Urine concentration

1. Loop of Henle
2. Countercurrent multiplier and distal tube
3. Variable reabsorption of water
4. mechanisms allowing urine to be concentrated to an osmolarity higher than that of plasma

Annotations:

• a countercurrent system is a system which expends energy to create a concentration gradient. e.g. production of hyperosmotic urine by the kidney.

1. loH essential for formation of concentrated urine
2. maximum urine concentration is related to length of loH
3. longer loH = higher concentrated urine
   1. loH function - recover water and sodium chloride from urine

1. Thick ascending limb
   1. impermeable to water and urea. NaCl actively reabsorbed
2. Thin descending limb
   1. Permeable to water
3. Thin ascending limb
   1. Impermeable to water. Permeable to Na+ and urea
4. urine in descending limb starts = 290 mosmol, in ascending limbs finishes 90 mosmol
5. Counter current flow - urine flow opposite to blood flow of capillaries supplied to tube
6. In ascending limb - movement of NA+, Cl- and K+ out of tubular lumen/into capillaries using ATPase's. Osmolarity decreases ~ 100 mosmol/L
7. In descending limb - movement of water into capillaries/ out of tubular lumen concentrates urine = increases osmolarity > 1000mosmol/L
8. Filtrate HYPERTONIC at turning point
9. Fluid HYPOTONIC in ascending limb
10. Fluid ISOTONIC as enters descending limb
11. Isotonic, hypertonic etc filtrate = compared to capillaries supplying it
    1. osmolarity estabilished in renal medulla

1. countercurrent blood supply - opposing flow direction to filtrate
2. prevents 'wash out' of solutes
3. 'picks up' water removed in the loH
   1. also, picks up Na, Cl and K leaving ascending limb

1. Low and constant permeability to water
2. Na+ and Cl- reabsorption into capillaries.
   1. via NaCl symporter in apical membrane driven by Na+K+ATPase in basal membrane
   2. K+ and H+ moved into the distal tubule
3. Modification of distal tubule where it passes close to glomerulus - JUXTAGLOMERULA APPARATUS
   1. regulates blood pressure and filtration rate of glomerulus

1. By now, filtrate is called urine
2. runs parallel to ascending limb of loH with flow in opposite direction
3. Both cortical and medullary sections are IMPERMEABLE to WATER, UREA and NACL
4. Permeability of water increases in prescense of ANTIDIURETIC HORMONE (ADH - vasopressin)
   1. approx 20% of glomerular filtrate reaches collecting duct
      1. 3/4 of water reaching collecting duct is abstracted in cortical collecting duct
      2. only 5% approx of glomerular filtrate is reabsorbed in the medulla
5. Collecting Duct - Principal cells
   1. Reabsorb Na+ into capillaries - apicial ion channels -> capillaries
      1. Reabsorb Cl- into capillaries - driven by luminal negative charge (Na+ actively moving out)
         1. Secrete K+ - taken into lumen - ion channels in apical membrane
            1. Variable permeability to water - ADH dependant

1. 1. Freely filtered in descending limb, loH
2. 2. 50% reabsorbed in proximal tubule
3. 3. Urea concentration increases in loH
4. 4. Tubular urea concentration increases with removal of water NOT urea
5. 5. Reabsorption in collecting duct in presence of ADH

1. either bicarbonate or H+ secreting
2. Important in pH regulation!
3. Intercalated cells can also reabsorb K+
4. Secretes HCO3- or H+ from capillary - tubule cell - lumen

1. 1. Countercurrentmultiplier in loH - HYPEROSMOTIC medulla
2. 2. Variable permeability of collecting duct to water
   1. osmotic reabsorption of water
   2. variable urine concentration
3. 3. Contribution of urea to hyper-osmolarity in medulla
4. 4. Cell types in collecting duct : principal cells and intercalated cells