Physiology - Renal Topic #3

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9spr
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70608
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Physiology - Renal Topic #3
Updated:
2011-03-04 21:15:14
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9spr
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Topic #3 – Proximal Tubular Reabsorption
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  1. Proximal Tubular Reabsorption
    • occurs in EPITHELIUM (1 cell layer thick, have polarity)
    • different transporters on each side of cell (luminal/basolateral sides)
    • lumenal side have microvilli

    • routes for substrate reabsorption:
    • transcellular =through cells
    • paracellular =between cells through the tight junctions
  2. Passive Transport
    • involves diffusion = molecules cross epithelium from higher to low concentration
    • no energy needed
    • lipid soluble molecule cross easily
    • ions diffuse down electrical gradient
  3. Carrier-Mediated Transport
    • eg. facilitated diffusion
    • Symport: same direction (Na/glucose co transport)
    • Antiport: opposite direction/exchange (Na/H co transport)

    facilitates SOLVENT DRAG =
  4. Active Transport
    • needs a carrier
    • goes against concentration gradient
    • needs energy/atp
    • Secondary Active Transport:
    • indirect use of energy for transport
    • uses Na gradient set up by Na/K ATPase (basolateral)
    • solvent drag - small solutes carried in water flow (SGLT)
    • sodium glucose linked transporter-SGLT
  5. Transport Capacity
    • Tmax = max rate of transport capacity (due to difficulty for agent to find transporter)
    • if Tmax is exceeded excess substrate stays in filtrate and is excreted
    • eg. Tmax for glucose is exceeded in: diabetes or mellitus
    • high GFR in pregnancy
  6. Proximal Tubular Function
    • solutes reabsorbed: sodiume glucose amino acids bicarbonate
    • tubule is permeable to water, so water follows solutes osmotically
    • 67% of water and Na+absorbed here
    • fluid remaining in filtrate is iso osmotic
    • in EARLY tubule:
    • Na+absorbed with HCO3-and organic molecules (amino acids, glucose) (eventually too much is absorbed and the reabsorption tails off)
    • in LATE tubule
    • Na+absorbed with Cl-
  7. Organic Ion Secretion
    • organic ions (eg. penicillin, aspirin) are actively secreted into the filtrate
    • un-ionized substances first converted to ionized form in liver and then secreted
    • ionized substances remain in filtrate

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