Osmosis and tonicity: If human red blood cells (RBCs) are placed in 0.9% NaCl (physiological saline), what change occurs to the cells?

Introduction / Context:Tonicity determines water movement across semipermeable membranes. This question reinforces the concept of isotonic solutions commonly used in clinical settings to preserve red blood cell integrity during handling and transfusion-related procedures.

Given Data / Assumptions:

• RBC membrane is permeable to water but not to NaCl on the timescale considered.
• Physiological saline is 0.9% NaCl (~154 mM), isotonic with plasma.

Concept / Approach:In an isotonic environment, extracellular osmolarity equals intracellular osmolarity. Therefore, there is no net water movement across the membrane, and cell volume remains stable—no hemolysis or crenation occurs.

Step-by-Step Solution:

Verification / Alternative check:In hypotonic solutions (e.g., pure water), RBCs swell and hemolyze; in hypertonic solutions (e.g., >0.9% NaCl), RBCs crenate. Neither happens in 0.9% NaCl, confirming isotonicity.

Why Other Options Are Wrong:

• Bursting occurs in hypotonic, not isotonic, media.
• Shrinkage occurs in hypertonic media.
• Chemical destruction is not expected at physiologic NaCl concentration.
• Pump-mediated corrections are not needed when no gradient exists.

Common Pitfalls:Confusing isotonicity (osmotic effect) with iso-osmolarity versus permeability nuances—here, NaCl behaves effectively as non-penetrating on the relevant timescale.

Final Answer:No net change; cell volume remains essentially the same