Updated: 3/13/2018

Relative Concentrations along Proximal Tubule

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Overview

This illustrates the relative concentrations of various molecules along the proximal tubule and depicts under which osmotic conditions those molecules are secreted or reabsorbed.

Introduction
  • Most small substances are freely filtered by the glomerulus meaning
    • filtration is in general a non-selective process
    • a diverse set of substances enter the proximal tubule
  • Substances are subsequently secreted or reabsorbed by the proximal tubule so that
    • toxins can be cleared faster than filtration alone would allow
    • useful substances can be retained by the body despite filtration
  • The balance between secretion and reabsorption of each substance controls
    • the concentration of that substance at the end of the proximal tubule
    • the concentration that is presented to downstream portions of the nephron
  • The proximal tubule also absorbs the majority of filtered water
 Relative Tubular Concentrations
  • Substance concentrations in the proximal tubule can be defined relative to the plasma concentration of the same substance (tubular fluid/plasma = TF/P) so that
    • the action of the proximal tubule on the substance can be easily determined
    • substances can be analyzed relative to those with known behavior
Interpretation of TF/P
TF/P Meaning
Causes Examples
> 1
  • Solute is reabsorbed less quickly than water
  • Active secretion
  • No reabsorption
  • PAH
  • Inulin
  • Urea
= 1
  • Solute and water are reabsorbed at the same rate
  • Solute reabsorption drives water reabsorption
  • Sodium
< 1
  • Solute is reabsorbed more quickly than water
  • Active reabsorption
  • Cotransport
  • Glucose
  • Amino acids
  • Bicarb 
 
Key Proximal Tubule Changes
  • Inulin is neither secreted nor reabsorbed in the proximal tubule so
    • it ↑ in concentration along the proximal tubule
    • its concentration can be viewed as a proxy for water reabsorption
  • Na+ reabsorption is the main driver of H2O reabsorption so
    • sodium TF/P remains close to 1 throughout the proximal tubule
  • Cl- reabsorption has two phases including a
    • slower rate than Na+ in the proximal 1/3 of the proximal tubule
    • equivalent rate to Na+ in the rest of the proximal tubule
  • Cl- TF/P therefore ↑ before it plateaus

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