Fluid and Electrolyte Homeostasis and Imbalance

Comprehensive vocabulary flashcards covering fluid compartments, oncotic and hydrostatic pressures, fluid regulation hormones, and clinical electrolyte imbalances from Chapter 24.

Intracellular Fluid (ICF)

The fluid contained within cell membranes throughout the body, comprising approximately $2/3$ of total body water (TBW). It is rich in potassium ($K^+$) and magnesium ($Mg^{2+}$) ions, and low in sodium ($Na^+$) and chloride ($Cl^-$) ions.

Extracellular Fluid (ECF)

The body fluid located outside the cells, comprising approximately $1/3$ of total body water (TBW). It is rich in sodium ($Na^+$), chloride ($Cl^-$), and bicarbonate ($HCO_3^-$) ions, and low in potassium ($K^+$), magnesium ($Mg^{2+}$), and phosphate ($PO_4^{3-}$) ions.

Total Body Water (TBW)

All water contained within the human body, encompassing intracellular fluid ($2/3$), extracellular fluid ($1/3$), and water present within the gastrointestinal and urinary tracts.

Interstitial Fluid

Extracellular fluid that surrounds the tissue cells, occupying the microscopic spaces between cells rather than being contained inside blood vessels.

Intravascular Fluid

The fluid portion of extracellular water residing specifically within blood vessels, commonly identified as blood plasma.

Hydrostatic Pressure

The mechanical pushing force that drives fluid OUT of the blood vessels and capillaries into the surrounding interstitial tissue space (measured at approximately $35\,mmHg$ at the arterial end of a capillary).

Oncotic Pressure (Colloid Osmotic Pressure)

The pulling force exerted by large plasma proteins (primarily albumin) that draws water into the blood vessels (approximately $25\,mmHg$ at the venous end), preventing fluid from leaking into interstitial tissues.

Albumin

The major plasma protein synthesized by the liver that stays inside the bloodstream and acts like a sponge to maintain vascular oncotic pressure; decreased levels (from cirrhosis or nephrotic syndrome) cause fluid leakage into tissues, leading to edema.

Total Body Water Variation by Age

The proportion of total body weight that is water decreases across the lifespan: approximately $80\%$ in preterm infants, $75\%$ in neonates, $65\%$ in young children, $60\%$ in adult males ($50\%$ in adult females), and $50\%$ in older adult males ($45\%$ in older adult females).

Insensible Water Loss

Invisible and non-measurable water loss that occurs continuously through evaporation from the skin and exhalation of water vapor through the lungs with each breath.

Antidiuretic Hormone (ADH / Vasopressin)

A hormone produced by the hypothalamus (supraoptic and paraventricular nuclei) and secreted by the posterior pituitary gland that signals the renal collecting ducts to reabsorb water into the bloodstream, thereby concentrating urine, reducing water loss, and regulating thirst.

Aldosterone

A mineralocorticoid hormone secreted by the adrenal cortex referred to as the 'salt water' hormone; it stimulates the kidneys to reabsorb sodium and water into the bloodstream while promoting the excretion of potassium ions ($K^+$) in a $1:1$ exchange.

Aquaporins

Specialized transmembrane protein channels embedded within the cell membrane that allow water molecules to move rapidly and selectively across lipid bilayers between the intracellular and extracellular compartments.

Sodium-Potassium Pump

An active transport membrane protein mechanism that hydrolyzes $ATP$ into $ADP + P_i$ to move three sodium ions ($Na^+$) out of the cell and two potassium ions ($K^+$) into the cytoplasm against their concentration gradients.

Hypovolemia

An extracellular volume (ECV) deficit resulting from body fluid loss via vomiting, diarrhea, excessive sweating, hemorrhage, burns, excessive diuretic use, or inadequate oral fluid intake.

Hypervolemia

An extracellular volume (ECV) excess caused by pathophysiologic states such as heart failure, kidney failure, excessive intravenous fluid administration, excessive dietary sodium intake, liver cirrhosis, or hyperaldosteronism.

Total Parenteral Nutrition (TPN)

A nutritionally complete intravenous infusion solution administered directly into the vascular system, serving as a significant parenteral source of electrolytes, water, and nutrients.

Atrial Natriuretic Peptide (ANP)

A peptide hormone released by the cardiac atria in response to elevated blood volume and atrial stretch; it signals the kidneys to excrete sodium and water, resulting in vasodilation and decreased blood pressure.

B-type Natriuretic Peptide (BNP)

A peptide hormone synthesized and released by the cardiac ventricles during volume overload and ventricular distention that promotes renal excretion of sodium and water, reducing vascular fluid volume and blood pressure.

Sodium ($Na^+$)

The major cation of extracellular fluid with a normal serum reference range of $135\text{--}145\,mEq/L$; it maintains extracellular osmolality, functions in acid-base balance, and initiates action potentials from neurons to muscle fibers.

Hyponatremia

A serum sodium concentration below $135\text{--}145\,mEq/L$ (specifically $< 135\,mEq/L$) that impairs action potential generation, leading to decreased neuromuscular excitability, malaise, confusion, disorientation, headaches, decreased deep tendon reflexes ($\downarrow DTRs$), and muscle weakness.

Hypernatremia

A serum sodium concentration exceeding $145\,mEq/L$, causing increased action potential generation, heightened neuromuscular excitability, agitation, increased deep tendon reflexes ($\uparrow DTRs$), muscle cramping, headaches, dry mucous membranes, fever, and tachycardia.

Potassium ($K^+$)

The major cation located inside the cell with a normal serum concentration of $3.5\text{--}5.0\,mEq/L$; it regulates neuromuscular excitability, intracellular osmolality, and critical cardiac electrophysiology and contractility.

Hypokalemia

A serum potassium level below $3.5\,mEq/L$ caused by low intake, diuretics, eating disorders, diarrhea, or chronic alcoholism; manifestations include muscle weakness, cramps, paresthesias, hyporeflexia, constipation, ileus, palpitations, and cardiac arrhythmias.

Hyperkalemia

A serum potassium level exceeding $5.0\,mEq/L$ caused by renal failure or overly rapid potassium administration; clinical signs include paresthesias, muscle twitching, irritability, nausea, diarrhea, abdominal cramping, palpitations, life-threatening arrhythmias, and cardiac arrest.

Calcium ($Ca^{2+}$)

An essential cation with a normal total serum concentration of $9\text{--}11\,mg/dL$ ($4.5\text{--}5.5\,mEq/L$); it stabilizes neuromuscular cell resting membrane potentials, facilitates blood clotting, strengthens cardiac contractions, and participates in bone formation.

Calcium-Phosphorus Relationship

An inverse clinical relationship regulated by parathyroid hormone (PTH) and calcitonin, wherein high serum calcium levels correspond to low phosphate levels, and elevated phosphate levels drive down serum calcium concentrations.

Chvostek Sign and Trousseau Sign

Two hallmark clinical indicators of hypocalcemia (or hypomagnesemia) reflecting latent tetany: Chvostek sign is facial twitching elicited by tapping the facial nerve anterior to the ear; Trousseau sign is carpopedal spasm induced by inflating a blood pressure cuff above systolic pressure.

Hypocalcemia

A serum total calcium level below $9\,mg/dL$ (or $< 8.5\,mg/dL$ / ionized $Ca^{2+} < 4.5\,mg/dL$) caused by renal failure, hypoparathyroidism, vitamin D deficiency, or hyperphosphatemia; characterized by muscle cramps, tetany, laryngospasm, hyperreflexia, paresthesias, and positive Chvostek and Trousseau signs.

Hypercalcemia

A serum calcium level greater than $10.5\,mg/dL$ (or $> 11\,mg/dL$) caused by hyperparathyroidism, malignancy, thiazide diuretics, or immobilization; symptoms include muscle weakness, decreased reflexes, bone pain, lethargy, confusion, constipation, nausea, vomiting, anorexia, and dysrhythmias.

Phosphate ($PO_4^{3-}$)

An essential intracellular anion with a normal adult serum concentration of $2.5\text{--}4.5\,mg/dL$; vital for bone formation, $ATP$ synthesis, DNA and RNA production, acid-base buffering, and normal functioning of RBCs, WBCs, and platelets.

Hypophosphatemia

A serum phosphate level below $2.5\,mg/dL$ caused by intestinal malabsorption, chronic alcoholism, antacids binding phosphate (magnesium/aluminum/calcium), or malnutrition; leads to depleted intracellular $ATP$ and cardiac dysrhythmias.

Hyperphosphatemia

A serum phosphate level greater than $4.5\,mg/dL$ caused by chronic renal failure, hypoparathyroidism, muscle breakdown, or phosphate-containing enemas/laxatives; manifests as increased neuromuscular excitability, soft tissue calcium phosphate deposition, and cardiac dysrhythmias.

Magnesium ($Mg^{2+}$)

The second most abundant intracellular cation, with a normal serum concentration of $1.5\text{--}2.5\,mEq/L$; designated 'The Great Stabilizer' because it stabilizes cardiac and neuromuscular membranes, operates the $Na^+/K^+$ pump, relaxes smooth muscle, and treats asthma, pre-eclampsia, preterm labor, and ventricular tachycardia.

Hypomagnesemia

A serum magnesium level below $1.5\,mEq/L$ (or $< 1.7\,mg/dL$) caused by chronic alcoholism, malnutrition, diarrhea, or prolonged nasogastric suction; causes increased neuromuscular excitability, tremors, muscle cramps, tetany, hyperreflexia, positive Chvostek and Trousseau signs, and dysrhythmias.

Hypermagnesemia

A serum magnesium level greater than $2.5\,mEq/L$ (or $> 2.5\,mg/dL$) caused by renal failure or excess intake of magnesium-containing antacids and laxatives; manifests as muscle weakness, diminished deep tendon reflexes, flaccid paralysis, drowsiness, coma, hypotension, and bradycardia.