Gastrointestinal Tract: Overview and Functions

Introduction to the Gastrointestinal Tract

The gastrointestinal (GI) tract, also known as the alimentary canal, is a continuous muscular tube that digests and absorbs food. It extends from the mouth to the anus and includes several specialized organs and accessory structures.

• Main organs: mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon, rectum, anus)

• Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, pancreas

Major Functions of the GI Tract

• Mastication: Mechanical and chemical breakdown of food in the mouth

• Ingestion: Intake of food and swallowing

• Digestion: Mechanical and chemical breakdown of food into absorbable units

• Absorption: Movement of nutrients from the gut into the blood

• Excretion (Defaecation): Removal of undigested material as feces

Structural Organization of the GI Tract

Layers of the GI Tract

The wall of the GI tract is composed of four main layers, each with distinct functions:

• Mucosa: Innermost layer; consists of epithelium (mainly simple columnar), lamina propria (connective tissue), and muscularis mucosae (smooth muscle). Functions in secretion, absorption, and protection.

• Submucosa: Dense connective tissue containing blood vessels, lymphatics, glands, and the submucosal plexus (controls secretion).

• Muscularis Externa: Smooth muscle responsible for peristalsis and segmentation; contains the myenteric plexus (controls motility).

• Serosa: Outermost layer; visceral peritoneum covering the GI tract in the abdominal cavity.

Movements of the GI Tract

GI tract motility is essential for moving and mixing food:

• Peristalsis: Coordinated, wave-like contractions of circular and longitudinal muscles that propel food distally.

• Segmentation: Alternating contractions of nonadjacent segments, mixing food and enhancing absorption.

Peritoneum and Peritoneal Folds

Structure and Function of the Peritoneum

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

• Parietal peritoneum: Lines the abdominal wall

• Visceral peritoneum: Covers the organs

• Peritoneal cavity: Space between the two layers, containing lubricating fluid

Peritoneal Folds and Mesenteries

• Mesentery: Double layer of peritoneum that suspends the small intestine from the posterior abdominal wall, providing a pathway for blood vessels, nerves, and lymphatics.

• Mesocolon: Attaches the large intestine to the posterior abdominal wall.

• Greater omentum: Large apron-like fold of visceral peritoneum that hangs from the stomach and covers the intestines, containing fat and immune cells.

Functional Anatomy of the Mouth

Oral Cavity Structure

The mouth (oral cavity) is the entry point for food and is lined with stratified squamous epithelium. Major structures include:

• Cheeks, hard and soft palate, uvula, tongue, teeth

Salivary Glands and Saliva

• Major glands: Parotid, submandibular, sublingual

• Saliva: Contains water, mucus, and the enzyme salivary amylase (initiates starch digestion)

• Salivation is controlled by the parasympathetic nervous system (CN VII and IX)

Teeth: Structure and Types

Teeth are specialized for mechanical digestion (mastication):

• Structure: Crown (covered by enamel), neck, root (covered by cementum), pulp cavity (contains nerves and vessels), periodontal ligaments anchor teeth to bone.

• Types: Incisors (cutting), canines (tearing), premolars and molars (crushing and grinding)

• Dental formula: Deciduous (20), permanent (32)

Tongue: Structure and Function

• Composed of intrinsic and extrinsic muscles

• Functions: Manipulates food, forms bolus, initiates swallowing, essential for speech and taste

• Papillae: Filiform (friction), fungiform and vallate (taste buds)

• Contains lingual tonsil (immune function)

Digestion in the Mouth

• Mechanical digestion: Chewing breaks food into smaller pieces and mixes it with saliva

• Chemical digestion: Salivary amylase begins starch breakdown; process called hydrolysis

• Amylase is inactivated by stomach acid

Oesophagus: Structure and Function

Anatomy of the Oesophagus

The oesophagus is a muscular tube that transports food from the pharynx to the stomach. It passes through the mediastinum and diaphragm (at the oesophageal hiatus, T10).

• Four layers: mucosa, submucosa, muscle layer, outer connective tissue

• Upper and lower oesophageal sphincters regulate passage of food

• Peristalsis moves the bolus toward the stomach

Stomach: Structure, Histology, and Function

Gross Anatomy of the Stomach

• Regions: Cardia, fundus, body, pyloric antrum, pyloric canal, pylorus (with pyloric sphincter)

• Borders: Lesser curvature (medial), greater curvature (lateral)

• Located under the diaphragm in the left upper abdomen

Stomach Wall and Gastric Mucosa

• Mucosa: Simple columnar epithelium with rugae (folds) and gastric pits leading to gastric glands

• Submucosa: Contains blood vessels and submucosal plexus

• Muscularis externa: Multiple smooth muscle layers for mixing

• Serosa: Visceral peritoneum

Gastric Gland Cell Types and Secretions

• Surface mucous and mucous neck cells: Secrete mucus for protection

• Parietal cells: Produce hydrochloric acid (HCl) and intrinsic factor (for vitamin B12 absorption)

• Chief cells: Secrete pepsinogen (converted to pepsin by HCl) and gastric lipase

• G cells: Endocrine cells that secrete gastrin (stimulates gastric activity)

Digestion in the Stomach

• Mechanical digestion: Mixing waves churn food into chyme

• Chemical digestion: HCl denatures proteins, activates pepsinogen; pepsin digests proteins

• Salivary amylase is inactivated by stomach acid

Regulation of Gastric Secretion and Motility

• Cephalic phase: Triggered by sight, smell, taste, or thought of food; stimulates gastric secretion via vagus nerve (CN X)

• Gastric phase: Initiated by food in the stomach; stretch and chemoreceptors increase secretion and motility

• Intestinal phase: Begins as chyme enters the duodenum; enterogastric reflex and hormones (secretin, CCK, VIP) inhibit gastric activity

Small Intestine: Structure and Function

Gross Anatomy of the Small Intestine

• ~6 meters long; extends from pyloric sphincter to ileocecal valve

• Subdivisions: Duodenum (retroperitoneal), jejunum, ileum (both attached by mesentery)

• Major site of digestion and absorption

Duodenum, Jejunum, and Ileum

• Duodenum: Shortest part (~25 cm), receives bile and pancreatic ducts, contains Brunner's glands (alkaline mucus)

• Jejunum: ~2.5 m, main site of nutrient absorption

• Ileum: ~3.6 m, joins large intestine at ileocecal valve, contains Peyer's patches (immune function)

Blood Supply and Innervation

• Arterial supply: Superior mesenteric artery

• Venous drainage: Superior mesenteric vein → hepatic portal vein → liver

• Innervation: Vagus nerve (parasympathetic), thoracic splanchnic nerves (sympathetic)

Histology of the Small Intestine

• Epithelium: Simple columnar (enterocytes) specialized for absorption; goblet cells secrete mucus

• Surface area adaptations: Circular folds (plicae circulares), villi, microvilli (brush border)

• Intestinal crypts: Secrete intestinal juice, contain enteroendocrine cells (hormones), Paneth cells (antimicrobial), stem cells

Digestion in the Small Intestine

• Mechanical digestion: Segmentation and peristalsis mix and move chyme; bile aids fat digestion

• Chemical digestion: Enzymes from pancreas and brush border complete digestion of carbohydrates, proteins, and lipids

Summary Table: Layers of the GI Tract

Key Equations and Concepts

• Hydrolysis (general reaction for digestion):

• Enzyme action (example: starch breakdown):

Additional info: The notes above integrate and expand upon the provided lecture content, ensuring a comprehensive, exam-ready summary for college-level Anatomy & Physiology students.