FIVE MAJOR PROCESSES OF THE DIGESTIVE TRACT

• Motility of digestive products through the digestive tract.
• Secretion of enzymes and fluids into the digestive tract.
• Digestion of food breakdown.
• Absorption of nutrients and water.
• Barrier from the external environment and microorganisms.

Minor Process

• Elimination of waste and undigested material from the body.

STRUCTURES

Alimentary canal and glands/organs

• Alimentary canal
  – Runs from the mouth to the anus.
  – We can imagine the alimentary canal as a donut hole within a giant donut.
• Accessory glands/organs
  – Secrete digestive juices into the alimentary canal
  – To imagine the accessory glands, we draw a donut and show jelly ooze into the donut hole to represent the accessory gland secretions.

Accessory organs

• Salivary glands secrete saliva
• Liver secretes bile into the gallbladder
• Gallbladder secretes stored bile into the duodenum after a meal
• Pancreas secretes pancreatic juice into the duodenum

Liver, gallbladder, and pancreatic secretions pass through individual ducts

– All merge at duodenum → secretions secrete into duodenum

Additional features of alimentary canal:

• Smooth muscle lining allows peristalsis
  – Peristalsis is rhythmic contractions that push food down the alimentary canal.
  – Sphincters are modified, one-way valves that comprise smooth muscle. They contract and relax to regulate movement through the alimentary canal.

DIGESTIVE TRACT PATHWAY

Oral cavity

• Site of mastication (chewing).

Esophagus

• Transports the food bolus to the stomach (via peristalsis).
• Gastroesophageal sphincter: regulates the movement of food into the stomach
  – Prevents “heartburn”: the reflux of food back into the esophagus.

Stomach

• Churns and converts food into liquid chyme.
• Pyloric sphincter regulates the movement of chyme into our small intestine; it releases chyme slowly to allow the duodenum time to absorb food nutrients.

Small intestine

• The major site of nutrient absorption.
• Three major divisions: duodenum, jejunum, ileum.
• Most nutrient absorption in duodenum.

Large intestine

• Absorbs water
• Three major divisions: cecum, colon, rectum.

Anus

• Excretes waste.

DIGESTION OF THE FOUR MAJOR MACROMOLECULES

Aided by luminal secretions/enzymes – promote digestion of macromolecules to absorbable nutrients

Oral cavity

• Luminal secretions: Saliva = amylase, mucus, lysozyme
• Digests: Polysaccharides → smaller polysaccharides, maltose
  – Enzyme is salivary amylase

Esophagus

• Luminal secretions: Mucus, which lubricates passage of food to the stomach.  NO digestion

Stomach

• Luminal secretions: Gastric juice = HCl, pepsin, mucus
• Digests: Polypeptides → small polypeptides
  – Enzyme is pepsin

Small intestine

• Luminal secretions are:
  – Bile
  – Pancreatic enzymes: trypsin, chymotrypsin, carboxypeptidase, amylase, lipase and nuclease
  – Brush border enzymes (not secreted): dissaccharidases, aminopeptidases
• Digests:
  – Polysaccharides → disaccharides, maltose. Enzyme: pancreatic amylase
  – Disaccharides → monosaccharides. Enzyme: Disaccharidases (brush border)
  – Polypeptides → smaller polypeptides. Enzyme: trypsin & chymotripsin
  – Smaller polypeptides → amino acids. Enzymes: carboxypeptidases & aminopeptidases (brush border)
  – DNA/RNA → nucleotides. Enzyme: nuclease
  – Fat globules → glycerol, fatty acids. Enzyme: lipase and bile salts

Large Intestine

• Luminal secretions: mucus.  NO digestion. Instead absorbs water and electrolytes.