Week 4 - Volume Control Flashcards
What percentage of sodium and water is reabsorbed in the thin descending limb of LoH?
No sodium
10-15% water
Hyperosmotic filtrate
What percentage of sodium and water is reabsorbed in the ascending thin and thick limb of LoH
Sodium is 25%
Water is none
Starts to make hypoosmotic
What percentage of sodium and water is reabsorbed in the distal convulated tubule?
Sodium about %5
Water is none
So more hypoosmotic
What percentage of sodium and water is reabsorbed in the collecting duct system?
About 3% sodium
Water varies depending on dehydration/water loading levels
Explain reabsorption in s1 of the proximal convoluted tubule
The sodium potassium atpase on the basolateral membrane sets up a gradient for sodium to move from the lumen in to the cell dragging glucose with it via a symporter. The glucose will continue down its concentration gradient in to the interstitium and then into the peritubular capillary. The sodium that has been moved by the 3sodium2potassium atpase in to the interstitium also moves along concentration gradient to capillary. Potassium leaks back out of cell.
NaH which pumps h out of cell therefore setting gradient to allow organic cation antiporter.
Amino acids lost
Concentration of urea and chloride increase compensating for loss of glucose providing a concentration gradient for cl reabsorption in s2/3
What happens in the PCT at s2/3?
Basolateral is sodium potassium atpase
Which pulls chloride through from lumen to cap (bc of sodium)
Chloride also moves paracellulary due to concentration gradient.
No glucose sodium symporter so NaH exchanger moves sodium out of lumen for use by nak atpase
Water wants to move in to the capillarys because of the high oncotic pressure in peritubules.
What is the point of the glomerulotubular balance system?
2nd line of defence - blunts sodium ones to any GFR changes which occur despite autoregulation. Always reabsorb a proportion or percentage of the sodium rather than a concentration.
What is count current multiplication?
Descending limb - has aquaporin channels but no sodium potassium choride channels. Water leaves because interstium is at a higher osmolarity (due to ascending limb)
As filtrate reaches the tip of LoH it is increasingly concentrated.
As filtrate passes in to the thin ascending limb, aquaporins disappear meaning there will be a passive movement of solutes down concentration gradient.
The thick ascending limb has sodium potassium chloride channels which actively pump ions out therefore increasing osmolarity in interstitium which makes full circle - drawing water out of descending limb
Along with the NaKCC2 channel in the thick ascending limb, what other channels are important on both apical and basolateral sides
On apical side there is the NaKcc2 which drives 2 chloride, 1 potassium and one sodium in to cell. There is a ROMK on apical which allows potassium back in to lumen. There are chloride channels on basolateral side to allow chloride out in to capillary. Along with nak atpase which drives the sodium in the first place.
Why is the location of the ROMK important in the thick ascending limb?
It is located on the apical membrane and It drives potassium back from the cell in to the lumen allowing the nakcc2 to continue to work.
Which part of the nephron uses the most energy?
The TAL. Therefore particularly sensitive to hypoxia.
The fluid leaving the loop of henle is what?
Hypoosmotic compared to plasma because all th solutes have been pumped out in the TAL.
Explain the different transporters in the distal convoluted tubule
Nak atpase drives
Sodium chlorid of ncc transporter on apical membrane.
Choride channel on basolateral membrane
Also important site for calcium reabsorption, therefore sodium calcium exchanger, sodium in to cell and calcium in to capillary. Couple to a proposed calcium channel on the apical membrane. Affected by parathyroid hormone.
Filtrate is therefore further diluted, more hypoosmotic
Explain what happens in that distal convoluted tubule and the collecting duct?
Fine tuning. Two cell types, principle = 70% responsible for reabsorption of sodium via ENaC. Type B intercalated cells which reabsorb chloride actively and secret hydrogen and bicarbonate.
There is also a potassium channel on the apical membrane which potassium moves through in to the lumen
Again all driven by nak atpase.
Variable water uptake through aquaporin, dependent on ADH.
What percentage of the sodium AND WATER is filtered in the PCT
67 % sodium
65 % water
Results in iso osmotic reabsorbtion
