Why don't blood cells shrink or swell in an isotonic sodium chloride solution \((0.9 \%\) saline)?

Osmosis is a fundamental biological process where water molecules move from a region of lower solute concentration to one of higher solute concentration through a semi-permeable membrane. This movement is driven by a natural phenomenon aiming to equalize solute concentrations on both sides of the membrane, and it occurs spontaneously without the need for energy.

The importance of osmosis in biological systems cannot be overstated. It is essential for maintaining the hydration level and proper functioning of cells. For example, when human red blood cells are placed in an isotonic sodium chloride solution, which is commonly used in medical practices, water molecules will move across the cell membrane at an equal rate in both directions. This movement occurs because the solute concentration inside the cells is the same as that in the surrounding solution, preventing any net water flow and maintaining the cells' structural integrity.

A semi-permeable membrane is a selective barrier that allows certain molecules or ions to pass through it by diffusion and occasionally by more specialized processes of facilitated diffusion, ion channels, or active transport. These membranes are vital components of cell walls.

In practice, a semi-permeable membrane might be thought of as a gatekeeper, determining which substances can enter or leave the cell. For red blood cells in an isotonic solution, the membrane permits water to move freely in and out, but it restricts the movement of sodium chloride and other larger molecules. This selectivity plays a crucial role in processes like osmosis, ensuring that only water moves in response to solute concentration differences.

The equilibrium state is a condition where there is no net change in the system despite ongoing processes that strive to balance conditions on either side of a membrane. When a cell is in an isotonic environment, it has reached an equilibrium state with its surroundings, meaning that there is no net movement of water into or out of the cell.

This balance is crucial for the stability of cells, as any deviation could cause cells to either shrink or swell, potentially leading to cell damage or death. In the scenario of red blood cells in a 0.9% saline solution, this equilibrium state is what prevents them from changing in volume, ensuring they can continue to function optimally in the bloodstream.