Digestion is a process of breaking down larger insoluble food particles into smaller water-soluble molecules that can be easily absorbed by the body cells.

• Alimentary canal/Digestive tract/ Gastrointestinal (GI) tract is a continuous tube from mouth to anus.
• Six basic processes performed by digestive system are:
  1. Ingestion: Taking food and liquids into mouth (eating).
  2. Secretion: Releasing water, acid, buffers, and enzymes into lumen (interior space) of alimentary canal.
  3. Mixing and propulsion: Alternate contractions and relaxations of smooth muscle in the walls of alimentary canal leads to churning and movement of food through canal. This capability of tract is called as motility.
  4. Digestion: Mechanical and chemical breakdown of food.
  5. Absorption: Passage of digested products from GI tract into blood and lymph.
  6. Defecation: Elimination of feces/stool from GI tract.

Digestion in Mouth

• Buccal cavity performs two major functions: Mastication (chewing) of food and help in Swallowing of food.
• Sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.
• Teeth and the tongue with the help of saliva masticate and mix up the food thoroughly into a soft, flexible, easily swallowed mass called a bolus (lump).
• Mucus in saliva helps in lubricating and adhering food into a bolus.
• Now, food molecules begin to dissolve in water present in saliva, as enzymes can react with food molecules in a liquid medium only.
• After mechanical digestion of food by teeth and tongue, saliva contributes to chemical digestion in the mouth.
• Saliva contains electrolytes (Na+, K+, Cl–, HCO–), salivary amylase and lysozyme.
• Lysozyme acts as an antibacterial agent that prevents infections.
• Salivary amylase performs hydrolytic action on dietary carbohydrates like monosaccharide, disaccharide sugars or complex polysaccharides such as starch present in food.
• About 30 per cent of starch is hydrolyzed here by this enzyme (optimum pH 6.8) into a disaccharide maltose, maltotriose and α-dextrins
• Bolus is then conveyed into the pharynx and then into the esophagus by Swallowing or Deglutition.
• Deglutition is a mechanism that moves food from the mouth into the stomach.
• During swallowing, tongue rises against the palate, soft palate and uvula are drawn superiorly, to close nasopharynx, Larynx rises, the epiglottis seals off the larynx and prevent the entry of swallowed foods into nasal cavity.
• Bolus further passes down through esophagus by successive waves of muscular contractions called Peristalsis.
• Esophagus secretes mucus and transports food into the stomach. It does not produce digestive enzymes, and it does not carry on absorption.
• Gastro-esophageal sphincter/ cardiac sphincter/ Lower esophageal sphincter control the passage of food from esophagus into the stomach.