ABSTRACT
This chapter reviews the methods and interpretations of water permeability measurements. Water permeates biological membranes through several routes. Water appears to move through specific water channels, at least in the red cell membrane, in the toad bladder, and presumably the distal portions of the renal nephron. Osmotic water permeability is measured by subjecting cells to a sudden change in the osmotic pressure of the suspending medium and recording the subsequent change in cell volume. At steady-state conditions, no net movement of water takes place in the system. The nuclear magnetic resonance method relies on the fact that nuclear magnetic moments in disordered water molecules become oriented in a magnetic field. The fact that osmotic water permeability is larger than diffusional water permeability in human red cells as demonstrated repeatedly by different methods and techniques emphasizes the concept of pores in the human erythrocyte membrane.
