Water, Electrolytes, and Acid-Base Balance

Vocabulary practice flashcards covering water balance, fluid compartments, electrolytes, clinical disorders, and acid-base regulation.

Total Body Water (TBW)

The total volume of fluid in the body, accounting for approximately $60\%$ of body weight in a normal adult ($70\%$ in infants, and $50\text{--}55\%$ in elderly persons).

Intracellular Fluid (ICF)

The fluid compartment located inside cells that constitutes $2/3$ ($40\%$) of total body water, with potassium ($K^+$) and magnesium ($Mg^{2+}$) as principal cations and phosphate ($HPO_4^{2-}$) and proteins as principal anions.

Extracellular Fluid (ECF)

The fluid compartment located outside cells that constitutes $1/3$ ($20\%$) of total body water, characterized by sodium ($Na^+$) as the principal cation and chloride ($Cl^-$) and bicarbonate ($HCO_3^-$) as the principal anions.

Transcellular Fluid (Third Space)

A subcompartment of extracellular fluid that makes up approximately $1\%$ of total body water, comprising cerebrospinal fluid, intraocular fluids, serous fluids, and fluids within the gastrointestinal, respiratory, and urinary tracts.

Obligatory Water Loss

The continuous, unavoidable loss of water from the body of approximately $2500\,mL/\text{day}$, occurring via urine, feces, and insensible perspiration.

Antidiuretic Hormone (ADH)

A hormone released by the posterior pituitary in response to hypothalamic osmoreceptor stimulation during high osmotic concentration that promotes water conservation at the renal collecting ducts and stimulates the thirst center.

Aldosterone

A mineralocorticoid released by the adrenal cortex in response to high $K^+$, low $Na^+$, or renin-angiotensin system activation that increases $Na^+$ reabsorption and $K^+$ secretion in the distal convoluted tubules and collecting ducts.

Natriuretic Peptides (ANP and BNP)

Hormones released in response to stretching of the heart wall that reduce thirst and inhibit the release of ADH and aldosterone, promoting renal diuresis and sodium excretion.

Syndrome of Inappropriate Hypersecretion of Antidiuretic Hormone (SIADH)

A clinical condition characterized by excessive loss of sodium through the urine secondary to hypersecretion of ADH.

Diabetes Insipidus

A disorder caused by a deficiency of ADH and characterized by the excretion of copious volumes of dilute urine.

Isotonic Dehydration

A type of dehydration characterized by a balanced, proportional loss of water and sodium, commonly resulting from severe diarrhea or vomiting.

Hypertonic Dehydration

A type of dehydration characterized by the loss of proportionally more water than sodium, typically seen in conditions such as fever.

Hypotonic Dehydration

A type of dehydration characterized by the loss of proportionally more sodium than water, commonly caused by the excessive use of diuretics.

Anasarca

Severe, generalized edema characterized by widespread fluid accumulation throughout the body tissues.

Bilateral Pedal Edema

Symmetrical swelling caused by excess fluid accumulation in the interstitial tissues of both feet and lower legs, commonly assessed clinically via depression of tissue for pitting.

Hyponatremia

A subnormal plasma sodium level below $135\,mEq/L$ caused by diarrhea, vomiting, sweating, diuretics, or Addison's disease, manifesting as nausea, vomiting, muscle cramps, hypotension, and personality changes.

Hypernatremia

An elevated plasma sodium concentration exceeding $145\,mEq/L$ caused by high sodium ingestion, renal failure, or Conn's disease, manifesting as thirst, dry mouth, dry skin, restlessness, and convulsions.

Hyperkalemia

An elevated serum potassium concentration exceeding $5.0\,mEq/L$ caused by chronic renal failure, crush injuries, or burns, presenting with an irregular slow pulse, weakness, tented T waves, prolonged P-R intervals, widened QRS complexes, and ST-segment elevation.

Hypokalemia

A serum potassium level below $3.5\,mEq/L$ caused by potassium-wasting diuretics, hyperaldosteronism, severe vomiting, or diarrhea, presenting with hypotension, paralytic ileus, flattened/inverted T waves, prominent U waves, and ST-segment depression.

Chvostek's and Trousseau's Tests

Clinical diagnostic tests used to identify latent tetany and neuromuscular irritability characteristic of hypocalcemia.

Parathyroid Hormone (PTH)

A hormone released by the parathyroid glands in response to low ECF calcium that increases renal calcium reabsorption, stimulates phosphate excretion, promotes bone resorption, and enhances intestinal calcium absorption via $1,25(\text{OH})_2\text{D}$.

Volatile Acid

Carbonic acid ($H_2CO_3$) formed from the hydration of carbon dioxide produced during cellular aerobic metabolism ($15{,}000\,mmol\,\text{CO}_2/\text{day}$), which is eliminated from the body by the lungs.

Fixed (Non-volatile) Acids

Acids produced from incomplete metabolism of carbohydrates (lactate), lipids (ketones), and proteins ($H_2SO_4$, $H_3PO_4$) totaling $50\text{--}100\,mEq/\text{day}$, which cannot be exhaled and must be excreted by the kidneys.

Chemical Buffer System

The body's first line of defense against pH shifts, consisting of bicarbonate, phosphate, and protein systems that act within seconds to reversibly bind or release $H^+$ without physically eliminating it from the body.

Henderson-Hasselbalch Equation

The formula $pH = pK' + \log_{10}\left(\frac{[HCO_3^-]}{0.03 \times P_{CO_2}}\right)$ describing blood pH, where in health $[HCO_3^-] = 24\,mmol/L$ and $P_{CO_2} = 40\,mm\,Hg$, resulting in a normal pH of $7.4$.

Phosphate Buffer System

A major intracellular and tubular buffer system consisting of $HPO_4^{2-}$ and $H_2PO_4^-$ that accounts for approximately $40\%$ of the buffering of nonvolatile acids in the urine.

Ammonia Buffer System

A renal buffering mechanism accounting for $60\%$ of urine acid buffering, where proximal tubular glutamine metabolism produces $NH_3$ that secretes into the lumen to form and excrete ammonium ($NH_4^+$).

Respiratory Acidosis

An acid-base disturbance characterized by increased $P_{CO_2}$, increased $[H^+]$, and decreased pH ($<7.35$) caused by hypoventilation, compensated renally by increased $H^+$ excretion and $HCO_3^-$ reabsorption.

Respiratory Alkalosis

An acid-base disturbance characterized by decreased $P_{CO_2}$, decreased $[H^+]$, and elevated pH ($>7.45$) caused by hyperventilation, compensated renally by decreased $H^+$ excretion and decreased $HCO_3^-$ reabsorption.

Metabolic Acidosis

An acid-base disorder defined by decreased plasma $[HCO_3^-]$, increased $[H^+]$, and decreased pH ($<7.35$) caused by diarrhea, ketosis, or renal disease, compensated by respiratory hyperventilation and renal proton excretion.

Metabolic Alkalosis

An acid-base disorder defined by elevated plasma $[HCO_3^-]$, reduced $[H^+]$, and elevated pH ($>7.45$) caused by vomiting, diuretic therapy, or alkaline drug intake, compensated by respiratory hypoventilation and renal bicarbonate excretion.