The Digestive System: Anatomy, Physiology, and Regulation

Vocabulary practice flashcards covering the anatomical structures, histological features, hormonal regulation, and physiological processes of the digestive system.

Ingestion

The active process of taking food and liquids into the digestive tract through the oral cavity.

Mechanical Processing

Physical breakdown, shearing, and mixing of food particles (such as by teeth and stomach contractions) that increases surface area for enzymatic attack and facilitates movement.

Digestion

The chemical breakdown of ingested materials into smaller absorbable molecules mediated by digestive enzymes (e.g., proteins to amino acids, triglycerides to glycerol and fatty acids, and polysaccharides to monosaccharides).

Peristalsis

Coordinated waves of smooth muscle contraction that propel a bolus along the tract; circular muscles contract behind the bolus while longitudinal muscles ahead of the bolus contract.

Segmentation

Cycles of localized smooth muscle contractions that fragment and mix the bolus with intestinal secretions without propelling food forward in any particular direction.

Crop

An expanded pouch in the avian esophagus specialized for temporary storage and softening of ingested feed prior to digestion.

Proventriculus

The glandular stomach of birds that secretes hydrochloric acid and digestive enzymes to chemically process feed before it enters the gizzard.

Gizzard (Ventriculus)

The thick, muscular stomach compartment of birds that mechanically grinds feed in the absence of teeth.

Enamel

A densely packed crystalline calcium phosphate layer covering the crown of the tooth, representing the hardest biologically manufactured substance.

Dentin

A mineralized matrix resembling bone that constitutes the principal bulk and structure of a tooth beneath the enamel and cementum.

Pulp Cavity

The central vascularized and innervated internal chamber of a tooth that receives blood vessels and nerve fibers through the root canal.

Zygomatic Glands

Specialized salivary glands found exclusively in carnivorous species such as dogs and cats, situated close to the orbit and zygomatic arch.

Rugae

Prominent, temporary folds in the mucosal lining of the empty stomach that flatten out to allow expansion as the stomach fills with food.

Parietal Cells

Gastric secretory cells that secrete intrinsic factor (necessary for vitamin $B_{12}$ absorption) and hydrochloric acid ($\text{HCl}$) to maintain gastric juice at approximately $\text{pH } 2.0$.

Chief Cells

Gastric secretory cells located in gastric glands that produce and secrete pepsinogen, an inactive proenzyme converted by $\text{HCl}$ into protein-digesting pepsin.

G Cells

Enteroendocrine cells situated in the pyloric glands that secrete the peptide hormone gastrin into the bloodstream to stimulate gastric motility and acid production.

Cephalic Phase

The initial stage of gastric regulation initiated by the sight, smell, taste, or anticipation of food; mediated by parasympathetic fibers in the vagus nerve ($N\,X$) to prepare the stomach for food intake.

Gastric Phase

The regulatory phase of digestion lasting $3\text{--}4\text{ hours}$ stimulated by the physical presence of food, stomach distension, and elevated $\text{pH}$, driving neural reflexes and gastrin release.

Intestinal Phase

The phase of gastric regulation initiated when acidic chyme enters the duodenum, triggering hormonal releases and neural reflexes that slow gastric emptying and reduce stomach secretions.

Enterogastric Reflex

A long neural reflex triggered by duodenal distension and acidity that inhibits myenteric plexus activity, slows gastric motility, and contracts the pyloric sphincter.

Gastric Inhibitory Peptide (GIP)

A duodenal hormone released upon chyme entry that inhibits gastric secretion and motility while stimulating insulin secretion by the pancreas.

Cholecystokinin (CCK)

A duodenal hormone released in response to lipids and proteins that stimulates gallbladder contraction, relaxes the hepatopancreatic sphincter, and prompts pancreatic digestive enzyme secretion.

Secretin

A peptide hormone released by the duodenum when acidic chyme enters, stimulating pancreatic duct cells to secrete an alkaline, bicarbonate-rich buffer solution.

Vasoactive Intestinal Peptide (VIP)

A duodenal peptide hormone that stimulates regional capillary vasodilation and inhibits gastric acid production to optimize intestinal absorption.

Plicae Circulares

Permanent, macroscopic transverse circular folds in the mucosa and submucosa of the small intestine that increase the luminal surface area approximately threefold ($3\times$).

Intestinal Villi

Fingerlike mucosal projections covered by simple columnar epithelium that amplify small intestinal absorptive surface area tenfold ($10\times$) and contain capillaries and a central lacteal.

Lacteals

Specialized lymphatic capillaries situated inside intestinal villi that absorb dietary lipids (packaged as chylomicrons) that cannot enter blood capillaries directly.

Paneth Cells

Specialized defense cells located at the base of intestinal crypts (Crypts of Lieberkühn) that secrete antimicrobial peptides.

Taeniae Coli

Three distinctive longitudinal ribbons of smooth muscle running along the exterior surface of the colon whose resting tone puckers the wall into haustra.

Haustra

A series of visible pouches along the walls of the colon that permit distension and elongation as intestinal contents are compacted.

Pancreatic Acini

The exocrine gland units of the pancreas consisting of cell clusters and associated ducts that make up roughly $99\%$ of pancreatic volume and secrete digestive enzymes.

Hepatopancreatic Sphincter (Sphincter of Oddi)

A muscular valve encircling the junction of the common bile duct and pancreatic duct at the duodenal ampulla that controls the discharge of bile and pancreatic juice into the duodenum.

Exocrine Pancreatic Insufficiency (EPI)

A pathological state characterized by inadequate secretion of pancreatic digestive enzymes due to chronic organ damage, presenting with diarrhea, bloating, and difficulty gaining weight.

Porta Hepatis

The hilum or doorway to the liver through which the hepatic portal vein, hepatic artery proper, and common hepatic bile duct enter and leave.

Liver Lobules

The basic functional and histological units of the liver, approximately $1\,\text{mm}$ in diameter, containing radiating plates of hepatocytes, sinusoids, and a central vein.

Kupffer Cells

Resident stellate macrophages lining liver sinusoids that engulf pathogens, cell debris, and damaged erythrocytes, as well as act as antigen-presenting cells.

Bilirubin

A yellowish pigment derived from the breakdown of the heme component of hemoglobin, extracted from circulation by hepatocytes and excreted into bile.

Microvilli

Microscopic projections of the intestinal epithelial cell membrane that form the brush border, host digestive enzymes, and increase absorptive surface area twentyfold ($20\times$).

Duodenum

The initial segment of the small intestine acting as a mixing bowl that receives chyme from the stomach and neutralizes acids with secretions from the pancreas and liver.

Jejunum

The middle segment of the small intestine responsible for the vast majority of chemical digestion and nutrient absorption.

Ileum

The final and longest segment of the small intestine that controls the flow of remaining chyme into the cecum through the ileocecal valve.

Oblique Muscle Layer

An innermost third smooth muscle layer unique to the muscularis externa of the stomach that assists in churning and mixing gastric contents.

Internal Anal Sphincter

A circular ring of smooth muscle in the anal canal that operates under involuntary autonomic control.

External Anal Sphincter

A distal ring of skeletal muscle encircling the anal canal that is subject to voluntary conscious control.

Parotid Glands

Paired salivary glands located near the ears that produce a serous secretion containing salivary amylase for carbohydrate digestion.

Sublingual Glands

Salivary glands situated under the tongue that primarily produce a mucous secretion that acts as a buffer and lubricant.

Pancreatic Duct (Duct of Wirsung)

The primary channel delivering exocrine secretions from pancreatic acinar and duct cells to the duodenal ampulla.

Gallbladder

A muscular, pear-shaped sac on the posterior surface of the liver that stores, modifies, and concentrates bile until stimulated to contract.

Hepatic Sinusoids

Highly permeable, fenestrated blood channels running between hepatocyte plates that convey blood from portal triads toward the central vein.

Hepatic Portal Vein

A major vessel delivering approximately two-thirds ($\frac{2}{3}$) of the liver's blood supply, transporting deoxygenated, nutrient-rich blood from the digestive tract.

Hepatic Artery Proper

A systemic arterial vessel that delivers approximately one-third ($\frac{1}{3}$) of the liver's total blood supply as oxygenated arterial blood.

Urea Synthesis

The metabolic process in the liver that detoxifies ammonia produced by amino acid deamination by converting it to urea for renal excretion.

Cecum

The expanded, pouch-like initial segment of the large intestine that receives material from the ileum and participates in bile salt reabsorption.

Cloaca

The terminal chamber of the avian digestive system serving as a shared exit for feces, urine, and gametes.

High-Density Lipoprotein (HDL)

A circulating lipoprotein synthesized in the liver that returns peripheral cholesterol back to hepatocytes for reuse or excretion, widely recognized as good cholesterol.

Enterocytes

Simple columnar absorptive cells of the intestinal epithelium that express brush border enzymes and transport water, electrolytes, and nutrients.

Falciform Ligament

A fold of peritoneum on the anterior liver surface that stabilizes the organ and demarcates its right and left anatomical lobes.