Fluid and electrolyte problems are common in hospitalized patients. Most of these problems are secondary to other diseases or are undesirable side effects of therapy. There are a few diseases regularly associated with certain pH or electrolyte alterations that can help suggest the diagnosis and can be used to monitor therapy.

Fluid and electrolytes in one form or another make up nearly all of the human body. It is useful to think of these constituents as though they were contained in three separate compartments between which are variable degrees of communication: individual cells, containing intracellular fluid; vascular channels, containing blood or lymph; and the extracellular nonvascular tissue containing the interstitial fluid. Shifts of fluid and electrolytes between and within these compartments take place continually as the various activities concerned with homeostasis, cell metabolism, and organ function go on. To some degree, these changes can be monitored clinically by their effects on certain measurable parameters, including the pH and concentration of certain ions (electrolytes) in a fairly accessible substance, the blood. This chapter covers blood pH and its disturbances; the next chapter discusses certain electrolyte and fluid disorders.