<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2012) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2012) (Learn how and when to remove this template message) </Td> </Tr> <P> Tonicity is a measure of the effective osmotic pressure gradient, as defined by the water potential of two solutions separated by a semipermeable membrane . In other words, tonicity is the relative concentration of solutes dissolved in solution which determine the direction and extent of diffusion . It is commonly used when describing the response of cells immersed in an external solution . </P> <P> Unlike osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an effective osmotic pressure . Solutes able to freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane . It is also a factor affecting imbibition . </P>

Relative to a surrounding hypertonic solution the contents of a red blood cell is