Chapter 25: The Digestive Tract

Introduction to the Digestive System

• The digestive system comprises the muscular digestive tract and accessory organs.

  • Think of it as one long tube or canal that starts at the mouth and ends at the anus, facilitating the entire process of food ingestion, digestion, absorption of nutrients, and elimination of waste.

• The digestive tract & accessory organs are responsible for:

  1. Ingestion— Bringing foods & liquids into the mouth

  2. Mechanical Processing— Chewing & Swallowing

  3. Digestion— Chemical & enzymatic breakdown of food into nutrients

  4. Secretion— Acids, enzymes, & buffers form GI lining or accessory organs

  5. Absorption— Movement of nutrients from the small intestine to the bloodstream regressive

  6. Compaction— Dehydration of indigestible materials & organic wastes

  7. Excretion— removal of wastes from the digestive tract

25.1 An Overview of the Digestive System

Histological Organization of the Digestive Tract

• Major layers of the digestive tract:

  • Mucosa: inner lining, consisting of mucosal epithelium and lamina propria.

    • Plicae— Folds of the mucosa that increase surface area for maximum absorption and secretion, particularly prominent in the small intestine.

  • Submucosa: Composed of areolar tissue (blood supply)

    • Submucosal Neural Plexuses— networks of neurons located in the submucosa and muscular layers that regulate digestive functions including motility and secretion.

  • Muscularis Externa (Muscular Layer): Contains smooth muscle fibers.

    • Myenteric Plexus— a network of nerves located between the muscular layers of the gastrointestinal tract that plays a crucial role in regulating digestive motility and coordinating peristalsis.

  • Serosa: A serous membrane in the peritoneal cavity.

Muscular Layers and Movement of Digestive Materials

• Smooth muscle cells in the digestive tract exhibit plasticity, tolerating extreme stretching.

• Contains visceral smooth muscle tissue arranged in sheets with no motor innervation.

• Pacesetter cells cause rhythmic contraction waves—

  • Peristalsis— waves of muscular contractions propel materials through the digestive tract

    • Essentially pushes things forward through “the tube”

  • Segmentation— Movements that churn and fragment material with no movement towards a particular area

    • Essentially smashing materials to increase the efficiency of nutrient absorption and facilitate digestion before they are further broken down in the intestines.

The Peritoneum

• The serosa (visceral peritoneum) is continuous with the parietal peritoneum lining the body wall.

• Abdominal organs may be:

  • Intraperitoneal— Organs that are completely enveloped by visceral peritoneum

    • ie stomach, liver, Ileum

  • Retroperitoneal— Organs that are located behind the peritoneum and not completely surrounded by it

    • ie the kidneys and ureters

  • Secondarily Retroperitoneal— Organs that were originally formed in the intraperitoneal cavity but later become retroperitoneal during embryonic development,

    • ie the duodenum (first portion of small intestines) and the pancreas

• Mesenteries are fused double sheets of peritoneal membrane that suspend digestive tract portions, found within the peritoneal cavity.

  • Stabilizes organs & blood vessels

25.2 The Oral Cavity

Functions of the Oral Cavity

1. Sensory analysis of food.

2. Mechanical processing (teeth, tongue).

3. Lubrication with mucus and saliva.

4. Limited digestion via salivary enzymes.

Anatomy of the Oral Cavity

• Stratified Squamous Epithelium lines the oral cavity

• The oral cavity consists of 2 Palates:

  • Hard Palate— Separates oral cavity from nasal cavity

  • Soft Palate— Lies posterior to the hard palate, separates the oral cavity from the nasopharynx, closes off the nasopharynx during swallowing.

    • Uvula— hangs in the posterior center of soft palate

• The tongue facilitates food manipulation and sensory analysis; dorsum of tongue covered in papillae.

  • Anterior body, posterior root

  • Frenulum of the tongue— Thin fold of mucous membrane connects the body of the tongue to the mucosa of the oral floor.

• Glands: Salivary glands secrete salivary amylase, secretions controlled by the autonomic nervous system

  • Parotid— The largest of the salivary glands, located near the ear

  • Sublingual— The smallest salivary glands, situated beneath the tongue

  • Submandibular— The glands located beneath the jaw

• Teeth: Designed for mastication

  • The Crown (enamel) surrounds the Dentine, which is the hard tissue beneath the enamel and forms the bulk of the tooth structure.

  • Roots: Embedded in the jawbone, providing stability and support to the tooth

  • Periodontal ligament— CT that attaches the tooth to the surrounding alveolar bone, allowing for slight movement and providing cushioning during chewing.

• Types of teeth:

  • Incisors: Cutting

  • Canines (cuspids): Tearing

  • Premolars (bicuspids): Crushing

  • Molars: Grinding

    • Deciduous Teeth (20) are replaced by Permanent Teeth (32)

25.3 The Pharynx

Skeletal Muscles in Swallowing

• Pharyngeal constrictor muscles, palatopharyngeus, stylopharyngeus aid swallowing.

The Swallowing (Deglutition) Process

1. Buccal Phase: The Bolus of food is compressed against the hard palate, tongue forces the bolus into the pharynx, isolating the nasopharynx

2. Pharyngeal Phase: The palatopharyngeus and stylopharyngeus muscles elevate the larynx, the epiglottis folds and the bolus passes the closed glottis toward the esophagus

3. Esophageal Phase: The upper esophageal sphincter opens, and perostalic waves push the bolus through the esophagus. The bolus reaches the lower esophageal sphincter and enters the stomach

25.4 The Esophagus

• A hollow, muscular tube transporting food and liquids to the stomach.

  • Lies posterior to the trachea, slightly left of the midline

  • Esophageal Hiatus— the opening in the diaphragm through which the esophagus passes, allowing it to connect to the stomach.

Histology of the Esophageal Wall

• Composed of mucosa, submucosa, muscular layer, and adventitia.

25.5 The Stomach

Major Functions

1. Temporary storage of ingested matter.

2. Mechanical breakdown of food.

3. Chemical digestion utilizing acids/enzyme disruption.

Anatomy of the Stomach

• The anterior & posterior surfaces are smooth and rounded, typically extending from T7-L3

• Divided into:

  1. Cardia— Superior medial portion of the stomach that connects to the esophagus.

  2. Fundus— The upper portion of the stomach, contacts inferior & posterior portions of the diaphragm

  3. Body (Corpus)— The largest region of the stomach that functions as a mixing tank for ingested food and gastric secretions

  4. Pyloric Part (Plyorus)— The lower portion of the stomach, connecting to the duodenum of the small intestine. Regulates the passage of partially digested food (chyme) into the duodenum via the:

     1. Pyloric Antrum— The wider proximal portion, that connects the stomach to the duodenum

     2. Pyloric canal: The narrower distal portion leading to the pyloric sphincter , which controls the flow of chyme from the stomach into the small intestine.

• Muscular layer has three muscle bands:

  1. Longitudinal

  2. Circular

  3. Inner Oblique

• Supplied by left gastric, splenic, and common hepatic arteries.

Histology of the Stomach

• Lined by simple columnar epithelium and shallow gastric pits.

• There are four primary cells within the stomach:

  1. Parietal cells: Secrete intrinsic factor and hydrochloric acid—

     • Lowers pH, denatures proteins, kills microorganisms

  2. Chief cells: Only in the fundus, Secret pepsinogen, is converted to pepsin by acids.

     • Pepsin is responsible for the enzymatic digestion of proteins into peptides

  3. G cells: Secrete hormone gastrin—

     • Stimulates the secretory activity of both parietal and chief cells

  4. Mucous Neck Cells— Produce mucus to protect the gastric lining—

     • Protects it from the corrosive effects of hydrochloric acid

Regulation of Gastric Activity

• Gastric secretions are controlled by the CNS and local hormonal signals.

25.6 The Small Intestine

Regions

• Comprises the:

• Duodenum— The shortest and widest segment of the SI, it is responsible for the initial phase of digestion and the mixing of chyme with digestive juices from the pancreas and bile from the liver.

• Jejunum— The middle section of the SI, where the bulk of chemical digestions occurs

• Ileum— The final and longest segment of the SI, controls the flow of material from the ileum to the cecum of the LI

Support of the Small Intestine

• The small intestine is specialized for absorption, with nearly 90% of bodily absorption taking place here

• Supplied by the superior mesenteric artery and vein.

• Mesentery proper supports blood, lymphatics, and nerves.

Histology of the Small Intestine

• Circular folds and intestinal villi increase absorption surface area.

• Lined by intestinal glands (enterocrine, goblet, stem cells).

• Duodenum: Contains submucosal glands and receives bile and pancreatic secretions.

• Ileum: Contains Peyer's patches (aggregated lymphoid nodules).

Regulation of the Small Intestine

• Intestinal juice buffers acid and dissolves digestive enzymes/products.

25.7 The Large Intestine

Main Functions

1. Reabsorb water and compact feces.

2. Absorb vitamins from bacteria.

3. Store fecal material prior to defecation.

   • The horsed-shaped large intestine begins at the junction with the ileum and ends at the anus

Structure

• Divided into cecum, colon, and rectum.

Cecum and Appendix

• Cecum: Collects and stores materials from the ileum, begins process of compaction

• Appendix: A slender, hollow organ attached to the posteromedial surface of the cecum— primarily associated with the lymphatic system.

The Colon

• Larger diameter, thinner walls, features haustra (pouches that permit elongation), teniae coli, omental appendices.

• Subdivided into:

  • Ascending— The section that travels upward from the cecum to the hepatic flexure

  • Transverse— The section that crosses the abdomen from the hepatic flexure to the splenic flexure

  • Descending— The section that moves downward from the splenic flexure to the sigmoid colon

  • Sigmoid Regions— The section that leads from the descending colon to the rectum, storing fecal matter until it is ready to be expelled.

The Rectum

• Contains internal and external anal sphincters, controls fecal passage.

• Rectal wall distension triggers the defecation reflex.

Histology of the Large Intestine

• Abundant goblet cells, lack of villi, mucus-secreting intestinal glands.

• Unlike the SI, the LI:

  • Lacks villi

  • Has an increased number of goblet cells and deeper intestinal glands

  • Possesses teniae coli within the muscular layer

Regulation of the Large Intestine

• Slow peristalsis and haustral churning in the cecum to transverse colon.

25.8 Accessory Digestive Organs

The Liver

• The largest visceral organ, responsible for regulating metabolism, serves as blood resivor , bile production

• Divided into left, right, quadrate, and caudate lobes.

• Hepatic artery and portal vein supply blood; veins drain it.

• Basic unit is the liver lobule, containing portal triads and bile canaliculi.

The Gallbladder

• A hollow, pear-shaped, muscular sack that stores and concentrates bile before releasing it to the SI— bile salts aid lipid digestion.

• Divided into three regions:

  1. Fundus

  2. Body

  3. Neck

     • Cystic duct connects it to the bile duct.

The Pancreas

• Lies posterior to the stomach, extending laterally from the duodenum toward the spleen

• Divided into

  • Head

  • Body

  • Tail

• The pancreas is a mixed gland with both exocrine and endocrine functions

  • Pancreatic duct— delivers exocrine secretions to the duodenal ampulla Exocrine function secretes digestive enzymes, endocrine function produces hormones (insulin, glucagon).

25.9 Aging and the Digestive System

• Digestion and absorption remain functional in elderly; however, age-related changes may occur, such as decreased epithelial turnover, changes in smooth muscle tone, and increased cancer rates.