crystalloid solution. Transfusion medicine A balanced isotonic solution–eg, Ringer's lactate or saline fluid solution, used for volume expansion. Cf Colloid. of crystalloid or colloid as the optimal resuscitation fluid.1,2 There are inherent differences between colloids and crystalloids that contribute to their effects. A volume expander is a type of intravenous therapy that has the function of providing volume for the circulatory system. It may be used for fluid replacement. Contents. [hide]. 1 Physiology; 2 Types. Crystalloids. Physiology · Types · Crystalloids.