🍽️ Digestion, Absorption & Nutrition

Digestion is the mechanical and biochemical hydrolysis of complex macromolecular food substances into simple, diffusible, and absorbable nutrient monomers.

---

1. 🔬 Alimentary Canal Histology (Outer to Inner)

  1. Serosa (Visceral Peritoneum: Mesothelium + areolar connective tissue)
        │
        ▼
  2. Muscularis (Outer Longitudinal + Inner Circular smooth muscles)
     [Auerbach's / Myenteric Nerve Plexus: Controls GI Peristalsis]
        │
        ▼
  3. Submucosa (Loose connective tissue, blood vessels, lymphatics, Brunner's glands in duodenum)
     [Meissner's Submucosal Nerve Plexus: Controls GI Secretions]
        │
        ▼
  4. Mucosa (Muscularis mucosae + Lamina propria + Epithelium with Goblet cells & Crypts of Lieberkühn)

---

2. 🧪 Digestive Secretions & Enzyme Cascades

                                  ENZYMATIC DIGESTION CASCADE
                                               │
         ┌─────────────────────────────────────┼─────────────────────────────────────┐
         ▼                                     ▼                                     ▼
   [SALIVARY AMYLASE (Ptyalin)]        [GASTRIC JUICE (pH 1.5 - 2.0)]        [PANCREATIC JUICE (pH 7.8 - 8.4)]
   ├── Starch -> Maltose (30%)         ├── Pepsinogen --[HCl]--> Pepsin      ├── Trypsinogen --[Enterokinase]--> Trypsin
   └── Optimum pH = 6.8                ├── Prorennin -> Rennin (Infants)     ├── Chymotrypsin, Carboxypeptidase
                                       └── Castle's Intrinsic Factor (B12)   └── Pancreatic Amylase, Lipase (Steapsin)

2.1 Enteric Hormonal Regulation

1. Gastrin (G-cells of stomach): Stimulates gastric glands to secrete $HCl$ and pepsinogen.
2. Secretin (Duodenal S-cells, First discovered hormone by Bayliss & Starling): Stimulates pancreas to secrete water and bicarbonate ions ($HC{O}_3^{-}$) to neutralize gastric acid.
3. Cholecystokinin (CCK-PZ): Stimulates gallbladder contraction to release bile and pancreatic acinar cells to secrete pancreatic digestive enzymes.
4. Gastric Inhibitory Peptide (GIP / Enterogastrone): Inhibits gastric secretion and motility.

---

3. 🧈 Digestion & Absorption of Lipids

Lipids are insoluble in water and require sequential emulsification, micellar solubilization, and chylomicron packaging:

$$\text{Triglycerides}\underset{\text{Bile Salts (Sodium taurocholate/glycocholate)}}{\overset{\text{Emulsification}}{\to }}\text{Fine Lipid Droplets}\stackrel{\text{Pancreatic Lipase}}{\to }\text{Monoglycerides}\mathbf{+}\text{Free Fatty Acids}$$

  Intestinal Lumen: Micelles (Fatty acids + Bile salts)
         │
         ▼ [Passive Diffusion into Enterocyte]
  Enterocyte Cytoplasm: Resynthesized Triglycerides + Protein Coat ──► Chylomicrons
         │
         ▼ [Exocytosis]
  Lacteals (Intestinal Lymphatic Capillaries) ──► Thoracic Duct ──► Systemic Venous Blood