Digestive System Overview

The Digestive System

• The digestive system is a complex process with multiple parts and steps.
• The alimentary canal (GI tract) is the passageway for food.
• The organs of the alimentary canal include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine, rectum, and anus.
• Accessory digestive organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
• The digestive process includes ingestion, propulsion (swallowing and peristalsis), mechanical and chemical digestion, absorption, and defecation (egestion).
• The digestive system breaks down food into smaller molecules that can be absorbed into the bloodstream.

Learning Outcomes

• Identify locations of digestive organs and mesenteries in the abdominal cavity.
• Describe the structure and functions of organs in the alimentary canal.
• Compare/contrast microscopic anatomy of regions in the alimentary canal
• Trace the path of food through the alimentary canal
• Describe accessory digestive organs and their functions.

Overview of the Digestive System

• The alimentary canal includes the mouth, pharynx, esophagus, stomach, small intestine (jejunum, ileum, duodenum), large intestine (colon), rectum, and anus
• Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Digestive Processes

• Ingestion: Taking food into the body
• Propulsion: Moving food through the digestive tract via swallowing and peristalsis.
• Mechanical Digestion: Physical breakdown of food (chewing, churning).
• Chemical Digestion: Chemical breakdown of food (enzymes).
• Absorption: Nutrients are absorbed into the bloodstream.
• Defecation: Undigested material is eliminated from the body.

Sources of Nutrients

• Macronutrients: carbohydrates, proteins, and fats
• Micronutrients: vitamins and minerals

Histology of the Alimentary Canal Wall

• Layers of the wall include epithelium, lamina propria, muscularis mucosae, submucosa, submucosal nerve plexus, circular muscle, myenteric nerve plexus, longitudinal muscle, and serosa/adventitia.
• Functions of each layer vary throughout the GI tract.

Nervous Control of the Gut

• Intrinsic (enteric nervous system): sensory neurons monitor tension, and visceral motor fibers can function without extrinsic nerves.
• Myenteric plexus controls GI motility, tonic contraction, frequency / intensity of contraction, and sphincter tone.
• Submucosal plexus controls secretion and absorption.
• Extrinsic (ANS): sympathetic and parasympathetic nervous systems.

Peritoneum and Mesenteries

• Peritoneum: a serous membrane that lines the abdominal cavity.
• Parietal peritoneum lines the abdominal cavity wall, while visceral peritoneum covers organs.
• Intraperitoneal organs are completely surrounded by peritoneum, whereas retroperitoneal organs lie behind the peritoneum.
• Mesenteries are double folds of peritoneum that attach organs to the abdominal wall.
• Examples include the greater omentum, lesser omentum, mesentery proper, and falciform ligament.

The Mouth and Salivary Glands

• Mouth/oral cavity is the beginning of the digestive tract.
• Salivary glands (parotid, sublingual, submandibular) produce saliva containing digestive enzymes.
• Teeth aid in mechanical digestion by tearing and grinding food.
• Tongue aids in chewing, swallowing, and tasting.

The Pharynx and Esophagus

• Pharynx: a passageway for both air and food.
• Esophagus: a muscular tube that transports food from the throat to the stomach.

Esophageal Hiatus

• The opening in the diaphragm through which the esophagus passes

The Stomach

• J-shaped organ inferior to the diaphragm.
• Functions:
• Storage
• Digestion (mechanical and chemical)
• Protein digestion begins in the stomach
• Absorption of water, alcohol, and drugs.
• Food enters as bolus and leaves as chyme (partially digested food and gastric juices.)

Wall Modifications of the Stomach

• Rugae
• Oblique muscle in muscularis.

Gastric Pits and Glands

• Gastric glands consist of mucous, parietal, chief, and enteroendocrine cells.
• Mucus cells protect the stomach lining.
• Parietal cells secrete HCl to activate pepsinogen.
• Chief cells secrete pepsinogen (activated to pepsin by HCl).
• Enteroendocrine cells secrete hormones like gastrin.

The Small Intestine

• Primary site of chemical digestion and absorption of 90% of nutrients.
• Subdivisions: duodenum, jejunum, and ileum
• Modifications (increased surface area): circular folds, villi, microvilli to enhance absorption.
• Nutrients absorbed include monosaccharides, amino acids, and fatty acids/glycerol.

Absorbing Nutrients

• Carbohydrates, lipids and proteins are absorbed in small intestine
• Simple diffusion, facilitated diffusion, and cotransport are methods of absorption.

The Large Intestine (Colon)

• Site of absorption of water and electrolytes.
• Subdivisions: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.

How Does Food Pass through the Alimentary Canal?

• Food travels from mouth to anus through the digestive tract.
• Organs involved and structure/substructures.
• Valves/sphincters between organs regulate food passage
• Describe how food is processed and modified as it passes through each organ. (functions of each organ)

Describe the function of the accessory digestive organs/structures

• Discuss the function of the various organs in the digestive process (exocrine, endocrine function).
• Salivary glands, liver, gallbladder, and pancreases.

Functions of Pancreas

• Exocrine pancreas secretes digestive enzymes and bicarbonate into the duodenum.
• Endocrine pancreas releases insulin and glucagon into the bloodstream to regulate blood glucose levels.

Gallbladder Function

• Stores and concentrates bile produced by the liver.

Bile duct Function

• Delivers bile from the liver and gallbladder to the duodenum.

Hormones of Digestion

• Gastrin (stomach), Cholecystokinin (CKK) (small intestine), Secretin (small intestine), GIP (Gastric Inhibitory Peptide)

Summary of Digestive System Regulation

• Digestive hormones and nervous signals regulate secretion by accessory organs and motility of the GI tract.

Explore the intricate workings of the digestive system, including the alimentary canal and its accessory organs. This quiz covers the structure, function, and processes involved in digestion, absorption, and excretion. Test your knowledge on the anatomy and physiology of the digestive system.