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FLUID COMPARTMENTS

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Fluid in the body is distributed between intracellular and extracellular compartments. Total body water (TBW) is the sum of the intracellular and extracellular compartments. In a 70-kg adult male, it comprises 60% of body weight or about 42 L. This value can vary with age, gender, and with the amount of adipose tissue versus lean muscle present in the body, as the latter has higher water content.

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The extracellular fluid compartment (EFC) is equal to approximately one-third of the TBW or about 14 L in a 70-kg adult male. The extracellular compartment is subdivided into vascular, interstitial fluid, and transcellular compartments. The vascular compartment accounts for about 5% of total body weight or 3.5 L. The interstitial fluid compartment accounts for about 15% of total body weight or 9 L. The interstitial fluid tends to be low in protein and thus has a low oncotic pressure as compared to the vascular compartment. The intracellular fluid compartment accounts for two-third of TBW or about 28 L in a 60-kg adult male.

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ELECTROLYTES

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Sodium

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Sodium is the major cation found in the ECF. Its normal concentration in serum is 135-145 mmol/L. Sodium concentration plays a large role in governing the ECF volume through osmotic forces. Additionally, sodium plays an important role in the ability of neuronal and cardiac tissue to generate an action potential.

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The main factors that control sodium balance in the body are renal function (glomerular filtration rate), renin-angiotensin-aldosterone system, antidiuretic hormone (ADH), and atrial natriuretic peptide. Changes in serum sodium concentration largely have to do more with imbalances of TBW rather than sodium itself.

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Hyponatremia is largely due to an excess of water relative to sodium in the setting of increased ADH secretion, either due to hypovolemia, decreased effective atrial volume, or inappropriate secretion of ADH (SIADH). Hyponatremic patients can present with symptoms, including vomiting, weakness, mental status changes, seizures, and coma. The severity of these symptoms is related to acuity of the changes in serum sodium concentration. In asymptomatic patients, sodium concentration should be corrected slowly with a rate of no greater than 0.5 mEq/L/h using isotonic fluids such as normal saline or lactated ringers. Correcting at too rapid a rate can cause fluid shifts from the intracellular compartment to the extracellular compartment, potentially leading to central pontine myelinolysis. In symptomatic patients, the rate of sodium correction should be faster, with a goal of 2 mEq/L/h for the first 2-3 hours, until symptoms begin to improve. Treatment for hyponatremia can vary depending on the etiology. In patients with hypovolemic hyponatremia, normal saline infusion will provide volume resuscitation, removing the stimulus for ADH secretion and allowing the kidneys to remove excess free water. In patients with SIADH, fluid restriction and treatment of the underlying cause is most effective. With hypervolemic hyponatremic, patients require loop diuretics to mobilize excess water and ...

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