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 equilibrate with equal concentrations on both sides of the membrane without net solvent movement.
<em>When the rod cells become more involved, affected individuals experience a decreased ability to see at night or in low light situations and may lose the ability to see clearly to the sides </em>
