Anatomy of Digestion and Salivation

Salivation and Mouth

• The parotid gland produces serous saliva to hydrate food.
• The sublingual gland produces mucus saliva to lubricate food.
• The submandibular gland produces serous saliva to hydrate food.
• The mouth aids in food lubrication and hydration, and contains digestive enzymes like lingual (salivary) amylase, lingual lipase, and lysozyme.

Gastric Digestion

• Gastric glands secrete acid, pepsinogen, lipase, and mucus.

Small Intestine - Acid Neutralization

• The duodenum receives chyme from the stomach.
• Brunner’s glands, pancreatic secretions, and gallbladder secretions neutralize chyme.

Motility in the Mouth

• Swallowing (deglutition) involves the epiglottis closing, the bolus being pushed back, the oropharynx relaxing, and peristalsis.

The Esophagus

• Propels digesting food to the stomach.
• The upper third is skeletal muscle, the lower two thirds are smooth muscle.
• Contractions of circular and longitudinal muscles create peristaltic waves.
• Peristalsis can be bidirectional.

Secretions Summary

• The mouth secretes saliva, enzymes, and mucus.
• The stomach secretes acid, pepsinogen, lipase, and mucus.
• The small intestine (duodenum) receives secretions from Brunner's glands, the pancreas, and the gallbladder.

Peristalsis

• Primary peristalsis begins at the pharynx, is regulated by the vagus nerve, involves skeletal muscle contraction, and circular muscle relaxations followed by contractions.
• Secondary peristalsis is initiated by the stretching of muscles by digesta, results in muscle contractions, and can be reversed.

Motility in the Stomach

• The lower esophageal sphincter (LOS) should remain closed.
• The pylorus regulates the flow of chyme from the stomach.
• Churning involves stomach muscle contractions, peristalsis, and the pylorus remaining closed.
• Emptying involves peristalsis, and the pylorus opening.

Motility Summary

• Autonomic motility, called peristalsis, begins in the mouth, involves the contraction of circular and longitudinal muscles, and the relaxation of circular muscles.

Digestion Before the Small Intestine

• Carbohydrates, proteins, and lipids are digested before entering the small intestine.
• A slow, constant flow of digesta to the small intestine is achieved even after a large meal.

Small Intestine Digestion

• The duodenum is where the majority of digestion occurs.
• The following enzymes aid in protein digestion in the duodenum: trypsin, chymotrypsin, elastase, carboxypeptidases, and aminopeptidases.
• Brush border aminopeptidases are located on the enterocytes, while others are secreted from the pancreas.
• Carbohydrate digestion in the duodenum involves the following enzymes: amylase, dextrinase, glucolyase, lactase, maltase, isomaltase, and sucrase.
• These brush border enzymes break down carbohydrates into monosaccharides for absorption.
• Lipid digestion in the duodenum involves the following components: lipase, bile salts, and bile acids.
• These components aid in the formation of lipid droplets.

Digestion Summary

• Protein digestion involves trypsin, chymotrypsin, elastase, carboxypeptidases, and aminopeptidases, which are secreted from the pancreas or the brush border of enterocytes.
• Carbohydrate digestion involves amylase, dextrinase, glucolyase, lactase, maltase, isomaltase, and sucrase. These enzymes are all located on the brush border of the small intestine.
• Lipid digestion involves lipase, bile salts, and bile acids, which are secreted from the pancreas and gallbladder.

Small Intestine Absorption: Proteins

• Absorption occurs in the duodenum and jejunum.
• Di- and tri-peptides, and amino acids are absorbed.
• Active transporters mediate absorption on the apical side of the intestinal epithelium.
• Passive transporters mediate absorption on the basolateral side.

Small Intestine Absorption: Carbohydrates

• Absorption occurs in the duodenum and jejunum.
• Monosaccharides are the form absorbed.
• Active transport occurs on the apical side of the intestinal epithelium.
• Passive transport occurs on the basolateral side.

Small Intestine Absorption: Lipids

• Absorption occurs in the duodenum, jejunum, and ileum.
• Lipid droplets are absorbed by endocytosis.
• Further digestion occurs in the intestinal lumen.
• Lipids are packaged into chylomicrons.
• Lacteals within the small intestine transport chylomicrons.

Small Intestine Absorption: Water and Electrolytes

• Absorption occurs in the jejunum and ileum.
• 90% of GI water is reabsorbed.
• Absorption is primarily driven by passive transport along an ion gradient.