The Digestive SystemCh23 Notes

The Digestive System

Structure and Functions of the Digestive System

The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. It consists of the gastrointestinal (GI) tract and accessory digestive organs.

• Major Functions: Ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.

• GI Tract vs. Accessory Organs: The GI tract (alimentary canal) is a continuous tube from mouth to anus (mouth, pharynx, esophagus, stomach, small intestine, large intestine). Accessory organs (teeth, tongue, salivary glands, liver, gallbladder, pancreas) assist in digestion but are not part of the tube.

• Mechanical vs. Chemical Digestion: Mechanical digestion physically breaks food into smaller pieces (e.g., chewing, churning in stomach, segmentation in intestines). Chemical digestion uses enzymes to break down macromolecules into absorbable units (e.g., amylase for carbohydrates, proteases for proteins).

• Segmentation: Localized contractions that mix food in the intestines.

• Peristalsis: Wave-like muscle contractions that move food along the GI tract.

• Pathway of Ingested Substances: Mouth → Pharynx → Esophagus → Stomach → Small Intestine (duodenum, jejunum, ileum) → Large Intestine → Rectum → Anus.

General Gross and Microscopic Anatomy of the GI Tract

The GI tract is composed of several organs and four main tunics (layers) that perform specialized functions.

• Organs of the GI Tract: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anal canal.

• Accessory Digestive Organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

• GI Tract Tunics:

  • Mucosa: Innermost layer; secretes mucus, absorbs nutrients, and protects against pathogens.

  • Submucosa: Connective tissue with blood vessels, nerves, and glands.

  • Muscularis (Muscularis Externa): Smooth muscle responsible for peristalsis and segmentation; typically two layers (circular and longitudinal).

  • Serosa/Adventitia: Outermost layer; serosa is a visceral peritoneum, adventitia is connective tissue (in esophagus and retroperitoneal organs).

Peritoneum and Mesenteries

The peritoneum is a serous membrane lining the abdominal cavity, supporting and protecting abdominal organs.

• Visceral Peritoneum: Covers external surfaces of most digestive organs.

• Parietal Peritoneum: Lines the body wall.

• Peritoneal Cavity: Space between the two peritoneal layers, containing serous fluid for lubrication.

• Mesentery: Double layer of peritoneum that anchors organs and provides a pathway for blood vessels, nerves, and lymphatics.

• Types of Mesenteries: Mesentery proper (small intestine), mesocolon (large intestine), lesser omentum (stomach to liver), greater omentum (stomach to transverse colon).

• Intraperitoneal vs. Retroperitoneal Organs: Intraperitoneal organs are surrounded by peritoneum (e.g., stomach, liver), while retroperitoneal organs are behind the peritoneum (e.g., pancreas, parts of the duodenum).

Oral Cavity

The oral cavity is the entry point for food and is involved in mechanical and chemical digestion.

• Boundaries: Lips (anterior), cheeks (lateral), palate (superior), tongue (inferior), oropharynx (posterior).

• Mastication: The process of chewing food to increase surface area for enzymes.

• Hard Palate: Bony anterior roof; Soft Palate: Muscular posterior roof; Uvula: Prevents food from entering the nasopharynx.

• Tongue: Contains taste buds (on papillae), aids in mixing food and swallowing.

• Teeth: Mechanically break down food; types include incisors, canines, premolars, molars.

• Salivary Glands: Parotid, submandibular, sublingual glands; secrete saliva containing enzymes (e.g., amylase), mucus, and antimicrobial agents.

• Saliva: Moistens food, begins starch digestion, cleanses mouth, dissolves chemicals for taste.

Anatomy of the Pharynx

The pharynx is a muscular tube that serves as a passageway for both air and food.

• Regions: Nasopharynx (air only), oropharynx (air and food), laryngopharynx (air and food).

Gross and Microscopic Anatomy of the Esophagus

The esophagus is a muscular tube that transports food from the pharynx to the stomach.

• Location: Posterior to the trachea, passes through the diaphragm at the esophageal hiatus.

• Functions: Propels food via peristalsis; secretes mucus for lubrication.

• Specializations: Mucosa contains stratified squamous epithelium; muscularis transitions from skeletal (upper) to smooth muscle (lower).

Gross and Microscopic Anatomy of the Stomach

The stomach is a muscular organ that stores, mixes, and digests food.

• Location: Upper left abdomen, inferior to the diaphragm.

• Functions: Mechanical and chemical digestion, food storage, production of chyme.

• Specializations: Three layers of muscularis (longitudinal, circular, oblique) for churning; mucosa contains gastric pits and glands.

• Gastric Glands:

  • Parietal Cells: Secrete hydrochloric acid (HCl) and intrinsic factor.

  • Chief Cells: Secrete pepsinogen (inactive enzyme).

  • Mucous Neck Cells: Secrete mucus.

  • Enteroendocrine Cells: Secrete hormones (e.g., gastrin).

• Sphincters: Inferior esophageal (cardiac) sphincter (prevents reflux), pyloric sphincter (controls emptying into duodenum).

• Rugae: Gastric folds that allow stomach expansion.

Gross and Microscopic Anatomy of the Small Intestine

The small intestine is the primary site for digestion and absorption of nutrients.

• Location: Extends from the pyloric sphincter to the ileocecal valve.

• Functions: Completes digestion, absorbs nutrients and water.

• Segments: Duodenum (shortest), jejunum (middle), ileum (longest).

• Specializations: Circular folds (plicae circulares), villi, and microvilli increase surface area for absorption.

• Structures:

  • Duodenal (Brunner) Glands: Secrete alkaline mucus.

  • Intestinal Glands (Crypts of Lieberkühn): Secrete intestinal juice.

  • Peyer Patches: Lymphoid nodules in the ileum for immune defense.

Gross and Microscopic Anatomy of the Large Intestine, Rectum, and Anal Canal

The large intestine absorbs water and electrolytes, forming and expelling feces.

• Location: Frames the small intestine; extends from ileocecal valve to anus.

• Segments: Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal; includes flexures (hepatic, splenic).

• Functions: Water absorption, feces formation, vitamin production (by bacteria).

• Specializations: Muscularis externa forms teniae coli (longitudinal bands), haustra (pouches).

• Anal Sphincters: Internal (smooth muscle, involuntary), External (skeletal muscle, voluntary).

Gross and Microscopic Anatomy of the Accessory Digestive Organs

Accessory organs secrete substances essential for digestion.

• Liver:

  • Functions: Produces bile, processes nutrients, detoxifies blood, stores vitamins.

  • Structure: Four lobes, ligaments (coronary, falciform, round), porta hepatis (entry/exit for vessels and ducts).

  • Histology: Classic hepatic lobule (hexagonal unit), hepatic triad (branch of hepatic artery, portal vein, bile duct).

• Gallbladder: Stores and concentrates bile; releases bile into the duodenum.

• Pancreas:

  • Structure: Head, body, tail; exocrine (pancreatic acini) and endocrine (islets of Langerhans) components.

  • Functions: Secretes digestive enzymes and bicarbonate (exocrine); hormones insulin and glucagon (endocrine).

• Biliary Apparatus: Network of ducts transporting bile and pancreatic juice.

  • Components: Left/right hepatic ducts, common hepatic duct, cystic duct, common bile duct, main pancreatic duct, hepatopancreatic ampulla (ampulla of Vater), sphincter of Oddi, major duodenal papilla.

  • Pathway: Bile from liver → hepatic ducts → common hepatic duct → cystic duct (to gallbladder) or common bile duct (to duodenum); pancreatic juice via main pancreatic duct joins at ampulla of Vater.

Motility in the GI Tract

Motility refers to the movement of food through the digestive tract by muscular contractions.

• Deglutition (Swallowing): Involves mouth, pharynx, esophagus; glottis and larynx move to prevent aspiration.

• Types of Movements:

  • Peristalsis: Propels food forward.

  • Mixing Waves: Churn food in the stomach.

  • Segmentation: Mixes contents in the small intestine.

  • Mass Movement: Moves feces in the large intestine.

• Regulation: Volume, chemical composition, and osmolarity of chyme affect motility.

• Defecation Reflex: Initiated by rectal distension; involves autonomic and somatic nerves.

Physiology of Digestion

Chemical digestion involves enzymatic breakdown of macromolecules into absorbable units.

• Enzymes, Sources, Substrates, and Products:

  Enzyme	Source	Substrate	Product
  Amylase	Salivary glands, pancreas	Starch	Maltose, glucose
  Pepsin	Stomach (chief cells)	Proteins	Peptides
  Lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides
  Trypsin	Pancreas	Proteins	Peptides
  Lactase	Small intestine	Lactose	Glucose, galactose
  Additional info: Sucrase, maltase, carboxypeptidase, etc.	Small intestine, pancreas	Various	Monomers

• Locations of Digestion: Carbohydrates (mouth, small intestine), proteins (stomach, small intestine), lipids (small intestine).

• Zymogen: Inactive enzyme precursor (e.g., pepsinogen); prevents autodigestion.

• Hydrochloric Acid (HCl): Produced by parietal cells; denatures proteins, activates pepsinogen, kills microbes; regulated by neural and hormonal signals.

• Emulsification: Bile salts break up fat globules in the small intestine, increasing surface area for lipase action.

• Microbiome: Bacteria in the large intestine aid in digestion (e.g., fiber fermentation), vitamin production (e.g., vitamin K).

Processes of Absorption

Absorption is the movement of nutrients from the GI tract into the blood or lymph.

• Organs and Structures: Small intestine (villi, microvilli), large intestine (water, electrolytes).

• Transport Processes: Passive diffusion, facilitated diffusion, active transport, endocytosis.

• Mineral and Vitamin Absorption: Calcium (active, vitamin D-dependent), iron (regulated by need), fat-soluble vitamins (A, D, E, K) absorbed with lipids, water-soluble vitamins (B, C) by diffusion or active transport, vitamin B12 requires intrinsic factor.

• Water Absorption: Occurs mainly in small intestine by osmosis; large intestine absorbs remaining water.

Hormonal and Neural Regulation of Digestive Processes

Digestive processes are regulated by the enteric nervous system and hormones.

• Enteric Nervous System (ENS): Intrinsic nerve network controlling GI motility and secretion.

• Autonomic Innervation: Parasympathetic (stimulates digestion), sympathetic (inhibits digestion).

• Phases of Digestion:

  • Cephalic Phase: Initiated by sight, smell, thought of food; stimulates gastric secretion.

  • Gastric Phase: Food in stomach increases secretion and motility.

  • Intestinal Phase: Chyme in duodenum modulates gastric activity.

• GI Hormones:

  • Gastrin: From stomach; stimulates HCl and pepsinogen secretion.

  • Cholecystokinin (CCK): From duodenum; stimulates gallbladder contraction, pancreatic enzyme secretion.

  • Secretin: From duodenum; stimulates bicarbonate secretion from pancreas, inhibits gastric secretion.

Application of Homeostatic Mechanisms

The digestive system maintains mass balance and contributes to body fluid homeostasis.

• Mass Balance: Fluid intake + digestive secretions = fluid absorbed + excreted.

• Homeostasis: Proper absorption and excretion maintain electrolyte and fluid balance in the body.