## Why 0.9% Normal Saline is correct 0.9% Normal Saline is isotonic and cannot cross the cell membrane. It distributes only within the extracellular fluid (ECF) compartment. Since plasma comprises approximately 1/4 of the ECF volume (about 5% of body weight or 3.5 L in a 70-kg adult), administration of 4 L of normal saline results in expansion of the ECF by 4 L, of which only 1 L (1/4) remains in the plasma/intravascular space. This is the fundamental principle underlying fluid resuscitation in hypovolemic shock — to expand plasma volume by 1 L requires approximately 4 L of isotonic crystalloid. This reflects the plasma-to-ECF ratio of 1:4 as described in Guyton & Hall. ## Why each distractor is wrong - **5% Dextrose in Water (D5W)**: Dextrose is metabolized rapidly, leaving free water that distributes across all body fluid compartments (TBW). Only 1/12 of administered D5W remains in the intravascular space, making it unsuitable for acute volume expansion. It is used for free water replacement, not plasma expansion. - **Albumin 20% (colloid)**: While colloids remain in the intravascular space longer than crystalloids, they are not the answer to this specific question about the 1:4 ratio principle. Albumin is reserved for specific indications and is not the first-line fluid for the described scenario. - **Ringer's Lactate with added dextrose**: The addition of dextrose converts this to a hypotonic solution that distributes partially into the ICF. The dextrose component violates the principle of isotonic crystalloid distribution limited to ECF. **High-Yield:** Plasma = 1/4 of ECF = ~5% body weight. Isotonic saline distributes only in ECF, so 4 L saline → 1 L plasma expansion. D5W distributes in TBW after metabolism — use for free water deficit, not volume shock. [cite: Guyton & Hall 14e Ch 25]
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