Dehydration Detective
A patient has normal serum sodium, normal osmolality, and proportional loss of sodium and water.
Isotonic dehydration
What analytical method is used clinically to assess urine solute concentration?
Refractometry.
Which nephron segment is impermeable to water but actively reabsorbs sodium and chloride?
Thick ascending limb of the loop of Henle.
Which class of diuretics inhibits the Na⁺-K⁺-2Cl⁻ symporter?
Loop diuretics (e.g., furosemide).
Which hormone primarily regulates extracellular fluid volume?
Aldosterone
A patient has decreased total body sodium, hyponatremia, and water loss that is less than sodium loss.
Hypotonic dehydration
A urine sample has a USG of 1.025. Estimate the urine osmolality.
Answer: ~750 mmol/kg (last two digits × 30 → 25 × 30).
Why does tubular fluid remain iso-osmotic to plasma as it leaves the proximal tubule?
Water and solutes are reabsorbed proportionally in the proximal tubule.
A dog has primary hyperaldosteronism with persistent hypokalemia. You want to reduce sodium retention without worsening potassium loss.
Potassium-sparing diuretic (spironolactone)
What two stimuli increase aldosterone secretion?
Hyponatremia and hyperkalemia.
A dog presents with Na⁺ 158 mmol/L, Cl⁻ 126 mmol/L, osmolality 318 mmol/kg, and USG 1.050.
Na⁺: ↑
Cl⁻: ↑
Osmolality: ↑
Hypertonic dehydration due to decreased water intake elevated sodium and chloride with concentrated urine indicate intact ADH and renal concentrating ability.
A urine sample contains 4+ glucose. Explain why USG overestimates true solute concentration.
Glucose increases refractive index without proportionally increasing osmolality, falsely elevating USG.
Why is the ascending limb of the loop of Henle called the diluting segment?
Sodium and chloride are reabsorbed without water, lowering tubular fluid osmolality.
A patient has hypercalciuria and mild hypertension. You want a diuretic that reduces calcium excretion while acting in the distal nephron.
Thiazide diuretic (e.g., hydrochlorothiazide)
A patient is hyponatremic and hypovolemic. Which hormonal system activates first and why?
RAAS , restoring circulating volume is prioritized over osmolality.
A dog presents with hypernatremia, hyperchloremia, osmolality 318 mmol/kg, and USG 1.002.
Na⁺: ↑
Cl⁻: ↑
Osmolality: ↑
Answer: Hypertonic dehydration due to diabetes insipidus. hyposthenuria indicates lack of ADH effect despite dehydration.
Two dogs have the same USG. One has proteinuria and one does not. Which dog has the higher true urine osmolality?
The dog without proteinuria , protein falsely elevates USG without increasing osmolality.
Which nephron segments reabsorb water under the influence of ADH?
Collecting duct (and late distal tubule); the ascending limb does not reabsorb water.
A dog has cerebral edema and increased intracranial pressure. Renal function is intact. You need a drug that increases urine output without relying on tubular transporters.
Osmotic diuretic (mannitol)
Explain how aldosterone increases potassium excretion while increasing sodium reabsorption.
Sodium reabsorption creates a negative luminal potential that drives potassium secretion via principal cells.
A diabetic patient has hyperglycemia, glucosuria, polyuria, and dehydration. Explain why hypertonic dehydration is less common than hypotonic dehydration in diabetes mellitus.
Osmotic diuresis initially causes water loss, but compensatory water intake and ADH-mediated retention dilute sodium, making hypotonic dehydration more common.
A dehydrated dog produces inadequately concentrated urine. Provide two possible pathophysiologic explanations.
Diabetes insipidus (central or nephrogenic) or loss of medullary osmotic gradient; USG helps differentiate renal vs hormonal causes.
Explain why urine remains dilute when the medullary osmotic gradient is lost, even if ADH is present.
Without a hyperosmotic medulla, water cannot be reabsorbed despite ADH-mediated aquaporin insertion.
A dog with glaucoma requires decreased aqueous humor production and mild diuresis. The drug should act in the proximal tubule and alter bicarbonate handling.
Carbonic anhydrase inhibitor (acetazolamide)
Aldosterone act son which nephron segment to increase Na+ reabsorption and K+ secretion?
Principal cells of distal tubule and collecting duct