Chapter 23: The Digestive System – Structure, Function, and Physiology

Overview of the Digestive System

Digestive System Functions and Organ Groups

The digestive system is responsible for breaking down food and absorbing nutrients for use by body cells. It consists of two main groups of organs:

• Alimentary canal: A continuous muscular tube including the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Its primary functions are digestion and absorption of food.

• Accessory digestive organs: These include the teeth, tongue, salivary glands (in the mouth), and the gallbladder, liver, and pancreas (connected by ducts). They assist in mechanical breakdown and produce secretions to aid digestion.

Major Processes of Digestive System Activity

Six essential activities occur during digestive system activity:

• Ingestion: Taking food into the GI tract.

• Propulsion: Moving food through the GI tract via swallowing and peristalsis.

• Mechanical breakdown: Increasing surface area for digestion and mixing with secretions.

• Digestion: Enzymatic catabolism of food into chemical components.

• Absorption: Movement of digested end products from the GI lumen into blood or lymph.

• Defecation: Elimination of indigestible substances via the anus.

Anatomy and Histology of the Digestive System

Peritoneum and Mesenteries

The peritoneum is a serous membrane lining the abdominopelvic cavity and covering digestive organs. It consists of:

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

• Parietal peritoneum: Lines the body wall.

• Peritoneal cavity: Space between the two layers, filled with serous fluid.

• Mesentery: Double layer of peritoneum that supports organs, allows passage of vessels, and stores fat.

• Retroperitoneal organs: Located posterior to the mesentery, against the dorsal abdominal wall.

Layers of the Alimentary Canal

The alimentary canal has four basic layers:

• Mucosa: Innermost, moist epithelial membrane; secretes, absorbs, and protects.

• Submucosa: Connective tissue with blood/lymph vessels, nerve fibers, and lymphoid follicles.

• Muscularis externa: Inner circular and outer longitudinal smooth muscle; responsible for peristalsis and segmentation.

• Serosa: Protective outer layer (visceral peritoneum).

Splanchnic Circulation and Hepatic Portal System

• Splanchnic circulation: Arteries branching from the abdominal aorta supply digestive organs.

• Hepatic portal circulation: Drains nutrient-rich blood from digestive organs to the liver for processing.

Neural Control of the Digestive System

Enteric Nervous System

The GI tract has its own nervous system, the enteric nervous system, composed of enteric neurons forming two main plexuses:

• Submucosal plexus: Innervates the submucosa.

• Myenteric plexus: Innervates smooth muscle layers, controlling motility.

Regulation occurs via:

• Short reflexes: Mediated by enteric neurons.

• Long reflexes: Involve CNS integration and autonomic nerves.

Digestive activity is governed by chemical/mechanical stimuli, effectors (smooth muscle and glands), and control by neurons/hormones.

Functional Anatomy of the Digestive System

The Mouth and Associated Organs

The mouth is lined with stratified squamous epithelium and bounded by lips, cheeks, palate, and tongue. Key features:

• Lips and cheeks: Skeletal muscle core, keep food between teeth.

• Palate: Hard (bony) and soft (muscular); aids in food manipulation and swallowing.

• Tongue: Skeletal muscle, mixes food, initiates swallowing, aids speech; papillae house taste buds.

• Salivary glands: Produce saliva (water, electrolytes, enzymes, mucins, lysozyme, IgA).

• Teeth: Deciduous (20) and permanent (32); classified as incisors (cutting), canines (tearing), premolars/molars (grinding).

Digestive processes in the mouth include ingestion, mechanical breakdown, propulsion, and initial digestion of polysaccharides.

Pharynx and Esophagus

The pharynx (oropharynx, laryngopharynx) and esophagus move food from the mouth to the stomach. Swallowing (deglutition) involves:

• Buccal phase: Voluntary, bolus forced into oropharynx.

• Pharyngeal-esophageal phase: Involuntary, food moves through pharynx and esophagus.

Stomach Structure and Function

The stomach temporarily stores food and begins protein digestion. Key features:

• Regions: Cardial, fundus, body, pyloric.

• Microscopic anatomy: Simple columnar epithelium, goblet cells, gastric glands (mucus, HCl, intrinsic factor, pepsinogen, hormones).

• Digestive processes: Protein denaturation, absorption of alcohol/aspirin, secretion of intrinsic factor.

• Regulation: Neural and hormonal mechanisms (cephalic, gastric, intestinal phases).

• Alkaline tide: HCO3– pumped into blood as H+ is secreted into stomach lumen, raising blood pH.

Liver, Gallbladder, and Pancreas

• Liver: Four lobes, composed of lobules (hepatocytes), secretes bile, processes nutrients, stores vitamins, detoxifies.

• Gallbladder: Stores and concentrates bile.

• Pancreas: Secretes pancreatic juice (enzymes and bicarbonate) into small intestine; enzymes are activated in the duodenum.

• Bile and pancreatic juice delivered to intestine via ducts; secretion regulated by hormones (cholecystokinin, secretin) and neural stimuli.

Small Intestine

The small intestine is the major site for digestion and absorption. Structural modifications enhance absorption:

• Divisions: Duodenum, jejunum, ileum.

• Wall features: Circular folds, villi, microvilli increase surface area.

• Cell types: Enterocytes (absorption), goblet cells (mucus), enteroendocrine cells (hormones), Paneth cells (antimicrobial).

• Digestive processes: Segmentation, peristalsis, feedback control of gastric emptying.

Large Intestine

The large intestine absorbs water and eliminates feces. Features include:

• Unique structures: Teniae coli, haustra, epiploic appendages.

• Subdivisions: Cecum, appendix, colon, rectum, anal canal.

• Mucosa: Thick, with mucus-producing goblet cells.

• Bacteria: Ferment indigestible carbohydrates, synthesize vitamins B and K, influence immunity.

• Digestive processes: Haustral contractions, mass movements, defecation reflex.

Physiology of Digestion and Absorption

Mechanism of Digestion and Absorption

Digestion hydrolyzes food into nutrients absorbed across the gut epithelium. Most digestion occurs in the small intestine via enzymatic hydrolysis. Absorption involves transport across epithelial cells into capillaries or lacteals.

Processing of Nutrients

• Carbohydrates: Digested by salivary amylase, pancreatic amylase, and brush border enzymes; absorbed as monosaccharides via co-transport and facilitated diffusion.

• Proteins: Digested by pepsin (stomach), pancreatic proteases, and brush border enzymes; absorbed as amino acids via co-transport and facilitated diffusion.

• Lipids: Emulsified by bile salts, digested by pancreatic lipases; absorbed as monoglycerides and fatty acids, reassembled into triglycerides, packaged as chylomicrons, transported via lacteals.

• Nucleic acids: Hydrolyzed by pancreatic nucleases to nucleotide monomers.

• Vitamins, electrolytes, water: Small intestine absorbs dietary vitamins, electrolytes, and water (by osmosis); large intestine absorbs vitamins B and K.

Developmental Aspects of the Digestive System

Embryonic Development

• Epithelial lining forms from endoderm; oral and cloacal membranes form mouth and anus.

• By week 5, alimentary canal is a continuous tube.

Digestive System After Birth and Aging

• Newborns: Rooting and sucking reflexes, small stomach, inefficient peristalsis.

• Aging: Decline in motility, juice production, absorption; increased risk of diverticulosis, fecal incontinence, GI cancers.

Recent Discoveries and Research Topics

• Mesentery classified as an organ.

• Brain tanycytes control appetite.

• "Good fats" may help inflammatory bowel conditions.

• Gut bacteria influence peristalsis.

Suggested research topics include Helicobacter pylori infections, hepatitis, weight loss surgeries, motility disorders, digestive system cancers, and Crohn’s disease.