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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
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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
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Carrier-Mediated Transport
- eg. facilitated diffusion
- Symport: same direction (Na/glucose co transport)
- Antiport: opposite direction/exchange (Na/H co transport)
facilitates SOLVENT DRAG =
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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
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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
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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-
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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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