ABSTRACT
This chapter describes how water is distributed in the body and how the movement of water within the body is controlled by osmotic gradients across cell membranes. Water is the most important component of the human organism, which no doubt stems from our evolutionary origins in an aquatic environment. It has unusual chemical and physical properties, with a powerful solvent capacity, and acts as a medium for the myriad biochemical reactions, physiological processes and anatomical structures that support life. Water, just like other constituents of living tissue, is in a state of dynamic equilibrium. A healthy individual at rest is in water balance— that is, the amounts of water lost are exactly balanced by the amounts consumed by eating and drinking. Water content in urine is under the control of vasopressin hormone, which is released from the pituitary gland into the blood in response to small differences in the osmotic concentration of plasma.
