Infants and children differ from mature adults in their metabolic requirements and turnover, composition of body fluids, and the degree to which regulation and/or adaptation occurs in normal or disordered states. All of these factors not only affect the growth and development of a child, but also, if deranged, may result in significant morbidity and mortality. Hence, a clear understanding of the composition of body fluids and their constituents in different compartments is essential to the optimal management in all children. Moreover, a disturbance in fluid, electrolyte, and/or acid-base balance commonly accompanies many other disorders. Thus, a pediatrician must be able to diagnose and appropriately manage such disturbances.
BODY FLUID AND ELECTROLYTE DISTRIBUTION AND REGULATION
Water is the major constituent of the body. Total body water ranges from 60% to 80% of body weight. A newborn infant has close to 80% of body weight as water, whereas a child over the age of 12 months has approximately 60%. Since adipose tissue has very little water and approximately 80% of muscle tissue is water, a muscular child will have proportionately more water when compared to an obese child.
Total body water (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 is approximately 1.5 times that of ICF. However, this changes rapidly to become almost equal to ICF by day 10 of life. ICF gradually increases in proportion to become twice as much as ECF by age 3 years, and this proportion (two-thirds of TBW is ICF and one-third of TBW is ECF) is maintained thereafter. The compositional difference between interstitial fluid and plasma is mainly related to plasma proteins. Plasma volume is maintained by the oncotic pressure exerted by plasma proteins (primarily albumin). Thus, plasma measurement of electrolytes and acid-base is representative of the values for ECF.
ICF and ECF volumes maintain their proportions because water moves from one compartment to another through water channels (aquaporins) in the cell membrane to achieve osmotic equilibrium. When particles are restricted to 1 compartment, they will hold water with them so that water will not cross to the other compartment. Sodium (Na) and its coupled anions, mainly chloride (Cl) and bicarbonate (HCO3), are the particles restricted to ECF and thus control the ECF volume. Potassium (K) and its coupled anions, macromolecules such as organic phosphates, are restricted to ICF and control its volume. These macromolecules are essential for cellular function, and therefore, their concentration is nearly constant. Hence, change in the ratio of particle to water comes about largely as a result of change in the content of water in the ICF. Thus, water has to move into ...