Introduction

Red blood cells (RBCs) are highly sensitive to the osmotic environment in which they are suspended. Hemolysis, or the rupture of RBC membranes leading to the release of hemoglobin, occurs when RBCs are placed in solutions with inappropriate concentrations of sodium chloride (NaCl). This article explains how the concentration of NaCl influences RBC hemolysis, why hemolysis occurs specifically in hypotonic solutions, and identifies the NaCl concentration at which hemolysis typically starts, focusing on the options provided.

Osmolarity and Hemolysis

The normal concentration of NaCl in blood plasma is approximately 0.9%, which is isotonic to RBCs; this means that the osmotic pressure inside the cells and in the surrounding fluid is balanced, preventing water movement into or out of the cells.

• When RBCs are placed in a hypotonic solution (where the NaCl concentration is lower than 0.9%), water flows into the cells by osmosis due to the higher intracellular solute concentration.

• This influx of water causes the cells to swell and eventually burst (hemolyze) if the osmotic imbalance is severe.

Hemolysis Threshold for NaCl Concentrations

Research and experimental data indicate:

• Hemolysis begins at approximately 0.5% NaCl concentration. At this level, the solution is hypotonic enough for water to enter RBCs, causing swelling and rupture.

• Complete hemolysis can occur below this concentration.

• Concentrations approaching or exceeding 0.9% NaCl are isotonic or hypertonic, preventing swelling and hemolysis.

Explanation of Options

1. 2.0% NaCl: Hypertonic, shrinks RBCs (crenation), no hemolysis.

2. 1.5% NaCl: Still hypertonic, no hemolysis, RBC shrinkage.

3. 1.0% NaCl: Slightly hypertonic to isotonic, no hemolysis.

4. 0.5% NaCl: Hypotonic, causes water influx, initiates hemolysis.

Thus, among the options, RBC hemolysis occurs when they are suspended in 0.5% NaCl (Option 4).

Clinical Relevance

Understanding hemolysis due to osmotic changes is important in medical diagnostics and for preparing intravenous fluids. Infusions must be isotonic to avoid damaging RBCs and causing hemolysis.

Correct answer: (4) 0.5% NaCl.