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Updated: Mar 13 2018

Relative Concentrations along Proximal Tubule

  • Overview
  • 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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Private Note

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