Dehydration lowers the blood volume, decreasing renal blood flow, and the kidneys produce less of the ammonia needed to maintain acid-base balance. Severe dehydration may lower the blood volume enough to cause circulatory shock and oliguria (Collins & Claros, 2011).

Water is one of the most important nutrients for maintaining life and ensuing healthy aging. Body fluids move nutrients, gases, and wastes throughout the body; are essential in metabolizing food into energy; and assist in the body’s overall functions. The body loses water from the lungs, skin, gastrointestinal tract, and kidneys on a regular basis. There is a small, but significant, amount of water lost in the stool in normal conditions. The renal and stool loss of water is known traditionally as sensible water loss. Although the kidney can minimize the loss of fluid, it can never totally conserve water (Klotz, 1998).

The evaporative water loss that occurs through the lungs and skin is known as insensible water loss. Insensible water loss is independent of the body water and solute (electrolytes) and content. It does, however, depend on the patient’s body surface area and environmental temperature (Adelman & Solhung, 1996).

At least 80% of all fluids administered today contain some electrolytes. Because electrolyte therapy is often a lifesaving procedure, its safe and successful administration is essential. Knowledge of the fundamentals of fluid and electrolyte metabolism contributes to safe electrolyte therapy.

Abnormalities of body fluid and electrolyte metabolism present therapeutic problems. Treatment is complex when the mechanisms that normally regulate fluid volume, electrolyte composition, and osmolality are impaired. An understanding of these metabolic abnormalities enables the nurse to recognize the problems involved. Such clinical challenges arise in patients with renal insufficiency, adrenal insufficiency, adrenal hyperactivity, and other kinds of impaired organ function.

The two following examples illustrate how knowledge of fluid and electrolyte metabolism contributes to safe, successful therapy in the critically ill patient. Correction of a severe potassium deficit resulting from vomiting and diarrhea presents a problem in the dehydrated patient. Potassium replacement is imperative. However, potassium administered to patients with renal insufficiency results in potassium toxicity because the kidneys are unable to excrete electrolytes. The adverse effects of excess potassium on the heart muscle are arrhythmia and heart block. The nurse must recognize the importance of both hydrating the patient before potassium can be administered safely and watching for diminished diuresis that could necessitate a change in therapy. Once antidiuresis occurs, the potassium infusion must be interrupted and the physician/licensed independent practitioner (LIP) notified. Potassium is the chief cation of body cells (160 mEq/L of intracellular water) and is concerned with the maintenance of body fluid composition and electrolyte balance. Potassium participates in carbohydrate utilization and protein synthesis and is critical in the regulation of nerve conduction and muscle contraction, particularly in the heart. Chloride, the major extracellular anion, closely follows the metabolism of sodium, and changes in the acid-base of the body are reflected by changes in the chloride concentration.

Normally about 80% to 90% of the potassium intake is excreted in the urine and the remainder in the stools and, to a small extent, in the perspiration. The kidney does not conserve potassium well so that during fasting, or in patients on a potassium-free diet, potassium loss from the body continues resulting in potassium depletion. A deficiency of either potassium or chloride will lead to a deficit of the other.

Another example relates to therapeutic problems in patients with impaired liver function. When excessive loss of gastric fluid occurs, replacement fluid is necessary. Most deficits caused by gastric suction, unless severe, are treated with 0.9% sodium chloride in 5% dextrose in water or 0.33% sodium chloride in 5% dextrose in water. However, severe loss may call for gastric replacement fluids containing ammonium chloride, which can be potentially
dangerous when administered to patients with impaired liver function. Ammonium chloride administered to a patient with severe liver damage may result in ammonia intoxication because of the liver’s inability to convert ammonia to hydrogen ions and urea.

The total body water content of a person varies with age, weight, and sex. The amount of water depends on the amount of body fat. Body fat is essentially water free; the greater the fat content, the lower the water content.

In contrast to infants, who have a high body fluid content (approximately 70% to 80% of the body weight), approximately 60% of a typical adult’s weight consists of fluid (water and electrolytes; Figure 8-1). After 40 years of age, mean values for total body fluid in percentage of body weight decrease for both men and women; a sex differentiation, however, remains. After 60 years of age, the percentage may decrease to 52% in men and 46% in women. With aging, lean body mass decreases in favor of fat (Metheny, 2012).

The total body fluid is functionally allocated into two main compartments: the intracellular and the extracellular compartments (Figure 8-2). The intracellular compartment consists of the fluid inside the cells and comprises approximately two thirds of the body fluid, or 40% of the body weight. The extracellular compartment consists of the fluid outside the body cells—the plasma, representing 5% of the body weight, and the interstitial fluid (fluid in tissues), representing 15% of the body weight (Metheny, 2012).

In newborn infants, the proportion is approximately three-fifths intracellular and twofifths extracellular. This ratio changes and reaches the adult level by the time the infant is approximately 30 months of age.

There is one additional compartment, the transcellular compartment. The transcellular fluid is the product of cellular metabolism and consists of secretions such as gastrointestinal secretions and urine. An analysis of the secretions may assist the practitioner in tracing lost electrolytes and prescribing proper fluid and electrolyte replacement. Excessive fluid and electrolyte loss must be replaced to maintain fluid and electrolyte balance in the two main
compartments. The amount of body water loss is easily computed by weighing the patient and noting loss of weight: 1 L body water is equivalent to 1 kg, or 2.2 lb, of body weight. Up to 5% weight loss in a child or adult may signify moderate fluid volume deficit— > 5% may indicate severe fluid volume deficit. Weight changes are also valuable as indicators of body water gains—acute weight gain may indicate water excess.

Body fluid contains two types of solutes (dissolved substances): the electrolytes and the nonelectrolytes. Electrolytes serve two main purposes: to act in controlling body water volume by osmotic pressure and to maintain the proper acid-base balance of the body. The nonelectrolytes are molecules that do not break into particles in solution but remain intact. They consist of dextrose, urea, and creatinine.

Electrolytes are molecules that break into electrically charged particles called ions. The ion carrying a positive charge is called a cation, and the ion with a negative charge is called
an anion. Potassium chloride is an electrolyte that, dissolved in water, yields potassium cations and chloride anions. Chemical balance is always maintained; the total number of positive charges equals the total number of negative charges. The quantity of charges and their concentration are expressed as milliequivalents (mEq) per liter of fluid. Because the number of negative charges must equal the number of positive charges for chemical balance, the milliequivalents of cations must equal the milliequivalents of anions.

Each fluid compartment has its own electrolyte composition. The extracellular compartment (plasma and interstitial fluid) contains a high concentration of sodium, chloride, and bicarbonate and a low concentration of potassium. The composition of the intracellular fluid is quite different; the concentrations of potassium, magnesium, and phosphate are high, whereas the sodium and chloride concentrations are relatively low.

Electrolyte composition of the intracellular fluid is in part related to electrolyte composition of the plasma and interstitial fluids. Disturbances in the extracellular fluid (ECF) are reflected in the patient’s symptoms; this information, combined with an analysis of plasma, is thus a valuable guide to therapy.

Occasionally, however, the electrolyte determination of plasma may be misleading. For example, the concentration of potassium in plasma may be high while there is a body deficit. This surplus is due to the shift of potassium from intracellular to ECF in the process of large potassium losses through the kidneys. Determination of plasma sodium may also present a false picture. In the case of an edematous cardiac patient, the plasma concentration may be low despite excess body sodium because total body sodium is equal to the sum of the products of volume times concentration in the various compartments.

Fluids are described in various ways. Significant characteristics of fluids include their concentration, or osmolality, as well as their acidity or alkalinity (pH).

The body uses regulating mechanisms called homeostatic mechanisms to maintain the constancy of body fluid volume, electrolyte composition, and osmolality. These mechanisms function through the renocardiovascular, endocrine (adrenal, pituitary, and parathyroid), and respiratory systems. The kidneys, skin, and lungs are the main regulating agents (Metheny, 2012).

Electrolyte content of the intracellular fluid differs from that of the ECF. Because specialized techniques are required to measure electrolyte concentration in the intracellular fluid, it is customary to measure the electrolytes in the ECF, chiefly plasma. Plasma electrolyte concentrations may be used to assess and manage patients with a diversity of electrolyte imbalances. Although some tests are performed on serum, the terms serum electrolytes and
plasma electrolytes are used interchangeably. Table 8-1 identifies intracellular and extracellular concentrations and serum values. Table 8-2 lists plasma electrolyte values.