ChloeYeo
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My text book says water is reabsorbed from the loop of Henle, DCT and the collecting duct, and that the regulation of water potential also takes place in these 3 parts of the nephron.
However while I was doing some past paper questions, one of the question asked this:
In the proximal convoluted tubule, the glucose, most of the ............................................... and
some of the salts are reabsorbed into blood ....................................................... that surround
the nephron at this point.

I don't understand the first part of this question. I wrote most of the vitamins but the mark scheme says ignore vitamin, and I don't know why. The mark scheme says water is the answer and urea can also be accepted. Why is this? Also, I thought water was only reabsorbed in loop of Henle/DCT/collecting duct because that was what it said in the text book. How come MOST of water is absorbed in PCT? urea as well.
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mgi
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(Original post by ChloeYeo)
My text book says water is reabsorbed from the loop of Henle, DCT and the collecting duct, and that the regulation of water potential also takes place in these 3 parts of the nephron.
However while I was doing some past paper questions, one of the question asked this:
In the proximal convoluted tubule, the glucose, most of the ............................................... and
some of the salts are reabsorbed into blood ....................................................... that surround
the nephron at this point.

I don't understand the first part of this question. I wrote most of the vitamins but the mark scheme says ignore vitamin, and I don't know why. The mark scheme says water is the answer and urea can also be accepted. Why is this? Also, I thought water was only reabsorbed in loop of Henle/DCT/collecting duct because that was what it said in the text book. How come MOST of water is absorbed in PCT? urea as well.
" most of vitamins" is such a vague term that's why and sounds like a guess!
The PCT reabsorbs 65% of the filtered water, Na+, Cl−, and K+.
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Jpw1097
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(Original post by ChloeYeo)
My text book says water is reabsorbed from the loop of Henle, DCT and the collecting duct, and that the regulation of water potential also takes place in these 3 parts of the nephron.
However while I was doing some past paper questions, one of the question asked this:
In the proximal convoluted tubule, the glucose, most of the ............................................... and
some of the salts are reabsorbed into blood ....................................................... that surround
the nephron at this point.

I don't understand the first part of this question. I wrote most of the vitamins but the mark scheme says ignore vitamin, and I don't know why. The mark scheme says water is the answer and urea can also be accepted. Why is this? Also, I thought water was only reabsorbed in loop of Henle/DCT/collecting duct because that was what it said in the text book. How come MOST of water is absorbed in PCT? urea as well.
Most of the water is reabsorbed in the PCT, as is Na, K, Cl, glucose and most other things, because the concentration gradient is highest here, and water follows the electrolytes.
However, the amount of water reabsorption in the DCT/collecting ducts can be altered through aquaporins. So while it is not the site of most water reabsorption, it is the site which fine tuned how much water reabsorbed, to maintain plasma osmolality/blood pressure.
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macpatgh-Sheldon
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(Original post by ChloeYeo)
My text book says water is reabsorbed from the loop of Henle, DCT and the collecting duct, and that the regulation of water potential also takes place in these 3 parts of the nephron.
However while I was doing some past paper questions, one of the question asked this:
In the proximal convoluted tubule, the glucose, most of the ............................................... and
some of the salts are reabsorbed into blood ....................................................... that surround
the nephron at this point.

I don't understand the first part of this question. I wrote most of the vitamins but the mark scheme says ignore vitamin, and I don't know why. The mark scheme says water is the answer and urea can also be accepted. Why is this? Also, I thought water was only reabsorbed in loop of Henle/DCT/collecting duct because that was what it said in the text book. How come MOST of water is absorbed in PCT? urea as well.
Good morning Chloe,

I would strongly suspect that this is an Edexcel Q as the mark scheme is ridiculous deficient and even incorrect [I hope for your sake that your school has not chosen for your class the Edexcel board, as I have come across a number of instances of this board emanating bovine faeces].

If I may elaborate, think about renal function [as with anything else in nature] logically. The substances that need to be reabsorbed from the "urine compartment" back into the blood are obviously those substances that the body needs i.e. you don't want to "throw them away" in the loo. Therefore, you can work out that just about ALL the glucose, the vast majority of the amino acids, a lot of water, and the electrolytes mentioned by the final year [I think now Jpw1097 ??] medical student are reabsorbed. Urea is NOT, as it is the main substance that the body needs to excrete nitrogen [the other being creatinine], so it needs to end up in the loo. [please correct me if my knowledge here is outdated, Jpw1097].

OK why in the proximal tubule? Well, most of the solutes are reabsorbed by active transport, and the proximal tubule cells [of the cuboidal cell endothelium] are equipped with numerous large mitochondria [to provide ATP] and numerous microvilli [to increase surface area].

Be safe!
M.
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Jpw1097
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(Original post by macpatgh-Sheldon)
Good morning Chloe,

I would strongly suspect that this is an Edexcel Q as the mark scheme is ridiculous deficient and even incorrect [I hope for your sake that your school has not chosen for your class the Edexcel board, as I have come across a number of instances of this board emanating bovine faeces].

If I may elaborate, think about renal function [as with anything else in nature] logically. The substances that need to be reabsorbed from the "urine compartment" back into the blood are obviously those substances that the body needs i.e. you don't want to "throw them away" in the loo. Therefore, you can work out that just about ALL the glucose, the vast majority of the amino acids, a lot of water, and the electrolytes mentioned by the final year [I think now Jpw1097 ??] medical student are reabsorbed. Urea is NOT, as it is the main substance that the body needs to excrete nitrogen [the other being creatinine], so it needs to end up in the loo. [please correct me if my knowledge here is outdated, Jpw1097].

OK why in the proximal tubule? Well, most of the solutes are reabsorbed by active transport, and the proximal tubule cells [of the cuboidal cell endothelium] are equipped with numerous large mitochondria [to provide ATP] and numerous microvilli [to increase surface area].

Be safe!
M.
Just finished fourth year, I’ll be intercalating next year.

It sounds counterintuitive but most urea actually is absorbed in the PCT, even though it is important to excrete.
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nexttime
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(Original post by macpatgh-Sheldon)
Good morning Chloe,

I would strongly suspect that this is an Edexcel Q as the mark scheme is ridiculous deficient and even incorrect [I hope for your sake that your school has not chosen for your class the Edexcel board, as I have come across a number of instances of this board emanating bovine faeces].

If I may elaborate, think about renal function [as with anything else in nature] logically. The substances that need to be reabsorbed from the "urine compartment" back into the blood are obviously those substances that the body needs i.e. you don't want to "throw them away" in the loo. Therefore, you can work out that just about ALL the glucose, the vast majority of the amino acids, a lot of water, and the electrolytes mentioned by the final year [I think now Jpw1097 ??] medical student are reabsorbed. Urea is NOT, as it is the main substance that the body needs to excrete nitrogen [the other being creatinine], so it needs to end up in the loo. [please correct me if my knowledge here is outdated, Jpw1097].

OK why in the proximal tubule? Well, most of the solutes are reabsorbed by active transport, and the proximal tubule cells [of the cuboidal cell endothelium] are equipped with numerous large mitochondria [to provide ATP] and numerous microvilli [to increase surface area].

Be safe!
M.
Urea is actually reabsorbed quite a lot, especially in a dehydrated state. Its too big an osmolyte not to be.

There used to be a school of thought that disproportionately high urea (vs creatinine) meant that the alternation in renal function must be due to dehydration. It isn't actually that reliable though.
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