Stages of Digestion Overview

Stages of Digestion

• Digestion involves four stages: ingestion, digestion, absorption, and elimination.
• Ingestion is the intake of nutrients.
• Digestion is the breakdown of molecules.
• Absorption is the intake of digested molecules into the digestive tract.
• Elimination is the removal of waste food from the body.

Mouth

• Chemical digestion begins in the mouth.
• Salivary glands secrete saliva containing amylase.
• Amylase breaks down starch into simple sugars.
• Mechanical digestion occurs through teeth breaking down food.
• The tongue turns food into a soft bolus.

Teeth

• Different types of teeth exist, each with specific functions in mechanical digestion.

Esophagus

• A muscular tube that transports the bolus to the stomach using peristalsis.
• Peristalsis are wave-like muscular contractions.
• Mucus keeps the passage moist.
• The epiglottis prevents food from entering the lungs.

Stomach

• Food is stored temporarily for chemical and mechanical digestion.
• The muscular J-shaped stomach churns food into a thick liquid called chyme.
• Food triggers the secretion of gastric juice. Contains:
  • Hydrochloric acid (HCl) to break down food.
  • Mucus to protect the stomach lining from acid.
  • Pepsin (an enzyme) to break down proteins (inactive until acid released)
  • Salt and water

Small Intestine

• Covered with villi to increase surface area for nutrient absorption into the blood.
• Each villus is covered with microvilli.
• Nutrients are absorbed and carried via the bloodstream.
  • Monosaccharides are converted to glucose, excess stored as glycogen.
  • Amino acids are converted to waste or new proteins.
  • Glycerol and fatty acids are reassembled into triglycerides for absorption.
• Three parts: duodenum, jejunum, and ileum
  • Duodenum (0.5m): primary site for digestion due to pancreatic and gallbladder secretions.
  • Jejunum (2.5m): large folds for digestion and absorption.
  • Ileum (3m): fewest villi, pushing undigested material through .

Accessory Organs

• Pancreas: leaf-shaped; produces enzymes to break down food, bicarbonate to neutralize chyme, and insulin to control blood sugar.
• Liver: largest internal organ; produces bile to break down lipids (stored in gall bladder), stores glycogen, iron, and vitamins; disposes of waste products (alcohol, drugs).
• Gall bladder: stores bile from liver; releases it into the duodenum for fat digestion.

Large Intestine (Colon)

• Shorter but wider than the small intestine.
• Main function:
  • Re-absorption of water into the blood.
  • Housing anaerobic bacteria for continued food breakdown, vitamin production (folic acid, B, K).
  • Converts waste into feces which are pushed to the rectum and anus.

Digestive Disorders

• Diarrhea: insufficient water absorption in the large intestine.
• Constipation: excessive water absorption in the large intestine.