GI Tract Theme 4
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- Created by: Splodge97
- Created on: 23-05-17 11:27
What are the main functions of the kidney?
Elimination of foreign substances (drugs/by-products of metabolism), regulating ECF volume, maintaining ion conc, monitoring pH and producing renin
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What are the sections of the kidney?
Outer capsule, cortex (where blood flow is greatest, decreasing into the medulla) and the medulla (inner kidney observed as triangular sections - ureter papillae branch to it)
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What are the different types of nephron?
Superficial = present mainly in the cortex, minimal function as short loop. Juxtamedullary = Mainly in the medulla, main functional unit as long loop of Henle.
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What is Bowman's capsule?
At juxtamedullary nephron, leading into convoluted proximal tubule. Holds the glomerulus where bloodstream (afferent arteriole) and tubular system meet.
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What is the typical glomerular filtration rate?
Occurs according to molecule size and charge, typically 125ml/min (so 3L plasma cycled 60 times/day). Change indicates kidney disease/metabolic condition/adverse drug reaction.
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What are the barriers to glomerular filtration?
Fenestrated endothelium of afferent arteriole (pores 70nm), basement membrane (resists negative/more than 5.5Da) and podocyte filtration slits of Bowman's capsule (pores 4-14nm). Means smaller positive molecules pass most easily.
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Which molecules do/don't undergo glomerular filtration?
Do = H20, K+, Ca2+, Cl-, urea, glucose, inulin. Don't = RBC's, WBC's, serum albumin.
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What can occur due to glomerular filtration malffunction?
Proteinurea (proteins in filrate produces cloudy urine) or haematurea (blood in the urine)
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What ooccurs in the proximal convoluted tubule?
Apical epithelium forms micro-villi to increase SA. 70% water and most electrolytes (Na+ as an isosmotic solution, 70% via cotransport with glucose/AA's) reabsorbed. Drugs and large cations/anions also enter. Some pH maintenance in acidosis.
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What occurs in the descending limb of the loop of Henle?
Apsorption of H2O (no Na+ as no channels). Countercurrent system causes concentrated solution at the end.
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What occurs in the ascending limb of the loop of Henle?
Reabsorption of 25% filtered Na+ (no H20 reabsorption as no aquaporins). Means filtrate entering distal tubule relatively dilute. Na+ rebasorbed via Na+/K+/2Cl- at the apical membrane, Na+/K+/ATPase (and K+ and Cl- channels) at the basolateral.
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What occurs in the distal convoluted tubule and collecting duct?
8% NaCl filtered is reabsorbed, water normally cannot be. In DCT via Na+/Cl- at apical and Cl- at basolateral. In CD principal cells ENaCC and K+ channels at apical, Na+/K+/ATPase at basolateral.
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Why is it essential there are normally no aquaporins in the CD and DCT?
Because water would leave along its osmotic gradient as Na+/urea concentrated in interstitium, so unine permanently concentrated
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What is the myogenic response of glomerular filtration autoregulation?
Stretch of afferent arteriole through high BP causes smooth muscle to undergo reflex contraction; slows flow through glomerulus. Increases time for/efficiency of filtration so volume lost in urine increased (lowing BP).
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What is tubuloglomerular feedback (part of GFR autoregulation)?
Increased BP causes increased BF through tubular system. Detected by macula densa cells in ascending limb and DCT. Release paracrines to the juxtaglomerular cells surrounding afferent arteriole; narrow so resistance. Slows BF, increases H20 loss.
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What molecules can be used to measure GFR?
Those freely permeable at the glomerulus, aren't resorbed or secreted, not subject to metabolism, not produced by kidney and don't affect GFR. Inulin (injected) is an example.
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Why is creatinine not as efficient in measuring GFR?
Organic cation so secreted at the PCT and also product of muscle metabolism (so subject to muscle mass)
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What is the equation to calculate GFR?
U(inulin) x V(urine)/P(inulin) - all values in ml/min
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What is the clearance rate of a substance?
ml of plasma totally cleared of a solute in one min (units ml/min, same equation as GFR). Cx=GFR for inulin (100% filtered is excreted). Cx=0 if completely reabsorbed (glucose). Cx>GFR if more secreted (penicillin). Cx
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Why does diabetes alter the Cx of glucose?
Leads to some retention of glucose in the kidney (as more in plasma than resorptive capacity); means osmolarity of filtrate increased so water moves into it, increasing urination and activating thrist reflex (in lateral preoptic area of hypothalamus)
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What drugs can be used to reduce Na+ reabsorption?
Loop diuretics (inhibit Na+/K+/ATPase of TAL), thiazide diuretics (block Na+/Cl- in DCT) and amiloride (inhibits ENaCC in principal cells of CD). Used to treat oedema and high BP as increase water lost in urine (as lower interstitial osmolarity).
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How does the loop of Henle act as a countercurrent multiplier?
NaCl actively transported from TAL into interstitium (against its conc grad as was isosmotic); increased osmolarity causes more H20 reabsorption in DL (so conc). New filtrate creates osmotic grad (higher deeper in medulla). Triggers more Na+ pumping.
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How do the vasa recta act as a countercurrent exchanger?
In descending solutes move into blood and water out (as increasing osmolarity); in ascending solutes move out and water in (as decreasing osmolarity). Equal opposite actions mean O2 to loop of Henle without losing osmotic grad.
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How does urea act to concentrate urine?
Freely filtered at glomerulus, trapped until reaches CD where moved into interstitium (contributes 1/2 its osmolrity). Some enters TAL but recycled back to CD. Water reabsorption increased as interstitium more conc.
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How do A-intercalated cells of the CD maintain pH?
Work in acidosis. Low pH in interstitium (as buffering capacity HCO3- decreased) means more CO2 enters. Combines with H20 via carbonic anhydrase; H+ made excreted by H+/ATPase and H+/K+/ATPase, HCO3- reabsorbed via HCO3-/Cl- antiporters.
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How do B-intercalated cells of the CD maintin pH?
Act in alkalosis. H20 and CO2 in cells converted via carbonic anhydrase to H+ (reabsorbed via H+/ATPase and H+/K+/ATPase) and HCO3- (excreted by HCO3-/Cl- antiporters).
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Where is ADH released? What stimulates this?
Produced mainly in supraopti nuclei of the hypothalamus, then stored and secreted in posterior pituitary. Relase stimulated by >1% osmolarity increase (sensed by osmoreceptors of hypothalamus) in low BP, and nicotine (why smoking causes high BP).
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What is the action of ADH?
Acts rapidly (half life 18 mins), binds to basolateral surface of principal cells to activate adenylate cyclase. Creates PKA via cAMP, increases incorporation of aquaporin 2 in apical membrane (so water reabsorbed, concentrating urine/increasing BP).
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Where is aldoesterone released? What stimulates this?
Released by adrenal cortices (when acted upon by ACTH from anterior pitutary). Release stimulated by increased K+ (acts directly) or via renin-angiotensin system (indirectly via angiotensin II). Release inhibited by high osmolarity (though ADH not).
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Describe the action of aldosterone
Acts on DCT and CD, binds to receptor in principal cells (as steroid) causing transcription/translation of ENaCC and Na+/K+/ATPase. Increases Na+ reabsorption/K+ excretion (by Na+/K+/ATPase then K+ channels), lowering BV/BP. Cl- follows passively.
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How is the renin angiotensin system initiated?
Baroreceptors detect decrease in BP so stimulate the sympathetic system to dilate the afferent arteriole. Decreases blood flow past macula densa so juxtaglomerular cells signalled to secrete renin.
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What are the actions of angiotensin II? How does it cause negative feedback?
Angotensin II causes peripheral vasoconstriction, sympathetic to the heart, ADH and aldosterone; increases BP which is picked up by baroreceptors (switch off sympthetic response so renin release inhibited).
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How does ANP (atrial natriuretic peptide) act?
High BP increases pressure in atrial walls so release ANP. Increases GFR so macula densa don't stimulate renin, indirectly reducing aldosterone (also directly at adrenal cortex). Inhibits ADH at hypothalamus, acts directly on kidney. Lowers BP.
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Other cards in this set
Card 2
Front
What are the sections of the kidney?
Back
Outer capsule, cortex (where blood flow is greatest, decreasing into the medulla) and the medulla (inner kidney observed as triangular sections - ureter papillae branch to it)
Card 3
Front
What are the different types of nephron?
Back
Card 4
Front
What is Bowman's capsule?
Back
Card 5
Front
What is the typical glomerular filtration rate?
Back
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