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19 Cards in this Set
- Front
- Back
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What value is osmolarity maintained at?
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about 290 mOsm/L
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What is the hormone controlling absorption of water from distal segments?
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ADH
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Which part of "mOsm/L" does the body regulate?
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The denominator. The liters
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What happens in water deprivation? (plasma osmolarity increases, then what?)
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1) Plasma osmolarity increases
2) Osmoreceptors in hypothalamus sense increase in osmolarity (1% or more) 3) They release ADH from posterior pituitary and thirst is stimulated 4) Late distal tubule permeability to water is increased due to ADH action , and water drinking is initiated 5) Increase in water reabsorption 6) Increase in urine osmolarity due to more concentrated urine being produced and more water being reabsorbed 7) Plasma osmolarity decreases towards normal |
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What happens in water drinking (excessive)
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1) Plasma osmolarity decreases
2) Osmoreceptors sense this 3) Decrease in thirst and decrease in ADH secretion 4) Late distal tubule/collecting ducts become impermeable to water 5) Decrease in water reabsorption 6) Decrease in urine osmolarity 7) plasma osmolarity decreases towards normal |
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How does alcohol affect plasma osmolarity?
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It blocks effects of ADH and increases urine excretion
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What are the three components to forming concentrated urine?
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1) Corticopapillary (CP) gradient
2) Vasa recta - helps maintain the CP gradient by slowing blood flow 3) ADH - makes distal tubule and collecting duct cells permeable to water |
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How is CP gradient formed?
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0) At first, ISF, descending and ascending limbs have same osmolarity
1) NaCl is reabsorbed without water in thick ascending limb, leaving the ISF hyperosmotic and tubular fluid hyposmotic. Since the thing descending limb is permeable to water, it leaves the descending limb until descending limb tubular fluid has same osmolarity as ISF 2) New fluid is pushed down nephron with isomolarity to plasma as it gets filtered. Water leaves the descending limb to dilute the ISF. The ascending limb is still impermeable to water and is still dilute because solutes but NOT WATER are leaving the ascending limb 3) Process is continued about 17x |
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Explain urea recycling
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In presence of ADH, water is reabsorbed in late distal tubule/collecting ducts and urea is left behind ---> [urea] in tubular fluid increases.
In the <b>inner medullary collecting ducts </b> there is a special urea transporter UT1 that allows urea to run down its concentration gradient from tubular fluid into inner medulla. This is the only way ANY urea can get out. |
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Where is UT1?
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Inner medullary collecting ducts.
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How do vasa recta contribute to countercurrent exchange?
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They prevent dissipation of CP gradient in two ways:
1) Keep blood flow to this region low so solutes making up gradient remain high 2) Act as countercurrent exchangers passively. Water and solute can go thru them. Vasa recta blood is in osmotic equilibrium with interstitium and helps maintain gradient. |
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Where is the V1 receptor? What does it do?
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V1, a vasopressin (aka ADH) receptor is on arterioles.
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Where is the V2 receptor? What does it do?
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It's the vasopressin receptor that's on the basolateral side of principal cells of late distal tubule/collecting duct.
ADH binds. Gs-coupled receptor activated. Adenylate cyclase --> cAMP Ultimate action is aquaporins, water channels, get inserted into the luminal membrane. |
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Which is faster, ADH or aldosterone?
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ADH because it's a peptide hormone. Aldosterone is a steroid hormone.
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When there is very low/absent ADH, what happens to the CP gradient and why?
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1) The countercurrent multiplication portion of the CP gradient is decreased b/c ADH increases the activity of the Na+/K+/2Cl transporter in thick ascending limb. Without it, there isn't as much solute reabsorbed, making the difference in osmolarity much less. Less NaCl has been reabsorbed without water.
2) No urea recycling without ADH. Thus, no UT1 transporters inserted into inner medullary region. Without ADH the entire distal tubule and collecting ducts region becomes a diluting segment. No water can be reabsorbed, only solutes. |
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What is free water clearance?
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The amount of water which must be subtracted or added to urine to make is isosmotic with plasma.
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If ADH is low, will the free water be positive or negative?
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If ADH is low, urine will be very hyposmotic wrt plasma. Free water is positive (there is extra water to take away)
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If ADH is high, will free water be positive or negative.
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When ADH is high, urine will be very concentrated wrt plasma. You will have to add water to make it isosmotic. Free water is negative.
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How to calculate free water clearance?
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Remember Free Water Clearance is not a "real" Clearance.
C<sub>H20</sub> = V - C<sub>osm</sub> Free water clearance = urine flow rate - (U<sub>osm</sub> * V / P<sub>osm</sub>) |
