Renal IV Flashcards
How much water is filtered and reabsorbed?
It is freely filtered but 99% is reabsorbed
Where does the majority of water reabsorption occur?
The proximal tubule
Where does hormonal control of reabsorption of water occur?
In the collecting duct
Explain how water reabsorption works in the proximal tubule.
It depends on Na+ reabsorption. Since Na+ has been moved from the tubule to the interstitial fluid, the osmolarity of the tubule has INCREASED and the osmolarity of the interstitial fluid has DECREASED.
This osmolarity gradient drives water from the tubular lumen to the interstitial fluid. This can occur paracellularly and transcellularly
The water along with sodium and other things dissolved in it will then enter the peritubular capillaries via bulk flow.
Does water reabsorption happen transcellularly or paracellularly?
Both.
When water intake is small, the kidney will reabsorb [more/less] water. When water intake is large, the kidney will reabsorb [more/less] water.
More, less
Where does dynamic regulation of water reabsorption in response to intake occur in the tubule?
This takes place in the collecting duct.
What are the 2 components of the dynamic regulation of water reabsorption in the CD?
- High osmolarity of the medullary interstitium
- Permeability of CD to water (regulated by vasopressin)
What is the maximum concentration of the urine in the kidney?
1400 mOsm/L
Where does urinary concentration take place?
It takes place as tubular fluid flows through the medullary collecting ducts.
Urinary concentration depends on […]
The hyperosmolarity of the medullary collecting duct interstitial fluid.
Medullary interstitial fluid becomes hyperosmotic through the […], which occurs in […]
Countercurrent multiplier system, Henle’s loop
What is countercurrent flow?
It is the flow of fluid down and then top the descending and ascending limbs respectively of Henle’s loop.
Describe the ascending and descending limbs of Henle’s loop:
a) Reabsorption of NaCl
b) Permeability to water
Descending loop:
a) Does not reabsorb NaCl
b) Permeable to water
Ascending loop:
a) Actively reabsorbs NaCl
b) Impermeable to water
Describe how the countercurrent multiplier system works prior to a move.
- Isotonic water (300 mOsm) flows into the descending limb
- Once it reaches the ascending limb, NaCl gets reabsorbed. This leaves the water hypotonic and the interstitial fluid hypertonic.
- Since the descending limb is permeable to water, water will flow out and into the interstitial fluid to balance the osmolarity gradient.
Result: descending limb - 400 mOsm, ascending limb 200 mOsm, interstitial fluid - 400 mOsm
Describe how the countercurrent multiplier system works after a move (assume hypertonic descending limb = 400 mOsmand hypotonic ascending limb = 200 mOsm)
- New isotonic water (300 mOsm) flows into the descending limb. The hypertonic water (400 mOsm) has been partially shifted into the ascending limb. The interstitial fluid is still hypertonic (400 mOsm).
- There is now an osmolarity gradient in the descending limb (300 -> 400) and in the ascending limb (400 -> 200)
- Na+ will move out of the ascending limb into the interstitium at the bottom, causing it to become more hypertonic.
- Water will then move out of the descending limb, increasing its osmolarity to match the interstitium.
Describe the steady state of the countercurrent multiplier system.
The top of the loop is isoosmotic and it gets more hyperosmotic as you go down, eventually reaching 1200 mOsm. The ascending limb is more hypoosmotic than the interstitum and descending limb, which are equal.
In the countercurrent multiplier system, where do active transport and diffusion occur?
Diffusion occurs on the descending limb, while active transport occurs on the ascending limb.
What is the vasa recta? Describe its appearance and location.
The vasa recta is the blood vessels in the medulla. It has a hairpin-like structure that is very similar to that of the loop of Henle.
What is the function of the vasa recta?
it minimizes excessive loss of solute from the interstitium in the countercurrent multiplier system.
What is the function of urea in water balance in the urine?
Similar to NaCl, it also contributes to medullary hyperosmolarity.
What is the consequence of the hyperosmolarity of the interstitial fluid in the medulla?
Water wants to flow out of the tubule. However, they can only happen if the tubule is permeable to water.
Water permeability in the tubule is regulated by […]
The amount of aquaporins in the plasma membrane
Describe the permeability of different parts of the tubule to water reabsorption. Include whether this state is permanent or not.
Proximal tubule: 67% (always permeable)
Descending limb: 15% (always permeable)
Ascending limb: never permeable
Distal tubule: never permeable
CCD and MCD: depends - permeability is subject to hormonal control via vasopressin.
