Renal physiology: Integrated approach

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PCT

Normal function:

Pathology:

Type II/ Proximal RTA (renal tubular acidosis):

  • Defective HCO3 reabsorption in PCT
  • Phosphaturia may occur
  • Vitamin D is lost through urine, resulting in osteomalacia
  • Often asymptomatic
  • Rx: Oral vitamin-D.

Fanconi syndrome:

  • Inadequate reabsorption by PCT
  • Everything (Glucose, amino acids, uric acid, phosphate and bicarbonate) gets excreted in urine
  • Loss of bicarbonate results in type II RTA
  • Loss of phosphate results in rickets and osteomalacia
  • Rx: Replacement of lost substances in urine
  • Expired tetracycline may damage PCT and give rise to Fanconi syndrome like condition.

Descending limb of loop of Henle

Normal function: Water reabsorption only

Thick ascending limb (TAL) of loop of Henle

Normal function:

Drug inhibiting NKCC2 transporter: Furosemide

Pathology:

Barter syndrome:

  • Defect in the NKCC2 transporter
  • Presents at infancy
  • Associated with polyhydramnios
  • Severe depletion in plasma volume resulting in severe hypotension → Very high renin activity → Very high serum aldosterone level
  • Defect in paracellular transport of calcium → Calciuria → Renal calculi
  • Severe hypokalemia
  • Worst prognosis
  • Rx: Aggressive hydration.

DCT

Normal function:

Drug inhibiting Na-Cl transporter: Thiazide

Pathology:

Gitelman syndrome:

  • Defect in Na-Cl transporter of DCT
  • Mild diseaes, often remains asymptomatic till 20-30 years of age
  • BP of the patient is essentially normal
  • Presents with abdominal cramp and ileus due to associated defect in paracellular transport of Mg (causing hypomagnesemia)
  • Good prognosis
  • Rx: Mg supplementation.

Collecting duct

Principle cell: Normal function:

Pathology:

  1. Liddle syndrome:
    • Inheritance: AD
    • Dysregulation of the epithelial sodium channel (ENaC)
    • Unregulated Na absorption → Severe hypertension → Very low renin activity → Very low serum aldosterone level
    • So, there is a hyperaldosteronism-like state in presence of low aldosterone levels → It is termed as ‘pseudo-hyperaldosteronism’
    • Rx: Amiloride (ENaC blocker)
    • Good prognosis.
  2. Type IV RTA:
    • There is decreased level or resistance to aldosterone
    • Aldosterone increases ENaC activity
    • In decreased level/ action of aldosterone, ENaC activity decreases; resulting in hyperkalemia; which is mild in severity
    • Often caused by Aldosterone antagonists/ ACE-I/ ARB
    • Rx: Stop the offending drugs
    • Good prognosis.

Intercalated cell:

Normal function: Secretion of H+

Pathology:

Type I/ Distal RTA:

  • Failure of H+ secretion by H+ K+ ATPase pump
  • Presents early with chronic severe acidemia and hypokalemia
  • Urine is alkaline
  • High blood acid level → Bone resorption → Short stature
  • Bone resorption → Increased calcium release from bone → High serum calcium level → Polyuria & renal calculi
  • High blood acid level → Intracellular acidosis → Increased citrate uptake and metabolism in the cell → Low citrate level in blood → Less amount of citrate is excreted in urine → Hypocitraturia
  • Classically associated with Sjogren syndrome, may be seen with SLE & RA
  • Rx: Potassium and Citrate supplementation
  • Worst prognosis.

Action of ADH on collecting duct

ADH (Antidiuretic hormone) = AVP (Arginine vasopressin)

Normal function: Water absorption

Pathology:

Syndrome of inappropriate antidiuretic hormone secretion (SIADH):

  • Inappropriately increased secretion of ADH → Enhanced water reabsorption → Volume of total body water is increased but amount of total body Na is same → Dilutional hyponatremia/ Euvolemic hyponatremia (volume is not that increased as to cause clinical edema)
  • Volume expansion → Activation of atrial stretch receptors → Secretion of Atrial natriuretic peptide (ANP) → Natriuresis → High urinary Na
  • Treatment of choice (TOC): Water restriction
  • DOC:  Vasopressin receptor antagonists (Vaptans)
  • Vaptan of choice: Tolvaptan.

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