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LEARNING OBJECTIVES
Upon completion of the chapter, the reader will be able to:
Estimate the volumes of various body fluid compartments.
Calculate the daily maintenance fluid requirement for patients given their weight, and gender.
Differentiate among currently available fluids for volume resuscitation.
Identify the electrolytes primarily found in the extracellular and intracellular fluid compartments.
Describe the unique relationship between serum sodium concentration and total body water (TBW).
Review the etiology, clinical presentation, and management for disorders of sodium, potassium, calcium, phosphorus, and magnesium.
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BODY FLUID COMPARTMENTS
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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 Clinicians 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.
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The most fundamental concept to grasp is an assessment of total body water (TBW), which is directly related to body weight. 
TBW constitutes approximately 50% of lean body weight in healthy females and 60% of lean body weight in males. For clinical purposes, most clinicians generalize that TBW accounts for 60% of lean body weight in adults, regardless of gender. The percentage of TBW decreases as body fat increases and/or with age (75%–85% of body weight is water for newborns). Unless the patient is obese (body weight > 120% of ideal body weight [IBW]), clinicians typically use a patient’s actual body weight when calculating TBW.2 In obese patients, it is customary to estimate TBW using lean body weight or IBW as calculated by the Devine–Devine method: males’ lean body weight = 50 kg + (2.3 kg/in. × [height in inches − 60]) and females’ lean body weight = 45.5 kg + (2.3 kg/in. × [height in inches − 60]).3–5 Note that 1 kg is equivalent to 2.2 lb, 1 in. is equivalent to 2.54 cm, and 1 L of water weighs 1 kg (2.2 lb).
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The intracellular fluid (ICF) represents the water contained within cells and is rich in electrolytes such as potassium, magnesium, phosphates, and proteins. The ICF is approximately two-thirds of TBW regardless of gender. For a 70-kg person, this would mean that the TBW is 42 L and the ICF is approximately 28 L. Note that ICF represents approximately 40% of total body weight.
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The extracellular fluid (ECF) is the fluid outside the cell and is rich in sodium, chloride, and bicarbonate. The ECF is approximately one-third of TBW (14 ...