1. Functions of the Digestive System

1. Ingestion

• via mouth
• active process

1. Mechanical Processing
2. Secretion

• release of water, acids, enzymes, buffers and salts

1. Digestion
2. Absorption

• movement across epithelium and into interstitial fluid

1. Excretion
2. Protection

• corrosive effects of acids and enzymes
• mechanical stresses
• bacteria

2. Overview

• major components

1. oral cavity
2. salivary glands
3. esophogus
4. stomach
5. liver/gallbladder
6. pancreas
7. intestine (small & large)

1. Histological Organization

1. Mucosa

• inner lining
• epithelia + C.T. (lamina propria)

1. Digestive epithelium

• can be simple or stratified
• have folds to increase surface area and permit expansion of lumen
• change as progress through digestive tract

1. Submucosa

• similar to lamina propria
• Dense C.T. + blood/lymph vessels, nerves & exocrine glands

1. Muscularis Externa

• Inner, circular layer
• Outer, longitudinal layer

1. Serosa

• covers muscularis externa
• none covering oral cavity, pharynx, esophagus & rectum

1. Movement of Digestive Materials

1. Peristalsis

• Waves of muscular contractions - muscularis externa

1. Segmentation

• Churn & mix
• Does not produce directional movements

1. Control of Digestive Function

1. Neural mechanisms

• primary mechanism of regulation

1. Hormonal regulation
2. Local mechanisms

1. Peritoneum

• no bones to protect digestive organs - mesenteries
• Ventral mesenteries

1. Lesser Omentum - between stomach and liver
2. Falciform Ligament - Liver and abdominal wall

• Dorsal Mesenteries

1. Greater Omentum - Small intestine & abdominal wall
2. Mesenterial Sheet - small intestine
3. mesocolons - large intestine

3. Oral Cavity (Buccal Cavity)

1. Functions

1. Analysis
2. Mechanical processing
3. Lubrication
4. Digestion (limited)

1. The Tongue

• Functions:

1. mechanical processing
2. assist in chewing
3. sensory analysis
4. secrete mucus and an enzyme (lingual lipase)

• Two regions

1. Root - posterior portion - pharyngeal portion

• Lingual lipase - breakdown of lipid before swallowing

1. Salivary Glands

1. parotid glands - salivary amylase - break down carbohydrates
2. sublingual glands (mucous) - beneath floor of the mouth
3. Submandibular glands (mucous & salivary amylase) - floor of the mouth
4. saliva

• lubricate
• moisten
• dissolve
• digest carbohydrates

1. The Teeth

1. General Structure

1. Types of teeth

1. Incisors - cutting
2. Cuspids (Canines) - tearing
3. Bicuspids (Premolars) - crushing, grinding
4. Molars - crushing, grinding

4. The Pharynx

1. Functions

1. common passageway for food and air
2. mucous secretion
3. provide muscular contractions for swallowing

5. The Esophagus

1. Functions

1. passageway for food
2. muscular contractions for swallowing

1. Histology

• stratified squamous
• mucosa/submucosa folded for expansion
• presence of esophageal glands for production of mucus

1. Swallowing (Deglutition)

1. Buccal phase - hard palate to pharynx - voluntary
2. Pharyngeal phase - pharynx to esophagus
3. Esophageal phase - esophagus to stomach

6. The Stomach

1. Functions

1. storage of ingested substances
2. mechanical digestion
3. acidic and enzymatic action
4. production of intrinsic factor (absorption of B₁₂)

1. Anatomy

1. Four regions:

1. Cardia

• smallest
• mucus glands

1. Fundus
2. Body

• largest
• mixing tank
• gastric glands

• regulate release into duodenum
• gastrin

1. Musculature

• contain an oblique layer of cells in addition to the circular and longitudinal layers

1. Histology

• simple columnar

1. gastric glands - fundus and body

1. parietal cells

1. intrinsic factor - absorption of B₁₂
2. HCl - hydrogen & chlorine transported separately

1. Chief cells

1. Pepsinogen - inactive proenzyme - converted to pepsin
2. renin and gastric lipase
2. pyloric glands

1. G cells - gastrin
2. D cells - Somatotropin

1. Regulation of Stomach Activity

• CNS, hormones, reflexes

1. Cephalic phase

• begin with sight, smell, taste or thought of food
• short (minutes)
• directed by CNS
• prepare stomach to receive food

1. Gastric phase

1. Stimuli

1. control

1. Neural

1. Hormonal

1. Local response

1. Intestinal phase

1. Neural response

1. Hormonal response

7. The Small Intestine and Associated Glandular Organs

1. The small intestine

1. Functions

1. Digestion
2. Absorption

1. Divisions

1. Duodenum

• receive chyme from stomach and digestive enzymes from pancreas & liver
• Curves in a C - encloses pancreas

1. Jejunem

• about 8 feet long
• bulk of digestion and absorption

c) Ileum

• about 12 feet long
• last segment
• into large intestine

1. Histology

1. Plicae Circulares
2. Intestinal Villi
3. Intestinal crypts

1. The Pancreas

• behind the stomach
• nodular texture
• exocrine organ - produce digestive enzymes & buffers
• divided into head, body and tail regions

1. Histology

1. Pancreatic Acini (Exocrine)

• blind pockets
• cuboidal cells
• pancreatic juice (water, ions and enzymes)

1. Pancreatic Islets (Endocrine)

• scattered between acini
• secrete insulin and glucagon

1. Physiology

1. Hormonal regulation - the hormones that stimulate pancreatic activity are the same hormones that inhibit the activity of the stomach

1. Secretin
2. CCK