Water and electrolyte balance in disease.

Many medical and surgical emergencies are complicated by derangement of water and electrolyte balance, the successful management of which depends upon the rapid assessment of essential clinical and laboratory data, and the prompt institution of a certain pre-arranged routine of treatment. An attempt is made in this review to illustrate the application of physiological principles to specific clinical problems. The major part (35 1.) of the water content of the body is an integral part of the cell structure, the remainder (15 1.) is extracellular fluid comprising intersitial fluid and plasma. The composition of the fluid in the two compartments is different. Potassium and phosphate are the main cation and anion respectively of the cells, while sodium and chloride preponderate in the extracellular fluid. The difference in ionic composition between the two compartments is well maintained in health, the distribution of water between the cells and the fluid bathing them being regulated by the osmotic pressure of solutes. An increased concentration of substances on one side of the membrane will draw water from the other. The extracellular fluid acts as a buffer between the cells and the external environment with which it is in constant contact through the lungs, skin, gut and kidney. It suffers greater proportional changes in volume than the cellular fluid. The concentration of solutes in body fluids is regulated by receptors in the hypothalamus sensitive to certain changes in the osmotic pressure of the plasma. The reabsorption of water by the renal tubules is then controlled by the production of the antidiuretic hormone of the posterior pituitary.' The mechanism whereby the volume of body fluids, particularly the extracellular fluid, is regulated, is not known. A mechanism analogous to that controlling osmotic ptessure has been postulated,2 whereby a receptor sensitive to changes in blood volume or flow stimulates the production of a hormone, possibly of the adrenal cortex, which, by adjusting the amount of salt reabsorbed, will regulate blood volume and indirectly extracellular fluid volume. Disturbances of water and electrolyte balance will be considered under two headings: (a) Water and salt balance; (b) potassium balance.