Red Blood Cells in Hypotonic Solution: Osmotic Swelling and Hemolysis

In a hypotonic solution (lower solute concentration than cytosol), water enters the RBC by osmosis. The cell swells, loses its biconcave disc shape, becomes spherical, and eventually the membrane ruptures (hemolysis) because mammalian RBCs lack a cell wall to resist turgor pressure. The lysed cell releases hemoglobin into the supernatant.