Fluids, Electrolytes, Acid-Base Dynamics, and Urinary Physiology

Comprehensive practice flashcards matching vocabulary terms and precise definitions across fluid, electrolyte, and acid-base homeostasis and urinary physiology.

Total Body Water (TBW)

The total volume of water contained across all body compartments, averaging about $75\%$ of body weight in newborns, $55\%\text{--}60\%$ in young men, slightly less in women, as little as $45\%$ in obese and elderly individuals, and approximately $40\,L$ in a $70\,kg$ ($150\,lb$) young male.

Intracellular Fluid (ICF)

The major body fluid compartment located within cells, accounting for approximately $65\%$ of total body water, with potassium ($K^+$) salts serving as its principal cations and phosphate ($HPO_4^{2-}$) as its major anions.

Extracellular Fluid (ECF)

The fluid compartment located outside of cells, representing about $35\%$ of total body water, subdivided into tissue (interstitial) fluid ($25\%$), blood plasma and lymphatic fluid ($8\%$), and transcellular fluid ($2\%$), with sodium ($Na^+$) and chloride ($Cl^-$) serving as the predominant ions.

Transcellular Fluid

A catch-all subcategory of extracellular fluid representing about $2\%$ of total body water, which includes cerebrospinal fluid (CSF), synovial fluid, peritoneal, pleural, and pericardial fluids, vitreous and aqueous humors of the eye, bile, and fluids of the digestive, urinary, and reproductive tracts.

Capillary Starling Forces

The balance of hydrostatic and osmotic pressures driving fluid across blood capillary walls, resulting in net filtration at the arterial end where capillary hydrostatic pressure exceeds blood colloidal osmotic pressure, and net reabsorption at the venous end where blood colloidal osmotic pressure exceeds capillary hydrostatic pressure.

Milliequivalent (mEq)

A unit of measure describing an electrolyte's chemical activity and combining power, defined such that $1\,mEq$ of any cation will react with exactly $1\,mEq$ of an anion; in any fluid compartment, total cation concentration in $mEq/L$ always equals total anion concentration.

Osmole (osm)

A unit reflecting one mole ($1\,mol$) of dissolved particles in solution, taking into account whether a solute dissociates in water (e.g., $1\,M\text{ glucose} = 1\,osm/L$, whereas $1\,M\text{ NaCl} = 2\,osm/L$).

Osmolality

A solution's ability to create osmotic pressure and drive fluid movement, evaluated clinically in serum ($280\text{--}300\,mOsm/kg$, predominantly determined by $Na^+$) and urine ($50\text{--}1400\,mOsm/kg$).

Specific Gravity of Urine

A clinical measure of the kidney's ability to concentrate or dilute urine relative to distilled water ($1.000$), possessing a normal clinical range of $1.003\text{ to }1.030$.

Tonicity

The ability of an extracellular solution to alter cell volume and internal hydrostatic pressure by driving osmotic water movement across the plasma membrane.

Hypotonic Solution

A solution possessing an osmolality below $270\,mOsm/kg$ and a lower nonpermeating solute concentration than intracellular fluid (such as distilled water or $0.45\%\text{ NaCl}$), causing water to flow into cells and make them swell.

Hypertonic Solution

A solution possessing an osmolality above $300\,mOsm/kg$ and a higher concentration of nonpermeating solutes than intracellular fluid (such as $3.0\%\text{ NaCl}$), causing water to exit cells and make them shrivel (crenate).

Isotonic Solution

A solution with an osmolality of $275\text{--}295\,mOsm/kg$ (matching blood plasma and normal intracellular fluid, such as $0.9\%\text{ NaCl}$ or $5\%\text{ Glucose}$), causing no net osmotic fluid transfer across the cell membrane.

Lactated Ringer's (LR)

An isotonic fluid resuscitation solution containing lactate, which is converted to bicarbonate by the liver to counteract metabolic acidosis; it is contraindicated in patients presenting with liver dysfunction.

Contraindication

A specific medical circumstance or condition under which a drug, clinical procedure, or surgery should not be administered because it may be hazardous to the patient.

Aldosterone

A mineralocorticoid steroid hormone secreted by the adrenal cortex (zona glomerulosa) that acts on the thick ascending limb, distal convoluted tubule, and cortical collecting duct to stimulate transcription of $Na^+\text{--}K^+$ pumps, thereby promoting $Na^+$, $Cl^-$, and water reabsorption while enhancing $K^+$ and $H^+$ excretion.

Cortisol

A glucocorticoid steroid hormone from the adrenal cortex that increases glomerular filtration rate and renal plasma flow, enhances $Na^+$ and water retention, increases $K^+$ excretion, and exerts anti-inflammatory actions by inhibiting the transformation of B cells into plasma cells.

Vasopressin (Antidiuretic Hormone / ADH)

A peptide hormone synthesized in the hypothalamus and stored in the posterior pituitary (neurohypophysis) that targets $V_2$ receptors on late distal convoluted tubules and collecting ducts to stimulate cAMP production, promoting insertion of aquaporin water channels to increase water reabsorption.

Calcitonin

A hormone produced by the parafollicular C cells of the thyroid gland in response to elevated blood calcium, which acts to lower blood $Ca^{2+}$ by inhibiting osteoclasts, stimulating osteoblast bone deposition, and promoting renal calcium excretion.

Parathyroid Hormone (PTH)

A hormone secreted by the parathyroid glands in response to low blood calcium that raises serum $Ca^{2+}$ by stimulating osteoclast proliferation, increasing renal tubular calcium reabsorption, inhibiting renal phosphate reabsorption, and promoting the terminal activation step of calcitriol synthesis.

Calcitriol

The active form of vitamin D, synthesized sequentially involving the skin, liver, and kidneys, which raises blood calcium by promoting intestinal calcium absorption and weak renal calcium reabsorption.

Bicarbonate Buffer System

The primary extracellular chemical buffering system ($CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$) regulated jointly by the respiratory and renal systems, functioning at an equilibrium ratio of $20:1$ ($HCO_3^- : H_2CO_3$) at normal blood pH ($7.35\text{--}7.45$).

Respiratory Acidosis

An acid-base disturbance resulting from alveolar hypoventilation (e.g., in emphysema) where pulmonary $CO_2$ elimination fails to keep pace with metabolic production, causing $CO_2$ accumulation in the extracellular fluid and lowering blood pH below $7.35$.

Respiratory Alkalosis

An acid-base disturbance triggered by hyperventilation, where the rate of pulmonary ventilation exceeds metabolic $CO_2$ production, causing excessive elimination of $CO_2$ and elevating blood pH above $7.45$.

Metabolic Acidosis

A clinical state characterized by an abnormal accumulation of metabolic organic acids (lactic acid, ketone bodies), excessive ingestion of acidic compounds (such as aspirin), or the loss of base through chronic diarrhea or laxative overuse, reducing blood pH below $7.35$.

Metabolic Alkalosis

A relatively rare acid-base imbalance resulting from an excessive accumulation of bicarbonate (antacid or IV bicarbonate overuse) or chronic loss of stomach hydrochloric acid via persistent vomiting, raising blood pH above $7.45$.

Hyponatremia

An abnormally low serum sodium concentration ($< 135\,mEq/L$) that causes water to move via osmosis into cells, resulting in cellular swelling, fatigue, muscle cramps, seizures, and potential coma.

Hypernatremia

An abnormally elevated serum sodium concentration ($> 145\,mEq/L$) typically caused by water deprivation, excessive saline administration, or diabetes insipidus, reflecting a deficit in body water relative to sodium.

Hypokalemia

A serum potassium deficiency ($< 3.5\,mEq/L$) that hyperpolarizes cell membranes, making muscle and nerve tissues less excitable and manifesting as muscle weakness, depressed reflexes, and cardiac arrhythmias with flattened or inverted T waves.

Hyperkalemia

An elevated serum potassium concentration ($> 5.5\,mEq/L$); rapid increases cause partial depolarization and abnormal hyperexcitability predisposing to cardiac arrest with tall peaked T waves, whereas chronic elevations inactivate voltage-gated $Na^+$ channels and reduce tissue excitability.

Hypocalcemia

A serum total calcium deficit ($< 4.5\,mEq/L$) that increases cell membrane sodium permeability, causing neuromuscular hyperexcitability, muscle spasms, tetany, laryngospasm, and a positive Trousseau's sign.

Hypercalcemia

An excessive concentration of serum calcium ($> 5.5\,mEq/L$, especially dangerous above $12\,mEq/L$) that reduces cell membrane sodium permeability, inhibiting muscular and neural depolarization and causing muscle weakness, depressed reflexes, and cardiac arrhythmias.

Diabetes Insipidus

An endocrine disorder characterized by intense thirst (polydipsia) and the excretion of massive volumes ($3\text{ to }20\,L/day$) of dilute urine, manifesting in either neurogenic (impaired hypothalamic ADH synthesis or pituitary release) or nephrogenic (renal insensitivity to ADH) forms.

Aldosteronism

An endocrine disease resulting from autonomous overproduction of aldosterone by the adrenal glands, leading to excessive renal sodium retention, potassium wasting, chronic hypervolemic hypertension, and elevated risks for left ventricular hypertrophy and stroke.

Erythropoietin (EPO)

A glycoprotein hormone produced and secreted by the kidneys that acts on bone marrow progenitor cells to stimulate the production of red blood cells (erythrocytes).

Urea

The most abundant metabolic nitrogenous waste molecule ($H_2N\text{--}CO\text{--}NH_2$), produced by the liver via the conversion of toxic ammonia released during amino acid deamination.

Uric Acid

A nitrogenous waste product generated from the catabolism of nucleic acid purines; when pathologically elevated, it crystallizes into joints to cause gouty arthritis.

Creatinine

A nitrogenous metabolic breakdown product derived from muscle creatine phosphate metabolism, which is filtered freely at the glomerulus without being reabsorbed by the renal tubules.

Blood Urea Nitrogen (BUN)

The clinical diagnostic measurement of the concentration of nitrogenous waste in blood plasma, which normally ranges between $10\text{ and }20\,mg/dL$.

Azotemia

A diagnostic condition denoting an abnormal elevation of blood urea nitrogen (BUN) and serum creatinine, signaling compromised renal clearance or poor kidney perfusion.

Uremia

A toxic clinical syndrome resulting from severe nitrogenous waste accumulation, characterized by diarrhea, vomiting, dyspnea, cardiac arrhythmias, and neuromuscular disturbances, requiring hemodialysis or transplantation.

Ureter

A retroperitoneal muscular tube approximately $25\,cm$ long that conveys urine from the renal pelvis to the urinary bladder via peristalsis, lined with transitional epithelium and bounded by muscular and adventitial coats.

Detrusor Muscle

The thick muscular wall of the urinary bladder comprised of three interlacing layers of smooth muscle that contract under parasympathetic stimulation to expel urine during micturition.

Trigone

The smooth, triangular region located on the posterior floor of the urinary bladder delineated by the two ureteral orifices superiorly and the internal urethral orifice inferiorly.

Internal Urethral Sphincter

A ring of smooth muscle located at the bladder neck under involuntary autonomic nervous system (ANS) control, which relaxes in response to parasympathetic efferent stimulation to allow urine passage.

External Urethral Sphincter

A ring of skeletal muscle within the pelvic urogenital diaphragm under voluntary somatic control via motor fibers of the pudendal nerve, providing conscious continence over urination.

Micturition

The physiological act of voiding the urinary bladder, initiated as an autonomic spinal reflex when volume reaches $200\text{--}400\,mL$ and modulated voluntarily by the pontine micturition center and cerebral cortex.

Renal Fraction

The portion of cardiac output supplied to the kidneys for blood processing and filtration, which accounts for approximately $21\%$ of resting cardiac output.

Cortical Nephron

The predominant nephron type ($85\%$ of all nephrons), situated largely within the outer renal cortex with short loops of Henle that dip only shallowly into the medulla, responsible for the bulk of plasma filtration.

Juxtamedullary Nephron

The specialized class of nephrons ($15\%$ of total) whose renal corpuscles lie close to the corticomedullary junction, possessing long loops of Henle that extend deep into the renal pyramids to create the medullary osmotic gradient.

Podocyte

A specialized stellate epithelial cell forming the visceral layer of Bowman's capsule, extending primary and secondary processes (pedicels) that wrap around glomerular capillaries to form filtration slits.

Pedicels

The smallest, finger-like foot processes of podocytes that interdigitate with foot processes of adjacent podocytes to cover the glomerular capillary basement membrane.

Filtration Slits

Narrow, negatively charged gaps approximately $30\,nm$ wide between interdigitating pedicels of podocytes, constituting the final filtration barrier that excludes large anions and proteins.

Glomerular Filtration Membrane

The three-tiered physiological barrier separating blood in glomerular capillaries from capsular space, consisting of the fenestrated capillary endothelium ($70\text{--}90\,nm$ pores), the negatively charged proteoglycan basement membrane, and the podocyte filtration slits ($30\,nm$).

Proximal Convoluted Tubule (PCT)

The longest and most coiled segment of the renal tubule, lined by simple cuboidal epithelium with a dense brush border of microvilli that reabsorbs approximately $65\%$ of the glomerular filtrate, $100\%$ of glucose, and two-thirds of filtered $Na^+$, $Cl^-$, and water.

Thick Segment of Nephron Loop

The portion of the loop of Henle (initial descending and most of ascending limb) lined by simple cuboidal epithelium heavily engaged in the active transport of $Na^+$, $K^+$, and $Cl^-$ via the $Na^+\text{--}K^+\text{--}2Cl^-$ cotransporter, which is completely impermeable to water.

Thin Segment of Nephron Loop

The lower hairpin turn of the descending loop of Henle, lined by simple squamous epithelium that is highly permeable to water but impermeable to sodium and other solutes.

Net Filtration Pressure (NFP)

The net pressure driving fluid across the glomerular membrane into the capsular space, calculated as blood hydrostatic pressure ($60\,mm\,Hg$) minus capsular hydrostatic pressure ($18\,mm\,Hg$) minus blood colloid osmotic pressure ($32\,mm\,Hg$), yielding $10\,mm\,Hg$ outbound.

Glomerular Filtration Rate (GFR)

The volume of filtrate formed per minute by both kidneys combined, averaging approximately $125\,mL/min$ ($180\,L/day$) in adult males and $105\,mL/min$ ($150\,L/day$) in adult females, of which approximately $99\%$ is reabsorbed.

Renal Autoregulation

The intrinsic capacity of the kidneys to maintain a relatively stable glomerular filtration rate and renal blood flow across systemic mean arterial blood pressure variations between $70\text{ and }180\,mm\,Hg$ without external neurohumoral input.

Juxtaglomerular Apparatus (JGA)

A sensory regulatory structure positioned at the vascular pole of each nephron where the distal convoluted tubule contacts the afferent and efferent arterioles, consisting of the macula densa, granular (juxtaglomerular) cells, and extraglomerular mesangial cells.

Macula Densa

A densely packed cluster of specialized epithelial sensory cells within the distal convoluted tubule that monitors the $NaCl$ load of the tubular fluid and secretes ATP/adenosine to regulate arteriolar tone.

Granular (Juxtaglomerular) Cells

Modified smooth muscle cells situated in the wall of the afferent arteriole that contract in response to adenosine to constrict the arteriole, and synthesize, store, and secrete the enzyme renin in response to hypotension or sympathetic stimulation.

Renin

An enzymatic hormone released by granular cells of the juxtaglomerular apparatus that cleaves the liver-derived plasma globulin angiotensinogen ($453\text{ amino acids}$) into the decapeptide angiotensin I ($10\text{ amino acids}$).

Angiotensin II

A potent octapeptide hormone formed when angiotensin-converting enzyme (ACE) cleaves angiotensin I; it stimulates systemic vasoconstriction, efferent arteriolar constriction, aldosterone secretion, ADH release, hypothalamic thirst, and direct tubular $Na^+$ reabsorption.

Atrial Natriuretic Peptide (ANP)

A peptide hormone secreted by atrial myocytes in response to increased blood pressure and atrial stretch that dilates the afferent arteriole, constricts the efferent arteriole to elevate GFR, and inhibits sodium reabsorption in the distal nephron, promoting natriuresis and diuresis.

Countercurrent Multiplier

The mechanism operating in the loop of Henle that continually concentrates the medullary interstitium up to $1200\,mOsm/L$ through opposite fluid flows: water leaves the permeable descending limb while $Na^+\text{--}K^+\text{--}2Cl^-$ are actively pumped out of the water-impermeable ascending limb.

Countercurrent Exchange System

The passive capillary transport system formed by the vasa recta in the renal medulla that supplies blood to the medullary tissue without washing away the hyperosmolar salt and urea gradient.

Urea Recycling

The continuous movement of urea out of the inner medullary collecting duct into the medullary tissue and into the descending limb of the loop of Henle, contributing approximately $40\%$ of the hyperosmolarity of the inner renal medulla.

Renal Clearance ($C$)

The volume of blood plasma completely cleared of a given substance by the kidneys per unit time ($mL/min$), determined by the clinical equation $C = \frac{UV}{P}$, where $U$ is urine concentration, $V$ is urine flow rate, and $P$ is plasma concentration.

Inulin

An exogenous plant polysaccharide that is filtered freely at the glomerulus but is neither reabsorbed nor secreted by the renal tubules, serving as the gold standard for measuring true glomerular filtration rate ($GFR = C_{\text{inulin}}$).

Stress Incontinence

A form of urinary incontinence characterized by the involuntary leakage of small amounts of urine during coughing, laughing, sneezing, or lifting, resulting from weakness of the pelvic floor musculature.

Urge Incontinence

A type of urinary incontinence caused by involuntary, spastic contractions of the detrusor muscle, commonly resulting from bladder infections or neurological damage.

Overflow Incontinence

Involuntary urinary dribbling occurring when the bladder fails to empty completely, leading to continuous chronic distension commonly caused by an enlarged prostate or urethral obstruction.

Renal Calculus (Kidney Stone)

A hard crystalline concretion formed in the renal pelvis composed of calcium salts ($80\%$), uric acid ($5\text{--}10\%$), struvite ($10\%$), or cystine ($\le 1\%$), which can cause severe flank pain and hematuria if it lodges in the ureter.

Lithotripsy

A noninvasive medical procedure that utilizes high-energy focused ultrasound shock waves to pulverize renal calculi into minute fragments capable of passing through the urinary tract.

Hemodialysis

An extracorporeal medical procedure for managing renal failure where blood is passed through a semipermeable membrane against dialyzing fluid, artificially clearing nitrogenous wastes and excess fluid three times per week for $3\text{ to }4\,hours$ per session.