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Summary Chapter 29; Urine Concentration and Dilution; Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
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Summary of reabsorption in renal tubules
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Chapter 29; Urine Concentration and Dilution;Regulation of Extracellular Fluid Osmolarity and
Sodium Concentration
When a large amount of water is consumed, urine becomes very dilute. That is, the
concentration of solutes in urine decreases. However, the AMOUNT of solutes being
excreted remains constant.
The proximal tubules have an osmolarity similar to that of plasma, 300 mOsm/L.
The early distal tubules are hypo-osmotic, and have an osmolarity of around 100
mOsm/L.
When water levels are low, urine can be concentrated up to 1200-1400 mOsm/L.
We need to excrete at least 600 milliosmoles of metabolic waste solutes per day (e.g.
urea). And if urine can be concentrated to 1200 mOsm/L, that means that at the very
least, we have to excrete 600/1200 = 0.5 L of urine every day. This is called the
‘obligatory urine volume’.
Seawater has a NaCl osmolarity of 1200 mOsm/L. This, in addition to metabolic waste
means that 1200+600 = 1800 milliosmoles would have to be excreted. Therefore, for
every liter of seawater, the kidneys will excrete 1.5L (1800/3) of urine, leading to net
loss of fluid and so, dehydration.
The specific gravity of urine 1.002-1.028 g/ml.
Counter current multiplier mechanism:
1. The thick ascending limb of the loop of Henle actively transports ions out, but is
impermeable to water. This makes the urine in this portion dilute, while making
the medullary interstitium concentrated.
2. Urine entering the descending loop from the proximal tubules have an
osmolarity of 300 mOsm/L.
3. The descending loop isn’t impermeable to water. This means that the
concentrated medullary interstitium causes a lot of water to be reabsorbed by
osmosis, leaving the urine concentrated.
4. This urine enters the thick limb, where again solutes are reabsorbed but water
isn’t.
5. This makes the interstitium even more concentrated, causing even more water
reabsorption from urine entering the descending limb.
6. Thus, the cycle continues on and on (multiplying) until the urine has been
concentrated up to 1200-1400 mOsm/L.
, Urea Circulation:
1. Urea is mostly reabsorbed in the proximal tubules (40-50%).
2. Most of the rest of the tubular system doesn’t reabsorb urea, up until urine
reaches the inner medullary collecting ducts.
3. Here, urea is reabsorbed into the medullary interstitium to some extent (a lot of
if ADH levels are increased) by diffusion via the urea transporters UT-A1 and UT-
A3.
4. The urea in the interstitium diffuses (is secreted) into the thin loop via UT-A2.
5. This urea travels again all the way to the inner medullary collecting ducts,
where again a portion is reabsorbed and recirculated.
6. This process makes the urine concentrated and the renal medulla hyperosmotic.
The vasa recta are specialized blood vessels.
When the blood is descending, solutes move into it, and water moves out into the
interstitium. The opposite happens when the blood is ascending through them. They
both almost cancel each other out and the medullary interstitium remains
hyperosmotic.
The blood in the vasa recta also moves quite slowly, in order to reduce solute loss.
Concentration management of urine throughout the tubules:
1. The proximal tubules are able to reabsorb about 65% of the substances. A lot of
water is reabsorbed via the AQP-1 (aquaporin I channels).
2. Reabsorption in the descending limb will depend on how concentrated the
medullary interstitium is, which partly depends on how much urea was
reabsorbed from the collecting ducts, which depends on the levels of ADH.
3. NaCl is reabsorbed in the thin ascending limb.
4. Osmolarity drops to about 100 mOsm/L in the thick ascending limb, and further
down to 50 mOsm/L in the early distal tubule, due to no water reabsorption.
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