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Urine concentration
Descripción
AHS1. Renal Physiology. Countercurrent multiplier and distal tube/collecting duct. Frankie 4/6.
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physiology
ahs1
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Florence Papworth
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Florence Papworth
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Resumen del Recurso
Urine concentration
Learning objectives
Loop of Henle
Countercurrent multiplier and distal tube
Variable reabsorption of water
mechanisms allowing urine to be concentrated to an osmolarity higher than that of plasma
Countercurrent multiplier
Nota:
a countercurrent system is a system which expends energy to create a concentration gradient. e.g. production of hyperosmotic urine by the kidney.
loH essential for formation of concentrated urine
maximum urine concentration is related to length of loH
longer loH = higher concentrated urine
loH function - recover water and sodium chloride from urine
Loop of Henle
Thick ascending limb
impermeable to water and urea. NaCl actively reabsorbed
Thin descending limb
Permeable to water
Thin ascending limb
Impermeable to water. Permeable to Na+ and urea
urine in descending limb starts = 290 mosmol, in ascending limbs finishes 90 mosmol
Counter current flow - urine flow opposite to blood flow of capillaries supplied to tube
In ascending limb - movement of NA+, Cl- and K+ out of tubular lumen/into capillaries using ATPase's. Osmolarity decreases ~ 100 mosmol/L
In descending limb - movement of water into capillaries/ out of tubular lumen concentrates urine = increases osmolarity > 1000mosmol/L
Filtrate HYPERTONIC at turning point
Fluid HYPOTONIC in ascending limb
Fluid ISOTONIC as enters descending limb
Isotonic, hypertonic etc filtrate = compared to capillaries supplying it
osmolarity estabilished in renal medulla
Vasa recta
countercurrent blood supply - opposing flow direction to filtrate
prevents 'wash out' of solutes
'picks up' water removed in the loH
also, picks up Na, Cl and K leaving ascending limb
Distal tubule
Low and constant permeability to water
Na+ and Cl- reabsorption into capillaries.
via NaCl symporter in apical membrane driven by Na+K+ATPase in basal membrane
K+ and H+ moved into the distal tubule
Modification of distal tubule where it passes close to glomerulus - JUXTAGLOMERULA APPARATUS
regulates blood pressure and filtration rate of glomerulus
Collecting Duct
By now, filtrate is called urine
runs parallel to ascending limb of loH with flow in opposite direction
Both cortical and medullary sections are IMPERMEABLE to WATER, UREA and NACL
Permeability of water increases in prescense of ANTIDIURETIC HORMONE (ADH - vasopressin)
approx 20% of glomerular filtrate reaches collecting duct
3/4 of water reaching collecting duct is abstracted in cortical collecting duct
only 5% approx of glomerular filtrate is reabsorbed in the medulla
Collecting Duct - Principal cells
Reabsorb Na+ into capillaries - apicial ion channels -> capillaries
Reabsorb Cl- into capillaries - driven by luminal negative charge (Na+ actively moving out)
Secrete K+ - taken into lumen - ion channels in apical membrane
Variable permeability to water - ADH dependant
Urea
1. Freely filtered in descending limb, loH
2. 50% reabsorbed in proximal tubule
3. Urea concentration increases in loH
4. Tubular urea concentration increases with removal of water NOT urea
5. Reabsorption in collecting duct in presence of ADH
Intercalated cells
either bicarbonate or H+ secreting
Important in pH regulation!
Intercalated cells can also reabsorb K+
Secretes HCO3- or H+ from capillary - tubule cell - lumen
KEY POINTS
1. Countercurrentmultiplier in loH - HYPEROSMOTIC medulla
2. Variable permeability of collecting duct to water
osmotic reabsorption of water
variable urine concentration
3. Contribution of urea to hyper-osmolarity in medulla
4. Cell types in collecting duct : principal cells and intercalated cells
Recursos multimedia adjuntos
Countercurrent Flow And Urine Concentration, Loop Of Henle (image/jpeg)
Distal Tubule Function (image/jpeg)
Principal Cells Of Collecting Duct (image/jpeg)
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