Fluid compartments

[1] About two-thirds of the total body water of humans is held in the cells, mostly in the cytosol, and the remainder is found in the extracellular compartment.

Water and electrolytes are continuously moving across barriers (eg, cell membranes, vessel walls), albeit often in small amounts, to maintain this healthy balance.

Its extracellular fluid (ECF) contains about one-third of total body water.

The volume of the intravascular compartment is regulated in part by hydrostatic pressure gradients, and by reabsorption by the kidneys.

[5] Interstitial fluid provides the immediate microenvironment that allows for movement of ions, proteins and nutrients across the cell barrier.

[8] Examples of transcellular spaces include the eye, the central nervous system, the peritoneal and pleural cavities, and the joint capsules.

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.

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.

With pancreatitis or ileus, fluids may "leak out" into the peritoneal cavity, also causing depletion of the intracellular, interstitial or vascular compartments.

The precise volume of fluid in a patient's third spaces changes over time and is difficult to accurately quantify.