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Optimal management of fluids, electrolytes, and acid-base status in patients who are critically ill requires a general understanding of their normal composition and regulation. Disease processes, trauma, and surgery can all affect the manner by which the body controls its fluid balance and electrolytes.

Fluid Compartments
  1. The Total Body Water (TBW) ranges from 50% to 70% of the body mass and is determined by lean body mass, sex, and age (Table 8.1). There is an inverse relationship between TBW and percentage body fat because of the low water content of adipose tissues.
  2. Compartments of TBW
    1. The intracellular compartment is approximately 66% of TBW (~40% of body mass).
    2. The extracellular compartment is approximately 34% of TBW (~20% of body mass) and can be further divided into the following:
      1. The intravascular compartment, composed of plasma. It is approximately 5% of total body mass.
      2. The extravascular compartment, composed of interstitial fluid, lymph, bone fluid, fluids of the various body cavities, and mucosal/secretory fluids. The extravascular compartment represents approximately 15% of total body mass.
      3. The third space, an ill-defined space invoked to account for otherwise inexplicable fluid losses in the perioperative period or in patients who are critically ill. It may be considered “nonfunctional” interstitial fluid that cannot equilibrate with the circulating blood volume. However, many experts doubt the existence of a discrete third space.
  3. Ionic Composition of the Fluid Compartments: Various physiologic terms describe the concentrations of ions in a solution.
    1. Molarity: Moles of solute per liter of solution
    2. Molality: Moles of solute per kilogram of solvent
    3. Osmolarity: Osmoles per liter of solution. The number of osmoles is determined by multiplying the number of moles of solute by the number of freely dissociated particles from one molecule of solute. For example, 1 mole of NaCl will yield 2 osm in solution.
    4. Osmolality: Osmoles per kilogram of solvent
    5. Electrolytes in physiology are generally described in terms of milliequivalents per liter (mEq/L). The fluids of each compartment are electrically neutral. Average concentrations of each electrolyte in various compartments are given in Table 8.2.
  4. Movement of Water in the Body
    1. Water is generally readily permeable through cell membranes and moves freely throughout the different fluid compartments. Movement of water is largely determined by osmotic pressure and hydrostatic pressure. The osmotic pressure depends on the number of osmotically active molecules in solution.
    2. The movement of water between the interstitial and intravascular compartments within the extracellular space is described by Starling’s equation:

      3. where Qf is the fluid flux across the capillary membrane; K f is a constant reflecting the permeability of capillaries to water and their surface area; Pc and P i are the hydrostatic pressures in the capillary and interstitium, respectively; σ is the reflection coefficient (see subsequent text); and pic and pii are the colloid osmotic pressures in the capillary and interstitium, respectively.

      1. Large, negatively charged intravascular proteins to which vascular membranes are impermeable are responsible for the osmotic pressure gradient between intravascular and interstitial compartments. This component of osmotic pressure, known as oncotic pressure or colloid osmotic pressure, contributes a small amount to the total osmotic pressure of the fluids. The positive ions that are associated with the negatively charged proteins also contribute to the osmotic pressure. Albumin is the predominant type of protein responsible for the oncotic pressure, accounting for approximately two-thirds of the total oncotic pressure. Cells do not contribute to the oncotic pressure.
      2. The reflection coefficient (σ) describes the permeability of plasma proteins across a specific capillary membrane. It ranges from 0 (completely permeable) to 1 (impermeable) and varies in different disease states; it is approximately 0.7 in healthy tissues.
      3. Edema occurs when the rate of interstitial fluid accumulation is greater than the rate of interstitial fluid removal by the lymphatic system.