Body Fluid Compartments. Accurate replacement of fluid deficits necessitates an understanding of the distribution spaces of water, sodium, and colloid. Total body water approximates 60% of total body weight (42 L in a 70-kg adult). Total body water consists of intracellular fluid (ICF; 28 L) and extracellular fluid (ECF; 14 L). Plasma volume is about 3 L, and red blood cell volume is about 2 L. Whereas sodium is present principally in the ECF (140 mEq/L), potassium is present principally in the ICF (150 mEq/L). Albumin is the most important oncotically active constituent of ECF (4 g/dL).
Distribution of Infused Fluids. Conventionally, clinical prediction of plasma volume expansion after fluid infusion assumes that body fluid spaces are static. However, infused fluid does not simply equilibrate throughout an assumed distribution volume but is added to a highly regulated system that attempts to maintain intravascular, interstitial, and intracellular volume. Kinetic models of intravenous (IV) fluid therapy allow clinicians to more accurately predict the time course of volume changes produced by infusions of fluids of various compositions.
Regulation of ECF volume is influenced by aldosterone (enhances sodium reabsorption), antidiuretic hormone (enhances water reabsorption), and atrial natriuretic peptide (enhances sodium and water excretion).