Pathogenesis and Treatment of Hypoosmolar and Hyperosmolar States

Under usual conditions, the osmolality of the extracellular fluid (ECF) is maintained within an extremely narrow range. When this range is exceeded, the cells of the body are subject to an injurious influx or efflux of water, which preserves uniform tonicity of all body fluids. Thus, in situations where the osmolality of body fluids is disturbed, the principal pathophysiology is related to the inability of cells to maintain normal volume (and hence structure and function). Of course, each of the somatic cells of the body has some ability to regulate its own volume; when this regulatory ability is exceeded, symptoms referable to hypoosmolality or hyperosmolality appear. Before discussing specific clinical syndromes of deranged plasma tonicity, it is appropriate to review some important aspects of cell volume regulation.