Medical terms, defined.

Continuous capillaries have an uninterrupted endothelial lining with tight junctions and an intact basement membrane, allowing passage of only small molecules (water, ions, glucose) via intercellular clefts and transcytosis. They are the...

Sinusoidal (discontinuous) capillaries are the most permeable capillary type, with large intercellular gaps, an incomplete or absent basement membrane, and large fenestrations. They allow passage of proteins and even whole cells. They ar...

Alpha-intercalated cells are acid-secreting cells in the late distal tubule and collecting duct (cortical and medullary) that secrete H+ via apical H+-ATPase and H+/K+-ATPase and reabsorb HCO3- via basolateral Cl-/HCO3- exchanger (AE1)....

The Starling equation describes net fluid movement across a capillary wall: Jv = Kf[(Pc − Pi) − σ(πc − πi)]. Hydrostatic pressure pushes fluid out of capillaries, while plasma oncotic pressure (mainly albumin) pulls it back in. Disruptio...

The Frank-Starling curve describes the relationship between ventricular preload (end-diastolic volume/fiber length) and stroke volume: within physiologic limits, increased venous return stretches myocardial fibers and increases contracti...

Vital capacity (VC) is the maximum volume of air that can be forcibly exhaled after a maximal inspiration (VC = IRV + TV + ERV = TLC − RV). It is reduced in restrictive lung disease and neuromuscular weakness, and is often normal or only...

Inspiratory capacity (IC) is the maximum volume of air that can be inhaled after a normal, quiet expiration (from FRC to TLC). IC = tidal volume (TV) + inspiratory reserve volume (IRV), typically ~3 L in a healthy adult (~3.0–3.5 L in men).

West zones describe the regional distribution of pulmonary blood flow in an upright lung based on the relationships among alveolar (PA), pulmonary arterial (Pa), and pulmonary venous (Pv) pressures. Gravity makes perfusion greatest at th...

Pulmonary compliance is the change in lung volume per unit change in transpulmonary pressure (C = ΔV/ΔP), reflecting lung distensibility. It is decreased in restrictive diseases (pulmonary fibrosis, pulmonary edema, ARDS, atelectasis) an...

Laplace's law states that wall tension (T) is proportional to the transmural pressure (P) times radius (r), divided by wall thickness (T = P·r / 2h for a thin-walled cylinder; T = P·r / 2 for a sphere). Larger radius or higher pressure i...

The ventilation-perfusion (V/Q) ratio is the ratio of alveolar ventilation (V) to pulmonary capillary blood flow (Q). Normal overall is ~0.8 (V ≈ 4 L/min, Q ≈ 5 L/min). V/Q = 0 indicates shunt (perfusion without ventilation), and V/Q = ∞...

The Hering-Breuer (inflation) reflex is a protective vagally mediated reflex in which lung stretch receptors (slowly adapting pulmonary stretch receptors in airway smooth muscle) sense large tidal volumes and signal via the vagus nerve t...

Cardiac tamponade is accumulation of pericardial fluid (blood, effusion, pus, air) under enough pressure to impede diastolic filling, causing decreased stroke volume, cardiac output, and obstructive shock. Classic findings are Beck triad...

Osmolar clearance (Cosm) is the volume of plasma per unit time that is cleared of all osmotically active solutes by the kidney: Cosm = (Uosm × V) / Posm. It separates urine flow into osmolar clearance and free water clearance (CH2O = V −...

Sodium-glucose cotransporters (SGLTs) are secondary active transporters that move glucose across the apical membrane using the Na+ gradient set by the basolateral Na+/K+-ATPase. SGLT2 (early proximal convoluted tubule, S1/S2) reabsorbs ~...

Tubular secretion is the transfer of substances from peritubular capillaries (blood) into the tubular lumen, mainly in the proximal convoluted tubule (PCT), adding to what was filtered. It allows rapid elimination of organic acids/bases...

The Na+/K+ ATPase is a P-type ATPase in the plasma membrane that uses 1 ATP to pump 3 Na+ out and 2 K+ into the cell against their gradients. It maintains the resting membrane potential and low intracellular Na+, and it drives secondary...

The myogenic mechanism is the intrinsic ability of vascular smooth muscle (especially afferent arterioles of the kidney) to contract when stretched by increased intraluminal pressure and relax when pressure falls. It keeps blood flow and...

Na-K ATPase (Na+/K+ pump) is a P-type primary active transporter in the plasma membrane that pumps 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed, creating the electrochemical gradients essential for resting membrane potential, cell...

ADH (antidiuretic hormone, a.k.a. vasopressin/AVP) is a nonapeptide synthesized in the hypothalamus (supraoptic and paraventricular nuclei) and released from the posterior pituitary in response to increased plasma osmolality or decreased...

The renin-angiotensin system (RAS, or RAAS with aldosterone) is a hormonal cascade that regulates blood pressure, systemic vascular resistance, and Na+/water balance. Low renal perfusion, low distal tubular NaCl delivery, or sympathetic...

The migrating motor complex (MMC) is a cyclic, recurring pattern of electrical and contractile activity in the stomach and small bowel during fasting, occurring roughly every 90-120 minutes. It acts as the "intestinal housekeeper," sweep...

The gastrocolic reflex is a physiologic increase in colonic motility (mass movements/high-amplitude propagated contractions) triggered by food entering the stomach, leading to the urge to defecate soon after eating. It is most pronounced...

Brush border enzymes are hydrolases embedded in the microvillus membrane of small intestinal enterocytes that complete the final step of carbohydrate and peptide digestion (e.g., lactase, sucrase-isomaltase, maltase, glucoamylase/α-dextr...

Brush border enzymes are hydrolases anchored in the microvillus membrane of small intestinal enterocytes (duodenum/jejunum most) that complete the final digestion of carbohydrates and peptides into absorbable monosaccharides and amino ac...

Chymotrypsin is a pancreatic serine protease secreted as the inactive zymogen chymotrypsinogen and activated by trypsin. It is an endopeptidase that cleaves peptide bonds on the C-terminal side of large hydrophobic/aromatic amino acids (...

Enteropeptidase (formerly enterokinase) is a brush-border serine protease secreted by duodenal and jejunal enterocytes that converts inactive trypsinogen to active trypsin by cleaving its N-terminal hexapeptide. Trypsin then activates ot...

Enterokinase (enteropeptidase) is a brush-border serine protease secreted by duodenal and proximal jejunal enterocytes that converts trypsinogen to trypsin, initiating the pancreatic proenzyme activation cascade. Deficiency causes congen...

Pancreatic amylase is an alpha-amylase secreted by pancreatic acinar cells into the duodenum. It hydrolyzes internal alpha-1,4 glycosidic bonds of starch and glycogen into maltose, maltotriose, and alpha-limit dextrins. Serum amylase and...

Cholecystokinin (CCK) is a peptide hormone secreted by I cells of the duodenum and jejunum in response to fatty acids and amino acids. It stimulates gallbladder contraction, relaxes the sphincter of Oddi, and stimulates pancreatic enzyme...

VIP (vasoactive intestinal peptide) is a neuropeptide that relaxes smooth muscle, stimulates intestinal water and electrolyte secretion, and inhibits gastric acid secretion. Excess VIP from a VIPoma (non-beta pancreatic islet cell tumor)...

TSH (thyroid-stimulating hormone, thyrotropin) is an anterior pituitary glycoprotein hormone released by thyrotrophs in response to hypothalamic TRH. It stimulates thyroid follicular cells to synthesize and release T3/T4, and it is the b...

ACTH (corticotropin) is a 39-amino-acid peptide hormone secreted by corticotrophs of the anterior pituitary in response to CRH from the hypothalamus. It stimulates the adrenal cortex (mainly zona fasciculata and reticularis) to synthesiz...

Growth hormone-releasing hormone (GHRH) is a hypothalamic peptide (arcuate nucleus) that stimulates somatotrophs in the anterior pituitary to synthesize and release growth hormone (GH) via Gs/cAMP signaling. Its effects are opposed by so...

LH (luteinizing hormone) is a glycoprotein gonadotropin secreted by anterior pituitary gonadotrophs in response to pulsatile GnRH. In females, the mid-cycle LH surge triggers ovulation and corpus luteum formation. In males, it stimulates...

TRH (thyrotropin-releasing hormone) is a tripeptide (pGlu-His-Pro-NH2) secreted by the paraventricular nucleus of the hypothalamus. It stimulates thyrotrophs in the anterior pituitary to release TSH (and prolactin) via Gq/PLC/IP3/DAG sig...

GH (growth hormone, somatotropin) is a 191-amino-acid peptide secreted by somatotrophs (acidophils) of the anterior pituitary. It acts directly and via IGF-1 (somatomedin C) from the liver to promote linear growth and anabolism. Excess c...

FSH (follicle-stimulating hormone) is a glycoprotein gonadotropin secreted by anterior pituitary gonadotrophs under GnRH stimulation. It stimulates ovarian follicle growth and estrogen production (granulosa cells) in females and spermato...

CRH (corticotropin-releasing hormone) is a 41-amino-acid peptide secreted by the paraventricular nucleus of the hypothalamus that stimulates ACTH release from anterior pituitary corticotrophs. It is the top of the HPA axis (CRH → ACTH →...

Corticotropin-releasing hormone (CRH) is a 41-amino acid peptide secreted by the paraventricular nucleus of the hypothalamus that stimulates ACTH release from anterior pituitary corticotrophs via a Gs-coupled receptor (CRH-R1; cAMP/PKA)....

Prolactin-inhibiting factor (PIF) is hypothalamic dopamine, released from the tuberoinfundibular pathway into the hypophyseal portal system. It binds D2 receptors on anterior pituitary lactotrophs and tonically suppresses prolactin secre...

C cells (parafollicular cells) are neuroendocrine cells of the thyroid that secrete calcitonin, which lowers serum calcium by inhibiting osteoclast activity. They derive from neural crest (ultimobranchial body/4th-5th pharyngeal pouch) a...

Chromaffin cells are neuroendocrine cells of the adrenal medulla (and paraganglia) derived from neural crest. They are modified postganglionic sympathetic neurons innervated by preganglionic cholinergic fibers, and they secrete epinephri...

Parafollicular cells (C cells) are neuroendocrine cells of the thyroid that secrete calcitonin, which lowers serum calcium by inhibiting osteoclast activity. They derive from neural crest (via ultimobranchial body / 4th–5th pharyngeal po...

Saltatory conduction is the "jumping" propagation of action potentials between nodes of Ranvier along myelinated axons. Myelin increases membrane resistance and decreases capacitance, so depolarization spreads passively and rapidly betwe...

The synaptic cleft is the ~20–40 nm extracellular gap between the presynaptic terminal and the postsynaptic membrane at a chemical synapse. Neurotransmitter released from the presynaptic neuron diffuses across it to bind postsynaptic rec...

Synaptic vesicles are small (~40-50 nm) membrane-bound organelles in presynaptic terminals that store and release neurotransmitters by Ca2+-triggered exocytosis. Their fusion with the presynaptic membrane is mediated by SNARE proteins an...

Colloid is the protein-rich, iodinated thyroglobulin-containing material stored in the lumen of thyroid follicles; it serves as the storage reservoir for thyroid hormone precursors. More broadly, "colloid" also refers to large-molecule I...

The Golgi tendon organ (GTO) is an encapsulated proprioceptive mechanoreceptor at the muscle-tendon junction that senses muscle tension (force). Its Ib afferents drive the inverse myotatic reflex (autogenic inhibition), causing the homon...

The stretch (myotatic) reflex is a monosynaptic, ipsilateral spinal reflex in which muscle stretch activates muscle spindle Ia afferents, which directly excite alpha motor neurons of the same muscle, causing contraction. It also inhibits...