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Water and Na balance are closely interdependent. Total body water (TBW) is about 60% of body weight (ranging from about 50% in obese people to 70% in lean people). Almost 2⁄3 of TBW is in the intracellular compartment (intracellular fluid, or ICF); the other 1⁄3 is extracellular (extracellular fluid, or ECF). Normally, about 25% of the ECF is in the intravascular compartment; the other 75% is interstitial fluid.
Fig. 1: Fluid and Electrolyte Metabolism: Fluid compartments in an average 70-kg man. depicts the distribution of TBW.
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Fig. 1
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Fluid compartments in an average 70-kg man.
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Total body water = 70 kg × 0.60 = 42 L.
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The major intracellular cation is K, with an average concentration of 140 mEq/L. The extracellular K concentration is 3.5 to 5 mEq/L. The major extracellular cation is Na, with an average concentration of 140 mEq/L and an intracellular Na concentration of 12 mEq/L.
The concentration of combined solutes in water is osmolarity, which, in body fluids, is similar to osmolality. Serum osmolality can be measured in the laboratory or estimated according to the formula
Plasma osmolality (mOsm/kg) =
where serum Na is expressed in mEq/L and glucose and BUN are expressed in mg/dL. Osmolality of body fluids is normally between 275 and 290 mOsm/kg. Na is the major determinant of serum osmolality. Apparent changes in osmolality may result from errors in the measurement of Na with electrodes that are not ion-sensitive (see Fluid and Electrolyte Metabolism: Diagnosis). If measured osmolality exceeds estimated osmolality by ≥ 10 mOsm/L, then unmeasured osmotically active substances are probably present in the plasma (osmolar gap). The most common are alcohols (ethanol, methanol, isopropanol, ethylene glycol), mannitol , and glycine.
Water crosses cell membranes freely from areas of low solute concentration to areas of high solute concentration. Thus, osmolality tends to equalize across the various body fluid compartments, resulting primarily from movement of water, not solutes. Solutes such as urea that freely diffuse across cell membranes have little or no effect on water shifts (little or no osmotic activity), whereas solutes that are restricted to one fluid compartment, such as Na and K, have the greatest osmotic activity. Tonicity, or effective osmolality, reflects osmotic activity and determines the force drawing water across fluid compartments (the osmotic force). Osmotic force can be opposed by other forces. For example, plasma proteins have a small osmotic effect that tends to draw water into the plasma; this is normally counteracted by vascular hydrostatic forces that drive water out of the plasma.
The average daily fluid intake is about 2.5 L. The amount needed to replace losses from the urine and other sources is about 1 to 1.5 L/day in healthy adults. However, an average young adult with normal kidney function may ingest as little as 200 mL of water each day to excrete the nitrogenous and other wastes generated by cellular metabolism on a short-term basis. More is needed in people with any loss of renal concentrating ability (ie, the elderly; those with diabetes insipidus, certain renal diseases, hypercalcemia, severe salt restriction, chronic overhydration or hyperkalemia; and those ingesting ethanol, phenytoin , lithium , demeclocycline , or amphotericin B ) and those with osmotic diuresis (eg, due to high protein diets or hyperglycemia).
Other obligatory water losses are mostly insensible losses from the lungs and skin, averaging about 0.4 to 0.5 mL/kg/h body weight or about 650 to 850 mL/day in a 70-kg adult. With fever, another 50 to 75 mL/day may be lost for each degree C of temperature elevation above normal. GI losses are usually negligible, except with marked vomiting, diarrhea, or both. Sweat losses can be significant with environmental heat.
Water intake is regulated by thirst. Thirst is triggered by receptors in the anterolateral hypothalamus that respond to increased plasma osmolality (as little as 2%) or decreased body fluid volume. Hypothalamic dysfunction decreases the capacity for thirst.
Water excretion is regulated primarily by arginine vasopressin , also known as ADH. ADH is released by the posterior pituitary and results in increased water reabsorption in the distal nephron. ADH release is stimulated by increased plasma osmolality, decreased blood volume, decreased BP, or stress. ADH release may be impaired by certain drugs (eg, ethanol, phenytoin ) and diabetes insipidus (see Pituitary Disorders: Central Diabetes Insipidus).
Maximum daily fluid intake can be as much as 25 L. Greater amounts exceed the diluting capacity of the kidneys and quickly lower plasma osmolality.
Last full review/revision November 2005
Content last modified November 2005
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