renal physiology question

[predentgal]

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Why are the lower portions of the loop of Henle and collecting ducts selectively permeable to Nacl, urea, and/ or water?

Answer: To increase the osmalarity of the interstitai fluid in the iner medulla and help prevent water loss from the nephronic filtrate.

I understand the first part of the answer, but the second part doesn't make sense. The point of urine formation is to make the urine more concentrated, so wouldnt water loss from the filtrate be promoted so that water could get absorbed by the body and not end up in the urine? Also in collecting duct when sodium leaves the duct, doesn't water follow?

 

[Ibracadabra]

Why are the lower portions of the loop of Henle and collecting ducts selectively permeable to Nacl, urea, and/ or water?

Answer: To increase the osmalarity of the interstitai fluid in the iner medulla and help prevent water loss from the nephronic filtrate.

I understand the first part of the answer, but the second part doesn't make sense. The point of urine formation is to make the urine more concentrated, so wouldnt water loss from the filtrate be promoted so that water could get absorbed by the body and not end up in the urine? Also in collecting duct when sodium leaves the duct, doesn't water follow?

There's one more thing about the loop of Henle. The descending limb is both permeable NaCl and Urea. The ascending limb is NOT permeable to water. However, the ascending limb has Na+ pumps that actively transports sodium from the filtrate to the interstitial fluid (I think the ascending limb also pump some urea out).

This action creates a higher concentration gradient in the interstitial fluid of the medulla. So in the descending limb, water runs out to the interstitial fluid(which the vasa recta picks up right away), while Na+ flows into the filtrate by the laws of diffusion(going down its conc. gradient). When the filtrate reaches the ascending limb, the sodium pumps now actively pump out Na+, making the filtrate hypotonic(but no water is lost in here from the filtrate) when it reaches the distal convoluted tubule.

The collecting duct has the job of water reabsorption, in case of when we need it. The concentration gradient of the interstitial fluid of the medulla is created by the loop of henle, as described above. The outside is hypertonic, so water wants to flow out from the collecting duct.
However, there are special water channels(called aquaporins) that allow water to flow out from the collecting duct--in other words, make the collecting duct permeable to water.

The aquaporins are activated by ADH(antidiuretic H. or vasopressin). Remember that the hypothalamus makes ADH and keeps it in the posterior pituitary to store it.

When the body needs more water: the posterior pituitary releases ADH=>ADH works on the collecting duct=> which inserts/activates aquaporins into the membrane of the collecting duct=>b/c of the concentration gradient, water flows OUT of filtrate and into the interstitial fluid=>it gets sucked up right away by the vasa recta into the blood system.

I tried to be specific, hope it helps!

 

[predentgal]

That was extremely helpful..Thanksss.. but what confused me was that the answer choice I chose was this:

To increase the osmalarity of the interstitai fluid in the iner medulla and help PROMOTE water loss from the nephronic filtrate.

Not prevent. You even said in the end that collecting ducts reabsorb the water FROM the filtrate so how is that preventing water loss from the filtrate? Am I misinterpreting the question?

 

[Ibracadabra]

That was extremely helpful..Thanksss.. but what confused me was that the answer choice I chose was this:

To increase the osmalarity of the interstitai fluid in the iner medulla and help PROMOTE water loss from the nephronic filtrate.

Not prevent. You even said in the end that collecting ducts reabsorb the water FROM the filtrate so how is that preventing water loss from the filtrate? Am I misinterpreting the question?

Sorry I misread your question I thought you were confused on the first part! My bad.

The lower portions (I think this term is quite confusing... I don't think the question is not being specific here) of the loop of henle and the ascending limb is impermeable to water and prevents loss of water FROM filtrate.
About the collecting duct, the permeability depends on the body needs of water reabsorption.
1. If you have enough water and want to produce hypotonic urine, you don't secrete ADH and have the collecting duct less/not permeable to water=> prevents water loss from filtrate.
2. If you need more water in the body and want to produce hypertonic urine, you secrete ADH and have the collecting duct permeable to water =>promoting the water flow from filtrate to blood.

I'm actually not 100% sure on this one but it seems that "selectively" permeable is the keyword of the question... But I see your point. I don't think the question is being specific to be honest.
I hope other SDNers can help more :'(

I think this concept of the loop of henle is quite hard actually to visualize nor explain.. you might also want to look up the "counter-current exchanger/multiplier"

 

[dentalWorks]

Why are the lower portions of the loop of Henle and collecting ducts selectively permeable to Nacl, urea, and/ or water?

Answer: To increase the osmalarity of the interstitai fluid in the iner medulla and help prevent water loss from the nephronic filtrate.

you see, the collecting duct is NORMALLY impermeable to water. This is an important piece cause without it, your urine would turn into solids and OUCH !!!!! (you see my point)

You have already established that the lower portions of the loop of henle produce a huge concentration gradient..... okay thats great.... this concentration gradient is capable of pulling MOST of the water out (thnx to osmosis) of the collecting ducts if they weren't impermeable to water and basically your urine would turn into solid salt crystals (OUCH!!!)

So this is ultimately why loop of henle and more importantly the collecting duct is impermeable to water.

but the like Ibracadabra explained, ADH is one of those hormones that promote water being reabsorbed from the collecting duct.... its usually the case when you get extremely thirsty, loose alot of blood, need to increase blood pressure, or few other reasons that aren't coming to mind

Hope this helps

 

[vindicate]

Why are the lower portions of the loop of Henle and collecting ducts selectively permeable to Nacl, urea, and/ or water?

Answer: To increase the osmalarity of the interstitai fluid in the iner medulla and help prevent water loss from the nephronic filtrate.

I understand the first part of the answer, but the second part doesn't make sense. The point of urine formation is to make the urine more concentrated, so wouldnt water loss from the filtrate be promoted so that water could get absorbed by the body and not end up in the urine? Also in collecting duct when sodium leaves the duct, doesn't water follow?

Here is how it works:

Filtrate passes through: PCT, DLoH, ALoH, DCT, Collecting duct.

.....1. PCT - permeable to water
AA + Glucose actively pumped out. If you don't pump it out (too much glucose in your blood, your diabetic and have sweet pee)
Na+, K+ is actively pumped out.

What this means: Water, through Osmosis, will diffuse to interstitial space. You keep what you need: salts, AA, and glucose.


.....2. DLoH - permeable to water, not permeable to salts and Urea

What this means: Water, through Osmosis, will diffuse to interstitial space.

.....3 ALoH (has thin and thick components)-
a. thin - permeable to urea, not permeable H20.
b. thick - not permeable to H20 and urea
Na+, K+ is pumped from thick portion.

What this means: Since H20 can't diffuse out, this makes interstitial region hyperosmotic (water wants to diffuse out, but it cant 🙁)
In other words: Filtrate is dilute (hyposmotic), interstitial space is concentrated (hyperosmotic).
Low osmolarity = high concentration of water.
High osmolarity = low concentration of water.

.....4. DCT - not permeable to H20 nor Urea, slightly permeable to Na+ and K+. Also actively transport Na+ and K+.

What this means: Filtrate is getting more dilute!


Now let me ask you two questions...
1. What is the goal of species that live on land? Conserve water and salts.
2. Where are we at now? We have dilute filtrate (meaning were going to lose a lot of water). But we have saved the salts.

.....5. Collecting ducts - permeable to H20, not permeable to Urea.

What this means: H20 will diffuse into interstitial space, filtrate becomes very concentrated.

Aldosterone (mineralcorticoid (mineral = Na+)) - increases absorption of Na in collecting ducts, which will make water diffuse out from collecting duct, into interstitial space.

ADH - makes collecting duct MORE permeable to H20, so you absorb more H20.


Now, your question was, why do we increase osmolarity of water in the interstitial space?

This is so that in the collecting duct, water will diffuse out from the duct into interstitial space.

"so wouldnt water loss from the filtrate be promoted so that water could get absorbed by the body and not end up in the urine?"

It is the ultimate goal. So what the nephron does is have selective permeability to salts, h20, urea, to create a high osmolarity interstitial space. At the final step, the permeability of collecting duct to h20 will uptake that water, making your urine more concentrated.

" Also in collecting duct when sodium leaves the duct, doesn't water follow?"

Yes. It does. This makes your urine even more concentrated.

So the Nephrons goal is to create a scenario in which, the interstitial space has a higher osmolarity than filtrate, and use osmosis, in a area where only H20 is permeable, as the final step. (to conserve water through diffusion)

 

[vindicate]

you see, the collecting duct is NORMALLY impermeable to water. This is an important piece cause without it, your urine would turn into solids and OUCH !!!!! (you see my point)

You have already established that the lower portions of the loop of henle produce a huge concentration gradient..... okay thats great.... this concentration gradient is capable of pulling MOST of the water out (thnx to osmosis) of the collecting ducts if they weren't impermeable to water and basically your urine would turn into solid salt crystals (OUCH!!!)

So this is ultimately why loop of henle and more importantly the collecting duct is impermeable to water.

but the like Ibracadabra explained, ADH is one of those hormones that promote water being reabsorbed from the collecting duct.... its usually the case when you get extremely thirsty, loose alot of blood, need to increase blood pressure, or few other reasons that aren't coming to mind

Hope this helps

This is wrong for several reasons:

1. Descending loop of Henle is permeable to H20
2. Collecting duct is permeable to H20
3. Never heard of solid salt crystals forming. Also, you do pump some K+ in at collecting ducts. Moreover urea will keep some water in your filtrate, so nothing will ever completely diffuse out.

 

[dentalWorks]

my response = blue

This is wrong for several reasons:

1. Descending loop of Henle is permeable to H20
I never said or indicated other wise

2. Collecting duct is permeable to H20
wrong.... I don't have my text book next to me to scan it, but wiki should help me out here:

3. Never heard of solid salt crystals forming. Also, you do pump some K+ in at collecting ducts. Moreover urea will keep some water in your filtrate, so nothing will ever completely diffuse out.
Well of course you've never heard of crystals forming, the whole point I was trying to make is that it DOESN'T happen.