The initial approach to acutely ill children includes an assessment of their fluid and electrolyte status. The ability to maintain homeostasis and correct disturbances requires knowledge of the composition of the fluid spaces of the body and their changes with age and disease. This chapter discusses the physiologic basis of fluid management, some of the common disturbances, and an approach to management.

FLUID COMPARTMENTS

Total body water (TBW) is divided into the intracellular and extracellular compartments, with the extracellular compartment subdivided into intravascular and extravascular compartments. The relative size of these compartments varies with age (Fig. 81-1).¹ TBW is approximately 79% of body weight at birth, decreasing to the adult proportion of 55% to 60% over the first year of life. This primarily relates to a drop in extracellular fluid (ECF). Postnatal diuresis, as well as growth in cellular tissue, is responsible for the majority of the change. In addition, blood volume decreases from 80 mL/kg at birth toward the adult value of 60 mL/kg. By the time the child is 1 year of age, TBW comprises approximately 60% of body weight and is approaching the adult distribution of one-third in the extracellular compartments and two-thirds in the intracellular compartments. Body water exists as a complex solution of salts, organic acids, and proteins. The exact composition varies with body compartment (Fig. 81-2).²

MOVEMENT OF FLUID

Cellular membranes form the barrier between the extracellular and intracellular spaces (Fig. 81-3). They are freely permeable to water but impermeable to electrolytes and proteins, except by active transport. Although the specific osmoles differ in the two compartments, the osmolality is equal. Water distributes across this barrier by osmotic pressure. A rise in extracellular osmolality, as occurs with a sodium load, results in movement of water from the intracellular space to the extracellular space. Conversely, water intoxication leads to the movement of water from the extracellular space to the intracellular space.

The vascular endothelium forms the barrier between the intravascular and interstitial spaces. It is permeable to water and electrolytes but not to protein. Two forces regulate fluid movement. Hydrostatic pressure, created by the propulsion of blood through vessels, favors movement of fluid from the intravascular space to the interstitial space. This pressure falls as blood travels from the arterioles through the capillary bed to the lower-pressure veins. Oncotic pressure, exerted primarily by albumin found in the vascular space, favors water movement from the interstitium into the vascular space. Under normal conditions, there is a balance in the movement of water and electrolytes from the vascular space to the endothelium at the arteriolar side and in the reverse direction at the venous side.