Questions: Fluid and Electrolyte Regulation and Osmolarity

Question 1

A healthy person drinks 3 liters of pure water over one hour, well beyond their normal intake. Assuming normal kidney function, which hormonal response best describes what will happen?

• A. ADH secretion increases, causing the kidneys to concentrate urine and retain the extra water
• B. ADH secretion falls because plasma osmolarity drops; the kidneys produce large volumes of dilute urine to excrete the excess water
• C. Aldosterone secretion rises to retain sodium and compensate for the dilution of plasma
• D. Both ADH and aldosterone are suppressed equally, since both hormones regulate general fluid balance

Answer: (B)

Drinking pure water dilutes plasma — osmolarity falls below the normal ~290 mOsm/kg threshold. Hypothalamic osmoreceptors detect this decrease and suppress ADH secretion. Without ADH, aquaporin-2 channels are not inserted into the collecting duct, water cannot be reabsorbed, and large volumes of dilute urine are produced. Aldosterone responds to volume status and angiotensin II rather than osmolarity directly and would not be significantly affected by a pure water load. The ADH-osmolarity feedback is sensitive: even a 1–2% rise in osmolarity triggers ADH release, and a corresponding fall suppresses it.

Question 2

A patient has SIADH — ADH levels are persistently elevated despite normal blood volume and normal sodium intake. Why does this cause hyponatremia (low plasma sodium)?

• A. Excess ADH directly causes the kidneys to excrete sodium in the urine
• B. Excess ADH causes inappropriate water retention, diluting the sodium already present in plasma — the sodium is not lost, it is diluted by the excess retained water
• C. Excess ADH activates aldosterone, which suppresses sodium reabsorption in the distal tubule
• D. SIADH suppresses thirst, reducing fluid intake and secondarily decreasing sodium consumption

Answer: (B)

ADH does not act on sodium directly — it acts on water. By inserting aquaporin-2 channels into the collecting duct, excess ADH causes the kidneys to reabsorb more water than needed. Total body water increases, but total body sodium stays roughly constant (normal intake, normal excretion). The result is that sodium, which was normal, is now dissolved in a larger volume — its concentration falls. This is dilutional hyponatremia: sodium per liter decreases not because sodium was lost but because the denominator (total water) grew. Treatment is water restriction, not sodium administration, precisely because the problem is excess water.

Question 3

ADH and aldosterone regulate distinct aspects of fluid homeostasis: ADH primarily controls plasma osmolarity by adjusting water reabsorption, while aldosterone primarily controls extracellular fluid volume by adjusting sodium reabsorption.

• T. True
• F. False

Answer: True

This functional distinction is the organizing principle of fluid-electrolyte physiology. ADH responds to osmolarity (via hypothalamic osmoreceptors) and adjusts water balance — high osmolarity triggers ADH, which increases water reabsorption, restoring osmolarity. Aldosterone responds to volume signals (via the renin-angiotensin system triggered by low renal perfusion) and adjusts sodium balance — low volume triggers aldosterone, which retains Na⁺, and water follows osmotically to restore volume. Because water follows sodium, aldosterone indirectly affects water, but the primary regulatory loop is volume. The two systems can be activated together (dehydration) or independently (SIADH activates ADH without necessarily affecting aldosterone).

Question 4

Diabetes insipidus — the inability to produce or respond to ADH — primarily causes hyponatremia because the kidneys fail to retain sodium.

• T. True
• F. False

Answer: False

Diabetes insipidus causes hypernatremia (high plasma sodium), not hyponatremia. Without ADH, the collecting duct lacks aquaporin-2 channels and cannot reabsorb water. The kidneys produce massive volumes of dilute urine (up to 20 L/day). If fluid intake does not keep pace with these losses, total body water falls while total body sodium is relatively preserved — so sodium concentration rises. ADH regulates water, not sodium directly; losing ADH function means losing water, which concentrates sodium. This is the opposite of SIADH, which retains water and dilutes sodium.

Question 5

Why does SIADH cause hyponatremia despite normal sodium intake? Explain the mechanism in terms of what ADH actually controls and what goes wrong when it is dysregulated.

Model answer: ADH controls water reabsorption in the collecting duct by inserting aquaporin-2 water channels. Normally ADH is secreted only when plasma osmolarity rises above ~290 mOsm/kg or blood volume falls, allowing the kidneys to retain water when needed. In SIADH, ADH is secreted persistently regardless of osmolarity — from ectopic production (e.g., a lung tumor) or dysregulated hypothalamic secretion. The kidneys continue to reabsorb water even when plasma osmolarity is already normal or low. Total body water expands, but total body sodium stays roughly constant (normal intake equals normal urinary excretion of sodium). The plasma sodium concentration — sodium per liter of plasma — falls as the denominator (total water) grows without a corresponding increase in total sodium. The result is dilutional hyponatremia: too much water relative to sodium, not a sodium deficit. Treatment targets the cause: water restriction reduces total body water and restores the sodium-to-water ratio.

This mechanism also explains why simply giving more sodium in SIADH does not fix the problem: the kidneys, under the influence of excess ADH, will retain the extra water that comes with sodium administration, failing to raise the concentration.