MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A patient is admitted to the emergency room with severe dehydration. To rapidly restore blood volume and electrolyte balance, medical personnel administer an intravenous (IV) solution. Which of the following IV solutions would be most appropriate to prevent further cellular dehydration and effectively rehydrate the patient?
- AA hypotonic solution (e.g., 0.45% NaCl)
- BA hypertonic solution (e.g., 3% NaCl)
- CAn isotonic solution (e.g., 0.9% NaCl)
- DPure distilled water
Show answer & explanationAnswer & explanation
Correct answer: C. An isotonic solution (e.g., 0.9% NaCl)
An isotonic solution, such as 0.9% NaCl (normal saline), has an osmolality similar to that of the body's extracellular fluid. Administering an isotonic solution helps to increase blood volume without causing significant fluid shifts into or out of cells, thus effectively rehydrating the patient without damaging cells.
Why the other options are wrong
- A. A hypotonic solution would cause water to move into cells, potentially leading to cell swelling and lysis, and not effectively increasing intravascular volume without cellular damage.
- B. A hypertonic solution would draw water out of cells, exacerbating cellular dehydration.
- D. Pure distilled water is severely hypotonic and would cause red blood cells to swell and burst (hemolysis).
Isotonic Solution
A solution that has the same solute concentration (osmolality) as another solution, typically referring to the extracellular fluid of the body. When cells are placed in an isotonic solution, there is no net movement of water across the cell membrane.
- Same solute concentration as body fluids.
- No net water movement into or out of cells.
- Used for fluid replacement to avoid cell damage.
- Example: 0.9% NaCl (normal saline).
Memory trick: Isotonic is 'I-SO-perfect' for cells, no swelling, no shrinking!