# Lecture 13: The Digestive System — Anatomy and Overview

## Anatomy and Physiology II

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## Learning Objectives

By the end of this lecture, students will be able to:

1. List the major functions and processes of the digestive system
2. Describe the general histology of the GI tract wall (four tunics)
3. Describe the anatomy and function of the oral cavity, pharynx, and esophagus
4. Describe the gross anatomy and histology of the stomach
5. Describe the peritoneum and mesenteries
6. Explain the basic neural and hormonal regulation of digestion

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## Lecture Content

### I. Overview of the Digestive System

The digestive system consists of the gastrointestinal (GI) tract (alimentary canal) and its accessory organs. The **GI tract** is a continuous muscular tube approximately 9 meters long extending from the mouth to the anus, passing through the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anal canal. **Accessory organs** include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

#### Six Major Digestive Processes

Digestion involves six interrelated processes. **Ingestion** is the taking of food into the mouth. **Mechanical digestion** is the physical breakdown of food through chewing, churning, and segmentation. **Propulsion** moves food through the GI tract via swallowing (deglutition), which has both voluntary and involuntary phases, and peristalsis, waves of smooth muscle contraction that push food forward. **Chemical digestion** is the enzymatic breakdown of macromolecules into absorbable units. **Absorption** transfers digested nutrients from the GI lumen into the blood or lymph. **Defecation** eliminates indigestible residues from the body.

### II. General Histology of the GI Tract Wall

The wall of the GI tract consists of four layers, or tunics, arranged from the lumen outward.

#### 1. Mucosa (innermost)

The mucosa has three sublayers. The **epithelium** varies by region: the mouth, pharynx, esophagus, and anal canal are lined with stratified squamous epithelium for protection against abrasion, while the stomach and intestines are lined with simple columnar epithelium optimized for secretion and absorption. The **lamina propria** is a layer of loose connective tissue containing capillaries, lymphatic tissue (MALT), and nerve fibers. The **muscularis mucosae** is a thin layer of smooth muscle that creates local folds to increase surface area.

#### 2. Submucosa

The submucosa consists of dense connective tissue containing blood vessels, lymphatic vessels, and nerves. It houses the **submucosal plexus (Meissner's plexus)**, part of the enteric nervous system that regulates glandular secretions and blood flow. In certain regions, such as the duodenum, the submucosa contains glands (Brunner's glands).

#### 3. Muscularis Externa

The muscularis externa typically contains two layers of smooth muscle (three in the stomach). The **inner circular layer** constricts the lumen and produces segmentation movements. The **outer longitudinal layer** shortens the tube and produces peristalsis. Between these layers lies the **myenteric plexus (Auerbach's plexus)**, part of the enteric nervous system that controls GI motility.

#### 4. Serosa (or Adventitia)

The outermost layer is the **serosa** where the GI tract lies within the peritoneal cavity. The serosa is the visceral peritoneum, consisting of simple squamous epithelium on connective tissue. Where the GI tract is retroperitoneal (the esophagus, parts of the duodenum, ascending and descending colon, and rectum), the outer layer is instead an **adventitia** of connective tissue that blends with surrounding structures.

<image>A cross-sectional diagram of the general GI tract wall. Panel A: A cylindrical segment of the GI tract cut open and unrolled to show all four layers from innermost to outermost — mucosa (with epithelium, lamina propria, and muscularis mucosae labeled), submucosa (with submucosal plexus of Meissner highlighted), muscularis externa (inner circular and outer longitudinal layers with myenteric plexus of Auerbach between them), and serosa. Blood vessels, lymphatic vessels, and mesenteric attachment are shown. Panel B: An inset showing the enteric nervous system — the submucosal plexus controlling secretion and the myenteric plexus controlling motility, with connections from the autonomic nervous system (sympathetic decreasing motility, parasympathetic increasing motility via the vagus nerve).</image>

### III. The Peritoneum

The peritoneum is the largest serous membrane of the body. The **parietal peritoneum** lines the abdominal wall, while the **visceral peritoneum** covers the abdominal organs as their serosa. The **peritoneal cavity** between these layers is a potential space containing serous fluid. **Mesenteries** are double-layered peritoneal folds that suspend organs from the body wall and contain blood vessels, lymphatics, nerves, and fat. The **greater omentum** is an apron-like fold hanging from the greater curvature of the stomach that stores fat and contains immune cells, earning it the nickname "fatty policeman." The **lesser omentum** connects the liver to the lesser curvature of the stomach. The **mesentery proper** suspends the jejunum and ileum, and the **mesocolon** suspends parts of the large intestine. **Retroperitoneal organs**, including the kidneys, pancreas, parts of the duodenum, ascending and descending colon, aorta, and IVC, lie behind the peritoneum. **Peritonitis**, inflammation of the peritoneum from infection or perforation, is a serious and potentially life-threatening condition.

### IV. Oral Cavity (Mouth)

The oral cavity is bounded by the lips, cheeks, palate, and tongue. The **lips and cheeks** keep food between the teeth during chewing and contain the buccinator muscle. The **palate** has two portions: the hard palate anteriorly, formed by the maxillae and palatine bones, and the soft palate posteriorly, a muscular structure that elevates during swallowing to close off the nasopharynx. The uvula hangs from the posterior edge of the soft palate. The **tongue** is a mass of skeletal muscle that manipulates food during chewing and forms the bolus for swallowing. It contains taste buds on its papillae, and the lingual frenulum attaches it to the floor of the mouth.

#### Teeth

Humans develop two sets of teeth. The **deciduous (primary) teeth** number 20, while the **permanent (secondary) teeth** number 32, comprising incisors for cutting, canines for tearing, and premolars and molars for grinding. Each tooth consists of a crown above the gum line, a root below it, and a neck at their junction. **Enamel**, the hardest substance in the body, covers the crown. **Dentin**, a bone-like material, forms the bulk of the tooth. The **pulp cavity** contains blood vessels and nerves. **Cementum** covers the root and anchors the tooth to the periodontal ligament. **Dental caries** (cavities) result from bacterial acid eroding the enamel and dentin.

#### Salivary Glands

Three pairs of major salivary glands produce approximately 1 to 1.5 liters of saliva per day. The **parotid glands**, the largest, sit anterior to the ear and produce a serous (watery, enzyme-rich) secretion. The **submandibular glands**, beneath the mandible, produce a mixed serous and mucous secretion. The **sublingual glands**, beneath the tongue, produce primarily mucous secretion. Numerous minor salivary glands are scattered throughout the oral mucosa.

**Saliva** is 97 to 99.5% water and contains several important components. **Salivary amylase** (ptyalin) begins the digestion of starch. **Lingual lipase** begins fat digestion but is activated only in the stomach's low-pH environment. **Mucin**, a glycoprotein, lubricates food and protects the oral mucosa. **Lysozyme** and **IgA** provide antimicrobial defense. **Bicarbonate** buffers acids. Saliva also moistens food, dissolves chemicals for taste, and cleanses the mouth. Salivation is controlled by the parasympathetic nervous system via cranial nerves VII and IX and is stimulated by food in the mouth, or even by the sight, smell, or thought of food.

### V. Pharynx and Esophagus

#### Deglutition (Swallowing)

Swallowing proceeds through three phases. The **buccal (oral) phase** is voluntary: the tongue pushes the bolus against the hard palate and toward the oropharynx. The **pharyngeal phase** is involuntary and is triggered when the bolus contacts the pharyngeal wall. The soft palate elevates to close the nasopharynx, the epiglottis closes over the larynx to protect the airway, pharyngeal constrictor muscles propel the bolus through the pharynx, and the upper esophageal sphincter relaxes. The **esophageal phase** is involuntary, with peristalsis propelling the bolus through the esophagus to the stomach.

#### Esophagus

The esophagus is a muscular tube approximately 25 cm long that connects the pharynx to the stomach, passing through the diaphragm at the esophageal hiatus. Two sphincters guard its ends: the **upper esophageal sphincter (UES)**, composed of skeletal muscle, prevents air entry during breathing, and the **lower esophageal sphincter (LES)** or cardiac sphincter, composed of smooth muscle, prevents gastric reflux. The LES is not a true anatomical sphincter but rather a physiological constriction. The muscularis of the esophagus transitions from skeletal muscle in the upper third, to mixed in the middle third, to smooth muscle in the lower third. The esophagus is lined with non-keratinized stratified squamous epithelium and secretes mucus for lubrication but produces no digestive enzymes.

### VI. Stomach

#### Gross Anatomy

The stomach is a J-shaped muscular organ in the left upper quadrant, divided into four regions. The **cardia** surrounds the cardial orifice where the esophagus enters. The **fundus** is the dome-shaped superior portion. The **body** is the large central region. The **pylorus** is the funnel-shaped inferior region leading to the duodenum, consisting of the pyloric antrum and pyloric canal, and terminating at the **pyloric sphincter**, a thick ring of smooth muscle that controls gastric emptying. The **greater curvature** is the convex lateral surface, and the **lesser curvature** is the concave medial surface. **Rugae**, large mucosal folds visible when the stomach is empty, flatten as the stomach expands to accommodate food.

#### Histology of the Stomach Wall

The muscularis externa of the stomach is unique in having three layers: an inner oblique, a middle circular, and an outer longitudinal layer. This arrangement enables the churning and mixing movements that are characteristic of gastric digestion. **Gastric glands** in the body and fundus extend into the lamina propria and line **gastric pits**.

These glands contain several specialized cell types. **Mucous neck cells** secrete thin, acidic mucus. **Parietal (oxyntic) cells** secrete hydrochloric acid, which activates pepsinogen, denatures proteins, and kills bacteria, creating a gastric pH of 1.5 to 3.5. They also secrete intrinsic factor, which is essential for vitamin B12 absorption in the ileum. **Chief (zymogenic) cells** secrete pepsinogen, the inactive precursor of pepsin. HCl converts pepsinogen to pepsin, a protein-digesting enzyme active at low pH. **Enteroendocrine cells** include G cells (which secrete gastrin to stimulate HCl secretion and gastric motility), D cells (which secrete somatostatin to inhibit gastric secretion), and enterochromaffin-like (ECL) cells (which release histamine to stimulate parietal cells).

**Surface mucous cells** line the stomach surface and secrete a thick, alkaline mucus barrier that protects the mucosa from acid and enzymatic self-digestion, forming the essential mucosal barrier.

<image>A diagram of stomach anatomy and histology. Panel A: External view of the stomach showing the cardia, fundus, body, pylorus (pyloric antrum and canal), pyloric sphincter, greater and lesser curvatures, rugae visible through a window cut in the wall, and the connection to the esophagus above and duodenum below. The three muscle layers (inner oblique, middle circular, outer longitudinal) are shown in a cutaway. Panel B: A magnified cross-section of the gastric mucosa showing a gastric pit leading into a gastric gland. Each cell type is labeled with a distinct color: surface mucous cells lining the pit opening, mucous neck cells in the neck region, parietal cells (large and round) secreting HCl and intrinsic factor, chief cells (basophilic, at the base) secreting pepsinogen, and enteroendocrine cells (G cells, D cells, ECL cells) scattered throughout. Arrows indicate secretion products.</image>

### VII. Regulation of Digestive Activity — Overview

#### Neural Control

The **enteric nervous system (ENS)**, often called the "brain of the gut," can function independently of the central nervous system. The submucosal plexus (Meissner's) controls secretion, while the myenteric plexus (Auerbach's) controls motility. **Parasympathetic** input via the vagus nerve stimulates digestion by increasing motility, secretion, and blood flow. **Sympathetic** input inhibits digestion by decreasing motility and secretion and constricting the blood supply. **Short reflexes** are local reflexes within the enteric nervous system, while **long reflexes** involve CNS processing through vagal pathways.

#### Hormonal Control

GI hormones released by enteroendocrine cells into the blood coordinate digestive activity. **Gastrin** from G cells in the stomach stimulates HCl and pepsinogen secretion and gastric motility. **Secretin** from S cells in the duodenum stimulates pancreatic bicarbonate and bile secretion in response to acid. **Cholecystokinin (CCK)** from I cells in the duodenum stimulates pancreatic enzyme secretion and gallbladder contraction while inhibiting gastric emptying, in response to fats and proteins. **Gastric inhibitory peptide (GIP)** from K cells in the duodenum inhibits gastric secretion and stimulates insulin release. **Motilin** stimulates the migrating motor complex, the interdigestive motility pattern that cleans the GI tract between meals.

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