References:
- Saunders Comprehensive Review for the NCLEX-RN Examination, 9th Edition, ISBN 978-032-37-9530-2, by Linda Anne Silvestri, Angela E. Silvestri, and Jessica Grimm (Ch. 8, pp. 88-105)
Electrolytes are substances that, on dissolving in solution, ionizes; that is, some of its molecules split or dissociate into electrically charged atoms or ions. The metric system is used to measure volumes of fluids; liters (L) or milliliters (mL). The unit of measure that expresses the “combining activity” of an electrolyte is the milliequivalent (mEq). One mEq of any cation (positive ion) reacts chemically with one mEq of any anion (negative ion). They provide information about the number of anions or cations available to combine with other anions or cations.
Fluids are stored in different body compartments, and each compartment contains electrolytes. To function normally, body cells must have fluids and electrolytes in the right compartments in the right amounts. Homeostasis must exist; the number of cations and anions must be the same. Whenever an electrolyte moves out of a cell, another electrolyte moves in to take its place. Each compartment is divided by a semipermeable membrane:
- Intravascular compartment: fluids inside blood vessels. This contains ~6% of total body fluid.
- Intracellular compartment: fluids inside cells. This compartment has the most volume at ~70% of total body fluid.
- Extracellular compartment: fluids outside cells. This includes the interstitial fluid (~22%), which is fluid between cells (sometimes called the third space), blood, lymph, bone, connective tissue, and water; and transcellular fluid (~2%).
Body Fluids
Body fluids are responsible for transporting nutrients and removing waste products from cells.
- Total body fluid (intracellular and extracellular) amounts to about 60% of body weight in adults, 55% in older adults, and 80% in infants. Due to lowered and heightened fluid composition of the elderly and infants, they are at higher risk for fluid-related problems.
Fluid Imbalance
Infants and older adults need to be closely monitored for fluid imbalances when at risk.
- Body fluids consist of water and dissolved substances. The largest single fluid constituent of the body is water. Some substances, such as glucose, urea, and creatinine, do not dissociate in solution; that is, they do not separate from their complex forms into simpler substances when they are in solution. Other substances do dissociate, such as sodium chloride which separates into two elements in solution.
Fluid Shifting
- Third-Spacing refers to the accumulation and sequestration of trapped extracellular fluid in an actual or potential body space as a result of disease or injury. This trapped fluid represents a volume loss and is unavailable for normal physiological processes. These spaces include the pericardial, pleural, peritoneal, or joint cavities; the bowel; the abdomen; or within soft tissues after trauma or burns.
- Assessing intravascular fluid loss caused by this type of shifting is difficult to assess. Fluid is not excreted, so alterations in weight, intake, and output is not apparent until after organ malfunction occurs.
- Edema: the excess accumulation of fluid in the interstitial space; results from alterations in oncotic pressure, hydrostatic pressure, capillary permeability, and lymphatic obstruction (discussed in Fluid Transport).
- Localized edema can result from trauma (from accidents, surgery), localized inflammatory processes, or burns.
- Generalized edema, also called anascara, is an accumulation of fluid in the interstitial space throughout the body and results as a result of cardiac or renal conditions, or liver failure.
Fluid Transport
- Diffusion: a solute spreads through a solution or solvent, moving from higher concentration to lower concentration. A permeable membrane allows substances to pass through without restriction. A selectively permeable membrane allows some solutes to pass through without restriction, but prevents other solutes from passing freely. This occurs within and between fluid compartments if the membranes are permeable with the diffusing substances.
- Osmosis
- Filtration: the movement of solutes and solvents through hydrostatic pressure, moving from an area of higher pressure to an area of lower pressure.
- Hydrostatic Pressure: a force exerted by the weight of a solution. This is the driving force for filtration, where high pressure compartments “push” solutes and solvents into lower pressure compartments.
- A prime example of hydrostatic pressure is at the arterial end of capillaries, where hydrostatic pressure exceeds osmotic pressure and fluids and diffusible solutes move out of the capillary (e.g. oxygen from hemoglobin fuels cells) and the venous end of capillaries, where osmotic pressure exceeds hydrostatic pressure and pulls solutes and fluids into the capillary (e.g. waste products exit cells and enter the blood).
- Osmolality: the number of osmotically active particles per kilogram of water; it is the concentration of a solution. In the body, osmotic pressure is measured in milliosmoles (mOsm). The normal osmolality of plasma is 275 to 295 mOsm/kg (275 to 295 mmol/kg). This is the reference point for isotonicity of IV solutions.
Movement of Body Fluid
Cell membranes and capillary walls separate body compartments. Cell membranes are selectively permeable, and permit the movement of water and solutes through several forces. Higher concentrations of solutes produce more forces that move fluid and solutes from the compartment.
- In the case of isotonic solutions, both sides of a selectively permeable membrane have equal in concentration. Very little osmosis occurs as both sides have the same osmolality.
- In the case of hypotonic solutions: a solution, if lower in osmolality than another, is hypotonic to that solution; for example, IV solutions with an osmolality of less than 275 mOsm/L is considered hypotonic. Osmosis occurs, “hydrating” the cells.
- In the case of hypertonic solutions: a solution, if higher in osmolality than another, is hypotonic to that solution; for example, IV solutions with an osmolality of more than 295 mOsm/L is considered hypertonic. Osmosis occurs, “dehydrating” the cells.
- Osmotic pressure is produced by the concentration of solutes in a solution. These are the “fighting” pressures across a semipermeable membranes.
- Active transport is utilized by the body when moving electrolytes against the concentration gradient or osmosis. Metabolic processes in the cell utilize energy to allow this transport to occur. This includes sodium, potassium, calcium, iron, and hydrogen; some of the sugars, and the amino acids.
Fluid Intake and Output
Water normally enters the body through three sources: oral intake of fluids, foods (which contain water), and through the oxidation of ingested foods, e.g., the formation of 10 mL of water as a byproduct of the metabolism of 100 calories of fat, carbohydrates, or proteins. Water exits the body primarily through urine, via the kidneys. The kidneys play a major role in regulating fluid and electrolyte balance by adjusting the amount of fluid and electrolytes in the urine. It excretes the largest quantity of fluid.
- Water loss that cannot be measured is collectively coined as insensible loss, mostly attributed to water lost through moisture released from the lungs when breathing. This rate of loss varies based on depth and respiration.
- A large volume of electrolyte-containing liquids moves into the gastrointestinal tract, but this is almost entirely reabsorbed. A problem arises when severe diarrhea causes the loss of large quantities of fluids and electrolytes.
- Many organs contribute to balancing fluid content, and normal functioning for all of them is necessary to achieve and maintain fluid balance.
Diarrhea
The client with diarrhea is at high risk for a fluid and electrolyte imbalance.
| Source | Amount | Output | Amount |
|---|---|---|---|
| Ingested Water | 1200 - 1500 mL | Kidneys | 1500 mL |
| Ingested Food | 800 - 1100 mL | Insensible Loss (Skin) | 600 - 800 mL |
| Metabolic Oxidation | 300 mL | Insensible Loss (Lungs) | 400 - 600 mL |
| GI Tract | 100 mL | ||
| Total | 2300 - 2900 mL | Total | 2600 - 3000 mL |
Fluid and Electrolyte Homeostasis
The concentration and composition of body fluids must be nearly constant. Fluids and electrolytes exist together in solution; when either fluids or electrolytes are deficient, substances must be replaced through the intake of food and water or by therapy (e.g. IV solutions and medications). When excessive, therapy focuses on aiding in excretion (e.g. diuretics).
- The kidneys plays a major role in controlling this balance.
- The adrenal glands atop the kidneys also contribute to controlling extracellular fluids by regulating the amount of sodium reabsorbed by the kidneys.
- Antidiuretic hormone from the pituitary gland regulates the osmotic pressure of extracellular fluid by regulating the amount of water reabsorbed by the kidneys.
Nursing Intervention
If the client has a fluid or an electrolyte imbalance, the nurse must closely monitor the client’s cardiovascular, respiratory, neurological, musculoskeletal, renal, integumentary, and gastrointestinal status.