7 For example, any increase in ECF sodium concentration () will cause water to shift from the ICF to ECF until the ICF and ECF osmotic pressures are equal, thereby effectively distributing the Na + across extracellular and intracellular water. 6 An important consequence of this thermodynamic law is that the volume of distribution of body Na + and K + is actually the TBW rather than just the ECF or ICF volume, respectively. Thus, water will flow across membranes into a compartment with a higher solute concentration until a steady state is reached and the osmotic pressures have equalized on both sides of the cell membrane. This is because most biologic membranes are semipermeable (i.e., freely permeable to water but not to all aqueous solutes). Urea is unique among the major naturally occurring solutes in that it diffuses freely across most cell membranes 4 therefore, it is present in similar concentrations in almost all body fluids, except in the renal medulla, where it is concentrated by urea transporters (see Chapter 10).ĭespite very different solute compositions, both the ICF and ECF have an equivalent osmotic pressure, 5 which is a function of the total concentration of all solutes in a fluid compartment. HCO 3 − is present in both compartments but is approximately three times more concentrated in the ECF. Glucose, which requires an insulin-activated transport system to enter most cells, is present in significant amounts only in the ECF because it is rapidly converted intracellularly to glycogen or metabolites. 3 Similar transporters effectively result in confining Cl − largely to the ECF, and Mg 2+, organic acids, and phosphates to the ICF. Thus, membrane-bound Na +-K +-ATPase maintains Na + in a primarily extracellular location and K + in a primarily intracellular location. The solute composition of the ICF and ECF differs considerably because most cell membranes possess multiple transport systems that actively accumulate or expel specific solutes. 15.1 summarizes the estimated body fluid spaces of an average weight adult. Approximately 75% of the ECF compartment is interstitial fluid, and only 25% is intravascular fluid (blood volume). Estimates of the relative sizes of these two pools differ significantly, depending on the tracer used to measure the ECF volume, but most studies in animals and humans have indicated that 55% to 65% of TBW resides in the ICF, and 35% to 45% is in the ECF. Total body water (TBW) is distributed between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments. Water constitutes approximately 55% to 65% of body weight, varying with age, gender, and amount of body fat, and therefore constitutes the largest single constituent of the body. Yu MB, BChir, in Brenner and Rector's The Kidney, 2020 Body Fluids: Compartmentalization, Composition, and Turnover
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