Mechanisms to produce dilute/concentrated urine (Monday 06/11/17)

Question 1

How much water at each part of the nephron does a hydrated/de-hydrated /over-hydrated person re-absorb?

Answer 1

1. PCT: 70%
2. Descending L.O.H: 20%
3. Ascending LOH: 0%
4. DCT: 5%
5. Collecting Tubule: 4%

So urine is 1% or 1.5 L/day

In over-hydrated/volume expanded person:

1. PCT: 70%
2. Descending L.O.H: 20%
3. Ascending LOH: 0%
4. DCT: 4%
5. Collecting Tubule: 0 to 4%

So can lose 6% of water as urine or 12 L/day

In dehydrated person:

1. PCT: 70%
2. Descending L.O.H: 20%
3. Ascending LOH: 0%
4. DCT: 5%
5. Collecting Tubule: 5% = re-absorbs more

So can have as little as 0.2% lost as water

Question 2

Describe tubular urea handling in the ascending limb of the LOH?

Answer 2

• An osmotic gradient is generated by urea.
• The thin ascending limb secretes urea. It is water impermeable (water can’t enter or leave). However it has a very high sodium chloride concentration, so sodium chloride moves out of the thin ascending limb, into the interstitium.
• The thick ascending limb is also impermeable to water. Even more sodium chloride is pumped from the tubule into interstitum.
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Question 3

What are the properties of the thin descending limb?

Answer 3

• Permeable to H2O
• Impermeable to NaCl and urea

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