Notes on Digestion

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Digestion Facts:

Gastric Motility:

• During the intestinal phase, gastric motility is inhibited by distention of the small intestine.

• Gastrin speeds up gastric motility (emptying of stomach).

Stomach:

• The stomach is the most distensible part of the gastrointestinal tract.

Pancreatic Juice:

• Pancreatic juice contains: [pancreatic amylase (carbohydrates)], [chymotrypsin, elastase, carboxypeptidase (protein)], [pancreatic lipase (triglycerides)], and [ribonuclease and deoxyribonuclease (for nucleic acids)].

Constipation:

• Constipation can be caused my several reasons including emotional stress, spasm of the colon, and lack of exercise.

Galactose Absorption:

• Galactose is absorbed by primary active transport.

Monoglyceride Absorption:

• Monoglycerides are absorbed from the lumen of the GI tract by simple diffusion.

Anabolism:

• Anabolism is not a function of the digestive system.

Carbohydrate Digestive Enzymes:

• Carbohydrate digestive enzymes include: sucrase, amylase, maltase, lactase.

Saliva Secretion:

• Saliva secretion is not ceased after food is swallowed. Saliva secretion is stimulated by parasympathetic impulses.

Secretin:

• Secretin promotes the secretion of bicarbonate ions present in pancreatic juice.

Peristalsis:

• Mass peristalsis: a strong peristaltic wave that begins in the middle of the transverse colon and quickly drives the colonic contents into the rectum.

Protein Digestion:

• Brush border of the small intestine produces enterokinase.

• Carboxypeptidase digests peptides from the acid end of the molecule.

Salivary Glands:

• Salivary glands that consist of serous acini are parotid glands.

Parietal Cell Secretions:

• Parietal cells in gastric glands are stimulated by H2 receptors, histamine, acetylcholine, and gastrin.

Digestion in Mouth:

Saliva Composition:

• Saliva contains 99.5% water and the rest solutes.

• Solutes include Na, K, Cl, HCO3-, phosphate, urea, uric acid, mucus, IgA, lysozyme, and salivary amylase.

• Water is used to dissolve food.

• Cl ions activate saliva amylase.

• HCO3- and phosphate buffer food, so saliva is only slightly acidic.

Mucus:

• Mucus lubricates food.

• IgA prevents microbes from attaching to the epithelium.

Lysozyme:

• Lysozyme kills bacteria, preventing infection.

Saliva Secretion:

• Saliva is secreted by parasympathetic stimulation.

• Dryness in mouth is caused by stress, triggered by sympathetic stimulation.

• Salivation is controlled by parasympathetic neurons of the facial nerve (CN VII) and glossopharyngeal nerve (CN IX).

Digestion in Stomach:

Chyme:

• Minutes after a meal, peristaltic movements called mixing waves begin in the stomach, which mix food reducing it to chyme.

• Mixing waves are less intense in the fundus, and more intense at the pylorus.

Salivary Amylase:

• Action of salivary amylase continues until stopped by gastric juices.

Parietal Cells:

• Parietal cells secrete HCl which partially denatures proteins and promotes the action of gastric juices.

Pepsinogen:

• Chief cells secrete pepsinogen, which in active form - pepsin - digests protein.

Acetylcholine:

• Increased pH stimulates release of acetylcholine which stimulates secretion of gastrin.

Gastrin:

• G cells secrete gastrin which stimulates parietal cells and chief cells, stimulates growth of gastric glands, and strengthen contractions of the lower esophageal sphincter.

• Small amounts of gastrin is also released by the small intestine.

• Histamine and caffeine can stimulate release of gastrin.

Phases of Digestion:

• Gastric digestion occurs in three phases: cephalic, gastric, and intestinal phase.

• Only alcohol, water, ions, short-chain fatty acids and some drugs are absorbed in the stomach.

Additional Reading:

Basic Gastroenterology

1. Basic Gastrointestinal Physiology
2. Digestion FAQ, Defecation reflex, etc.
3. Digestion
4. Notes on Functions of the Liver
5. Notes on Jaundice
6. Types of Jaundice
7. Diagram of Gastric Blood Supply
8. FAQ on Gastric Digestion
9. Usage of the D-xylose Absorption Test

Gastroenterology Videos

1. Video of Abdominal Examination in a Clinical Setting

Related Topics

1. Gastrointestinal Disorders
2. Hepatobiliary and Pancreatic Disorders
3. Histology of the Digestive Tract I: Oral Cavity
4. Histology of the Digestive Tract II: Esophagus through Intestines
5. Histology of the Liver, Pancreas, and Gall Bladder
6. Abdominal Examination for Internal Medicine

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Useful Medical Images & Diagrams (link opens in a new window)

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