Amount and Composition of Body Fluids

Fundamental Concepts

The nurse needs to understand the physiology of fluid and electrolyte balance and acid–base balance to anticipate, identify, and respond to possible imbalances in each. The nurse also must use effective teaching and communication skills to help prevent and treat various fluid and electrolyte disturbances.

AMOUNT AND COMPOSITION OF BODY FLUIDS

Approximately 60% of a typical adult’s weight consists of fluid (water and electrolytes). Factors that influence the amount of body fluid are age, gender, and body fat. In general, younger people have a higher percentage of body fluid than older people, and men have proportionately more body fluid than women. Obese people have less fluid than thin people because fat cells contain little water.

Body fluid is located in two fluid compartments: the intra-cellular space (fluid in the cells) and the extracellular space (fluid outside the cells). Approximately two thirds of body fluid is in the intracellular fluid (ICF) compartment and is located primarily in the skeletal muscle mass.

The extracellular fluid (ECF) compartment is further divided into the intravascular, interstitial, and transcellular fluid spaces. The intravascular space (the fluid within the blood vessels) contains plasma. Approximately 3 L of the average 6 L of blood volume is made up of plasma. The remaining 3 L is made up of erythrocytes, leukocytes, and thrombocytes. The interstitial space contains the fluid that surrounds the cell and totals about 11 to 12 L in an adult. Lymph is an example of interstitial fluid. The transcellular space is the smallest division of the ECF compartment and contains approximately 1 L of fluid at any given time. Examples of transcellular fluid are cerebrospinal, pericardial, synovial, intraocular, and pleural fluids; sweat; and digestive secretions.

Body fluid normally shifts between the two major compart-ments or spaces in an effort to maintain an equilibrium between the spaces. Loss of fluid from the body can disrupt this equilibrium. Sometimes fluid is not lost from the body but is unavailable for use by either the ICF or ECF. Loss of ECF into a space that does not contribute to equilibrium between the ICF and the ECF is referred to as a third-space fluid shift, or “third spacing” for short.

An early clue of a third-space fluid shift is a decrease in urine output despite adequate fluid intake. Urine output decreases be-cause fluid shifts out of the intravascular space; the kidneys then receive less blood and attempt to compensate by decreasing urine output. Other signs and symptoms of third spacing that indicate an intravascular fluid volume deficit include increased heart rate, decreased blood pressure, decreased central venous pressure, edema, increased body weight, and imbalances in fluid intake and output (I&O). Third-space shifts occur in ascites,burns, peritonitis, bowel obstruction, and massive bleeding into a joint or body cavity.

Electrolytes

Electrolytes in body fluids are active chemicals (cations, whichcarry positive charges, and anions, which carry negative charges).The major cations in body fluid are sodium, potassium, calcium,magnesium, and hydrogen ions. The major anions are chloride,bicarbonate, phosphate, sulfate, and proteinate ions.

These chemicals unite in varying combinations. Therefore,electrolyte concentration in the body is expressed in terms of milliequivalents (mEq) per liter, a measure of chemical activity, rather than in terms of milligrams (mg), a unit of weight. More specifically, a milliequivalent is defined as being equivalent to the electrochemical activity of 1 mg of hydrogen. In a solution, cations and anions are equal in mEq/L.

Electrolyte concentrations in the ICF differ from those in the ECF, as reflected in Table 14-1. Because special techniques are required to measure electrolyte concentrations in the ICF, it is customary to measure the electrolytes in the most accessible por-tion of the ECF, namely the plasma.

Sodium ions, which are positively charged, far outnumber the other cations in the ECF. Because sodium concentration affects the overall concentration of the ECF, sodium is important in reg-ulating the volume of body fluid. Retention of sodium is associ-ated with fluid retention, and excessive loss of sodium is usually associated with decreased volume of body fluid.

As shown in Table 14-1, the major electrolytes in the ICF are potassium and phosphate. The ECF has a low concentration of potassium and can tolerate only small changes in potassium con-centrations. Therefore, release of large stores of intracellular potassium, typically caused by trauma to the cells and tissues, can be extremely dangerous.

The body expends a great deal of energy maintaining the high extracellular concentration of sodium and the high intracellular concentration of potassium. It does so by means of cell mem-brane pumps that exchange sodium and potassium ions. Normal movement of fluids through the capillary wall into the tissues de-pends on hydrostatic pressure (the pressure exerted by the fluid on the walls of the blood vessel) at both the arterial and the ve-nous ends of the vessel and the osmotic pressure exerted by the protein of plasma. The direction of fluid movement depends on the differences in these two opposing forces (hydrostatic versus osmotic pressure).

In addition to electrolytes, the ECF transports other sub-stances, such as enzymes and hormones. It also carries blood com-ponents, such as red and white blood cells, throughout the body.