A 70 kg man loses 3 liters of isotonic sweat during a marathon. Which body fluid compartment will show the GREATEST absolute volume decrease?

• A Extracellular fluid (ECF) compartment, specifically the interstitial component ✓
• B Intracellular fluid (ICF) compartment
• C Plasma volume equally to interstitial fluid
• D Transcellular fluid compartment

Explanation

Sweat is derived from plasma via eccrine gland secretion, primarily reducing ECF volume. Isotonic sweat loss does not change plasma osmolality (unlike pure water loss or hypertonic loss), so there is no osmotic driving force to redistribute fluid between ICF and ECF compartments. The ECF volume shrinks proportionally—roughly 75% of ECF loss is from the interstitial compartment (which constitutes ~75% of ECF volume ~11L), and 25% from plasma volume (~3L). ICF remains essentially unchanged because with isotonic loss, no osmotic gradient develops to drive water out of cells. Plasma volume does decrease but is a smaller fraction of total ECF loss.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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