The surgical patient is at risk for several derangements of body fluid volume and composition, some of which may be iatrogenic. Understanding the physiologic mechanisms that regulate the composition and volume
of the body fluids and the principles of fluid and electrolyte therapy
is essential for optimal patient management.
Total body water comprises 45–60% of body weight; the percentage in any individual is influenced by age and the lean body
mass, but in healthy individuals it remains remarkably constant
from day to day. Table 9–1 lists
the average values of total body water as a percentage of body weight
for men and women of different ages. Total body water is divided
into intracellular fluid (ICF) and extracellular fluid (ECF) compartments.
Intracellular water represents about two-thirds of total body water,
or 40% of body weight. The remaining one-third of body
water is extracellular. ECF is divided into two compartments: (1)
plasma water, comprising approximately 25% of ECF, or 5% of
body weight; and (2) interstitial fluid, comprising 75% of
ECF, or 15% of body weight.
Table 9–1. Total Body Water (as Percentage of Body Weight) in Relation to Age and Sex. |Favorite Table|Download (.pdf)
Table 9–1. Total Body Water (as Percentage of Body Weight) in Relation to Age and Sex.
The solute composition of the ICF and ECF compartments differs markedly (Figure 9–1). ECF contains principally sodium, chloride, and bicarbonate, with other ions in much
lower concentrations. ICF contains mainly potassium, organic phosphate, sulfate,
and various other ions in lower concentrations.
Electrolyte composition of human body fluids. Note that the values are in meq/L of water, not of body fluid. (From Leaf A, Newburgh LH: Significance of the Body Fluids in Clinical Medicine, 2nd ed. Thomas,
1955. Reproduced by permission from Blackwell Publishing.)
Even though plasma water and interstitial fluid have similar electrolyte compositions, plasma water contains more protein than
interstitial fluid. This results in slight differences in electrolyte
concentrations, as governed by the Gibbs-Donnan equilibrium. The
plasma proteins, chiefly albumin, account for the high colloid osmotic pressure of plasma, which is an important determinant of the distribution
of fluid between vascular and interstitial compartments, as defined
by the Starling relationships.
The kidneys maintain constant volume and composition of body fluids by two distinct but related mechanisms: (1) filtration and
reabsorption of sodium, which adjusts urinary sodium excretion to
match changes in dietary intake, and (2) regulation of water excretion
in response to changes in secretion of antidiuretic hormone. These two mechanisms allow the kidneys to keep the volume and osmolality
of body fluid constant within a few percentage points despite wide
variations in intake of salt and water. ...