PHS 3342 Lecture Notes - Lecture 11: Transcellular Transport, Distal Convoluted Tubule, Passive Transport
28 Apr 2018
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February 27, 2018
Tubular Transport Processes
Transport Properties of the Proximal Tubule
Has leaky epithelium
-Ions can be moved by passive diffusion between the cells or by paracellular (transcellular) transport
•NB: transcellular transport = across the cells
Will reabsorb sodium, iron, water, and urea
-Majority of sodium will be reabsorbed - only 100 mmole/day is excreted (0.4% of the filtered load) - active transport
•Remaining sodium will be reabsorbed in the distal tubule
-~80% of the filtered water will be reabsorbed - passive transport
-Reabsorbs ~50% of urea - passive transport
-Makes highly concentrated urine
Transport of sodium:
-Co-transport system: sodium is reabsorbed with glucose
-Antiport system: sodium is reabsorbed, H+ is excreted
-NB: the sodium being reabsorbed will cause water to follow (via osmosis)
HCO3- reabsorption by the proximal tubule
-Proximal tubule and collecting duct respond to pH of ECF and alter the rate of H+ secretion
-In the tubule lumen:
•HCO3- will combine with H+ (transported with sodium), forming H2CO3
•H2CO3 will be broken down into H2O and CO2
•CO2 can be transported across the lumen membrane back to the tubule cell, where it will be converted
back into HCO3- and be reabsorbed
-Helps promote more H+ secretion - which furthers Na+ secretion
-Majority of HCO3- is reabsorbed in the proximal tubule (~80%)
Thin Descending Limb
Very permeable to water
Thin Ascending Limb
Highly permeable to sodium, impermeable to water
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Document Summary
Ions can be moved by passive diffusion between the cells or by paracellular (transcellular) transport: nb: transcellular transport = across the cells. Majority of sodium will be reabsorbed - only 100 mmole/day is excreted (0. 4% of the ltered load) - active transport: remaining sodium will be reabsorbed in the distal tubule. ~80% of the ltered water will be reabsorbed - passive transport. Reabsorbs ~50% of urea - passive transport. Co-transport system: sodium is reabsorbed with glucose. Antiport system: sodium is reabsorbed, h+ is excreted. Nb: the sodium being reabsorbed will cause water to follow (via osmosis) Proximal tubule and collecting duct respond to ph of ecf and alter the rate of h+ secretion. Helps promote more h+ secretion - which furthers na+ secretion. Majority of hco3- is reabsorbed in the proximal tubule (~80%) Reabsorbs via na+/k+/2cl- symport system with the help of the na+/k+ atpase. Has tight junctions - no passive diffusion between the cells allowed.
