August Krogh Lecture. The renal concentrating mechanism in insects and mammals: a new hypothesis involving hydrostatic pressures.

Water moves from compartments of higher to compartments of lower water potential. Osmotically active solutes and negative hydrostatic pressure both lower water potential by stretching the hydrogen bonds between water molecules (Hammel-Scholander hypothesis). In trees the negative hydrostatic pressure in the sap is balanced by the osmotic pressure of the leaves. In response to differences in water potential, water flows across biological membranes through water-filled pores. Protein molecules, aquaporins, forming hourglass-shaped pores have been identified, cloned, and located in plasma membranes in mammalian as well as other tissues. Water molecules flow single file through aquaporins. Insects concentrate the urine in the rectum. Mammals concentrate the urine in the collecting ducts in the inner medulla. In both, a compartment with a high osmotic concentration is created through ion transport. Both have a muscular coat surrounding the tissue, which shows peristaltic contractions. In insects it is the muscular layer around the rectum; in mammals it is the renal pelvic wall that surrounds the papilla. Mechanisms are proposed whereby these peristaltic contractions, through the creation of positive and negative hydrostatic pressures in the tissues, can lead to hyperosmotic excreta.