7.1 - Body Fluid Compartments

Abstract

This chapter describes the indicator dilution method for determining the volume of distribution of otherwise inaccessible fluid compartments. Total body water is marked by deuterium oxide and averages around 60% of body weight. The extracellular volume can be determined by inulin as a marker, and it averages 20% of body weight. The plasma volume can be found using Evan’s blue dye as a marker, and it is typically 5% of body weight. The main fluid compartments are not these, but the intracellular fluid, interstitial fluid (ISF), and plasma, which can be obtained from the volumes of distribution of these markers. The plasma and ISF are nearly identical except for the protein concentration and its attendant Gibbs–Donnan effect. The basis of the Gibbs–Donnan distribution ratio is discussed. The rest of the chapter considers the effects of three interventions on the intracellular and extracellular fluid volumes and osmolarity: drinking 1 L of water; eating 10 g of NaCl, and infusing 1 L of isotonic saline. The point is made that the kidney regulates the volume and composition of body fluids by operating on the plasma.