Reabsorption & Secretion In Collecting Ducts

Reabsorption & Secretion In Collecting Ducts


REABSORPTION & SECRETION IN COLLECTING DUCTS

Collecting ducts mainly 2 portions:

  • Cortical portion of collecting duct.
  • Medullary portion/Medullary Collecting Duct (MCD).

PROCESS IN CORTICAL COLLECTING DUCT:

  • Functionally similar to late segment of distal tubule.

Facultative absorption type:

  • Reabsorption of solutes/water depends on body water/electrolyte balance.
  • Hence, facultative type.

Cells involved in reabsoprtion:

  • Principal (P) cells.
  • Intercalated (I) cells.

FUNCTIONS OF ‘P’ CELLS:

1. Sodium & water reabsorption:

  • Reabsorbed from lumen.
  • Occurs through Na2+ channels (ENaC channels). 
  • Approximately 3% Na reabsorption occurs.

2. Potassium secretion:

  • Secreted into lumen.
  • K+ both secreted & reabsorbed.
Note: Na2+ reabsorption & K+ secretion depends on,
  • Na2+-K+-ATPase activity in basolateral membrane.
  • Aldosterone.

3. Water reabsorption:

  • ADH Controlled.
  • Stimulates water channel, aquaporin-2.

FUNCTIONS OF ‘I’ CELLS:

  • Reabsorb potassium ion.
  • Secrete hydrogen ions into tubular lumen.

OTHER REABSORPTIONS:

Urea – 

  • Completely impermeable.
  • Hence, complete urinary urea excretion.

Cl reabsorbed – 

  • Because of electrochemical gradient, caused by Na2+ reabsorption.

Bicarbonate reabsorption –

  • Associated with H+ secretion.

PROCESS IN MEDULLARY COLLECTING DUCTS (MCD):

  • Final site for urine processing.
  • Extremely important site for determining final urinary water & solutes output.

Effect on water & sodium reabsorption:

  • Reabsorb less than 10% filtered water & sodium.
  • Water permeability – ADH controlled.
  • Through aquaporin-2.

On urea:

  • Permeable.
  • Minimal tubular urea is reabsorbed into medullary interstitium.
  • Raises osmolality & forms concentrated urine.

On H+ ion:

  • MCD secrets H+ ions.
  • Influences acid-base balance.
Exam Question
 

REABSORPTION & SECRETION IN COLLECTING DUCTS

  • Cortical collecting duct is functionally similar to late segment of distal tubule.
  • Principal (P) cells reabsorb sodium & water from lumen & secrete potassium into lumen.
  • Intercalated (I) cells reabsorb potassium & secrete hydrogen into lumen.
  • Cl is reabsorbed into cortical collecting duct.
Don’t Forget to Solve all the previous Year Question asked on Reabsorption & Secretion In Collecting Ducts

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