Chapter 24 Notes
The Digestive System

Mechanical and Chemical Digestion

1. Fundamental distinctions

• Chemical digestion - catabolic breakdown using hydrolytic enzymes
• Mechanical digestion - mastication, peristalsis, segmentation
• Absorption - uptake of broken down foods from the intestinal cells
  to blood stream to liver to blood stream to cells
• online overview

2. Histology of digestive tract

• Tunica mucosa

- surface epithelium, basement membrane
- underlying connective tissue called the lamina propria

• Submucosa

- blood vessels, and nerve plexuses

• Tunica muscularis

- inner circular smooth muscle makes tube thinner and longer when contracted
- outer longitudinal smooth muscle makes tube shorter and thicker when contracted

• Serosa

- visceral peritoneum
- membrane covers all organs and continuous with parietal peritoneum

1. mesentery
2. lesser omentum
3. greater omentum
 

3. Structures of the oral cavity

• hard palate: formed by maxilla and palatine bones
• soft palate: suspended uvula blocks nasal passageway during swallowing
• palatine tonsils: two lateral masses of lymphatic tissue
• salivary glands

parotid

1. "mumps"
2. between mastoid process and rami of mandible
3. drained by Stenson’s duct
4. secretes mostly amylase

submandibular

1. posterior under tongue
2. drained by Wharton’s duct
3. secretes amylase and mucus

sublingual

1. under tongue
2. secretes mucus through many small ducts

• SALIVA

- 99% water, 1200 ml per day, secreted
- continuous secretion except during dehydration
- mucin, a protein for lubrication and protection
- electrolytes, HCO₃^-, HPO₄^-, Cl^-
- lysozyme inhibits bacteria
- AMYLASE an enzyme that digests carbohydrates

• Teeth

- central and lateral incisors
- cuspids or canine
- first and second premolars or bicuspids
- first, second, and third molars
- dental caries: a problem before "crest"
- brush, floss, irrigate fluoride

4. Esophagus

• Swallowing online
• collapsible tube for transport of food
• circular muscles contract squeezing food down
• longitudinal muscles contact to receive food

5. Stomach  

• cardia-->fundus-->body-->pyloris
• closed ends through esophageal and pyloric sphincters
• rugae are folds that can increase size
• gastric pits are folds of columnar cells with the following secretions:

- chief cells secrete pepsinogen
- parietal cells secrete HCl to activate pepsinogen to pepsin and Vitamin B12 intrinsic factor
- PEPSIN hydrolyzes proteins to peptides
- mucous cells secrete mucus to protect gastric lining
- enteroendocrine cells secrete gastin to continue stomach secretions and motility

• the stomach online
   
• Three Phases of Gastric Secretion

CEPHALIC PHASE:

1. see smell taste initiate a reflex
2. nerve impulses from cerebral cortex to hypothalamus to medulla to      
    vagus nerve
3. parasympathetic stimulus of gastric secretions, pepsinogen and HCl

GASTRIC PHASE

1. partially digested proteins and caffeine stimulate release of the hormone gastrin
2. THE ROLES OF THE HORMONE GASTRIN ARE:

- continues secretions of gastric glands
- gastrin increases stomach motility
- contraction of GE sphincter
- relaxation of pyloric sphincter and ileocecal valve

INTESTINAL PHASE:

1. enterogastric reflex inhibits stomach activity
2. GASTRIC INHIBITORY PEPTIDE blocks stomach secretions and motility
3. SECRETIN

- same role as GIP
- stimulates secretions of small intestine
- stimulates pancreas to secrete bicarbonate
- stimulates liver to release bile

4. CCK

- same role as GIP
- stimulates pancreas to secrete enzymes
- stimulates contraction of the gall bladder

Phases of secretion online