Digestive System Anatomy and Physiology

Comprehensive vocabulary flashcards covering the histological tunics, neural regulation, salivary and gastric secretions, GI hormones, motility cycles, and nutrient digestion pathways across the human digestive system.

Mucosa

The innermost histological tunic of the GI tract wall, composed of a mucosal epithelium, lamina propria, and muscularis mucosa, functioning primarily in secretion and absorption.

Lamina Propria

The sub-layer of loose connective tissue within the mucosa that contains blood and lymph vessels for absorption and mucosa-associated lymphoid tissue ($\text{MALT}$) nodules for immune defense.

Muscularis Mucosa

A thin layer of smooth muscle within the mucosa that provides structural support and maintains the folds of the mucosal layer.

Submucosa

A highly vascular tunic of loose or areolar connective tissue containing blood and lymphatic vessels, lymph nodes, mucous glands, and the submucosal nerve plexus ($\text{Meissner's plexus}$).

Muscularis Externa

The muscular tunic responsible for motility, comprised of skeletal muscle in the mouth, pharynx, upper/middle esophagus, and external anal sphincter, two smooth muscle layers in the small and large intestines, and three smooth muscle layers in the stomach.

Serosa

The outermost serous membrane (visceral peritoneum) composed of connective tissue and simple squamous epithelium that secretes serous fluid to lubricate organs and reduce friction.

Submucosal Plexus (Meissner's Plexus)

An enteric nerve plexus situated between the mucosa and submucosa containing chemoreceptors and proprioceptors that monitor luminal conditions and motor neurons that regulate glandular secretions.

Myenteric Plexus (Auerbach's Plexus)

An enteric nerve plexus located between the circular and longitudinal smooth muscle layers of the muscularis externa whose motor neurons regulate the frequency and force of gastrointestinal motility.

Pacesetter Cells (Interstitial Cells of Cajal)

Specialized cells in the muscularis layer that trigger rhythmic waves of smooth muscle contraction to coordinate peristalsis and segmentation throughout the digestive tract.

Dentin

The calcified connective tissue that forms the bulk of the internal structure of a tooth beneath the enamel.

Periodontal Ligament

The fibrous connective tissue structure that anchors the root of a tooth into the bony socket of the mandible or maxilla.

Parotid Glands

A pair of major salivary glands that produces approximately $25\%$ of daily saliva volume, secreting fluid rich in salivary amylase.

Submandibular Glands

A pair of major salivary glands accounting for approximately $70\%$ of daily salivary output, secreting a mixture of buffers and mucus.

Lingual Lipase

A triacylglycerol-digesting enzyme secreted by lingual glands on the tongue that initiates fat breakdown.

Deglutition

The act of swallowing, divided into three stages: the voluntary oral phase, the involuntary pharyngeal phase, and the involuntary esophageal phase.

Upper Esophageal Sphincter (UES)

A voluntary sphincter composed of skeletal muscle that relaxes to permit food passage from the pharynx into the esophagus.

Lower Esophageal Sphincter (LES)

An involuntary smooth muscle sphincter located at the gastroesophageal junction that prevents acid reflux from the stomach into the esophagus.

Pyloric Sphincter

An involuntary smooth muscle sphincter regulating the squirt-wise passage of acidic chyme from the stomach into the duodenum (~$3\,\text{mL}$ per squirt).

Sphincter of Oddi

An involuntary smooth muscle sphincter that regulates the release of bile and pancreatic juice into the duodenum.

Ileocecal Sphincter

An involuntary smooth muscle sphincter that controls the movement of chyme from the ileum of the small intestine into the cecum of the large intestine.

External Anal Sphincter (EAS)

A voluntary sphincter composed of skeletal muscle that provides conscious control over defecation.

Parietal Cells

Gastric epithelial cells that secrete hydrochloric acid ($\text{HCl}$ via $H^+$ and $Cl^-$ ions, maintaining gastric $\text{pH}$ at $1.5\text{--}2.0$) and Intrinsic Factor.

Chief Cells

Gastric secretory cells that produce the inactive proteolytic zymogen pepsinogen as well as gastric lipase.

Intrinsic Factor (IF)

A glycoprotein secreted by gastric parietal cells that is required for the absorption of Vitamin $B_{12}$ in the terminal ileum.

Pepsin

An active proteolytic enzyme formed in the gastric lumen when pepsinogen is cleaved by hydrochloric acid ($\text{HCl}$) under acidic conditions ($\text{pH} < 2.0$).

Cephalic Phase

The initial digestive phase triggered by the sight, smell, taste, or thought of food, mediated by cranial nerves VII, IX, and X to prime salivary and gastric secretions.

Gastroileal Reflex

A short neural reflex triggered by stomach distension that stimulates peristalsis in the ileum and relaxes the ileocecal sphincter to empty ileal contents into the cecum.

Gastrocolic Reflex

A physiological reflex initiated by gastric distension that triggers mass movements (intense, sustained peristaltic contractions) in the colon.

Enterogastric Reflex

A regulatory reflex stimulated by acidic, fatty chyme entering the duodenum that suppresses gastric motility and turns off gastrin secretion.

Gastrin

A peptide hormone secreted by G cells of the gastric pits in response to stomach distension, peptides/amino acids, or vagal tone that stimulates parietal cell $\text{HCl}$ secretion, chief cell pepsinogen release, and gastric motility.

Somatostatin (SST)

An inhibitory peptide hormone released by D cells when gastric luminal $\text{pH} < 2.0$ that acts as a universal endocrine brake to inhibit gastrin, $\text{HCl}$, insulin, and glucagon release.

Secretin

A peptide hormone produced by duodenal S cells in response to low duodenal $\text{pH}$ that stimulates pancreatic secretion of bicarbonate-rich fluid ($\text{pH } 7.5\text{--}8.8$) and liver bile production while inhibiting parietal cell $\text{HCl}$ secretion.

Cholecystokinin (CCK)

A peptide hormone secreted by duodenal and jejunal I cells in response to fatty acids and amino acids that induces gallbladder contraction, relaxes the Sphincter of Oddi, and stimulates pancreatic enzyme output.

Glucose Insulinotropic Peptide (GIP)

An incretin peptide released by K cells in the duodenum and jejunum in response to saccharides that stimulates insulin secretion from pancreatic $\beta$-cells and promotes lipogenesis.

Glucagon-Like Peptide-1 (GLP-1)

An incretin hormone released by intestinal L cells in response to luminal glucose that stimulates insulin secretion, suppresses glucagon release, slows gastric emptying, and induces satiety.

Motilin

A peptide hormone produced by M cells in the duodenum and jejunum during fasting states or acidic duodenal entry that stimulates the Migrating Motor Complex ($\text{MMC}$) and increases lower esophageal sphincter tone.

Ghrelin

An orexigenic peptide hormone produced by ghrelinergic cells in the stomach/duodenum and pancreatic $\epsilon$-cells that rises prior to food intake and acts on the hypothalamus to stimulate appetite and growth hormone release.

Leptin

An anorexigenic peptide hormone produced by adipocytes during lipogenesis that binds hypothalamic receptors to suppress hunger and stimulate energy expenditure.

Enteropeptidase

A brush border enzyme (also known as enterokinase) on duodenal enterocytes that cleaves inactive pancreatic trypsinogen into active trypsin.

Kupffer Cells

Specialized phagocytic macrophages lining liver sinusoids that remove worn-out red blood cells, pathogens, and toxins, and act as antigen-presenting cells ($\text{APCs}$).

Hepatocytes

The primary functional cells of the liver responsible for synthesizing bile and urea, storing glycogen and fat-soluble vitamins ($A, D, E, K$), and synthesizing plasma proteins and coagulation factors.

Bile Salts

Amphipathic steroids synthesized from cholesterol and conjugated with taurine or glycine in hepatocytes that break large fat globules into water-soluble emulsion droplets known as micelles.

Plicae Circulares

Permanent circular folds within the mucosa and submucosa of the small intestine that spiral chyme and enhance surface area for digestion and absorption.

Brunner's Glands

Submucosal glands restricted to the duodenum that produce an alkaline, bicarbonate-rich mucus to protect the duodenal epithelium and neutralize acidic gastric chyme.

Peyer's Patches

Aggregated lymphoid nodules containing leukocytes located predominantly in the ileum that safeguard the gut against microbial pathogens.

Paneth Cells

Phagocytic cells located at the base of the crypts of Lieberkühn that secrete antibacterial lysozyme and $\alpha$-defensins.

Migrating Motor Complex (MMC)

A periodic pattern of sweeping peristaltic waves occurring during the interdigestive housekeeping phase every $60\text{--}90\,\text{minutes}$ (6–8 waves per night) to clear residual debris and bacteria from the stomach to the ileum.

Sodium-Glucose Co-transporter 1 (SGLT1)

A symport carrier on the apical membrane of intestinal enterocytes that drives the secondary active transport of glucose and galactose into the cell coupled to a $Na^+$ gradient.

Chylomicrons

Lipoprotein packages synthesized within enterocytes by assembling re-esterified triglycerides, cholesterol, and phospholipids with a protein coat, which exit into lymphatic lacteals.

Lacteals

Specialized lymphatic capillaries situated inside intestinal villi that absorb large chylomicrons and fat-soluble vitamins ($A, D, E, K$) for transport to the thoracic duct.

Haustral Contractions

Slow, segmenting contractions ($1\text{--}5\,\text{per minute}$) in the large intestine that expose chyme to the mucosal wall to maximize water, electrolyte, and vitamin absorption.

Defecation Reflex

A parasympathetic reflex triggered by rectal distension that causes involuntary relaxation of the internal anal sphincter ($\text{IAS}$) and conscious relaxation of the external anal sphincter ($\text{EAS}$) to eliminate feces.

Chemoreceptor Trigger Zone (CTZ)

A brainstem center located in the medulla oblongata that coordinates the emetic (vomiting) reflex upon stimulation by toxins, drugs, vestibular disturbances, or visceral distension.