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The surgical patient is at risk for several derangements of body fluid volume and composition, some of which may be iatrogenic. Understanding the physiologic mechanisms that regulate the composition and volume of the body fluids and the principles of fluid and electrolyte therapy is essential for optimal patient management.

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Total body water comprises 45–60% of body weight; the percentage in any individual is influenced by age and the lean body mass, but in healthy individuals it remains remarkably constant from day to day. Table 9–1 lists the average values of total body water as a percentage of body weight for men and women of different ages. Total body water is divided into intracellular fluid (ICF) and extracellular fluid (ECF) compartments. Intracellular water represents about two-thirds of total body water, or 40% of body weight. The remaining one-third of body water is extracellular. ECF is divided into two compartments: (1) plasma water, comprising approximately 25% of ECF, or 5% of body weight; and (2) interstitial fluid, comprising 75% of ECF, or 15% of body weight.

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Table 9–1. Total Body Water (as Percentage of Body Weight) in Relation to Age and Sex.
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The solute composition of the ICF and ECF compartments differs markedly (Figure 9–1). ECF contains principally sodium, chloride, and bicarbonate, with other ions in much lower concentrations. ICF contains mainly potassium, organic phosphate, sulfate, and various other ions in lower concentrations.

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Figure 9–1.
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Electrolyte composition of human body fluids. Note that the values are in meq/L of water, not of body fluid. (From Leaf A, Newburgh LH: Significance of the Body Fluids in Clinical Medicine, 2nd ed. Thomas, 1955. Reproduced by permission from Blackwell Publishing.)

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Even though plasma water and interstitial fluid have similar electrolyte compositions, plasma water contains more protein than interstitial fluid. This results in slight differences in electrolyte concentrations, as governed by the Gibbs-Donnan equilibrium. The plasma proteins, chiefly albumin, account for the high colloid osmotic pressure of plasma, which is an important determinant of the distribution of fluid between vascular and interstitial compartments, as defined by the Starling relationships.

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The kidneys maintain constant volume and composition of body fluids by two distinct but related mechanisms: (1) filtration and reabsorption of sodium, which adjusts urinary sodium excretion to match changes in dietary intake, and (2) regulation of water excretion in response to changes in secretion of antidiuretic hormone. These two mechanisms allow the kidneys to keep the volume and osmolality of body fluid constant within a few percentage points despite wide variations in intake of salt and water. ...

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