(1) Relating to isotonicity or isotonia. (2) Having equal tension; denoting solutions possessing the same osmotic pressure; more specifically, limited to solutions in which cells neither swell nor shrink. Thus, a solution that is isosmotic with intracellular fluid will not be isotonic if it includes solute, such as urea, that freely permeates cell membranes. (3) In physiology, denoting the condition when a contracting muscle shortens against a constant load, as when lifting a weight. Cf.: auxotonic, isometric (2)
