Digestive Physiology - Auto-enzymatic & Allo-enzymatic Digestion & Nutrient Absorption (Vocabulary Flashcards)

Vocabulary flashcards covering key concepts from the lecture notes on digestive physiology, including digestion types, anatomy, and absorption mechanisms across livestock species.

Autoenzymatic digestion

Digestion carried out mainly by enzymes secreted by the animal’s own GI tract (e.g., humans, pigs, dogs, cats, rats, poultry).

Alloenzymatic digestion

Digestion largely accomplished by enzymes produced by microbes in the GI tract, in addition to animal secretions (typical of herbivores like ruminants and some others).

Monogastric

A simple, single-compartment stomach; typical of humans, pigs, dogs, cats; limited ability to digest roughage.

Ruminant

Mammal with a foregut composed of four compartments (rumen, reticulum, omasum, abomasum) enabling foregut fermentation and deriving most energy from volatile fatty acids.

Hindgut fermenter

Monogastric herbivore with a large hindgut (cecum and colon) where post-gastric microbial fermentation provides energy from fibrous feeds.

Foregut

The stomach region in ruminants where microbial fermentation occurs before true gastric digestion.

Simple stomach

Stomach of monogastric animals; regions include esophageal, cardiac, fundic, and pyloric; stores and processes food.

Cardiac region

Upper stomach region with mucus secretion to protect the stomach lining.

Fundic region

Middle stomach region where major gastric secretions (HCl, pepsinogen) occur and protein digestion begins.

Pyloric region

Stomach region near the pyloric sphincter; secretes mucus and gastrin; regulates entry of digesta into the small intestine.

Parietal cells

Stomach cells that secrete hydrochloric acid (HCl) and intrinsic factor.

Chief cells

Stomach cells that secrete pepsinogen.

Pepsinogen

Inactive proteolytic enzyme activated by HCl to pepsin; initiates protein digestion.

Small intestine

Primary site of digestion and absorption; contains duodenum, jejunum, and ileum; lined with villi and microvilli (brush border) and enterocytes.

Duodenum

First section of the small intestine; receives bile and pancreatic juice; site of most active nutrient digestion; contains brush border enzymes like enterokinase/enteropeptidase.

Jejunum

Main site of nutrient absorption; high enzyme secretion; major site for many nutrient absorptions; pH around 7.0–7.5.

Ileum

Final section of the small intestine; reabsorbs bile acids and absorbs nutrients that escape earlier digestion; higher pH (~7.5–7.9).

Villi

Finger-like projections on the mucosa that increase surface area for absorption; contain enterocytes, arteriole, venule, and lacteal.

Enterocytes

Absorptive epithelial cells lining the villi; primary cells responsible for nutrient absorption.

Glycocalyx

Glycoprotein layer on microvilli that houses digestive enzymes; part of the brush border.

Brush border

Combination of microvilli and glycocalyx on enterocytes that traps substrates and aids digestion/absorption.

Microvilli

Tiny projections on enterocytes that massively increase absorptive surface area.

Lacteal

Lymphatic capillary in the villus for the transport of absorbed fats into the lymphatic system.

Mucosa

Innermost lining of the GI tract consisting of epithelium, glands, and lamina propria; site of secretion and absorption.

Submucosa

Dense connective tissue layer housing blood vessels, lymphatics, and sometimes exocrine glands; supports mucosa.

Muscularis externa

Two smooth muscle layers (inner circular and outer longitudinal) responsible for GI motility.

Serosa

Visceral peritoneum covering most of the digestive tract; adventitia in some regions; provides protection and lubrication.

Mesentery

Double sheet of peritoneal tissue that contains blood vessels, nerves, and lymphatics; stabilizes the digestive tract.

Enterocyte phospholipid bilayer

The cell membrane of absorptive enterocytes through which nutrients pass during absorption.

Simple diffusion

Nutrient transport down a concentration gradient with no transporter and no energy.

Facilitated diffusion

Diffusion that requires a membrane transporter but no energy; moves down the concentration gradient.

Active transport

Transport requiring a transporter and energy; moves against the concentration gradient.

Secondary active transport

Transport that uses the gradient of a second substance (often energy-requiring) to move nutrients; indirect energy use.

Blood vs. lymph transport

Water-soluble nutrients enter the blood via capillaries (portal circulation) and liver; fat-soluble nutrients enter the lymph via lacteals and then reach general circulation.

Liver

Accessory organ that produces bile and processes nutrients; bile emulsifies fats.

Gallbladder

Stores and concentrates bile (except in horses).

Pancreas

Produces pancreatic juice with bicarbonate and digestive enzymes (proteolytic, amylolytic, lipolytic, nucleases).

Bile

Digestive fluid produced by the liver; emulsifies fats and contains bile salts and cholesterol; recycled via enterohepatic circulation.

Enterokinase/Enteropeptidase

Duodenal enzymes that activate pancreatic enzymes (e.g., activates trypsinogen to trypsin).

Foregut fermentation (pre-gastric)

Fermentation occurring before the true stomach, as in ruminants, enabling microbial breakdown of fibrous feeds.

Rumen

Large foregut fermentation chamber in ruminants where microbes produce VFAs, vitamins, microbial protein; papillae increase surface area.

Reticulum

Honeycomb foregut compartment; aids cud regurgitation and protects against foreign materials; contains reticular groove in young ruminants.

Omasum

Foregut compartment with folds and papillae; filters particles and absorbs water.

Abomasum

True gastric stomach of ruminants; secretes HCl, mucus, pepsinogen, rennin; highly acidic.

Cecum

Blind pouch in hindgut fermenters where microbial fermentation occurs; slow passage and production of VFAs.

Colon (large intestine)

Water absorption and some fermentation; houses microbial ecosystem; participates in VFAs and vitamin production.

Cloaca

Common posterior cavity in birds where digestive, urinary, and reproductive tracts discharge.

Proventriculus

Avian glandular stomach that secretes gastric juices (HCl and pepsin); precedes the gizzard.

Gizzard

Avian muscular grinding stomach with grit; primary site of mechanical digestion.

Crop

Avian esophageal outpocket used for storage and softening of food before digestion.

Avian digestion modifications

Beak for picking; crop for storage; proventriculus for chemical digestion; gizzard for mechanical digestion; no true lactase in small intestine.

Cloaca (avian)

Common exit cavity for digestive, urinary, and reproductive tracts in birds.

Hardware disease

Ingestion of metal leading to injury in the reticulum; bacteria and objects can cause disease if moved through GIT.

Ruminant energy source

Volatile fatty acids (VFAs) produced by microbial fermentation; primary energy for ruminants.

Hydrochloric acid (HCl)

Acid secreted by parietal cells in the stomach; denatures proteins, activates enzymes, kills bacteria.

Gastrin

Hormone released by the pyloric region that stimulates secretion of gastric mucus and gallbladder contraction.