<P> Fluid shifts occur when the body's fluids move between the fluid compartments . Physiologically, this occurs by a combination of hydrostatic pressure gradients and osmotic pressure gradients . Water will move from one space into the next passively across a semi permeable membrane until the hydrostatic and osmotic pressure gradients balance each other . Many medical conditions can cause fluid shifts . When fluid moves out of the intravascular compartment (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain, heart and kidneys; when it shifts out of the cells (the intracellular compartment), cellular processes slow down or cease from intracellular dehydration; when excessive fluid accumulates in the interstitial space, oedema develops; and fluid shifts into the brain cells can cause increased cranial pressure . Fluid shifts may be compensated by fluid replacement or diuretics . </P> <P> "Third spacing" is the abnormal accumulation of fluid into an extracellular and extravascular space . In medicine, the term is often used with regard to loss of fluid into interstitial spaces, such as with burns or edema, but it can also refer to fluid shifts into a body cavity (transcellular space), such as ascites and pleural effusions . With regard to severe burns, fluids may pool on the burn site (i.e. fluid lying outside of the interstitial tissue, exposed to evaporation) and cause depletion of the fluids . With pancreatitis or ileus, fluids may "leak out" into the peritoneal cavity, also causing depletion of the intracellular, interstitial or vascular compartments . </P> <P> Patients who undergo long, difficult operations in large surgical fields can collect third - space fluids and become intravascularly depleted despite large volumes of intravenous fluid and blood replacement . </P> <P> The precise volume of fluid in a patient's third spaces changes over time and is difficult to accurately quantify . </P>

When an excess of body fluid exists in the intravascular compartment