## **Core Concept**
The administration of isotonic saline (0.9% NaCl) intravenously increases the extracellular fluid volume without altering the osmotic balance between the intracellular and extracellular compartments. Isotonic solutions have the same osmolarity as blood, which is approximately 300 mOsm/L. This property allows isotonic solutions to expand the extracellular fluid volume without causing a shift of water into or out of cells.

## **Why the Correct Answer is Right**
The correct answer, , involves understanding that isotonic saline primarily expands the **extracellular fluid (ECF) volume**. The ECF includes both the plasma volume and the interstitial fluid volume. When 2.0 L of isotonic saline is infused, it increases the volume of the extracellular space, which can lead to an increase in blood pressure due to the increased venous return and cardiac preload. This is a common clinical practice to manage hypovolemia.

## **Why Each Wrong Option is Incorrect**
- **Option A:** - This option is incorrect because isotonic saline does not significantly alter the intracellular fluid volume. The osmolarity of isotonic saline is equal to that of the body's fluids, so it does not cause water to move into cells.
- **Option B:** - This option might seem plausible because one might think that increased fluid could lead to increased urine output. However, the direct and immediate effect of isotonic saline infusion is not an increase in osmoreceptors' activity; rather, it's an expansion of the extracellular fluid volume.
- **Option D:** - This option is incorrect because while increased fluid volume can eventually lead to increased cardiac output (through the Frank-Starling mechanism), the direct and immediate effect of isotonic saline infusion is the expansion of the extracellular fluid volume.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that isotonic solutions are used to expand the extracellular fluid volume without changing the osmotic balance. This makes **0.9% NaCl (normal saline)** a common choice for fluid resuscitation in hypovolemic patients. However, it's also important to note that while isotonic saline can effectively expand plasma volume, it does not contain any electrolytes other than sodium and chloride, which can lead to hyperchloremic metabolic acidosis with large volumes.

## **Correct Answer:** .