![]() Urine is the major source of electrolyte loss in the body except when there are fluid losses from the gastrointestinal tract. Typically this occurs in gastroenteritis, where reduced oral intake is combined with excessive water and electrolyte loss in the stool. High solute load and limited urine concentrating ability makes them prone to significant ECF contraction (dehydration) when there are excessive amounts of water loss. In addition, the infant’s high metabolic rate and the solute load from enteral feeding formula means they require more water excretion per unit solute amount. 2 Infants are somewhat disadvantaged compared to the older child and adult in that they cannot maximally dilute (infant 200 mOsm/L versus adult 80 mOsm/L) and concentrate urine (infant 800 mOsm/L versus adult 1200 mOsm/L). The average osmolar excretion in newborn infants receiving infant formula is 16 to 20 mOsm/kg/d. Obligate water excretion in the urine is dependent upon solute load and the ability to concentrate and dilute urine. TABLE 113-2 Water Losses in Normal Children (mL/100 kcal/24 h) 4 – 7 Further discussion of fluid and electrolyte physiology in the preterm infant is beyond the scope of this chapter. ![]() 3 In addition, glomerular filtration rate is lower than in the term infant, and the large surface area–to–body weight ratio leads to considerable evaporative losses. Fractional excretion of sodium is inversely correlated with age in the preterm, who is susceptible to both sodium loss and sodium and volume overload. The preterm infant has a relative expansion of both TBW and ECF volume expansion, and a diuresis in the first few days of postnatal life is a common finding. By term, ECF and intracellular fluid (ICF) volume has fallen to 45% and 30% of TBW, respectively ( Figure 113-1). During early fetal life, TBW represents 90% of total body weight, with 65% being in the extracellular fluid (ECF) compartment. 1, 2 Total body water (TBW) is high in the fetus and preterm infant. Body water content changes significantly with age in children. ![]()
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