If your cells and body fluids are hypertonic to the water of swimming pool then why do you not swell and pop when you go for a swim?

Medical student's opinion:

The pressure acting on your cells has 2 forms:

- Oncotic pressure due to the difference in tonicity of the solution vs the cell

- Hydrostatic pressure due to the difference in physical pressures of outside vs cell

Swimming in hypotonic solution (your cells are hypertonic to it) will cause your cells to absorb some water. This is why your skin wrinkles (absorbing water and expanding your skin beyond the normal size).

However, absorbing that water increases the amount of water in your body/cells which have a limited ability to stretch. The more water absorbed the less they can stretch and therefor the greater the amount of hydrostatic pressure pushing back. (Think of blowing up a soccer ball, at some point it becomes very difficult to put more air in)

Eventually the hydrostatic pressure pushing back will equal the oncotic pressure pushing in and your body comes to equilibrium without popping.