Human Being: Digestion

The digestive tract:

1. Oral cavity
2. Pharynx
3. Esophagus
4. Stomach
5. Small intestine
6. Large intestine
7. Rectum

The small intestine begins with the duodenum.

• In the mouth, salivary amylase breaks down starch (alkaline medium).
• Gastric juice contains pepsin and hydrochloric acid (HCl): proteins are broken down in an acidic medium.
• In the small intestine, bile produced by the liver emulsifies fats.
• Pancreatic juice (trypsin, lipase, amylase) breaks down all nutrients.
• Nutrients are absorbed in the small intestine (through the villi). Water is absorbed in the large intestine.

Key Facts

• Starch is broken down into glucose, protein into amino acids, and fat into glycerol and fatty acids.
• The liver is the largest gland: it produces bile, neutralizes toxic substances, and stores glycogen.
• Gastric juice has a pH ≈ 1,5-2 (acidic). In the mouth and intestine, the medium is weakly alkaline.
• The length of the small intestine is ≈ 5-6 m. Its inner surface is large because of folds and villi.
• The esophagus pushes food along by peristalsis (wave-like contractions).