212 Fluid and Electrolyte Balance

Etiology

The underlying cause, originating factors, or disease mechanisms that give rise to a pathological condition or fluid/electrolyte imbalance.

Pathophysiology

The study of disordered, abnormal, or disrupted physiological processes that occur within the body as a result of disease, injury, or homeostatic imbalance.

Clinical manifestations

The observable physical signs (objective findings) and reported symptoms (subjective experiences) produced by a disease or state of fluid and electrolyte imbalance.

Intracellular compartment

The fluid space contained inside body cells holding approximately two-thirds of total body fluid, characterized by high concentrations of potassium, magnesium, and organic phosphates.

Extracellular compartment

The fluid space located outside body cells holding approximately one-third of total body fluid, characterized by high concentrations of sodium, chloride, and bicarbonate.

Interstitial compartment

The portion of extracellular fluid situated directly between tissue cells and outside blood vessels, containing very low concentrations of proteins.

Vascular compartment

The extracellular fluid space contained within blood vessels (blood plasma), rich in plasma proteins that exert capillary colloid osmotic pressure.

Osmolality

The degree of solute particle concentration in a body fluid measured as particles per kilogram of water, reflected clinically by the serum sodium concentration.

Osmosis

The passive movement of water across a semipermeable cell membrane from an area of lower particle concentration to an area of higher particle concentration until osmolality is equalized.

Filtration

The movement of fluid and small solutes between vascular and interstitial compartments across permeable capillary walls, driven by opposing hydrostatic and colloid osmotic pressures.

Diffusion

The passive movement of solute particles or gases down a concentration gradient from an area of higher concentration to an area of lower concentration.

Volume deficit

Extracellular fluid volume (ECV) deficit; an isotonic removal or loss of sodium-containing fluid from the extracellular compartment, leading to decreased vascular and interstitial fluid volumes.

Volume excess

Extracellular fluid volume (ECV) excess; an isotonic addition or retention of sodium-containing fluid in the extracellular compartment, causing expanded vascular and interstitial fluid volumes.

Dehydration

Clinical dehydration; a combined fluid disorder consisting of extracellular fluid volume deficit plus hypernatremia, resulting in decreased extracellular volume and cellular shriveling.

Edema

An excessive accumulation of fluid in the interstitial compartment caused by increased capillary hydrostatic pressure, increased interstitial colloid osmotic pressure, decreased capillary colloid osmotic pressure, or lymphatic blockage.

Hypernatremia

An elevated serum sodium concentration (>145 mEq/L) indicating hypertonic body fluids (water deficit), which pulls water out of cells by osmosis and causes brain cell shriveling.

Hyponatremia

A decreased serum sodium concentration (<135 mEq/L) indicating hypotonic body fluids (water excess), which drives water into cells by osmosis and causes brain cell swelling.

Plasma Electrolyte Imbalances

Hypokalemia

A decreased serum potassium concentration (<3.5 mEq/L) that hyperpolarizes muscle cell membranes, resulting in ascending skeletal muscle weakness, flaccid paralysis, abdominal distention, and cardiac dysrhythmias.

Hyperkalemia

An elevated serum potassium concentration (>5.0 mEq/L) that hypopolarizes muscle cell membranes, causing ascending flaccid paralysis, intestinal cramping, and severe cardiac dysrhythmias or cardiac arrest.

Hypocalcemia

A decreased serum total calcium concentration (<9.0 mg/dL) that lowers the threshold potential of excitable cells, producing increased neuromuscular excitability, tetany, and positive Chvostek/Trousseau signs.

Hypercalcemia

An elevated serum total calcium concentration (>11.0 mg/dL) that raises the threshold potential of excitable cells, resulting in decreased neuromuscular excitability, muscle weakness, diminished reflexes, fatigue, and constipation.

Hypomagnesemia

A decreased serum magnesium concentration (<1.5 mEq/L) that increases acetylcholine release at neuromuscular junctions, causing increased neuromuscular excitability, hyperreflexia, tetany, and positive Chvostek/Trousseau signs.

Hypermagnesemia

An elevated serum magnesium concentration (>2.5 mEq/L) that depresses acetylcholine release at neuromuscular junctions, causing decreased neuromuscular excitability, hyporeflexia, flaccid paralysis, hypotension, and respiratory depression.

Hypophosphatemia

A decreased serum phosphate concentration (<2.5 mg/dL) that impairs cellular ATP synthesis and tissue oxygen delivery, causing muscle weakness, respiratory failure from a weak diaphragm, paresthesias, and decreased cardiac contractility.

Hyperphosphatemia

An elevated serum phosphate concentration (>4.5 mg/dL) that typically induces secondary hypocalcemia with increased neuromuscular excitability or leads to soft-tissue calcium-phosphate crystal deposition.