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The composition of body fluids, metabolic requirements, and ability to adapt to change differs in infants and children compared to adults. Acute disruptions of fluids and electrolytes or acid-base balance can be life threatening, while chronic titubations interfere with normal growth and development, particularly of the brain and bone.

Water is the major constituent of the body. Total body water (TBW) ranges from 60% to 80% of body weight. A newborn infant has close to 80% of body weight as water, while a child over the age of 12 months has 60%. Since adipose tissue has very little water and approximately 80% of muscle tissue is water, a muscular child will have proportionately more water than an obese child. TBW is divided into 2 major compartments: intracellular fluid (ICF), which is 40% of body weight, and extracellular fluid (ECF), which is 20 % of body weight. ECF is further divided into interstitial (16% of body weight) and plasma (4% of body weight) volumes. At birth, ECF volume is approximately 1.5 times that of ICF volume. However, this balance changes rapidly such that ICF volume is equal to ECF volume by day 10 of life, and by 3 years of age, two thirds of TBW is ICF and one third is ECF. Plasma volume is maintained by the oncotic pressure exerted by plasma proteins (albumin). Plasma measurement of electrolytes and acid base is representative of the values for ECF.1

ECF and ICF volume is determined by the content of osmotically active particles within each compartment. Sodium (Na) and its coupled anions (chloride [Cl] and bicarbonate [HCO3]) are the osmotically active particles that control the ECF volume; potassium (K) and its coupled anions (from macromolecules such as organic phosphates) are the osmotically active particles that control ICF volume. Changes in ECF and ICF volume occur as water moves across the cell membrane through water channels (aquaporins) to achieve osmotic equilibrium. A change in the concentration of sodium in the ECF (plasma) will disrupt the osmotic equilibrium between the ICF and the ECF such that water moves across the cell membrane to achieve a new osmotic equilibrium. Extracellular osmolality is effectively determined by the ECF sodium concentration. This, in turn, determines ICF volume, since total body solute is relatively constant. ICF volume almost always increases (swollen cell) when plasma sodium concentration decreases (hyponatremia), and ICF volume decreases (shrunken cell) when the plasma sodium concentration increases (hypernatremia) (Fig. 466-1).

Figure 466-1.

The circle represents the cell membrane. Normal cell volume is depicted by the light red–shaded area. On the left side of the figure, the cell is placed in a hypotonic environment, which causes water to enter the cell, and the cell swells, as shown by the dark red–shaded area. On the right side of the figure, the cell is placed in a hypertonic environment with ...

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