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Fluid status, electrolyte homeostasis, and acid-base balance are clinical parameters of critical significance in surgical patients. Understanding normal physiology and pathophysiology related to these parameters is crucial.
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Surgical patients are at high risk for derangements of body water distribution, electrolyte homeostasis, and acid-base physiology. These disturbances may be secondary to trauma, preexisting medical conditions which alter normal physiology, or the nature of the surgery.
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BODY WATER DISTRIBUTION & COMPOSITION
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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 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.
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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.
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The body’s volume status and electrolyte composition are determined largely by the kidneys. The kidneys maintain a constant ...