Central Topic: Digestion & Absorption (HP-01)

DIGESTION & ABSORPTION
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├── ORGANS (Alimentary Canal)
│   ├── Mouth
│   │   ├── Teeth — mechanical breakdown
│   │   ├── Salivary glands (parotid, sublingual, submandibular)
│   │   └── Salivary amylase — starch → maltose (pH 6.8)
│   ├── Oesophagus
│   │   └── Peristalsis (enteric nervous system)
│   ├── Stomach
│   │   ├── Rugae — expansion
│   │   ├── HCl $\frac{parietal}{oxyntic cells}$ — activates pepsin, kills bacteria, pH 1.5–2.0
│   │   ├── Pepsinogen → Pepsin (chief cells)
│   │   ├── Gastric lipase (chief cells) — short-chain fats
│   │   └── Pyloric sphincter → controls chyme exit
│   ├── Small Intestine (principal site)
│   │   ├── Duodenum
│   │   │   ├── Enterokinase (duodenal mucosa) → activates trypsin
│   │   │   ├── Receives pancreatic juice + bile
│   │   │   └── Hormones: Secretin, CCK, GIP
│   │   ├── Jejunum
│   │   │   ├── Villi + microvilli (600-fold amplification)
│   │   │   ├── Brush border enzymes (maltase, sucrase, lactase, aminopeptidase, dipeptidases)
│   │   │   ├── Blood capillaries (glucose, amino acids)
│   │   │   └── Lacteals (fats → chylomicrons)
│   │   └── Ileum
│   │       ├── Completes absorption
│   │       ├── Vitamin B12 absorption
│   │       └── Bile salt reabsorption (enterohepatic circulation, 95%)
│   └── Large Intestine
│       ├── Haustra — sacculations
│       ├── Water absorption (osmosis)
│       ├── Mineral absorption
│       ├── Symbiotic bacteria → vitamins B + K
│       └── Faeces formation → rectum → anus
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├── ENZYMES
│   ├── Salivary amylase — starch — pH 6.8
│   ├── Pepsin — proteins — pH 1.5–2.0
│   ├── Gastric lipase — short-chain fats — pH 4.0–5.0
│   ├── Pancreatic enzymes (proenzymes activated by enterokinase → trypsin cascade)
│   │   ├── Trypsin — proteins/peptides — pH 7.5–8.5
│   │   ├── Chymotrypsin — proteins/peptides — pH 7.5–8.5
│   │   ├── Elastase — elastin — pH 8.0–8.5
│   │   ├── Carboxypeptidase — C-terminal — pH 7.5–8.0
│   │   ├── Lipase — emulsified triglycerides — pH 7.5–8.0
│   │   ├── Amylase — starch/glycogen — pH 7.0–8.0
│   │   └── Nucleases (DNase, RNase) — DNA/RNA — pH 7.5–8.0
│   └── Brush border enzymes
│       ├── Maltase — maltose → 2 glucose
│       ├── Sucrase — sucrose → glucose + fructose
│       ├── Lactase — lactose → glucose + galactose
│       ├── Aminopeptidase — N-terminal peptides → amino acids
│       ├── Dipeptidases — dipeptides → 2 amino acids
│       ├── Nucleotidases — nucleotides → nucleosides + phosphate
│       └── Nucleosidases — nucleosides → bases + sugars
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├── BILE (NOT an enzyme!)
│   ├── Produced: Liver
│   ├── Stored: Gallbladder
│   ├── Released by: CCK
│   └── Function: Bile salts EMULSIFY fats only (no enzymatic activity)
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├── HORMONES
│   ├── Gastrin — G-cells (pyloric stomach) — proteins → HCl + pepsinogen
│   ├── Secretin — S-cells (duodenum) — acid → bicarbonate (neutralization)
│   ├── CCK — I-cells (duodenum) — fats + AA → bile + enzymes
│   └── GIP — K-cells (duodenum) — fats + glucose → inhibit acid + insulin
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├── ABSORPTION ROUTES
│   ├── Glucose → active transport → blood capillaries → portal vein → liver
│   ├── Amino acids → active transport → blood capillaries → portal vein → liver
│   ├── Fatty acids → passive → chylomicrons → lacteals → thoracic duct → subclavian vein
│   └── Water → osmosis → large intestine → blood capillaries
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└── DISORDERS
    ├── Kwashiorkor — protein deficiency — oedema (low albumin)
    ├── Marasmus — total calorie deficiency — wasting (no oedema)
    ├── Jaundice — excess bilirubin — yellow skin + sclera
    ├── Diarrhoea — impaired water absorption — watery stools
    ├── Constipation — excess water absorption — hard stools
    ├── Hiatal hernia — acid reflux — heartburn (lying down)
    └── Indigestion — overeating/stress/enzyme deficiency — bloating