To have a thorough understanding of fluid and electrolyte issues, one must truly understand the body’s composition of solutes and water and how equilibrium is maintained in normal circumstances. Water is the most plentiful compound within the human body. Its percentage changes with age and body composition. It accounts for 80% of body weight in severely preterm infants, 70% in term infants, 65% in young children, and approximately 60% in older children and adolescents. There are two compartments for body water: intracellular and extracellular compartments.

Intracellular water (ICW) accounts for approximately 40% of body weight, or two thirds of total body water. Extracellular water (ECW) refers to interstitial and intravascular fluid volumes, collectively. Interstitial fluid is the water bathing the cells; serum is the water part of blood. ECW accounts for approximately 20% to 25% of body weight, or one third of total body water. Water movement between the interstitial and intravascular areas (i.e., water movement within ECW) is governed by starling forces. The equation is as follows:

Fluid movement = K* [(P_c – P_i) – (II_c – II_i)]

where K = capillary filtration coefficient, P_c = capillary hydrostatic pressure, P_i = interstitial hydrostatic pressure, II_c = capillary oncotic pressure, and II_i = interstitial oncotic pressure.

The movement of fluid across a capillary membrane is governed by the permeability of that membrane and the difference in hydrostatic and oncotic pressures on each side of the membrane. When normal homeostasis is disrupted (i.e., dehydration), the net movement of fluid is from the interstitial compartment into the intravascular compartment in the hopes of regaining homeostasis and maintaining blood pressure. On the other hand, the net
movement of water is from the intravascular space into the interstitium when there is intravascular volume overload or hypoalbuminemia.

Movement of water between fluid compartments (i.e., between the intracellular and extracellular space) occurs based on osmotic gradients. The osmolality of each compartment is reflective of the amounts and types of solutes in each space. Osmolality is defined as the number of milliosmoles of solute per kilogram of water. Extracellular homeostasis is regulated by the kidney. Intracellular homeostasis is regulated by a variety of transport mechanisms. Water flows freely (across semipermeable cell membrane) between the intracellular and extracellular compartments to equalize osmolality. Sodium and chloride are the major extracellular solutes. Glucose and urea nitrogen also play a significant role in extracellular osmolality, and the following equation estimates serum osmolality: