What defines normal tonicity in bodily fluids?

Normal tonicity in bodily fluids is fundamentally defined by the osmolarity of the fluid, which reflects the concentration of solutes. The typical range for normal tonicity in human plasma is between 275 to 295 mOsm/kg. This range is crucial for maintaining homeostasis within the body, as it ensures that cells neither swell due to excess fluid entering them nor shrink due to fluid leaving them.

When the osmolarity falls below 275 mOsm/kg, the solution is considered hypotonic, which can lead to cellular swelling and potential lysis. Conversely, an osmolarity above 295 mOsm/kg classifies a solution as hypertonic, which can cause cells to lose water and shrink. Maintaining a tonicity within the defined normal range is essential for various physiological processes, including nutrient transport, waste removal, and overall cellular health. This is why the range of 275-295 mOsm/kg is recognized as typical for normal bodily fluids.