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).
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 ...
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