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50 Cards in this Set
- Front
- Back
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What is osmolarity?
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Total solute concentration od a solution
Measure of water concentraion in that the higher the solution osmolarity, the lower the water concentration |
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What is a hypoosmotic solution?
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Has a total solute concentration less than that of normal extracellular fluid (< 300mOsm)
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What is an isoosmotic sollution?
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Has a total solute concentration equal to that of normal extracell flud (=300mOsm)
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What is a hyperosmotic solution?
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Has a total solute concentration grater than that of nmormal extracell fluid
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Is water freely filtered?
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Yes
but 99% is reabsorbed |
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Where does the majority of water reabsorption take place?
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Proximal tubule (2/3 of water reabsorption)
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Where does the major hormonal control of water reabsorption take place?
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Collecting duct
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What does water reabsorption depend on?
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Na reabsorption
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What are the steps for water reabsorption on the proximal tubule?
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1) Na+ reabsroption. Na diffuses from lume to tubular epithelial cels, then use Na+/K+ ATPase to get Na to ISF
2) Local osmolarity dec in tubular lummen and inc in ISF 3) Water goes to ISF by osmosis 4) Bulk flow: most things reabsorbed back to peritub caps |
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Describe step 1.
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Na uses active transport out of the proximal tubular cell toward the blood
Na is reabsorbed from the tubular lumen (urine) to the ISF across the epithelial cells |
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Describe step 2.
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Local osmolarity in the lumen decreases, while local osmolarity in the ISF increases
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Describe step 3.
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Difference in osmolarity causes water to follow rom the tubule into the ISF via tubular cells plasma mb through tight junctions
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Describe step 4.
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From the ISF, water, Na and other things dissolved in the ISF move together by bulk flow into peritubular caps (interstitial P promotes bulk flow)
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How is body water balance maintained?
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Wter intake small: kidney reabsorbs more water (less urine output, 0.4L/day)
Water ntake is large: Lidney reabsorbs less water: urine output 25L/day |
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Where does water regulation take place?
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Collecing duct
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What are the 2 critical components of regulation in the CD?
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1) Permeability of collecting duct to water (regulated by ADH/vasopressin)
2) High osmolarity of medullary interstitium |
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To what concentration can the kidney concentrate urine?
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1400mOsm/L
(filtrate starts at 300mOsm/L) |
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Where does urine concentration take place?
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MEDULLARY collecting ducts
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What does urinary concentration depend on?
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Hyperosmolarity of the ISF
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What happens in the presence of vasopressin?
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Water leaves the tubule and diffuses out into the ISF in the medulla, to be carried back to the blood
(Vasopressin= ADH =antidiuretic, .: don't urinate as much) |
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How does the medullary ISF become hyperosmotic?
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Function of the loop of Henle
->Countercurrent Multiplier System ->Flow in descending and ascending limbs run in opposite directions (countercurrent flow) |
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Describe step 1 of the countercurrrent multiplier system
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Initially, food is isoosmotic (300mOsm/L) in prximal area
TDL is impermeable to Na TAL: osmolarty of 200mOsm, actively reabsorbs NaCl and s IMPERMEABLE to water Osmolarity gradient between the ISF and TAL: 200mOsm/L --->Step 1 depends on what happens in Ascending Limb |
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Describe step 2.
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Descending limb does NOT reabsorb NaCl and is permeable to water
Water dragged out of tube to ISF .: btw Descending limb and ISF : isoosmotic, 400mOsm/L (equilibration) Consequently, lower part of ascending limb becomes 400mOsm/L |
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What happens after the move?
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Proximal part of the tubule is again at 300 mOsm/L (as is the top part of the descending limb)
Bottom part of loop is at 400mOsm as is all of the ISF Top part of ascending limb is 200mOsm Repeat everything: Na reabsorbed from TAL into ISF, inc ISF [solute] and making it hyperosmotc (need to maintain 200 mOsm gradient. .: Bottom of ISF become 500 and bottom of TAL is 300 mOsm) |
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What's the next step 2?
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Water diffuses out of the descending limb into the ISF
Top of ISF and DL: 400 mOsm Bottom of ISF and DL: 500 mOSM e Then the move etc |
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What is the countercurrent multiplier ystem?
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Repeat of 3 steps:
1) What happens in ascending limb 2)What happens in descending limb 3) Move |
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What happens as the flow goes up the TAL?
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Osmolarity decreases
By the time it leaves, it becomes hypotonic |
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What happens if the countercurrent multiplier system goes on for too long?
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End up in steady state
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How concentrated can the osmolarity in the descending limb be?
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1400mOsm
ISF also becomes very hyperosmotic |
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What is the vasa recta?
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Blood vessels in the medulla
Its hairpin loop minimizes excessive loss of solute gradient from the interstitium |
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What does the vasa recta do?
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Resorb net solute and water
-> 2x more volume in it at the end |
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What contributes to medullary hyperosmolarity?
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NaCl
Urea** |
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Describe the water permability of the tubules.
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Water reabsorption depends on the water permeability of the tubule
Permeability of the epithelium depends on the tubular segment (proximal tubule: highly permeable to water) Permeability depends a lot of aquaporins in the plasma mb |
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Where is water permeability subject to physiological control?
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MCD and CCD
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Which hormone is key to ths control?
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Vasopressin
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What is vasopressin?
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Peptide hormone
Anti-diuretic (also known as ADH) Produced by: hypothalamic neurons Released by: posterior lobe of pituitary gland Couple to GPCR of V1 (smooth muscle) and V2 (kidney) |
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What does vasopressin do?
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Stimulates the insertion of aquaporins in the luminal mb of the collecting duct cels to increase water permeability (inc absorption)
Vasopressin= collecting ducts permeable to water |
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What happens in the absence of vasopressin?
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Water diuresis cuz the collecting ducts become impermeable to water
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What is diabetes insipidis?
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Disease caused by the malfunction of the vasopressin system (vasopressin doesn't work cuz its either not released by the pituitary or there's no receptor in the tubule cell)
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What kind of urine is there without vasopressin?
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Very dilute urine
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What is the major regulator of water excretion?
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Vasopressin
Why: water excretion regulated by the rate of water reabsorption by the tubules. Vasopressin regulates this rate |
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What are the 2 mechanisms to cotrol vasopressin secretion?
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Osmoreceptors (important):osmolarity
Baroreceptors (less important): volume loss, need at least 5-10% change of extracelllar fluid |
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Describe osmoreceptor control of vasopressin secretion.
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Excess H2O ingested
-Dec body fluid osmolarity -Dec firing by hypothalamic osmoreceptors -Posterior piuitary: decrease vasopressin excretion -dec plasma vasopressin -Collecting duct: dec tubular permeability to H2O -> dec water reabsorption -End result: INC H2O excretion |
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Describe baroreceptor control of vasopressin secretion.
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Dec plasma volume
-Venous, atrial and arterial P dec (Reflexes mediated by CV baroreceptors) -Posterior pituitary: Inc vasopressin secretion -inc plasma vasopressin -Collecting ducts: inc tubular permeability to H2O and inc H2O reabsorption -End result: DEC H2O excretion |
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Why do we feel thirsty?
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Positive: Dec plasma volume, Inc osmolarity, dry mouth/throat
Negative: Metering water intake by GI tract |
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What do baroreceptors activate in thirst?
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Respond to dec in plasma volume
Activate AG II which is a ppositive factor for thirst |
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What happens in severe sweating?
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Loss of hypo-osmotic solution (Lose more water then NaCl)
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What does the loss f the hypoosmotc salt solution do?
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1) Dec plasma volume
2) Inc plasma osmolarity (dec H2O concentration) |
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What happens when the plasma volume decreases?
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Dec GFR and Inc plasma aldosterone --> Dec Na secretion
Inc plasma vasopressin -> Dec H2O excretion |
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What does increasing plasma osmolarity do?
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P(Na) increases to 150mEq/L (normally 140)
Increases plasma vasopressin Decreases amount of H2O excretion |
