The body continually attempts to maintain an internal balance, or homeostasis, in response to internal and external stressors. Fluid, electrolyte, and acid-base balance are vital for physiological functioning. The body’s fluid (water and electrolytes) is contained within the cells, blood vessels, and tissue (interstitial) space. The tissue space exists between the cells and blood vessels. The fluid within the cell is intracellular fluid (ICF). The fluid within blood vessels is intravascular fluid. The fluid that is between and surrounds cells is interstitial fluid. Intravascular and interstitial fluid are extracellular fluid (ECF), or fluid contained outside the cells.

Permeability is the ability of a substance, molecule, or ion to move across a membrane. Cells have semi-permeable membranes that allow some solutes to pass through the membrane but will prevent the passage of other solutes. A solute is a substance dissolved in a solution. A solvent is a liquid that contains a substance in solution. Blood vessels have semi-permeable membranes. Cells receive oxygen and nutrients from the intravascular fluid of arterioles. Venules carry waste away from the cells.

Approximately 45% to 75% of the body’s weight is made up of water, about two-thirds of which is intracellular. Water acts as a solvent, transports nutrients and oxygen, gives shaper and form to the cells, regulates body temperature, acts as a joint lubricant, and cushions body organs. Electrolytes are compounds that, when dissolved in water or another solvent, form or dissociate into ions (electrically-charged particles). Cations are positively-charged ions and anions are negatively-charged ions. The amount of cations or anions in a solution is expressed as milliequivalents per liter (mEq/L).

The main body electrolytes are sodium (Na+), potassium (K+), calcium, (Ca2+), and magnesium (Mg2+). Others are bicarbonate ions, phosphate, sulfate, and organic acid ions. Imbalances occur when the body has too much or too little of any of these electrolytes. The factors affecting fluid and electrolyte balance are age, diet, stress, exercise, or a warm or humid environment. The ECF contains the largest quantities of sodium, chloride, and bicarbonate ions. The ICF contains large quantities of potassium and phosphate ions and moderate quantitites of magnesium and sulfate ions.

Water moves within the body through several processes or forces. Diffusion is the continual movement of molecules in a solution or a gas. Osmosis is the movement of a solvent from an area of lesser concentration to an area of greater concentration. Active transport is the movement of a molecule or ion by a carrier and adenosine triphosphate (ATP), a source of energy. Hydrostatic pressure is the force exerted by a liquid against the sides of the container that holds it, governed by the rate of blood flow, the arterial blood pressure, and the venous blood pressure. Filtration is the movement of fluid through a semi-permeable membrane from an area with higher hydrostatic pressure to an area with lower hydrostatic pressure. Colloid osmotic pressure is movement of fluid between the intravascular and interstitial compartments, based on the number of solute particles on the concentrated side and the presence of a semi-permeable membrane.

The regulators of fluid balance are fluid and food intake, plus normal skin, lungs, gastrointestinal tract, and kidney function. Water enters the body through oral liquids, water in foods, and water formed by oxidation of foods. Water leaves the body through the skin, lungs, gastrointestinal tract, and kidneys. Water is lost through the skin by perspiration and through diffusion (insensible loss). Water is lost through the lungs through expired air. Water is lost from the gastrointestinal tract through the feces. Water is lost through the kidneys in urine. When there is a deficit of ECF, hormones, notably antidiuretic hormone (ADH), aldosterone, and renin, the kidneys conserve water. When there is a deficit in intravascular water, the renal tubules reabsorb more sodium. Since sodium promotes water retention, the water molecules go with the sodium ions back into the intravascular space.

Acid-base balance is the homeostasis of the hydrogen ion concentration in extracellular fluid. The greater number of hydrogen ions in a fluid, the more acidic it is. The fewer the number of hydrogen ions in a fluid, the more alkaline it is. The normal pH of extracellular fluid is 7.35 to 7.45. Three control systems regulate acid-base balance: buffer systems, respiration, and renal control. An acid-base buffer system is a solution that contains two or more chemical compounds that prevent marked changes in hydrogen ion concentration when either an acid or a base is added. The bicarbonate-carbonic acid system (carbonate system) is the body’s primary buffer system. It stabilizes the pH by combining/uncombining with hydrogen ions to keep the hydrogen level normal.

The respiratory system maintains acid-base balance by controlling the amount of carbon dioxide in the ECF. The rate of metabolism determines the formation of carbon dioxide. As carbon dioxide is removed, there is less available in the blood to combine with water to form carbonic acid. Renal control of ECF pH works by removing hydrogen or bicarbonate ions from body fluids. When bicarbonate ions are too high, the kidneys excrete bicarbonate ions, making the urine more alkaline. If hydrogen ions are too high, the urine becomes more acidic.

Important assessment data for fluid and electrolyte problems are the daily weight (to determine whether the client is retaining fluids), vital signs, intake and output, history of thirst, edema, skin and mucous membrane turgor, appearance of sunken eyes, flat jugular and hand veins, positive Chvostek’s sign, and positive Trousseau’s sign. Diagnostic tests include a low hematocrit and hemoglobin, serum osmolality (total concentration of dissolved particles or solutes per kilogram of water in serum), urine osmolality, urine pH, and serum albumin. Possible nursing diagnoses for clients with fluid and electrolyte alterations are Excess Fluid Volume, Deficient Fluid Volume, Risk for Deficient Fluid Volume, Impaired Gas Exchange, Decreased Cardiac Output, Risk for Infection, Impaired Oral Mucous Membrane, and Deficient Knowledge. Planning and implementation varies but should include daily weights, vital sounds, measurement of intake and output, oral hygiene, monitoring IVs, and client teaching.

IV fluids may be hypotonic (more dilute than the ECF), isotonic (the same osmolality as the ECF), or hypertonic (more concentrated than the ECF). Health care institutions stock and use a variety of administration sets, needles, dressings, and flow-control devices when administering IVs. Common complications of IV therapy are infiltration (leaking of fluids and medications into the surrounding tissue) and phlebitis (irritation of the vein).