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.

• 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.