Reabsorption and Secretion Along the Nephron Summary Table - Anatomy & Physiology

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Substance
Proximal Tubule
Loop of Henle
Distal Tubule and Collecting Duct
Hormonal Influences
Sodium
  • Passively enters the cell via ion channels or co-transporters in the apical membrane.
  • Is pumped out of the cell via the sodium/potassium ATPase in the basolateral membrane
  • Link to Article
  • Passively reabsorbed via diffusion from the thin ascending limb
  • Reabsorbed actively in the thick ascending limb via the Sodium/Potassium/2-Chloride symporter
  • It is then cleared from the cell via the sodium/potassium ATPase in the basolateral membrane
  • Link to Article
  • The remaining sodium is reabsorbed in the distal tubule via sodium chloride symporters
  • It is the cleared from the cells via the sodium/potassium ATPase
  • Link to Article
The reabsorption of sodium in the distal tubule is under the influence of two hormones:
Potassium
  • Potassium mainly moves across the membrane via the paracellular route.
  • It is then pumped into the cell in exchange for sodium via the sodium/potassium ATPase in the basolateral membrane
  • It then leaves the cell using a symporter with chlorine
  • Link to Article
  • Reabsorbed actively in the thick ascending limb via the Sodium/Potassium/2-Chloride symporter
  • It also enters the cell via the sodium/potassium ATPase in the basolateral membrane
  • Potassium from both these sources leaves the cell via a symporter with chlorine
  • Link to Article
  • Potassium is secreted into the lumen by the cells of the distal tubule
    • Potassium enters the cell in exchange for sodium
    • It is excreted passively through ion channels and using a symporter with chlorine
  • Potassium is on the whole excreted from the principal cells of the collecting duct
    • This occurs via the paracellular route and via ion channels
    • The gradient for the loss via the ion channels is maintained by the sodium/potassium ATPase in the basolateral membrane
  • The intercalated cells of the collecting duct absorb potassium using a potassium/hydrogen antiporter
  • Link to Article
Aldosterone acts of the sodium/potassium ATPase in the principal cells of the collecting duct increase potassium excretion.
Urea
  • Freely diffuses from the collecting duct into the medullary interstitium
  • Link to Article
ADH makes the collecting duct more permeable to Urea
H+ and HCO3-
  • The secretion of H+ in this section of the nephron is mainly a result of the Na+/H+ antiporter in the apical membrane using secondary active transport.
  • HCO3- is reabsorbed here in a different way to other substances. The presence of hydrogen in the lumen causes some amount of it to dissociate to carbon dioxide and water via the enzyme carbonic anhydrase. These enter the epithelial cells where they are reformed using the same enzyme. The HCO3- then leaves the cell using the basolateral 3HCO3-/Na+ symporter.
  • Link to Article
N/A
  • Hydrogen is excreted in the collecting ducts both in exchange for potassium and on its own using an ATPase.
  • Bicarbonate is reabsorbed.
  • Link to Article
N/A
Glucose
  • It is reabsorbed using a symporter with sodium via secondary active transport
  • Link to Article
N/A
N/A
N/A
Protein
N/A
N/A
N/A
Calcium
  • Bulk reabsorption but not regulation of calcium occurs in the proximal tubule
  • Link to Article
  • Calcium can passively move paracellulary into the vasa recta
  • Link to Article
The regulation of how much calcium is reabsorbed occurs in the athe distal tubule and collecting ducts. Parathyroid hormone stimulates reabsorption.
Organic Acids and Bases
N/A
N/A
N/A
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