Chapter 8: Digestive System — From Mouth to Metabolism

The digestive system breaks down food into absorbable nutrients, absorbs them into the bloodstream, and eliminates waste. Its dysfunction leads to some of the most common diseases in clinical practice.

GI Tract Anatomy

The alimentary canal is a 9-meter tube from mouth to anus, with accessory organs (liver, gallbladder, pancreas) providing essential digestive secretions.

Sequential structures:

1. Oral cavity — mechanical breakdown (teeth), salivary amylase (starch digestion begins)
2. Pharynx → Esophagus — peristaltic transport; lower esophageal sphincter (LES) prevents reflux
3. Stomach — HCl (parietal cells, pH 1–2) + pepsin (chief cells) for protein denaturation; churning forms chyme; intrinsic factor (for B12 absorption)
4. Duodenum — main site of digestion; receives pancreatic enzymes and bile; iron/calcium absorbed here
5. Jejunum — primary absorption of nutrients; rich brush border
6. Ileum — B12-intrinsic factor complex absorbed; bile acid reabsorption; lymphoid tissue (Peyer’s patches)
7. Large intestine (colon) — water/electrolyte absorption; gut microbiome; feces formation
8. Rectum → Anus — defecation

Key Digestive Enzymes

Enzyme	Source	Substrate	Product
Salivary amylase	Parotid glands	Starch	Maltose
Pepsin	Stomach chief cells (as pepsinogen)	Proteins	Peptides
Pancreatic amylase	Pancreas	Starch	Maltose, dextrins
Trypsin/Chymotrypsin	Pancreas (as zymogens)	Proteins	Amino acids/peptides
Lipase	Pancreas	Triglycerides	Fatty acids + monoglycerides
Lactase	Small intestine brush border	Lactose	Glucose + galactose

Liver Functions

The liver performs over 500 distinct functions:

• Metabolism: processes carbohydrates (glycogen storage), proteins (urea cycle), and lipids (lipoprotein synthesis)
• Detoxification: conjugates bilirubin, metabolizes drugs/toxins (CYP450 enzymes)
• Synthesis: clotting factors (I, II, V, VII, IX, X), albumin, complement proteins
• Bile production: bile salts emulsify dietary fats for lipase action
• Immune function: Kupffer cells phagocytose gut-derived pathogens

IBD vs IBS

Feature	IBD (Crohn’s/UC)	IBS
Pathology	Organic inflammation	Functional (no structural pathology)
Bloody stool	Common	Absent
Biomarkers	↑CRP, ↑calprotectin, ↑ESR	Normal
Endoscopy	Mucosal changes	Normal
Treatment	Aminosalicylates, steroids, biologics	Diet, antispasmodics, SSRIs

Crohn’s disease: transmural inflammation, skip lesions, any segment of GI tract, granulomas, associated with fistula/abscess. Ulcerative colitis: mucosal inflammation, continuous from rectum, pseudopolyps, increased colorectal cancer risk.

H. pylori and Peptic Ulcer Disease

Helicobacter pylori (gram-negative, urease-producing) colonizes the gastric mucosa, disrupting the protective mucus layer. It is found in ~50% of the world population.

• Diagnosis: urea breath test, stool antigen test, endoscopic biopsy + CLO test
• Treatment: triple therapy — proton pump inhibitor (PPI) + amoxicillin + clarithromycin for 10–14 days
• Complications of PUD: bleeding (most common), perforation, gastric outlet obstruction, malignancy (MALT lymphoma)

Key Checklist

• Traces the GI tract from mouth to anus and identifies each organ’s primary function
• Lists key digestive enzymes, their sources, and substrates
• Distinguishes IBD from IBS clinically and explains H. pylori eradication therapy