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Upon completion of the chapter, the reader will be able to:

  1. Estimate the volumes of various body fluid compartments.

  2. Calculate the daily maintenance fluid requirement for patients given their weight, and gender.

  3. Differentiate among currently available fluids for volume resuscitation.

  4. Identify the electrolytes primarily found in the extracellular and intracellular fluid compartments.

  5. Describe the unique relationship between serum sodium concentration and total body water (TBW).

  6. Review the etiology, clinical presentation, and management for disorders of sodium, potassium, calcium, phosphorus, and magnesium.




  • Image not available. Total body water (TBW) is approximately 50% of lean body weight in normal females and 60% of lean body weight in males. For clinical purposes, most clinicians generalize that total body water accounts for 60% of lean body weight in adults, regardless of gender. TBW is composed of the intracellular fluid (two-thirds of TBW) and the extracellular fluid (one-third of TBW). The extracellular fluid is made up of two major fluid subcompartments: the interstitial fluid and the intravascular fluid.

  • Image not available. Therapeutic fluids include crystalloid and colloid solutions. The most commonly used crystalloids include normal saline, dextrose/half-normal saline, hypertonic saline, and lactated Ringer's solution. Examples of colloids include albumin, the dextrans, hetastarch, and fresh-frozen plasma (FFP).

  • Image not available. The calculated serum osmolality helps determine deviations in TBW content.

  • Image not available. Concentrated electrolytes (potassium chloride [KCl], potassium phosphate, and sodium chloride [NaCl] greater than 0.9%) should not be stored in patient care areas as a patient safety measure.

  • Image not available. Hyponatremia is the most common electrolyte disorder in hospitalized patients and is defined as a serum sodium concentration below 136 mEq/L (136 mmol/L).

  • Image not available. IV potassium infusions running at rates of greater than 10 mEq/h (10 mmol/h) require cardiac monitoring.

  • Image not available. Calcium gluconate is the preferred peripherally infused calcium supplement because it is less irritating to the veins. Calcium chloride (CaCl) must be infused via a central line.

  • Image not available. Severe hypophosphatemia can result in impaired diaphragmatic contractility and acute respiratory failure.

  • Image not available. Serum magnesium concentrations do not correlate well with total body magnesium stores. For this reason, magnesium supplementation is often given empirically to critically ill patients.




A thorough understanding of the fundamentals of fluid and electrolyte homeostasis is essential given the frequency with which clinical disturbances are seen and the profound effects these disturbances can have on various aspects of patient care. However, the interplay of body fluids, serum electrolytes, and clinical monitoring is complex, and a thorough command of these issues is a challenging task even for advanced practitioners.1 Practitioners must be familiar with the key concepts of body compartment volumes, calculation of daily fluid requirements, and the various types of fluid available for replacement. The management of disorders of sodium, potassium, calcium, phosphorus, and magnesium integrates these concepts with issues of dose recognition and patient safety.


The most fundamental concept to grasp is an assessment of total body water (TBW), which is directly related to ...

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