Body fluid compartments

Total body water makes up approximately 60% of total body weight. Two-thirds of body water is intracellular, the remaining third is divided between the intravascular (plasma, 20%) and interstitial (80%) compartment, that is, 3L intravascular, 12L interstitial, and 25L intracellular.

Intracellular

The composition of the intracellular volume is maintained by a metabolically active membrane. It has a low sodium concentration (10 mmol.l–1) and a high potassium (150 mmol.l–1) concentration.

Interstitial

The interstitial volume is that part of the extracellular volume that is not present in the plasma – it is the fluid that bathes the cells. During illness or injury its membrane becomes leaky allowing immunological mediators access and the formation of oedema. It has an electrolyte composition that is similar to the plasma with a high sodium concentration (140 mmol.l–1) and a low potassium concentration (4 mmol.l–1). It has less protein than the plasma and therefore a lower oncotic pressure.

Intravascular

The intravascular compartment has a composition similar to that of the interstitial space. When the red cell volume is added, the total blood volume is derived. Clearly the main function of the red cell is to transport oxygen from the lungs to the tissues. The plasma has a number of key functions, which include providing the fluid volume necessary to suspend the red cells, clotting and immunological functions.

Fluid replacement

In order to make appropriate choices about fluid replacement it is essential that the processes of distribution between the three compartments are understood.