Formation of concentration gradient and urine formation

Question 1

how is the concentration gradient formed?

Answer 1

Descending limb
Stx = thin (flat cells with dew mitochondria)
no active Na+ transport
water permeable

Ascending limb thin
no active transport, low water permeability, high salt permeability

Ascending Limb thick
Mitocondria, low passive permeability to water and salt, Active Na+/Cl-/K+ pump out to interstitum

essentially –> Na+ is pumped out of Ascending loop of henle into the vasta recta and the intersitum, thus creating an environment that osmotically pulls out fluid from the descending loop of henle. the vasta recta absorbs both and travels in a counter current flow, thus maintains the process.

the greater the loop of henle the higher the osmotic gradient

Question 2

Explain the osmolarity change as the filtrate goes through the nephron

Answer 2

after the PCT the volume of the fluid maybe lowered but the ratio of water to solute does not change so the filtrate entering the descending limb of the loop of Henle is isometric to plasma

at the descending limb of the loop of henle the limb the filtrate becomes hyperosmotic as the water has been dragged out of the limb

then after the ascending limb of the loop of henle the filtrate becomes hypo-osmotic as the ions have been pumped out

in the distal tubule more NaCl s pumped out which causes the intersitum to become more concentrated but doe not change the hypo-osmotic of the filtrate

As the filtrate travels down the urea becomes more concentrated because the H20 moves out due the increasing concentration of NaCl in the intersitum

Question 3

explain the vasta recta

Question 4

explain the process of Na+ being transported out of the lumen

Answer 4

This is an active force

1, Na+/K+/2Cl- co transport in the endothelial cell via the Na+ concentration gradient driving it
2, generating the gradient is active Na+/K+ pumps on the basolateral side of the endothelial cell pumping out the Na+ and K+ in ( but the K+ leaks back out)
3. K+ and Cl+ leak out into the interstitium

Question 5

explain urine reabsorption

Answer 5

When the body decides it needs to reabsorb the
the terminal inner medullary collecting duct (IMCD) is highly permeable to urea via specific urea transporters

Thus, from the thick ascending limb distally, urea remains in the tubule fluid until it reaches the terminal IMCD deep in the medulla.
Because the thin limbs of Henle’s loop are permeable to urea, the high interstitial urea concentration drives urea into the thin limb luminal fluid, hence recycling the urea within the medulla.

Question 6

Summary

Answer 6

Counter current multiplication establishes an osmotic gradient using active Na/Cl/K co-transport in thick ascending limb
Gradient established by the loop of Henle
Requires energy so actively maintained
Counter current exchange by the vasa recta prevents dissolution of osmotic gradient
Filtrate passing through collecting duct passes through osmotic gradient, allowing a concentrated urine to form when water channels (aquaporins) open