Digestive System - Complete Notes
1. Overview
The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. It includes the alimentary canal and accessory digestive organs.
2. Alimentary Canal Organs
| Organ | Function |
|---|
| Mouth | Chews food, mixes with saliva (amylase) |
| Pharynx | Swallows food, passes to esophagus |
| Esophagus | Propels food to stomach using peristalsis |
| Stomach | Secretes acid and enzymes to digest proteins |
| Small Intestine | Main site for digestion and absorption (duodenum, jejunum, ileum) |
| Large Intestine | Absorbs water, forms feces |
| Rectum | Stores feces until defecation |
| Anus | Regulates fecal expulsion |
3. Accessory Organs
| Organ | Function |
|---|
| Salivary Glands | Produce saliva for carbohydrate digestion |
| Liver | Produces bile, metabolizes nutrients, detoxifies |
| Gallbladder | Stores and concentrates bile |
| Pancreas | Produces enzymes and bicarbonate for digestion |
4. Digestive Processes
- Ingestion
- Propulsion (swallowing, peristalsis)
- Mechanical digestion (chewing, churning)
- Chemical digestion (enzymatic breakdown)
- Absorption (nutrients into blood/lymph)
- Defecation (elimination of indigestible material)
5. Enzymes and Secretions
| Source | Secretion | Function |
|---|
| Salivary glands | Salivary amylase | Breaks down starch |
| Stomach | Pepsin, HCl | Breaks proteins, provides acidic pH |
| Pancreas | Trypsin, chymotrypsin, lipase, amylase | Digest proteins, fats, carbs |
| Liver | Bile | Emulsifies fats |
| Intestine | Maltase, lactase, peptidases | Final digestion |
6. Absorption Sites
- Stomach: Alcohol, aspirin
- Duodenum: Iron, calcium, magnesium
- Jejunum: Carbohydrates, amino acids, water-soluble vitamins
- Ileum: Bile salts, vitamin B12
- Colon: Water, electrolytes
7. Regulation of Digestion
- Nervous: Enteric nervous system, vagus nerve
- Hormonal: Gastrin (stomach acid), Secretin (bile/pancreatic juice), CCK (gallbladder contraction)
8. Gut Microbiome
- Trillions of bacteria aid in digestion, immunity, vitamin synthesis (e.g., B12, K)
- Dysbiosis can lead to inflammation, IBS, obesity
- Prebiotics (fiber) and probiotics (yogurt, fermented foods) support gut health
9. Digestive Reflexes
- Enterogastric Reflex: Inhibits stomach emptying when duodenum is full
- Gastrocolic Reflex: Stimulates colon movement after eating
- Gastroileal Reflex: Increases movement in ileum
10. Zymogens and Enzyme Activation
- Digestive enzymes are secreted in inactive form (zymogens) to prevent self-digestion
- Trypsinogen → Trypsin (activated by enterokinase)
- Chymotrypsinogen → Chymotrypsin
11. Role of Fiber and Water
- Fiber adds bulk, supports bowel regularity, and feeds gut bacteria
- Water is essential for digestion, nutrient transport, and stool softening
12. Common Digestive Disorders
- GERD (acid reflux)
- Peptic ulcers (H. pylori, NSAIDs)
- Gallstones (cholesterol/bile imbalance)
- Lactose intolerance (lactase deficiency)
- IBD (Crohn's, Ulcerative colitis)
- Celiac disease (gluten-triggered autoimmunity)
13. GLUT Transporters
- GLUT-2: Found in liver, pancreas, and basolateral membrane of enterocytes; transports glucose, galactose, and fructose
- GLUT-5: Apical surface of small intestine; facilitates fructose absorption
- Glucose absorption via SGLT1 (sodium-glucose linked transporter) in small intestine
14. Intrinsic Factor
- Secreted by parietal cells of the stomach
- Binds to Vitamin B12 and facilitates its absorption in the ileum
- Deficiency causes pernicious anemia
15. Additional Digestive Hormones
- GIP (Gastric Inhibitory Peptide): Stimulates insulin release, inhibits gastric acid
- Motilin: Regulates MMC (migrating motor complex) between meals
- Somatostatin: Inhibits all GI secretions (stomach acid, bile, insulin)
- Ghrelin: Stimulates appetite, secreted from the stomach
16. Brush Border Enzymes
- Located on the microvilli of intestinal epithelial cells
- Include disaccharidases: maltase, sucrase, lactase
- Include peptidases for breaking small peptides to amino acids
17. Pancreatic Juice Components
- Bicarbonate (HCO3⁻): Neutralizes stomach acid in duodenum
- Digestive enzymes: Amylase (carbs), Lipase (fats), Trypsin/Chymotrypsin (proteins)
18. Enteroendocrine Cells
- G cells: Secrete gastrin
- D cells: Secrete somatostatin
- I cells: Secrete CCK
- S cells: Secrete secretin
- These cells are scattered among mucosal lining of stomach and intestine
19. Overview of Metabolism
- Carbohydrate metabolism: Glucose → ATP via glycolysis, TCA cycle, and oxidative phosphorylation
- Protein metabolism: Amino acids used for repair, enzymes, or converted into glucose/fat (deamination in liver)
- Lipid metabolism: Fats broken to fatty acids and glycerol → ฮฒ-oxidation → ATP or ketone bodies
20. Digestive & Metabolic Disorders
- Malabsorption Syndromes: e.g., celiac disease, Crohn’s
- Pancreatitis: Inflammation affecting enzyme release
- Liver Cirrhosis: Reduces detoxification and bile production
- Metabolic Syndrome: Combo of obesity, insulin resistance, high BP
- Diabetes Mellitus: Impaired glucose metabolism
21. How to Eat for Better Digestion
- Chew thoroughly to initiate enzymatic digestion
- Eat in calm environments to enhance parasympathetic activity
- Eat small, balanced meals throughout the day
- Hydrate between meals, not during, to preserve enzyme function
- Include fiber for bowel movement and prebiotic effect
22. Path of Food: Step-by-Step Journey
- Mouth → chewed + mixed with saliva (amylase)
- Pharynx → Esophagus → Peristalsis to stomach
- Stomach → acid + pepsin begin protein digestion
- Duodenum → bile + pancreatic enzymes added
- Jejunum/Ileum → absorption into capillaries & lacteals
- Portal vein → Liver for detox, storage, metabolism
- Systemic circulation → Cells for usage/storage
23. Homeostasis & Feedback
- Negative feedback: Maintains pH, enzyme levels, glucose levels
- Liver: Regulates glucose, detoxifies ammonia → urea
- Pancreas: Releases insulin/glucagon based on blood sugar
- Hypothalamus: Regulates hunger and satiety via hormones
24. Blood Supply of Digestive Organs
- Celiac trunk: Supplies stomach, liver, spleen
- Superior mesenteric artery: Supplies small intestine and part of colon
- Inferior mesenteric artery: Supplies distal colon and rectum
- Hepatic portal vein: Transports absorbed nutrients to liver
25. Nerve Supply of Digestive Tract
- Parasympathetic: Vagus nerve (cranial nerve X) stimulates digestion
- Sympathetic: Inhibits digestion during stress
- Enteric Nervous System (ENS): "Gut brain" regulates motility, secretion, and reflexes
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