Medical Muse

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The loop of Henle is responsible for concentrating urine through the countercurrent multiplier. Basically, the descending portion is permeable to water, but the ascending portion isn't. As solute is reabsorbed in the ascending limb, it pulls water out in the descending limb. This leads to a high concentration of solute in the medullary interstitium which helps water reabsorption and the formation of concentrated urine.
 

MrNeuro

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The loop of Henle is responsible for concentrating urine through the countercurrent multiplier. Basically, the descending portion is permeable to water, but the ascending portion isn't. As solute is reabsorbed in the ascending limb, it pulls water out in the descending limb. This leads to a high concentration of solute in the medullary interstitium which helps water reabsorption and the formation of concentrated urine.
and the increase in osm in the medullar interstitum affects the osmosis out of the collecting duct
 

MedPR

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Yes, and the other key thing to remember is that the deeper you go into the medulla (longer loop of henle) the more concentrated it gets. So JM nephrons produce more concentrated urine than cortical nephrons.
 
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SaintJude

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Yes, and the other key thing to remember is that the deeper you go into the medulla (longer loop of henle) the more concentrated it gets. So JM nephrons produce more concentrated urine than cortical nephrons.


Mhhm interesting, never occurred to me...Will certainly keep this in mind.
 

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The ascending loop has Na/K ATPase pumps that pump sodium out, so the ascending loop is more dilute, especially at the cortex side. (also called the single effect)

Yes, the concentration increases towards the medulla, and decreases towards the cortex. And yes the descending loop runs counter current with the ascending loop of Henle.

The descending loop is permeable to both water and ions (ions move in, increasing the concentration as it moves down the loop of Henle).

The collecting duct has variable permeability which is controlled by ADH. More ADH, more aquaporin channels = more reabsorption of water and more concentrate urine.

Less ADH, fewer aquaporin channels = less reabsorption of water and more dilute urine.

The collecting duct also moved countercurrent to the ascending loop... Increasing the concentration of solutes as it moves towards the inner medulla.
 

MedPR

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The ascending loop has Na/K ATPase pumps that pump sodium out, so the ascending loop is more dilute, especially at the cortex side. (also called the single effect)

Yes, the concentration increases towards the medulla, and decreases towards the cortex. And yes the descending loop runs counter current with the ascending loop of Henle.

The descending loop is permeable to both water and ions (ions move in, increasing the concentration as it moves down the loop of Henle).

The collecting duct has variable permeability which is controlled by ADH. More ADH, more aquaporin channels = more reabsorption of water and more concentrate urine.

Less ADH, fewer aquaporin channels = less reabsorption of water and more dilute urine.

The collecting duct also moved countercurrent to the ascending loop... Increasing the concentration of solutes as it moves towards the inner medulla.


I'm pretty sure the descending loop is permeable only to water.
 

Charles_Carmichael

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I'm pretty sure the descending loop is permeable only to water.

:thumbup: Descending only permeable to water & Ascending portion only permeable to salt
No.

The thin descending limb is permeable to both water and ions (NaCl). IIRC, it's also permeable to urea. The thick ascending limb is permeable to NaCl, but impermeable to water.

Hope this helps.
 

rjosh33

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No.

The thin descending limb is permeable to both water and ions (NaCl). IIRC, it's also permeable to urea. The thick ascending limb is permeable to NaCl, but impermeable to water.

Hope this helps.

No, MedPR is right. While it is permeable to urea, the descending limb is virtually impereable to ions, especially Na and Cl.
 

Charles_Carmichael

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No.

The thin descending limb is permeable to both water and ions (NaCl). IIRC, it's also permeable to urea. The thick ascending limb is permeable to NaCl, but impermeable to water.

Hope this helps.

No, MedPR is right. While it is permeable to urea, the descending limb is virtually impereable to ions, especially Na and Cl.
It's how I learned it here in med school and that's what is stated in major physio books, like Guyton and Costanzo. Directly from Guyton's (bolded mine):

"The descending part of the thin segment is highly permeable to water and moderately permeable to most solutes, including urea and sodium. The function of this nephron segment is mainly to allow simple diffusion of substances through its walls."

I think, for MCAT purposes, you'll be fine thinking of the descending limb as only permeable to water.

:shrug:
 

rjosh33

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It's how I learned it here in med school and that's what is stated in major physio books, like Guyton and Costanzo. Directly from Guyton's (bolded mine):

"The descending part of the thin segment is highly permeable to water and moderately permeable to most solutes, including urea and sodium. The function of this nephron segment is mainly to allow simple diffusion of substances through its walls."

Interesting. In my physio book (Saladin, 5th edition), it says, "Most of the descending limb is very permeable to water but not to NaCl; therefore, water passes by osmosis from the tubule into the extracellular fluid, leaving the NaCl behind."

Wikipedia states this too, stating the descending limb has low permeability to ions.

I think, for MCAT purposes, you'll be fine thinking of the descending limb as only permeable to water.

:shrug:

Agreed. The fact that the ascending limb is impermeable to water is probably the other important take home message, too.
 

MedPR

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No.

The thin descending limb is permeable to both water and ions (NaCl). IIRC, it's also permeable to urea. The thick ascending limb is permeable to NaCl, but impermeable to water.

Hope this helps.

Technically this is correct; the descending limb is very slightly permeable to ions. To put this in perspective, we think of our internet firewall as being impermeable to viruses because it keeps the vast majority of them out, but we all certainly know it is not a perfect security system. The same is true for the descending limb and ions.

For the MCAT, though, the descending limb is permeable to water and the ascending permeable to ions.
 
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MrNeuro

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Technically this is correct; the descending limb is very slightly permeable to ions. To put this in perspective, we think of our internet firewall as being impermeable to viruses because it keeps the vast majority of them out, but we all certainly know it is not a perfect security system. The same is true for the descending limb and ions.

For the MCAT, though, the descending limb is permeable to water and the ascending permeable to ions.

EK states
Proximal Tubule is where most reabsorption occurs.
Solute exchange and
-secretion
-(Na H antiporter)
-urea
-bile pigments
-antibitotics
-other drugs
-absorption
-permeable to water
-apical membrane secondary active transport responsible for reabsorption of nearly all glucose, most proteins, and other solutes

NET result of PT is to reduce the amount of filtrate in the nephron while changing the solute composition without changing the osmolarity

Descending Loop of Henle
-fxn to increase solute concentration, and thus osmotic pressure, of the medulla (?? when the say medulla are they referring to the ECF or the lumen of the nephron in the medulla...pretty sure its the latter just want to double check)
-has avery low permeability to salt so filtrate osm goes up
 

SaintJude

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Maybe our classification should be more focused on understanding where in the kidney active and passive water transport occurs?

So passive transportation of Na+ Cl- solutes only occurs in the thin ascending limb, whereas every where else active transportation of Na+ causes the passive diffusion of H2O (can anyone verify this to make sure that's correct?)

I think this is why Kaplan notes that water is not usually pumped at all. Instead the kidney move ions (mostly Na+ and Cl-) by active transport to create gradients that water will follow by osmosis.
 
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LeilaFay

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Active transport only occurs in the ascending loop.
Na/K ATPase pumping ions out.

Everything else occurs via osmosis/diffusion via electrochemical gradients and osmotic gradients.
 

Charles_Carmichael

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Maybe our classification should be more focused on understanding where in the kidney active and passive water transport occurs?

So passive transportation of Na+ Cl- solutes only occurs in the thin ascending limb, whereas every where else active transportation of Na+ causes the passive diffusion of H2O (can anyone verify this to make sure that's correct?)

I think this is why Kaplan notes that water is not usually pumped at all. Instead the kidney move ions (mostly Na+ and Cl-) by active transport to create gradients that water will follow by osmosis.

Active transport only occurs in the ascending loop.
Na/K ATPase pumping ions out.

Everything else occurs via osmosis/diffusion via electrochemical gradients and osmotic gradients.
I think you guys are going way too in-depth for MCAT purposes. Keep it simple and think of the general concepts of what happens at each segment of the nephron (ex. most reabsorption occurs in the proximal tubule, the loop of Henle is designed to concentrate the ECF in the medulla, the late distal tubule/collecting duct is where urine is concentrated/diluted, etc). For the most part, that should get you through the renal questions that show up. And, if it gets more detailed than that, it's likely going to be passage-based rather than a discrete. Renal physiology is very tough. They're unlikely to ask you discrete questions on the details of renal physio.

Also, LeilaFay, I'm not sure if you referring to active transport only in the loop of Henle section of the nephron or the entire nephron. If you're talking about just the loop, the ascending limb uses an Na+/K+/2Cl- pump to actively transport Na+ out of the lumen, not an Na+/K+ pump. If you mean the entire nephron, active transport occurs in every segment of the nephron, not just the loop of Henle.
 

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EK states
Proximal Tubule is where most reabsorption occurs.
Solute exchange and
-secretion
-(Na H antiporter)
-urea
-bile pigments
-antibitotics
-other drugs
-absorption
-permeable to water
-apical membrane secondary active transport responsible for reabsorption of nearly all glucose, most proteins, and other solutes

NET result of PT is to reduce the amount of filtrate in the nephron while changing the solute composition without changing the osmolarity

Descending Loop of Henle
-fxn to increase solute concentration, and thus osmotic pressure, of the medulla (?? when the say medulla are they referring to the ECF or the lumen of the nephron in the medulla...pretty sure its the latter just want to double check)
-has avery low permeability to salt so filtrate osm goes up


The PCT is completely different than the descending loop. The gradient exists because of the high osmolarity in the medulla. At the bottom of the loop of henle (where the descending loop turns into the ascending loop) the filtrate is isoosmotic to the medulla, so in a way you could say that they are talking about both the fluid in the medulla and the filtrate.
 

Morsetlis

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Are you sure this is relevant to the MCAT?

Just remember how fluids flow following regions of high tonicity through interfaces permeable to that fluid (in this case, water). Tonicity is a property conferred by particles that interact with water and cannot freely diffuse through that interface (if they did, they would draw water with them, and equilibriate over time).
 

DrRichand1

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did you know the osmolarity of the filtrate in the PCT does not change until it enters the deasending loop of henly. Hea that's cuz solutes and water are reabsorbing in the same proportion, decreasing the total volume but not the osmolarity. ONce in the DSLH the tuble is permeable to water and solutes. Filtrate concentration increases until the hairpin turn is reach, the out flow of water and inflow of ions is causes by interstitium gradient.
 

bluedevil61212

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I'm still don't quite understand why the filtrate is more concentrated. If water leaves in the descending limb and salt leaves in the ascending limb, then both water and salt decrease, so shouldn't concentration stay the same?
 

ThesecretCh0rd

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I'm still don't quite understand why the filtrate is more concentrated. If water leaves in the descending limb and salt leaves in the ascending limb, then both water and salt decrease, so shouldn't concentration stay the same?
Great question.

I think you maybe talking about different areas of the loop. At the beginning of the loop (end of the proximal tubule) the filtrate is the same osmolality as plasma. At the bottom of the loop it is the same osmolality as the medulla--but this is very "concentrated." At the end of the loop the filtrate is dilute--more dilute than it started.

When the filtrate descends down the loop, the filtrate becomes more concentrated because the walls of the descending loop are permeable to water and there is a high tonicity (lots of solute) outside causing water to be "pulled" out. At the bottom of the loop, the filtrate is very concentrated relative to the tonicity of the filtrate when it left the proximal tubule. I think this is where you are referring to the filtrate being concentrated.

In the ascending limb, the walls are impermeable to water, but there are numerous pumps and channels to allow ion transport (so Na, K, Cl, Mg, and Ca can escape the lumen, but water can't). The result is that at the end of the ascending loop, the filtrate is dilute--even more dilute than when it started at the end of the proximal tubule (not more concentrated as you wrote). This is why the ascending limb is sometimes called the dilution segment.

The fact that it is so dilute gives you lots of flexibility depending on your volume status. You could a) excrete very dilute urine to lose volume or b) re-collect much of that water in the presence of hormones (ADH). Additionally, although we are talking about "concentrated" and "dilute" it is key to remember that even though you are "dilute" at the end--you have less total water in the filtrate than you started off with…because a purpose of this system is to passively recollect water (and then to use pumps and channels to collect ions on the ascending limb).

Hope this helps.
Oh, and GTHD
 
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