Fluid and electrolyte balance

The human body is made up principally of water, two-thirds of which is intracellular and one-third extracellular. Dissolved in body water are a variety of mineral and organic salts and proteins. These components are quantified in millimoles and are electrochemically balanced in each compartment. The relative quantities of each cation and anion are different in each compartment. Fluid motion through the body is determined by hydrostatic, osmotic and oncotic forces. The rate of extravasation is determined by the Starling equation. In perioperative medicine and critical care, temporal fluid shifts occur based on the stress response. Fluid management, in terms of volume and tonicity, should follow this response. Tissue oedema, particularly in the lungs, results from excess intravascular fluid or high venous pressures (hydrostatic pulmonary oedema) or damage to the capillary endothelium (capillary leak).