4. Producing a concentrated or dilute urine Flashcards
What are the vasa recta?
Capillaries that go deep into the medulla from the cortex
What are the vasa recta?
Capillaries that go deep into the medulla from the cortex
What is the normal osmolarity of the ECF?
300 mOsm/L when entering the kidney.
How does osmolarity in the tubule change?
Normal osmolarity of the extracellular fluid is 300 mOsm/L when entering the kidney.
This is reduced to about 100 mOsm/L by the time the fluid enters the distal convoluted tubule due to reabsorption of ions.
This osmolarity is further reduced in the DCT and collecting ducts as there is additional reabsorption of sodium chloride.
echo slide 5
Tubular transport in hydrated subject
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DCT: You have hypotonic solution. There is regulated NaCl reabsorption
What 2 factors do you need to make a concentrated urine?
- High level of ADH that allows the distal tubules and collecting ducts to become permeable to water and so reabsorbed.
- A gradient to pull this water out so there needs to be a high osmolarity of the renal medullary interstitial fluid.
How is the osmolarity in the medulla built up?
Passive absorption of ions across the epithelia of the thin ascending limb of the Loop of Henle
Active transport of sodium ions and co-transport of potassium, chloride and other ions out of the thick portion of the ascending limb of the loop of Henle.
Active transport of ions from the collecting duct.
Facilitate diffusion of urea from the medullary portion of the collection ducts into the medullary interstitium.
Diffusion of only small amounts of water from the medullary tubules into the medullary interstitium and far less than the reabsorption of ions that occurs there. This sets up an osmotic imbalance and gradient.
difference in permeability between thin descending and thin ascending limb?
TDL:
- H20
- No NaCl
- Urea
TAL:
- No H20
- NaCl +++++++
- Urea
Permeability to ions of thick ascending limb?
Active transport of NaCl ++
No H2O, NaCl and Urea
With the presence of ADH, which parts of the nephron become permeable to H2O?
DT (Distal tubule)
CCT (Cortical collecting duct)
IMCD (Inner medullary collecting duct)
Renal tubular transport in a dehydrated subject, stages?
- Descending limb: impermeable to salt, water moves passively
- Interstitial salt conc. increases as move towards hairpin bend because it is actively pumped out of the tubule
- Thin ascending:
Only found in long loops.
Passive Na+ movement out of the tubule.
No H2O movement - Thick ascending:
Active pumping against Na+ gradient.
No H2O movement - DCT: Removes ions due to active transport of NaCl and so the osmolarity of the tubule fluid continues to fall.
- Collecting tubule: H2O can move only if ADH present
How does the loop of henle generate an increasing conc gradient the deeper you go?
“Counter current multiplier”
Step 1. Fluid enters the loop of Henle from the PCT at 300 mOsm/L. The same as plasma.
Step 2. Active transport of ions from the thick ascending limb establishes a 200 mOsm/L gradient between the tubular fluid and the interstitial fluid. 400 mOsm/L occurs in the medullary interstitium compared to 200 mOsm/L in the ascending tubule fluid.
Step 3. The tubular fluid in the descending limb now equilibrates with the interstitial fluid as water moves out of the descending limb into the medullary interstitial fluid. Continued transport of ions but not water in the ascending limb maintains the gradient
Step 4. Flow of fluid into the loop of Henle from the PCT moves the fluid in the limbs on. The hyperosmotic fluid in the descending limb moves on into the ascending limb.
Step 5. Additional ions are pumped out of the fluid from the ascending limb until a 200 mOsm/L gradient is again established between the ascending limb tubule fluid and the medullary interstitium. This time the intersitial osmolarity rises to 500 mOsm/L and the ascending tubule falls to 300 mOsm/L.
Step 6. There is again a movement of water out of the descending limb of the loop of Henle to reach osmotic equilibrium with the medullary interstitial fluid. This increases the osmolarity of the tubule fluid in the descending limb up to 500 mOsm/L which moves on into the ascending limb for the processes of sodium and other ions movement to continue.
What is a benefit of the H20 reabsorption in the collecting ducts occurring in the cortex vs medulla?
In the presence of ADH the absorbed water is rapidly transported out of the kidney by the large blood flow through the kidney cortical peritubular capillaries.
This water absorption in the kidney cortex rather than the medulla helps preserve the osmotic gradient in the medulla.
Relevance of urea when forming concentrated urine?
Urea can contribute to the osmolarity of the medullary interstitial gradient when the kidney needs to form maximally concentrated urine in dehydration
At which points in the nephron is there movement of urea across the membrane?
Descending limb:
Urea conc rises as water is reabsorbed and urea moves into table due to passive and active secretion
Thick ascending limb, the DCT and cortical collecting ducts = Impermeable to urea.
Presence of ADH leads to further water reabsorption in these sections, so urea conc in tubule is higher.
Medullary collecting ducts: More water absorption, so urea is even more concentrated. High concentrations results in movement into medullary interstitial fluid (facilitated by specific urea transporters).
