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Study Guide: Lecture 31 - Transport & Excretion

Reabsorption of Na, Glucose, HCO2, etc.
Na/K pump reabsorbs Na> Na flows in via Na/Pi symport, Na/H antiport, Na/Glu,AA symport> Glu,AA reabsorb through channels> H in lumen makes CO2+H20> CO2 reabsorbed into cell> make HCO3- in cell> reabsorbed w/ 1Na/3HCO3- symport ↑[ions] in ECF cause water to follow, ↑[Cl] in lumen flows to ECF through gap junctions (2/3) and through cells (1/3) Proteins are not filtered, but small ones are degredaded into AAs or transported to transcytosis to ECF
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Secretion of H ions - prevent acidification
HCO3-/Cl- transporter, H2O dissociates into H and OH, H pumps pumps H out into distal tubule & collecting duct, also H/K pump Mechanisms will stop is gradient is too steep, they must be buffered to not make the pH too low
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Fate of secreted H ions: H buffering in urine
Bicarbonate: H+HCO3→ H2CO3→ CO2 (re-enters cell to make more HCO3)+H20 Diabastic phosphate → monobastic phosphate: H+NaNaHPO4→ NaH2PO4 + Na (reabsorbed) Ammonia→ammonium: H+NH3 (very toxic, diffuses everywhere)→ NH4 (trapped, cannot diffuse). In distal tubule & collecting duct, causes N to be removed
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K Homeostasis (most abundant ion in body)
excretion rate = intake rate constantly Proximal tubule: most filtered K is reabsorbed Distal Tubule: regulated K secretion (PRINCIPAL CELLS) or reabsorption (INTERCALATED CELLS) Rate of K secretion ∝ flow in area (if dehydrated, K does not get secreted ∴ hyperkalemia)
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Secretion of K Ions & Xenobiotics
Na/K pump, ROMK (renal outer medulla K channel I, inwardly rectifying K to lumen) OAT (organic Anion transporters) OCT (organic cation transporters)
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Excretion & Clearance
Excretion Rate = Filtration + Secretion - Reabsorption INULIN: not secreted or reabsorbed, so Excretion rate = filtration rate V = volume of urine formed, U = concentration in urine Rate of Excretion = VxU (mg/min) Rate of Filtration = GFR x [Inulin]plasma GFR = VxU/[inulin]plasma ex: 2 x 30 / 0.6 = 100
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Micturition/Urination
1. Stretch receptors activated by full bladder 2. Parasymp stimulation of muscle contraction, inhibition of MNs to external sphincter muscle 3. Contractions increase P, force internal sphincter open, urination
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