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Intravenous fluidsBody 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 intravascu-
lar (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 for-
mation 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 concentra-
tion (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 intersti-
tial 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
Inorder to make appropriate choices about fluid replacement it is essential that the
processes of distributionbetween the three compartments are understood. These
principles can then be used to guide the use of the different types of fluid replacement
available in order to achieve specific aims.