Step-by-Step Sequence: From Eating to Absorption
Step 1 — Mouth (Oral Cavity)
- Mechanical: Teeth chew food into smaller pieces; tongue mixes with saliva
- Chemical: Salivary amylase (pH 6.8) begins starch → maltose + limit dextrins
- Product formed: Bolus (moist, chewed food ball)
- Enzymes: Salivary amylase
- Duration: Seconds to minutes
Step 2 — Pharynx and Oesophagus
- Mechanical: Swallowing reflex propels bolus; peristalsis moves bolus down oesophagus
- Chemical: None (amylase still active but pH not optimal)
- Product: Bolus moves to stomach
- Enzymes: None newly added
- Duration: ~10 seconds
Step 3 — Stomach
- Mechanical: Churning movements mix food with gastric juice; rugae allow expansion
- Chemical: HCl (from parietal cells) activates pepsinogen → pepsin (pH 1.5–2.0). Pepsin digests proteins → large peptides. Gastric lipase digests short-chain fats.
- Salivary amylase INACTIVATED by acid
- Product formed: Chyme (semi-fluid acidic mass)
- Hormones released: Gastrin (G-cells → stimulates HCl + pepsinogen)
- Duration: 2–4 hours
Step 4 — Duodenum (First ~25 cm of Small Intestine)
- Chyme enters through pyloric sphincter (regulated, small amounts)
- Secretions arriving:
- Pancreatic juice (via pancreatic duct): trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase (activated by enterokinase → trypsin → cascade), plus active lipase, amylase, nucleases
- Bile (via bile duct from gallbladder): bile salts emulsify fats; no enzymes in bile
- Enterokinase (from duodenal mucosa) → trypsinogen → trypsin → cascades other proenzymes
- Hormones released: Secretin (acid → bicarbonate), CCK (fats + AA → bile + enzymes), GIP (fats + glucose → inhibit acid + insulin)
- Products: Starch → maltose; Proteins → smaller peptides; Fats → partially emulsified; DNA/RNA → nucleotides
Step 5 — Jejunum (Middle Small Intestine)
- Villi + microvilli provide enormous surface area
- Brush border enzymes complete digestion: maltase → 2 glucose; sucrase → glucose + fructose; lactase → glucose + galactose; aminopeptidase → amino acids; dipeptidases → 2 amino acids; nucleotidases + nucleosidases → bases + sugars + phosphate
- Absorption begins: glucose + amino acids → active transport → blood capillaries; fats → lacteals
- Duration: 1–4 hours transit
Step 6 — Ileum (Last Part of Small Intestine)
- Completion of absorption (glucose, amino acids, vitamin B12, fat-soluble vitamins)
- Bile salt reabsorption (~95%) into portal blood → liver → recycled (enterohepatic circulation)
- Ileocaecal valve controls entry into large intestine
Step 7 — Large Intestine / Colon
- Water absorption by osmosis (~1.3–1.8 L/day)
- Mineral absorption (sodium, potassium, chloride)
- Symbiotic bacteria synthesize vitamins B and K
- Undigested material compacted into faeces
- Duration: 12–24 hours
Step 8 — Rectum and Anus
- Storage of faeces (rectum)
- Defecation reflex (medulla oblongata + voluntary control)
- Faeces expelled through anus
- Duration: Variable