Body volumes and fluid kinetics

Abstract

The sizes of body fluid volumes have been measured under steady state conditions by the use of tracer methods. In an adult weighing 70 kg, they average 3 L for the plasma, 11 L for the interstitial fluid, and 28 L for the intracellular fluid (ICF) volume. Hence, the sum of the plasma and interstitial fluid volumes (the extracellular fluid, or ECF, volume) amounts to 14 L, or 20% of the body weight.Substances known to distribute solely within one body fluid compartment can be injected and the size of the compartment be calculated by means of dilution of the substance.The total body water (sum of ECF and ICF) can be measured with water isotopes, which include tritium (radioactive) and deuterium (not radioactive). The plasma volume has frequently been measured by radioactive iodated albumin.The indocyanine green (ICG) is a dye that binds to plasma globulins. The half-life is only 3 min due to rapid uptake by the liver. Therefore, ICG can be used both to measure the liver blood flow and the plasma volume.The volume effect of an infusion fluid implies how much of the infused volume that expands the blood volume. A simplistic approach to quantify the volume effect of an infusion fluid is to measure the Hb concentration before and after the infusion. Hb mathematics can also be elaborated upon to create a pharmacokinetic system for the analysis and simulation of the distribution and elimination if infusion fluids, an approach called volume kinetics.