Human Digestive System and Food Absorption

Digestion: The process by which complex food substances are broken down into simpler substances that can be absorbed by the body.
Ingestion: The process of introducing food into the mouth.

Mechanical digestion: Food is broken down into small particles by the teeth during chewing; the tongue turns the food into a bolus.
Chemical digestion: Presence, sight, or smell of food stimulates salivary glands to secrete saliva containing:
- Water: Provides a medium for chemical reactions and dissolves soluble food substances.
- Mucus: Lubricates food for easy swallowing and aids in bolus formation.
- Salivary amylase (Ptyalin): Digests starch in food into maltose.
Peristalsis: Wave-like motion of contraction and relaxation that pushes the bolus down the oesophagus into the stomach.

Presence of food stimulates the stomach wall to secrete gastric juice containing:
- Hydrochloric acid: Neutralises alkali from the mouth, creates an acidic environment for enzymes, kills bacteria, and activates pepsinogen and pro-rennin into pepsin and rennin.
- Mucus: Reduces friction during churning and protects stomach walls from hydrochloric acid corrosion.
- Pepsin enzyme: Digests proteins into peptides.
- Rennin enzyme: Coagulates milk protein (caseinogen) to casein (curd), which pepsin digests to peptides.
Churning: Muscular walls contract and relax to mix food into chyme, which enters the duodenum.
Direct Absorption: Soluble vitamins, water, alcohol, and soluble salts are absorbed directly into the bloodstream in the stomach without digestion.

Liver (Bile): Secretes bile (stored in the gall bladder, sent via bile duct). Bile salts neutralise stomach acid, provide an alkaline environment, and perform emulsification (breaking large fat droplets into tiny droplets for lipase).
Pancreas (Pancreatic juice): Secretes juice via the pancreatic duct containing:
- Pancreatic amylase: Breaks down starch into maltose.
- Pancreatic lipase: Breaks down fats to fatty acids and glycerol.
- Trypsin enzyme: Produced as inactive trypsinogen; breaks down proteins to peptides.
- Sodium hydrogen carbonate salt: Neutralises stomach acid and creates an alkaline environment.

Presence of food stimulates intestinal walls to produce succus entericus (intestinal juices) containing:
- Mucus: Lubricates intestinal walls for easy movement of chyle via peristalsis.
- Erepsin (peptidase) enzyme: Breaks down peptides into amino acids.
- Maltase enzyme: Breaks down maltose into glucose.
- Lipase enzyme: Breaks down fats and oils into fatty acids and glycerol.
- Sucrase/ invertase enzyme: Digests sucrose into glucose and fructose.
- Lactase enzyme: Breaks down lactose to glucose and galactose.
End Products of Digestion:
- Glucose (from carbohydrates).
- Amino acids (from proteins).
- Fatty acids and glycerol (from lipids).

Site of Absorption: Occurs in the ileum of the small intestine.
Villi (singular: villus): Finger-like projections greatly supplied with blood vessels that increase the surface area for absorption.
Villus Structural Components: Epithelial cells, blood capillaries, lacteal, blood vessel, and lymph ducts.