process of returning important substances from filtrate back into interstitium, then into renal blood vessels and eventually back to the body
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How much filtrate if reabsorbed in the PCT (approx.)?
70%
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Explain how the PCT is specialised for its function
Tubular cells in the PCT have:
- microvilli on apical membrane to increase SA for reabsorption
- large amounts of Na-K pumps on basolateral membrane
- large amounts of carbonic anhydrase enzyme
3 of 19
Describe the movement of Na ions across tubular cells of the PCT
- Na+ actively transported over basolateral membrane out of tubular cells towards peritubular capillaries
- sets up Na+ gradient that drives movement of Na+ over apical membrane of tubular cells (filtrate side)
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What does the active transport of Na+ across tubular cells cause?
- Negatively charge Cl to follow Na+ down its electrochemical gradient - sets up osmotic gradient which causes water to follow by osmosis
- bulk movement of water causes solvent drag + diffusion of solutes through selective ion + protein channels
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What is the difference between transcellular + paracellular absorption?
Is there a limit to how much glucose can be filtered how of the PCT? Explain.
No
- glucose= small molecule that easily passes through glomerular membrane
- glucose is freely filtered and does not saturate- filtration depends on plasma concentration
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Is there a limit to how much glucose can be reabsorbed from the PCT? Explain.
Yes- depends on:
- rate of flow of filtrate
- number of protein transporters
10 of 19
Describe phosphate reabsorption in the PCT
- linked to Na co-transport
- hormonally regulated by parathyroid hormone
- parathyroid hormone reduces phosphate reabsorption so increases phosphate excretion
11 of 19
Why are a large number of environmental toxins highly lipid soluble?
- lipid soluble substances can easily cross cell membranes + as water is absorbed from the filtrate a diffusion gradient is set up which promotes reabsorption
- difficult to excrete highly lipid soluble substances via urine
- liver converts many foreign
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What functional adaptations are there in the PCT that allows large volume reabsorption of water and solutes?
- large SA
- single layer epithelial cells
- high concentration of Na/K pumps
- asymmetrical distribution of Na/K pumps- allows diffusion gradient
- high concentration of carbonic anhydrase
- peritubular capillaries continuous with efferent arteriole have
13 of 19
Describe the unregulated secretion of H+ in the PCT
- secreted into filtrate by secondary active transport (sodium hydrogen exchanger)
- protons bind to non-bicarbonate buffers and excreted in urine
- or secreted by NH4-Na antiporter on apical membrane
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How is bicarbonate reabsorbed in the PCT?
- no protein carrier on apical membrane
- linked to proton secretion
- made possible by large quantities of carbonic anhydrase enzyme
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Describe and explain the key features that allow 80-90% of bicarbonate to be reabsorbed in the PCT
- apical membrane of tubular cells impermeable to bicarbonate
- basolateral membrane permeable to bicarbonate
- Na/H antiporter secretes H+ that binds to bicarbonate
- carbonic acid dissociates + CO2 moves into tubular cell
- CO2 + H2O = carbonic acid - d
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Apart from H+, what other substances are secreted in the PCT?
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