Digestive System Notes

Steps of Food Processing

• Ingestion: The first step of food processing.
• Digestion: The second step.
• Absorption: The third step.
• Elimination/Egestion: The final step (Figure 1).

Digestion

• Mechanical Digestion:
  • Physical grinding of molecules into smaller pieces.
• Chemical Digestion:
  • Involves enzymes specific to certain molecules.
  • Breaks them into smaller pieces (enzymatic hydrolysis).

Hydrolytic Enzymes

• Each cleaves a specific macromolecule.
• Amylase:
  • Hydrolyzes starch (macromolecule).
• Lipase:
  • Hydrolyzes lipids or fats.

Absorption

• The body takes up nutrients (amino acids, glucose) from food.
• Mostly via diffusion in the small intestine.
• Nutrients move from the small intestine into the circulatory system.
• Water absorption mainly occurs in the large intestine.

Elimination

• The final step of food processing.
• Undigested materials pass out from the digestive system.
• Undigested material goes from the large intestine to the outside via the anus.

Parts of the Digestive System

• Often referred to as the alimentary canal due to its tube-like structure.
• Average length in humans is about 30 feet.

Mouth, Pharynx, and Esophagus

• Mechanical Digestion in the Mouth:
  • Chewing.
  • Teeth grind food into smaller particles.
  • Dentition indicates the organism's diet (grinding plant matter or tearing flesh).
• Chemical Digestion in the Mouth:
  • Involves 3 types of salivary glands: parotid, sublingual, and submaxillary.
  • Salivary glands produce saliva (a watery mixture).
  • Saliva enters the mouth via ducts.
  • Water in saliva helps disperse food particles (hydrolysis).
  • Salivary amylase (enzyme) breaks down starch into maltose.
    • Starch is a macromolecule.
    • Maltose is a disaccharide (2 glucose molecules).
• Passage from Mouth to Esophagus:
  • Food passes from the mouth via the pharynx to the esophagus.
  • The pharynx is a chamber common to both the respiratory and digestive systems.
  • Epiglottis prevents food from entering the trachea.
  • During swallowing, the larynx is pulled upward, causing the epiglottis to cover the entrance to the respiratory system.
• Bolus and Peristalsis:
  • While chewing, food forms into a ball (bolus).
  • The bolus moves to the back of the throat, stimulating the pharynx.
  • Stimulation of the pharynx stimulates the esophagus.
  • Food moves down to the stomach via muscular contractions (peristalsis).
  • The esophagus is a muscular tube that passes food from mouth to stomach.
  • Entry to the stomach is via the cardiac orifice (opening).

Stomach

• A backwards J-shaped organ that can hold up to 4 liters when full.
• May store food for about 4 hours.
• Functions:
  • Mechanical and chemical digestion.
  • The stomach churns its contents while storing them.
  • Internal surface increased by folds called rugae.
  • Produces gastric juice when food is present.
    • Gastric juice contains hydrochloric acid (HCl) and enzymes.
    • HCl lowers the pH of the stomach, making it acidic.
    • Acid is produced by parietal cells.
    • Low pH activates pepsinogen into pepsin (enzyme).
    • Pepsinogen (inactive form) is secreted by chief cells.
    • Pepsin breaks peptide bonds of protein chains.
    • Amino acids move to the small intestine for absorption.
  • Guards against self-digestion via a coating of mucus.
    • Mucus is secreted by mucus cells of the stomach lining.
    • The stomach lining is constantly eroded, requiring cell regeneration by mitosis every 3 days.

Chyme and Movement to Small Intestine

• Contents from the stomach mixed with gastric juices are called chyme.
• Chyme moves from the stomach to the small intestine through the pyloric sphincter.
• The pyloric sphincter (band of smooth muscle) regulates food passage.

Small Intestine

• About 7 meters long in humans.
• Three parts:
  • Duodenum: Where much of the absorption takes place.
  • Jejunum
  • Ileum

Accessory Organs

• Located close to the duodenum, aiding in chemical digestion.
• Their secretions are drained into the duodenum via specific ducts.
• Pancreas:
  • A diffused glandular organ producing many digestive enzymes.
  • Enzymes include:
    • Pancreatic trypsin and chymotrypsin: break polypeptide chains into smaller amino acid chains.
    • Pancreatic amylase (maltase): breaks down disaccharides into monosaccharides (e.g., maltose into glucose).
    • Pancreatic lipase: breaks down fats into glycerol and fatty acids after emulsification.
  • Enzymes are released through the pancreatic duct into the duodenum.
• Liver:
  • Performs other functions such as detoxification and glucose storage (as glycogen).
  • Produces bile, which breaks down big fat droplets into smaller ones (emulsification).
  • Bile is moved into the gallbladder for temporary storage.
• Gallbladder:
  • Stores bile.

Absorption of Nutrients in the Small Intestine

• The surface area of the small intestine is increased by villi.
  • Villi are small, finger-like projections away from the wall.
  • Microvilli project off the villi to increase surface area even more.
• Villi contain many blood vessels (capillaries).
• Organic molecules are taken up into the circulatory system through capillaries.
• Nutrients diffuse into the bloodstream through villi and into capillaries across a cell membrane (absorption).

Jejunum and Ileum

• Hard to distinguish externally; require microscopic observation.
• Also responsible for nutrient absorption and reabsorbing water back into the body.

Large Intestine (Colon)

• Material moves from the small intestine to the large intestine at a T-shaped intersection with the cecum (a small sac).
• Appendix - finger-like projection on the cecum. Thought to increase the bacteria flora in the intestine.
• About 3 meters long.
• Three parts: ascending colon, transverse colon, and descending colon.
• Major Function:
  • Reabsorbs water added to the food tube with saliva, gastric juice, bile, pancreatic secretions, and intestinal juices.
  • Water diffuses across the villi.
  • Undigested material (feces) gets more solid as it moves through the colon.

Bacteria in the Large Intestine

• Many different kinds of bacteria present.
• Bacteria feed on waste and indigestible materials (e.g., cellulose).
• Waste products of bacteria include methane and hydrogen sulfate.
• Some bacteria produce vitamins including folic acid, vitamin K, and several B vitamins.

Other Components of the Large Intestine

• Epithelial cells secrete mucus.
• Mucus assists in the smooth passage of solid wastes.
• The final portion is the rectum, where wastes are stored.
• Wastes are eliminated via the anus, a muscular sphincter.The final portion is the rectum. Wastes are stored in the rectum and eliminated via anus, which is a muscular sphincter.