Fluid status, electrolyte homeostasis, and acid–base balance are clinical parameters of critical significance in surgical patients. As a surgeon, it is crucial to understand normal physiology and pathophysiology related to these parameters.
Surgical patients are at a high risk for derangements of body water distribution and electrolyte homeostasis. These disturbances may be secondary to trauma, surgical or medical conditions that may alter normal physiology, or the nature of the surgery itself.
Body Water Distribution & Composition
Total body mass is 45%-60% water. The percentage in any individual is influenced by age and lean body mass; therefore the percentage is higher in men compared to women, in children compared to adults, and in people of normal body habitus compared to the obese (Table 9–1). Two-thirds of total body water (TBW), 30%-40% of body mass, is intracellular; one-third, 15%-20% of total body mass, is extracellular. The extracellular fluid (ECF) is divided into two compartments, with 80% (12%-16% of total body mass) in the interstitial compartment and 20% (3%-4%) in the intravascular compartment. One-fifth of intravascular fluid is proximal to the arterioles; the remaining four-fifths is distal to the arterioles.
Table 9–1.Approximate percentage water of total body mass. |Favorite Table|Download (.pdf) Table 9–1. Approximate percentage water of total body mass.
|Patient ||% |
|Children ||60 |
|Non-elderly males ||60 |
|Non-elderly females ||50 |
|Elderly males ||50 |
|Elderly females ||45 |
The intracellular, interstitial, and intravascular compartments each hold fluid characterized by markedly different electrolyte profiles (Figure 9–1). The main intracellular cation is the potassium ion (K+), while the main extracellular cation is the sodium ion (Na+). Not only the electrolyte profile, but also the protein composition of the fluids, differs; intracellular cations are electrically balanced mainly by the polyatomic ion phosphate (PO43–) and negatively charged proteins, while extracellular cations are balanced mainly by the chloride ion (Cl−). The intravascular fluid has a relatively higher concentration of protein and lower concentration of organic acids than the interstitial fluid. This higher concentration of protein, chiefly albumin, is the main cause of the high colloid osmotic pressure of serum, which in turn is the chief regulator of the fluid distribution between the two extracellular compartments. The relationship between colloid osmotic pressure and hydrostatic pressure governs the movement of water across the capillary membrane, and is modeled by the Starling equation.
Electrolyte composition of human body fluids. Note that the values are in mEq/L of water, not of body fluid. (Reproduced with permission from Newburgh LH, Leaf A: Significance of the Body Fluids in Clinical Medicine. Springfield, IL: Charles C Thomas; 1950.)