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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.
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FLUIDS & ELECTROLYTES
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Surgical patients are at a high risk for derangements in acid–base and fluid status 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.
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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 than women, in children compared to adults, and in people of normal body mass index (BMI) compared to those with obesity (Table 11–1). Two-thirds of total body water (TBW), or 30%-40% of body mass, is intracellular; one-third, or 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.
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The intracellular, interstitial, and intravascular compartments each hold fluid characterized by markedly different electrolyte and protein composition (Figure 11–1). The main intracellular cation is potassium (K+), whereas the main extracellular cation is sodium (Na+). Intracellular cations are primarily electrically balanced by the polyatomic ion phosphate (PO43–) and negatively charged proteins, whereas extracellular cations are balanced mainly by the chloride ion (Cl–). Intravascular fluid has a relatively higher concentration of protein and lower concentration of organic acids than interstitial fluid. This higher concentration of protein, chiefly composed of albumin, is the main driver of the high colloid osmotic pressure of serum, which in turn regulates 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 regulated largely by the kidneys. The kidneys maintain a steady volume and osmolality by modulating how ...