If the red blood cells (RBCs) of human blood are isolated and are diluted in normal saline (an isotonic solution to blood), what will happen to the RBCs?

When red blood cells (RBCs) are placed in normal saline, which is an isotonic solution, there is no net movement of water across the cell membrane [1]. An isotonic solution has the same solute and water concentration as the intracellular fluid of the body [1]. Because the osmotic pressure is balanced, the rate of water entering the cell equals the rate of water leaving it, resulting in no net flow. Consequently, the cell maintains its stable shape and volume, and there is no change in the diameter of the RBCs. In contrast, a hypotonic solution would cause the cells to swell and potentially burst (hemolysis), while a hypertonic solution would cause them to shrink (crenation) [3]. Normal saline is specifically used in medical treatments because its compatibility with body fluids prevents such physiological damage.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK557609/
2. [3] https://www.khanacademy.org/science/biology/membranes-and-transport/diffusion-and-osmosis/v/hypotonic-isotonic-and-hypertonic-solutions-tonicity