The Digestive System

Chapter 24: The Digestive System

Introduction to Digestive Functions

• Nutrients: Foods contain various nutrients used for building and repairing body tissues, providing chemical energy.

• Digestion: Food must be broken down into smaller molecules suitable for absorption into body cells, a function accomplished by the digestive system.

• Homeostasis: The digestive system contributes to homeostasis by absorbing water, vitamins, and minerals while eliminating waste from the body.

Overview of the Digestive System

• The Digestive System is composed of two main parts:

  • Gastrointestinal (GI) Tract: A continuous tube from mouth to anus with organs including the mouth, esophagus, stomach, small intestine, and large intestine.

  • Accessory Digestive Organs: Include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Key Processes of Digestion

1. Ingestion: Taking food and liquids into the mouth.

2. Secretion: Release of digestive substances (water, acid, buffers, enzymes) into the GI tract over 7 liters daily.

3. Mixing and Propulsion: Churning and movements of food through the GI tract.

4. Digestion: Both mechanical (chewing) and chemical breakdown of food.

5. Absorption: Nutrients pass from the GI tract into blood/lymph.

6. Defecation: Elimination of indigestible substances and waste as feces.

Anatomy of the Digestive System

Layers of the GI Tract

• The GI tract contains four layers from deep to superficial:

  1. Mucosa: Epithelium, lamina propria, and muscularis mucosae (plays a role in absorption).

  2. Submucosa: Areolar connective tissue containing blood and lymphatic vessels.

  3. Muscularis: Smooth muscle layers that enable GI motility.

  4. Serosa/Adventitia: Outermost layer.

Detailed Look at Major Digestive Organs

1. Mouth

• Formed by lips, cheeks, hard/soft palates, and tongue.

• Saliva: Secreted by salivary glands; begins digestion of carbohydrates (salivary amylase) and lipids (lingual lipase).

2. Pharynx and Esophagus

• Pharynx: Passage shared by digestive and respiratory systems.

• Esophagus: Muscular tube connecting pharynx to stomach; moves food via peristalsis.

3. Stomach

• Connects the esophagus and duodenum.

• Processes food through mixing and chemical action of gastric juices (HCl, pepsin).

• Forms chyme, which is released into the small intestine.

4. Small Intestine (Duodenum, Jejunum, Ileum)

• Main site for digestion and absorption of nutrients.

• Villi and Microvilli: Increase surface area for absorption.

• Digestive Enzymes: Secreted pancreatic juice and intestinal juices complete digestion of carbs, proteins, and fats.

• Chyme spends 3-5 hours here.

5. Large Intestine

• Absorbs water, vitamins (B vitamins, vitamin K), and forms feces for elimination.

• Key structures: cecum, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal.

• Involves bacterial activity to further breakdown and absorb nutrients.

Hormonal Regulation of Digestion

• Gastrin: Released in response to food in stomach; stimulates gastric juice secretion.

• Cholecystokinin (CCK): Released when fatty acids and amino acids are present; stimulates enzyme secretion from pancreas and bile release from gallbladder.

• Secretin: Responds to acidic chyme in the duodenum; promotes bicarbonate secretion from pancreas.

Aging and the Digestive System

• Aging Effects: Decreased secretions, motility, and muscle tone; increased likelihood of constipation, diverticulitis, and other gastrointestinal issues.

• Common Disorders: Dental caries, peptic ulcers, and colorectal cancer.

Clinical Connections

• Appendicitis: Inflammation of the appendix, often requiring surgical removal.

• Hepatitis: Infection of the liver with various types leading to jaundice and potential liver failure.

• Colorectal Cancer: Regular screening recommended, as early detection is crucial.

Summary of Digestive Activities

• Digestive enzymes from various organs facilitate digestion, and absorption primarily occurs in the small intestine, where 90% of nutrients are absorbed.

• The large intestine further processes undigested materials, with feces formed for elimination.