Harrison's Internal Medicine Chapter 46. Fluid and Electrolyte Disturbances Sodium and Water Sodium and Water: Introduction Composition of Body Fluids Water is the most abundant constituent in the body, comprising approximately 50% of body weight in women and 60% in men. This difference is attributable to differences in the relative proportions of adipose tissue in men and women. Total body water is distributed in two major compartments: 55–75% is intracellular [intracellular fluid (ICF)], and 25–45% is extracellular [extracellular fluid (ECF)]. The ECF is further subdivided into intravascular (plasma water) and extravascular (interstitial) spaces in a ratio of 1:3. . | Chapter 046. Sodium and Water Part 1 Harrison s Internal Medicine Chapter 46. Fluid and Electrolyte Disturbances Sodium and Water Sodium and Water Introduction Composition of Body Fluids Water is the most abundant constituent in the body comprising approximately 50 of body weight in women and 60 in men. This difference is attributable to differences in the relative proportions of adipose tissue in men and women. Total body water is distributed in two major compartments 55-75 is intracellular intracellular fluid ICF and 25-45 is extracellular extracellular fluid ECF . The ECF is further subdivided into intravascular plasma water and extravascular interstitial spaces in a ratio of 1 3. The solute or particle concentration of a fluid is known as its osmolality and is expressed as milliosmoles per kilogram of water mosmol kg . Water crosses cell membranes to achieve osmotic equilibrium ECF osmolality ICF osmolality . The extracellular and intracellular solutes or osmoles are markedly different due to disparities in permeability and the presence of transporters and active pumps. The major ECF particles are Na and its accompanying anions Cl-and HCO3- whereas K and organic phosphate esters ATP creatine phosphate and phospholipids are the predominant ICF osmoles. Solutes that are restricted to the ECF or the ICF determine the effective osmolality or tonicity of that compartment. Since Na is largely restricted to the extracellular compartment total body Na content is a reflection of ECF volume. Likewise K and its attendant anions are predominantly limited to the ICF and are necessary for normal cell function. Therefore the number of intracellular particles is relatively constant and a change in ICF osmolality is usually due to a change in ICF water content. However in certain situations brain cells can vary the number of intracellular solutes in order to defend against large water shifts. This process of osmotic adaptation is important in the defense of cell volume and .
