Chapter 24: The Digestive System

Comprehensive vocabulary flashcards covering the anatomy, histology, enzymatic mechanisms, and physiological processes of the human digestive system from Chapter 24.

Gastrointestinal (GI) Tract

The continuous tube (also called the alimentary canal) through which food passes, extending from the mouth to the anus; consists of the mouth, most of the pharynx, esophagus, stomach, small intestine, and large intestine.

Accessory Digestive Organs

Organs that assist in mechanical breakdown or produce chemical secretions to aid digestion, but through which food does not directly pass; includes the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Ingestion

The digestive system function that involves taking food and liquids into the mouth (eating).

Secretion

The release of water, acid, buffers, and enzymes by cells of the GI tract wall and accessory organs into the lumen.

Mixing and Propulsion

The capability of the GI tract smooth muscle to alternate contracting and relaxing to churn food and propel it through the canal toward the anus.

Mechanical Digestion

The physical breakdown of food into smaller pieces by churning, grinding, and agitation, such as mastication in the mouth and mixing waves in the stomach.

Chemical Digestion

The enzymatic breakdown of large carbohydrate, lipid, protein, and nucleic acid molecules into smaller absorbable units via hydrolysis.

Absorption

The passage of filtered and digested fluids, ions, and nutritional products from the lumen of the GI tract into blood or lymph capillaries.

Defecation

The discharge of indigestible substances, unabsorbed materials, bacteria, and sloughed cells from the body via the anus as feces.

Mucosa

The innermost lining of the GI tract wall, composed of three sublayers: an inner epithelium for protection, secretion, and absorption; a middle lamina propria of loose connective tissue; and an outer thin layer of smooth muscle called the muscularis mucosae.

Lamina Propria

The middle layer of the mucosa, composed of loose connective tissue supporting blood vessels, lymphatic vessels, and nerves.

Muscularis Mucosae

A thin layer of smooth muscle within the mucosa that creates micro-folds to dramatically increase the surface area available for digestion and absorption.

Submucosa

The tunic of loose connective tissue that binds the mucosa to the muscularis externa; contains numerous blood and lymphatic vessels and houses the submucosal nerve plexus.

Submucosal Plexus

An intrinsic network of nerves located within the submucosa that regulates movements of the mucosa and innervates secretory cells in mucosal glands.

Muscularis

The muscular tunic of the GI tract wall; consists of voluntary skeletal muscle in the mouth, pharynx, upper two-thirds of the esophagus, and external anal sphincter, and involuntary smooth muscle arranged into inner circular and outer longitudinal fibers throughout the remainder of the tract.

Myenteric Plexus

An intrinsic nerve network situated between the circular and longitudinal muscle layers of the muscularis that serves as the major controller of GI tract motility.

Serosa

The outermost serous membrane covering organs suspended in the abdominopelvic cavity, composed of simple squamous epithelium and connective tissue; also known as the visceral peritoneum.

Adventitia

A fibrous connective tissue layer that forms the outer covering of organs lacking a serosa, such as the esophagus, anchoring them to adjacent structures.

Enteric Nervous System (ENS)

The intrinsic neural circuitry of the GI tract, referred to as the 'brain of the gut,' extending from the esophagus to the anus and comprising the myenteric and submucosal plexuses.

Parietal Peritoneum

The outer portion of the largest serous membrane of the body that directly lines the internal surface of the abdominopelvic wall.

Visceral Peritoneum

The inner layer of the peritoneum that wraps and forms the outer serosal covering of certain abdominal organs.

Peritoneal Cavity

The potential space enclosed between the parietal and visceral layers of the peritoneum that contains lubricating serous fluid to reduce friction.

Saliva

A fluid secreted into the oral cavity composed of $99.5\%$ water and $0.5\%$ solutes (ions, dissolved gases, urea, uric acid, mucus, lysozyme, and salivary amylase) that cleanses, lubricates, and initiates chemical digestion.

Salivary Amylase

A digestive enzyme found in saliva that initiates starch digestion in the mouth by cleaving starch polysaccharides into smaller sugars like maltose.

Lysozyme

An antimicrobial enzyme found in saliva and tears that destroys bacteria and helps protect the oral cavity against infection.

Parotid Gland

The largest major salivary gland, located inferior and anterior to the ear between the skin and masseter muscle, secreting saliva into the oral cavity via the parotid duct.

Submandibular Gland

A major salivary gland positioned in the floor of the oral cavity beneath the base of the tongue and medial to the body of the mandible.

Sublingual Gland

A major salivary gland located beneath the tongue and superior to the submandibular glands, discharging saliva through multiple small sublingual ducts.

Lingual Frenulum

A fold of mucous membrane along the midline of the undersurface of the tongue that anchors it to the floor of the mouth and restricts excessive posterior movement.

Papillae

Nipple-shaped projections on the dorsal and lateral surfaces of the tongue that are often covered in taste buds capable of detecting sweet, sour, bitter, and salty tastants.

Lingual Lipase

An enzyme secreted by lingual glands on the tongue that begins fat digestion by targeting triglycerides, becoming activated later in the acidic lumen of the stomach.

Enamel

The hard, calcium-rich, acellular outer covering of the tooth crown; the hardest substance in the human body.

Dentin

A calcified connective tissue that forms the primary mass and structural framework of both the crown and root of a tooth.

Pulp Cavity

The central chamber within the crown of a tooth containing pulp, a vascularized connective tissue supplied with blood vessels and sensory nerves.

Cementum

A bone-like substance covering the dentin of a tooth root that attaches the root securely to the periodontal ligament.

Mastication

The mechanical digestion process of chewing, in which food is cut and ground by teeth and manipulated by the tongue while mixing with saliva.

Bolus

A soft, flexible, rounded mass of chewed food lubricated with saliva that is prepared for swallowing.

Upper Esophageal Sphincter (UES)

A muscular ring of skeletal muscle located at the superior boundary of the esophagus that regulates the entry of food from the laryngopharynx into the esophagus.

Lower Esophageal Sphincter (LES)

A ring of smooth muscle near the cardiac orifice of the stomach that relaxes during swallowing to allow the bolus to enter the stomach and contracts to prevent acid reflux.

Deglutition

The act of swallowing food, organized into three successive stages: the voluntary buccal stage, the involuntary pharyngeal stage, and the involuntary esophageal stage.

Peristalsis

A progressive wave of coordinated, involuntary muscular contractions and relaxations of circular and longitudinal fibers that propels a bolus or chyme forward through the GI tract.

Rugae

Large macroscopic longitudinal folds of the mucosal and submucosal layers seen in an empty stomach that flatten out as the organ distends with food.

Gastric Pits

Narrow microscopic indentations and channels in the stomach mucosa into which gastric glands empty their secretions.

Mucous Neck Cells

Exocrine cells situated in the neck of gastric glands that secrete a protective layer of mucus into the gastric lumen.

Parietal Cells

Gastric gland cells that secrete hydrogen ($H^+$) and chloride ($Cl^-$) ions separately to synthesize hydrochloric acid ($HCl$), as well as intrinsic factor.

Chief Cells

Exocrine cells of the gastric glands that synthesize and secrete pepsinogen (the inactive proenzyme of pepsin) and gastric lipase.

G Cells

Enteroendocrine cells found within gastric glands of the pyloric antrum that secrete the hormone gastrin into the bloodstream to stimulate gastric motility and acid production.

Chyme

The acidic, soupy semifluid mass of partially digested food and gastric juice resulting from mechanical churning in the stomach.

Mixing Waves

Gentle, rippling peristaltic contractions of the stomach muscularis that macerate swallowed food, mix it thoroughly with gastric juice, and reduce it to chyme.

Pepsin

The principal protein-digesting enzyme of the stomach, produced when chief cell-derived pepsinogen is activated in the presence of acidic hydrochloric acid ($HCl$).

Gastric Lipase

An enzyme secreted by chief cells in the stomach that hydrolyzes triglycerides into fatty acids and monoglycerides.

Cephalic Phase

The initial phase of digestion triggered by the sight, smell, thought, or initial taste of food, which activates neural centers to prepare the mouth and stomach for feeding.

Gastric Phase

The digestive phase initiated once food reaches the stomach, driven by neural reflexes and gastrin to enhance gastric secretion and muscular motility.

Intestinal Phase

The regulatory phase of digestion that begins when chyme enters the small intestine, triggering responses that retard gastric emptying and stimulate the release of bile and pancreatic juice.

Pancreatic Acini

Clusters of exocrine cells making up $99\%$ of the pancreas that synthesize and secrete pancreatic juice containing digestive enzymes.

Pancreatic Islets

Endocrine clusters of cells (islets of Langerhans) constituting $1\%$ of the pancreas that secrete hormones directly into the blood, including insulin, glucagon, somatostatin, and pancreatic polypeptide.

Pancreatic Juice

A clear, alkaline secretion consisting of water, sodium bicarbonate (which buffers acidic chyme), and enzymes including pancreatic amylase, trypsin, chymotrypsin, and pancreatic lipase.

Hepatocytes

The major specialized functional and metabolic cells of the liver, responsible for bile production, metabolic transformations, and secretion.

Bile Canaliculi

Small narrow intercellular channels between adjacent hepatocytes that collect newly secreted bile and drain it toward bile ducts.

Kupffer's Cells

Resident stellate macrophages located in liver sinusoids that engulf and destroy worn-out blood cells, microbes, and foreign particulate debris.

Gallbladder

A small pear-shaped muscular sac situated on the posterior surface of the liver that stores and concentrates bile produced by hepatocytes until signaled to release it into the cystic duct.

Bilirubin

The primary yellow-orange bile pigment derived from the breakdown of the heme component of recycled red blood cells.

Emulsification

The mechanical dispersal of large lipid globules into tiny suspended droplets by amphipathic bile salts, creating an extensive surface area for pancreatic lipase activity.

Duodenum

The shortest, most proximal segment of the small intestine (approx. $10\text{ inches}$ long) that receives chyme from the stomach along with bile and pancreatic secretions.

Jejunum

The middle section of the small intestine (approx. $8\text{ feet}$ long), extending between the duodenum and the ileum, specialized for nutrient absorption.

Ileum

The final and longest segment of the small intestine (approx. $12\text{ feet}$ long), terminating at the ileocecal valve where it connects to the large intestine.

Circular Folds (Plicae Circulares)

Permanent, spiral macroscopic ridges formed by the mucosa and submucosa of the small intestine that slow chyme transit and force it to spiral, expanding the surface area.

Villi

Fingerlike projections of the intestinal mucosa containing an arteriole, venule, capillary network, and central lacteal for the uptake of digested nutrients.

Microvilli

Microscopic projections of the apical plasma membrane of intestinal absorptive cells that form a distinct brush border to maximize digestive and absorptive surface area.

Brush Border Enzymes

Digestive enzymes embedded within the plasma membrane of microvilli that perform terminal digestion of carbohydrates, proteins, and nucleotides on the absorptive cell surface.

Segmentations

Localized, alternating mixing contractions of the small intestine circular smooth muscle that slosh chyme back and forth against the mucosa without producing net forward movement.

Migrating Motility Complex (MMC)

A pattern of slow, wave-like peristalsis that begins in the lower stomach and moves progressively along the small intestine to clear remaining chyme and debris between meals.

Micelles

Small spherical lipid packages surrounded by bile salts that ferry hydrophobic long-chain fatty acids and monoglycerides to the apical surface of absorptive enterocytes.

Chylomicrons

Large lipoprotein transport complexes assembled in enterocytes, enclosing resynthesized triglycerides and cholesterol with a protein coat, released via exocytosis into lacteals.

Teniae Coli

Three narrow, thickened longitudinal bands of smooth muscle running the length of the colon's muscularis externa that bunch the wall into haustra.

Haustra

A series of characteristic pouch-like sacs lining the large intestine produced by tonic contractions of the teniae coli.

Haustral Churning

A mechanical movement of the colon in which a relaxed haustrum distends as it fills, eventually contracting to push its contents into the next haustrum.

Mass Peristalsis

A strong, coordinated wave of contraction beginning in the transverse colon that quickly propels fecal matter through the colon into the rectum.