all interstium (tissue fluid) and tubular fluid (what is in the loop) is at 400 Osm initially. This would result in concentrations of 500 (medulla) and 300 (tubule) respectively after action of the acsending limb.

What is required is a method of converting this small transverse gradient into a large lonigtudinal gradient - this is essentially what a counter-current multiplier is. This is achieved in conjunction with the descending limb. It is water permeable and does not actively transport solute. The medulla is hypertonic (because of the ascending limb's activity - 500 in our example) and thus water is drawn out of the descending limb (tubular fluid is arriving at 400), raising the osmolarity of the tubule fluid from 400 to say nearer to 500. This fluid now enters the ascending limb at 500 Osm (instead of 400 as before), and as we said, we can generate a medullary-tubule pressure difference 200 (max transverse gradient), so we now achieve 600 Osm (instead of 500) in the medulla as the fluid passes through the ascending limb.

So the counter-current multiplier allows us to achieve a very much higher medullary interstitial pressure.

Try rawing out a LoH with these concentrations and you'll see what i mean.

Concerning the urea, the concentration of urea becomes higher because the loop is relatively urea impermeable (at least when there is little ADH) and water is leaving - so its like having more solute in less water => higher concentration. This effect is more true of the whole tubule rather than just the loop of henle because of its relatively low water reabsorption (10% of GF).

The OP said that they didn't follow what they had in the first post. Going into more scientific depth isn't going to help that; just boost your ego.

That said, I do like the advice of drawing it out and annotating.

Firstly, sometimes you need to go into a little depth to understand something well. Depth doesn't necessarily equate to complexity. Its actually very useful to think of the kidney in the way that I have because if you understand what it is trying to achieve, you can see how the components work. I also tried to explain all the language I had used. Because the OP mentioned Cl- ion movements and what they said was wrong, I felt obliged to correct it.

Finally, what you said was wrong so thats the most useless of all.

Depth doesn't necessarily equate to complexity. Maybe not, but in this case it equates to how easy it is to understand. The deeper you go into something, generally the more difficult to understand and the more components to think about. The level of depth you went into is unlikely to help someone if they don't really understand the basics (ie: those statements mentioned in the first post)

I doubt the OP understands more about it now.

Well if that is the case, then that wasn't intended. IMO, as someone whoes been through alevel, I felt the explanation I gave, if a bit in-depth, explains quite well the whole process.

Indeed it does. Rep for you tomorrow, that was very interesting to read.

Hi could you please explain the loope of Henle for me in detail as I do not really understand it right know