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Loop of Henle
- responsible for 25% of Na+, 15% of water reabsorbed by kidney
- PROBLEM: high Na+concentration in urine (up to 1200 mM)BUT Na+/K+ATPase can only pump against a 200 mM gradient
- SOLUTION: Loop of Henle creates an OSMOTIC GRADIENT in the kidney ISF
- osmolarity of outer cortex is isotonic
- inner medulla is hypertonic
- loop structure is central to creating this gradient
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countercurrent multiplier effect:
- created by combining:
- a loop structure
- different permeabilities to Na and water in different parts of the loop
- Effect: osmotic gradient is multiplied
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parts of loop of henle
- descending loop: permeable to water but not sodium
- ascending loop: permeable to sodium but not water
- Na/K ATPasepumps Na into interstitium
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Diuretics
- inhibits transport of Na out of tubule
- this retains more fluid in tubule than urine
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Distal Tubule / Collecting Duct
- distal tubule fluid is hypotonic (100mOsM)
- 20% of water, 8% of Na+, absorbed here
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role of Anti-diuretic Hormone (ADH) = vasopressin
- binds to membrane receptor, ↑ cAMP
- makes membrane permeable to water
- promotes membrane insertion of water channels (aquaporins)
- ADH is released from: posterior pituitary
- osmolarity receptors located in: hypothalamus
- release stimulated by stress
- release inhibited by: alcohol
- non functional in diabetes insipidus
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response to changes in osmolarity vs. blood pressure
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Urea reabsorption
- Urea – waste product formed from Nitrogenous wastes
- formed in: collecting duct
- uremia = build up of too much urea
- Cortex:
- impermeable to urea
- urea concentration gradually increases as water is removed
- Medulla:
- permeable to urea
- some urea is transported out of CD and into ascending loop
- effect: further water retention
- clinical implications: helps extend life for those in cases of severe dehydration
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Countercurrent Exchange
- vasa recta
- function: reabsorbtion of Na and urea
- rapid equilibration (exchange) of Na+, water in blood with ISF
- loop shape ensures: further concentrate urine
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