In: Anatomy and Physiology
Tonicity is the ability of an extracellular solution to
transport water in an out of the cell by phenomenon of osmosis.
Hence, tonicity measures the osmotic pressure gradient that exist
between the cell and the extracellular medium/solution.
Isotonic solution has same solute concentration as living cell. Hypertonic solution has more solute concentration (less water) than the living cell (more water) while in hypotonic solution, there is more solute present (less water) in the living cell than in the extracellular solution (more water). Sea water has a high concentration of salt/solute. Hence, it is a hypertonic solution.
The salt or solute concentration will be higher in the sea water than in the red blood cells (RBC). As a result, water will move from the interior of red blood cell to the sea water in an attempt to equalize the salt concentration on both sides of the plasma membrane of RBC. This will lead to water loss in the RBCs, causing cell shrinkage.