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gastrointestinal tract anatomy layers wall diagram

A 3D reconstruction of the human esophagogastric junction (EGJ) wall, obtained via synchrotron-radiation X-ray phase-contrast imaging. The image displays the distinct anatomical layering of the gastrointestinal tract in high detail. Three primary layers are labeled: the mucosa (muc), characterized by a relatively smooth surface; the submucosa (sub), exhibiting a highly textured, honeycomb-like appearance; and the muscular layer (mus), which shows longitudinal strips and folds. Panel A reveals a deep crevice within the muscular layer, indicated by a double arrow, likely representing a fixation artifact. The 3D model demonstrates clear demarcation and spatial arrangement between these tissue planes. This imaging modality provides superior soft-tissue contrast compared to conventional CT or barium studies, allowing for the precise visualization of the internal architecture of the EGJ. It serves as an educational tool for advanced anatomy and diagnostic radiology students to understand the micro-structure of the esophageal wall.

**Imaging Modality:** Electronic radial Endoscopic Ultrasound (EUS). **Anatomical Region:** Upper gastrointestinal tract, specifically the stomach lumen and gastric wall, with visualization of the retroperitoneal space. **Key Landmarks and Visual Features:** * **Gastric Wall:** The image demonstrates the characteristic five-layer sonographic architecture of the gastric wall. These layers include the hyperechoic superficial mucosa, hypoechoic deep mucosa (muscularis mucosae), hyperechoic submucosa, hypoechoic muscularis propria, and the outer hyperechoic serosa/adventitia. * **Gastric Folds:** Intraluminal protrusions of the gastric wall are visible, representing normal rugae. * **Extraluminal Structures:** Adjacent to the gastric wall, the pancreatic body is identified by its homogeneous, finely granular, salt-and-pepper echotexture. The splenic vein is visible as an anechoic, tubular vascular structure positioned posterior to the pancreas. **Clinical Context/Pathology:** The image shows a normal physiological appearance of the gastric wall and surrounding pancreaticosplenic anatomy without evidence of wall thickening, masses, or lymphadenopathy. **Diagnostic Features:** Preservation of the five-layer wall stratification is a key diagnostic indicator used to exclude transmural infiltration by malignancy or deep inflammatory processes.

This diagnostic ultrasound image presents a transverse scan of the upper abdomen, specifically illustrating the normal wall stratification of the stomach. The visual demonstrates the characteristic 'gut signature' through three distinct echogenic layers: a hyperechoic inner layer representing the mucosa and submucosa (labeled 1), a middle hypoechoic layer representing the muscularis propria (labeled 2), and a hyperechoic outer layer corresponding to the subserosa (labeled 3). Additionally, the image displays prominent ring-down artifacts, indicated by yellow arrows, which are classic ultrasonographic signs of intraluminal gas. This comparison is useful in gastroenterology and radiology for differentiating normal gastric anatomy from pathological wall thickening or loss of stratification. The image serves as an educational reference for transabdominal sonographic evaluation of the gastrointestinal tract.

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small intestine villi absorption histology

This histopathology image depicts a paraffin-embedded small intestinal mucosa section stained with Hematoxylin and Eosin (H&E), captured at a low-to-mid magnification to emphasize overall villous architecture. The tissue shows finger-like villi emanating from the lamina propria, with closely apposed, tall mucosal folds consistent with villous mucosa of the small intestine. The surface epithelium appears as simple columnar enterocytes with scattered goblet cells, and the absorptive cell brush border is not resolved at this scale. The lamina propria contains loose connective tissue with a sparse vascular network, and the muscularis mucosae is present as a thin boundary beneath the mucosal layer. No overt architectural distortion, crypt branching, or dysplastic glands are evident in the visible fields; there is no clear inflammatory infiltrate or edema highlighted at this magnification. The image emphasizes normal or near-normal mucosal pattern, suitable as a baseline reference for comparison with samples showing villous atrophy, blunting, or neoplastic villiform growth. This slide is relevant for routine evaluation of malabsorption syndromes, celiac disease workups, polyps with villous features, or educational demonstrations of normal intestinal histology. Accurate interpretation requires correlating endoscopy findings, patient history, and additional sections to differentiate normal variants from pathology.

Light microscopy of hematoxylin and eosin stained small intestinal mucosa, prepared as longitudinal sections, displaying the mucosal folds or villi characteristic of the jejunal/ileal region. Each villus presents a tall, brush-border–bearing epithelium with basally positioned nuclei in enterocytes and scattered goblet cells producing mucin. The apical surface is densely stained, while goblet cells appear pale to vacuolated. The villous cores comprise loose lamina propria with scant capillaries and occasional lymphocytes, and a thin muscularis mucosae underneath is not readily resolved at this magnification. The overall architecture is preserved, with uniform crypts at the base, no crypt hyperplasia, and no plasmacytosis or lymphocytic infiltrate. There is no submucosal edema or inflammatory exudate; vessels and stromal tissue appear normal, and there is no evidence of villous blunting or villous atrophy. These features correspond to normal mucosal histology of the small intestine in health or in control specimens. This image serves as a reference standard for educational comparison, enabling recognition of normal mucosal architecture and the distinction from enteropathies such as celiac disease, inflammatory bowel disease, infectious enteritis, or malabsorption syndromes. It can assist in diagnostic training, multidisciplinary case discussions, and slide-based pathology curricula, and provides a baseline for morphologic correlation with clinical symptoms and laboratory tests.

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stomach anatomy gastric glands peptic ulcer

Clinical photograph of a macro-surgical specimen following a quasi-total gastrectomy. The specimen displays the gross anatomy of the stomach, characterized by a predominantly reddish-brown, glistening serosal surface with irregular texture. Centrally located in the median third of the gastric body is a prominent, large, benign chronic giant ulcer, measuring approximately 5 cm in diameter. The ulcer is deeply excavating and penetrative, exhibiting well-defined but irregular, slightly raised borders and a dark, necrotic-appearing base. The surrounding gastric wall shows signs of secondary inflammatory changes. This visual demonstrates a severe complication of peptic ulcer disease where the depth of the lesion indicated penetration into adjacent structures, such as the liver, necessitating radical surgical intervention for management. The specimen serves as an educational example of macroscopic pathology in chronic gastric ulceration and the indications for extensive gastric resection.

This endoscopic image shows the internal surface of the stomach, specifically focusing on the gastric antrum and prepyloric region. The central feature is a large, irregularly shaped, deep gastric ulcer. The ulcer bed is covered with a yellowish-white fibrinous exudate and exhibits numerous air bubbles, which are common in gastric secretions. The surrounding gastric mucosa is erythematous and edematous, indicating active inflammation (chronic active gastritis). In the background, the pylorus is visible but appears distorted and narrowed, suggestive of a potential stricture or functional obstruction caused by the proximity of the large ulcer and associated inflammatory swelling. This clinical photograph is highly relevant for teaching about Peptic Ulcer Disease (PUD), gastroduodenal anatomy during esophagogastroduodenoscopy (EGD), and the visual manifestations of severe gastric infections such as Cytomegalovirus (CMV) in the gastrointestinal tract.

Gastrointestinal System - Complete Notes

Sources: Guyton & Hall Textbook of Medical Physiology (14th ed.), Ganong's Review of Medical Physiology, Medical Physiology (Boron & Boulpaep), Sleisenger & Fordtran's GI & Liver Disease

1. OVERVIEW AND FUNCTIONS

The alimentary tract provides the body with a continual supply of water, electrolytes, vitamins, and nutrients. Five key processes make this possible:

Movement of food through the tract (motility)
Secretion of digestive juices
Digestion of food into absorbable molecules
Absorption of water, electrolytes, and digestion products
Regulation by local, nervous, and hormonal systems

Each segment is structurally adapted to its role: the esophagus for passage, the stomach for temporary storage and initial digestion, the small intestine for digestion and absorption, and the large intestine for water/electrolyte recovery and waste compaction. - Guyton and Hall Textbook of Medical Physiology, p. 771

2. ANATOMY OF THE GI WALL

The GI wall has 5 concentric layers (outer to inner):

Layer	Key Features
Serosa	Outermost; connective tissue + mesothelium
Longitudinal muscle	Outer smooth muscle; shortens gut segments
Circular muscle	Inner smooth muscle; narrows lumen; main layer for peristalsis
Submucosa	Dense connective tissue; contains submucosal (Meissner's) plexus, blood vessels, lymphatics
Mucosa	Innermost; epithelium + lamina propria + muscularis mucosae

The myenteric (Auerbach's) plexus lies between the longitudinal and circular muscle layers.

GI wall layers - EUS showing gastric wall stratification

GI Smooth Muscle as a Syncytium

Individual smooth muscle fibers: 200-500 µm long, 2-10 µm diameter
Connected by gap junctions - low-resistance channels allowing ion flow between cells
Functions as a syncytium: action potentials propagate throughout the muscle mass
Travels faster longitudinally than circumferentially
Guyton and Hall, p. 772

3. ENTERIC NERVOUS SYSTEM (ENS)

The ENS is the "gut brain" - it can control GI function entirely independently of the CNS. Two major plexuses:

Myenteric (Auerbach's) Plexus

Located between longitudinal and circular muscle layers
Controls GI movements (motility)

Submucosal (Meissner's) Plexus

Located in the submucosa
Controls secretion and local blood flow

Autonomic Control

Division	Origin	Effect
Parasympathetic (cranial)	Vagus nerve (CN X) - esophagus to mid-colon	Increases ENS activity → increases motility + secretion
Parasympathetic (sacral)	S2-S4 → pelvic nerves	Supplies distal colon, rectum, anus; mediates defecation reflex
Sympathetic	T5-L2 → celiac & mesenteric ganglia	Inhibits motility and secretion; norepinephrine release

Key point: Strong sympathetic stimulation can block food movement through the entire GI tract. - Guyton and Hall, p. 779

Afferent sensory fibers from the gut respond to: mucosal irritation, excessive distension, and specific chemicals. Up to 80% of vagal fibers are afferent (gut → brain), not efferent.

4. GASTROINTESTINAL MOTILITY

Types of Movement

Type	Description	Location
Peristalsis	Sequential contraction behind + relaxation ahead of a bolus; propels content aborally	Throughout GI tract
Segmentation	Rhythmic, non-propulsive contractions that mix content	Small intestine
Haustration	Slow segmenting contractions	Large intestine
Mass movements	Powerful propulsion 1-3x/day	Large intestine
Migrating Motor Complex (MMC)	"Housekeeper wave" between meals; 90-min cycles	Stomach → ileum

Electrical Activity

Slow waves (basic electrical rhythm): set by interstitial cells of Cajal (ICC) in the circular muscle layer
- Stomach: ~3/min
- Duodenum: ~12/min
- Ileum: ~8/min
Spike potentials: superimposed on slow waves; trigger actual muscle contraction
Slow waves do NOT cause contraction themselves - they only set the maximum frequency

5. HORMONAL CONTROL OF THE GI TRACT

The major GI hormones act via the portal circulation on target cells with specific receptors:

Hormone	Cell Type	Stimulus	Major Actions
Gastrin	G cells (antrum, duodenum)	Protein, distension, vagal (ACh); inhibited by acid	↑ Gastric acid secretion; ↑ mucosal growth
Cholecystokinin (CCK)	I cells (duodenum, jejunum, ileum)	Fat, protein, acid	↑ Pancreatic enzymes; ↑ bile release (gallbladder contraction); ↓ gastric emptying
Secretin	S cells (duodenum, jejunum, ileum)	Acid, fat	↑ Pancreatic bicarbonate; ↑ biliary bicarbonate; ↓ gastric acid; ↓ gastric emptying
GIP (Glucose-dependent Insulinotropic Peptide)	K cells (duodenum, jejunum)	Protein, fat, carbohydrate	↑ Insulin secretion; ↓ gastric acid; ↓ food intake
Motilin	M cells (duodenum, jejunum)	Fat, acid, nervous	↑ Gastric + intestinal motility; triggers MMC
GLP-1	L cells (duodenum → colon)	Fat, protein, carbohydrate	↑ Insulin (glucose-dependent); ↓ gastric emptying; ↓ food intake

Source: Guyton and Hall, Table 63.1, p. 780

6. DIGESTION

All three major macronutrients are digested by hydrolysis - the addition of water to break condensation bonds.

Carbohydrate Digestion

Stage	Location	Enzyme	Substrate → Product
Salivary	Mouth	Salivary amylase (ptyalin)	Starch → dextrins, maltose
Gastric	Stomach	(amylase inactivated by acid)	—
Pancreatic	Small intestine	Pancreatic amylase	Starch → maltose, maltotriose
Brush border	Small intestine	Maltase, sucrase, lactase	Disaccharides → monosaccharides

Final products absorbed: Glucose (from starch and sucrose), fructose, galactose (from lactose).

Protein Digestion

Stage	Location	Enzyme/Source	Notes
Gastric	Stomach	Pepsin (from pepsinogen, activated by HCl)	Endopeptidase; cleaves at aromatic AAs
Pancreatic	Small intestine	Trypsin, chymotrypsin, elastase (endopeptidases); carboxypeptidases A & B (exopeptidases)	Trypsin activates other zymogens
Brush border	Small intestine	Aminopeptidases, dipeptidases	→ Free amino acids, di/tripeptides

Fat (Lipid) Digestion

Lingual lipase (minor, mouth/stomach)
Emulsification by bile salts in duodenum - breaks large fat droplets into smaller ones (increases surface area)
Pancreatic lipase (with colipase) cleaves triglycerides → 2-monoglycerides + 2 fatty acids
Micelle formation - bile salts surround products; transport them to brush border
Absorption into enterocytes → re-esterification → chylomicrons → lymphatics (lacteals)

7. ABSORPTION

The small intestine is the primary site of absorption, with three specializations that multiply surface area ~600-fold:

Circular folds (plicae circulares) - 3x increase
Villi - 10x increase
Microvilli (brush border) - 20x increase

Nutrient Absorption Sites

Nutrient	Primary Site	Mechanism
Glucose, galactose	Duodenum, jejunum	SGLT-1 (Na-coupled active transport)
Fructose	Duodenum, jejunum	GLUT-5 (facilitated diffusion)
Amino acids	Duodenum, jejunum	Na-coupled cotransporters (multiple)
Fat-soluble vitamins (A, D, E, K)	Small intestine	Micellar absorption (passive)
Vitamin B12	Terminal ileum	Requires intrinsic factor (from parietal cells)
Iron	Duodenum	Ferrous (Fe²⁺) form via DMT-1; hepcidin regulated
Calcium	Duodenum	Calcitriol-dependent active transport
Bile salts	Terminal ileum	Active transport (enterohepatic circulation)
Water	Small + large intestine	Osmosis following solute absorption

Electrolyte and Water Absorption

Small intestine absorbs ~7-8 liters/day
Large intestine absorbs ~1.5-2 liters/day (max capacity ~5-6 L)
Na⁺ absorption drives most water absorption osmotically
Aldosterone increases Na⁺ absorption in the colon

8. GASTRIC SECRETION

The stomach secretes ~2L of gastric juice per day.

Cell Type	Location	Secretion	Function
Chief (peptic) cells	Fundus/body	Pepsinogen	Precursor to pepsin (activated by HCl)
Parietal (oxyntic) cells	Fundus/body	HCl, Intrinsic factor	Acid: kills bacteria, activates pepsin; IF: B12 absorption
G cells	Antrum	Gastrin	Stimulates parietal cells
D cells	Antrum	Somatostatin	Inhibits gastrin (paracrine)
Mucous cells	Neck + surface	Mucus, HCO₃⁻	Protects mucosa from acid

Phases of Gastric Secretion

Phase	Trigger	Mediator	% of Total Secretion
Cephalic	Sight, smell, thought, taste of food	Vagus (ACh)	~30%
Gastric	Food in stomach: distension, protein	Gastrin, ACh, histamine	~60%
Intestinal	Acid/fat/hypertonic chyme in duodenum	Secretin, CCK, GIP, enterogastrone	~10% (inhibitory)

HCl secretion mechanism: Parietal cells use H⁺/K⁺-ATPase (proton pump) on apical membrane. Stimulated by histamine (H2 receptors), ACh (M3), and gastrin (CCK-B). Blocked by PPIs (irreversible H⁺/K⁺-ATPase inhibitors) or H2 blockers.

9. PANCREATIC AND BILIARY SECRETION

Pancreas

Acinar cells: secrete digestive enzymes (lipase, amylase, proteases as inactive zymogens)
Ductal cells: secrete bicarbonate-rich fluid (neutralizes duodenal acid)
Trypsinogen activated by enterokinase (from duodenal brush border) → trypsin → activates other zymogens
Stimulated by: CCK (enzyme secretion), secretin (bicarbonate)

Bile

Produced by hepatocytes, stored and concentrated in gallbladder
Contains: bile salts, lecithin, cholesterol, bilirubin, water, electrolytes
Bile salts: emulsify fats, form micelles; recycled via enterohepatic circulation (terminal ileum)
Released by gallbladder contraction in response to CCK
~95% of bile salts reabsorbed; 5% lost in feces (compensated by hepatic synthesis)

10. LARGE INTESTINE - COLON

Functions

Water and electrolyte absorption
Compaction of feces
Microbial fermentation (short-chain fatty acids from fiber)
Storage and controlled evacuation

Motility

Haustrations: segmental contractions; mix content; slow
Mass movements: strong propulsive contractions, 1-3x/day; triggered by gastrocolic reflex (eating → colonic activity)

Defecation Reflex

Rectal distension → afferent signals via pelvic nerves
Reflex relaxation of internal anal sphincter (smooth muscle, involuntary)
Voluntary relaxation of external anal sphincter (skeletal muscle)
Valsalva maneuver + levator ani contraction expels feces

11. GI BLOOD FLOW

GI tract receives ~20-25% of cardiac output postprandially
Splanchnic circulation: celiac artery (stomach, liver, spleen, duodenum), superior mesenteric artery (small intestine, proximal colon), inferior mesenteric artery (distal colon, rectum)
Local metabolic factors (↓O₂, ↑CO₂, adenosine) cause functional hyperemia after a meal
Sympathetic stimulation causes vasoconstriction → can divert blood away from gut in "fight or flight"

12. GI MICROBIOTA

~100 trillion microorganisms; predominantly in large intestine
Composition: ~1000 species; mainly Firmicutes and Bacteroidetes
Functions:
- Ferment undigested carbohydrates → short-chain fatty acids (SCFAs: butyrate, propionate, acetate)
- Synthesize vitamin K and some B vitamins
- Competitively inhibit pathogens
- Train and modulate immune system (70% of immune cells in gut-associated lymphoid tissue [GALT])
- Influence gut-brain axis
Disruption (dysbiosis): linked to IBD, IBS, obesity, diabetes, liver disease

13. PATHOPHYSIOLOGY OF COMMON GI DISORDERS

Achalasia

Failure of lower esophageal sphincter (LES) to relax during swallowing
Cause: damage to myenteric plexus neurons in the distal esophagus → loss of inhibitory innervation
Features: dysphagia (solids + liquids), megaesophagus, regurgitation, aspiration risk
Treatment: LES dilation, botulinum toxin injection, Heller myotomy, POEM

Gastroesophageal Reflux Disease (GERD)

LES incompetence → acid reflux into esophagus
Symptoms: heartburn, regurgitation, water brash, chronic cough
Complications: erosive esophagitis, Barrett's esophagus (metaplasia: squamous → columnar), adenocarcinoma risk
Treatment: lifestyle changes, PPIs (omeprazole, lansoprazole), anti-reflux surgery

Peptic Ulcer Disease (PUD)

Ulceration of mucosa due to imbalance between aggressive (HCl, pepsin) and protective (mucus, HCO₃⁻, prostaglandins) factors
H. pylori (most common cause - 80-90% of duodenal ulcers): urease-producing gram-negative rod; damages mucosal barrier
NSAIDs: inhibit COX-1 → ↓ prostaglandin synthesis → ↓ mucus/HCO₃⁻ production
Gastric ulcers: usually on lesser curvature; pain worsened by food
Duodenal ulcers: most common; pain relieved by food (antacids); nocturnal pain
Complications: bleeding, perforation, gastric outlet obstruction
Treatment: PPI + clarithromycin + amoxicillin (triple therapy for H. pylori)

Gastritis

Type A (autoimmune): anti-parietal cell antibodies; fundus/body involved; leads to pernicious anemia (no IF → no B12 absorption); risk of gastric carcinoma
Type B (H. pylori): antrum predominant; most common type; risk of MALT lymphoma, gastric carcinoma

Inflammatory Bowel Disease (IBD)

Feature	Crohn's Disease	Ulcerative Colitis
Distribution	Anywhere mouth to anus; skip lesions	Rectum upward; continuous
Depth	Transmural	Mucosal/submucosal only
Gross appearance	Cobblestone; "string sign" on barium; fistulae, strictures	Pseudopolyps; lead-pipe colon
Histology	Non-caseating granulomas	Crypt abscesses, goblet cell depletion
Bowel involvement	Terminal ileum most common	Colon only
Complications	Fistulae, abscesses, strictures, malabsorption	Toxic megacolon, colon cancer (risk ↑ with pancolitis and duration)
Treatment	Steroids, azathioprine, anti-TNF (infliximab), vedolizumab	5-ASA (mesalazine), steroids, anti-TNF, surgery (curative)

Celiac Disease

Immune-mediated enteropathy triggered by gluten (gliadin fraction) in genetically susceptible individuals (HLA-DQ2/DQ8)
Villous atrophy, crypt hyperplasia, increased intraepithelial lymphocytes
Symptoms: malabsorption, steatorrhea, weight loss, bloating, iron-deficiency anemia, fatigue
Extraintestinal: dermatitis herpetiformis, osteoporosis, infertility, peripheral neuropathy
Diagnosis: anti-tissue transglutaminase (anti-tTG) IgA antibodies; confirmed by duodenal biopsy
Treatment: strict lifelong gluten-free diet

Irritable Bowel Syndrome (IBS)

Functional disorder: altered gut motility + visceral hypersensitivity (no structural pathology)
Rome IV criteria: recurrent abdominal pain ≥1 day/week for 3 months, associated with ≥2 of: change in stool frequency, change in stool form, relief/change with defecation
Subtypes: IBS-C (constipation predominant), IBS-D (diarrhea predominant), IBS-M (mixed)
Treatment: dietary (low-FODMAP diet), antispasmodics, loperamide (for IBS-D), linaclotide (IBS-C)

Colorectal Cancer (CRC)

Second most common cancer death in Western countries
Adenoma-carcinoma sequence: normal mucosa → hyperplastic polyp → tubular adenoma → villous adenoma → carcinoma
Key mutations: APC (familial adenomatous polyposis), KRAS, p53, SMAD4
Symptoms: change in bowel habit, rectal bleeding, abdominal pain, iron-deficiency anemia, weight loss
Right-sided: occult bleeding, anemia, bulky tumors; Left-sided: obstruction, change in stool caliber
Screening: colonoscopy from age 45 (or 40-45 in high-risk groups); fecal occult blood test (FOBT)
Treatment: surgery ± adjuvant chemotherapy (FOLFOX, FOLFIRI); targeted: bevacizumab, cetuximab

14. QUICK REFERENCE: LAYERS & NERVE PLEXUSES

Lumen
  ↓
Mucosa         ← Epithelium + Lamina propria + Muscularis mucosae
Submucosa      ← Meissner's plexus (secretion + blood flow)
Circular muscle
  ↕             ← Auerbach's (myenteric) plexus (motility)
Longitudinal muscle
Serosa

15. KEY FORMULAS AND MNEMONICS

GI Hormones - "G-Cells SCRAM"

Gastrin - G cells - antrum
Secretin - S cells - duodenum
CCK - I cells - duodenum/jejunum
Motilin - M cells - duodenum/jejunum

GERD complications - "BEA"

Barrett's esophagus
Erosive esophagitis
Adenocarcinoma

Crohn's vs. UC - "Crohn's is FULL (transmural); UC is SUPER (superficial)"

PUD protective vs. aggressive - "CAMP vs. HEP"

Protective: Cyclin (prostaglandins), Alcaline mucus, Mucus, Prostaglandins
Aggressive: HCl, Encyme (pepsin), P. pylori/NSAIDs

Summary Table: Digestive Enzymes

Enzyme	Source	Substrate	Product
Salivary amylase	Parotid glands	Starch	Dextrins, maltose
Pepsin	Chief cells (pepsinogen + HCl)	Proteins	Polypeptides
Pancreatic amylase	Pancreatic acini	Starch	Maltose
Trypsin	Pancreas (trypsinogen + enterokinase)	Proteins	Polypeptides
Chymotrypsin	Pancreas	Proteins	Polypeptides
Pancreatic lipase	Pancreas	Triglycerides	Fatty acids + 2-monoglyceride
Lactase	Brush border	Lactose	Glucose + galactose
Sucrase	Brush border	Sucrose	Glucose + fructose
Maltase	Brush border	Maltose	Glucose + glucose
Enterokinase	Duodenal brush border	Trypsinogen	Trypsin (activates cascade)

These notes cover the full GI system at a medical student level, drawn from Guyton and Hall Textbook of Medical Physiology, Ganong's Review of Medical Physiology, and Sleisenger & Fordtran's GI & Liver Disease. Let me know if you'd like any section expanded - e.g., liver physiology, GI pharmacology, specific disease management, or embryology.

Gi physiology

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parietal cell HCl secretion mechanism proton pump

This composite figure illustrates the endoscopic and histopathological features of a proton pump inhibitor-related lesion (PPI-L) with low-grade dysplasia. (a) White-light imaging (WLI) reveals a 6 mm, homogenous reddish, protruded lesion located on the greater curvature of the upper gastric third. (b) Magnifying endoscopy with narrow-band imaging (ME-NBI) demonstrates a regular microvascular (MV) pattern and a regular microsurface (MS) pattern, bordered by a distinct demarcation line. (c, d) H&E stained sections at 100x magnification show parietal cell hyperplasia, foveolar epithelial dilation and elongation, and stromal vascularity. (e–i) Immunohistochemical analysis (100x) displays strong MUC5AC positivity in foveolar epithelial cells, while MUC6 (f), pepsinogen-I (g), and H+/K+-ATPase (h) are negative. (i) Ki-67 immunostaining reveals a high labeling index (80%), indicating significant cellular proliferation. The image serves as an educational reference for differentiating benign-appearing gastric protruded lesions from early gastric cancer using the MESDA-G diagnostic algorithm.

This is a bright-field light microscopy image of fixed gastric mucosa section stained with Hematoxylin and Eosin (H&E). The tissue shows gastric pits with an isthmus and neck populated by conspicuous parietal (oxyntic) cells with abundant eosinophilic cytoplasm and central nuclei. Parietal cells secrete hydrochloric acid and intrinsic factor, essential for digestion and B12 absorption. Deeper in the gland, chief (peptic) cells possess basophilic cytoplasm and secrete pepsinogen. Interspersed neuroendocrine cells release hormones; a stem cell zone resides at the neck, supporting constant epithelial renewal. The overall architecture corresponds to fundic-type oxyntic mucosa with tubular glands arranged in distinct isthmus/neck and base compartments. No overt inflammatory infiltrate or architectural distortion is evident, consistent with normal histology. This image is valuable for teaching gastric histology, differentiating mucous neck cells, parietal cells, chief cells, enteroendocrine cells, and stem cell niches, and for contextualizing pathologies affecting acid secretion, intrinsic factor production, or gastric carcinogenesis in educational and research settings.

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intestinal absorption glucose amino acid sodium cotransport enterocyte

A pathophysiology diagram illustrating the mineral absorption mechanisms within the intestinal lumen and enterocyte, specifically focusing on zinc (Zn/Zn+2). The diagram delineates three primary pathways for ingested minerals. 1) Organic absorption: Zinc (Zn) complexed with amino acids (AA) utilizes a specific Amino Acid Transporter on the enterocyte membrane, leading to successful absorption. 2) Inorganic absorption: Free ionic zinc (Zn+2) utilizes a dedicated Inorganic Metal Transporter to enter the enterocyte. 3) Non-absorption: Ionic zinc (Zn+2) that remains free or interacts with dietary antagonists (e.g., phytates or oxalates, labeled as 'Antagonist') fails to be transported, resulting in a 'Not Absorbed' outcome. The visual contrasts the efficiency of chelated organic minerals versus inorganic forms and highlights the role of competitive inhibition by antagonists. Key anatomical landmarks include the lumen and the apical membrane of the enterocyte. This illustration serves educational purposes in gastroenterology and clinical nutrition regarding bioavailability and trace element metabolism.

A two-part schematic diagram detailing the preparation and absorption mechanism of paclitaxel-loaded polymeric micelles for oral delivery. (A) Preparation phase: Illustrates the synthesis of PTX-loaded LC-SA/CS-SA micelles using the solvent evaporation-hydration method. Individual components including Chitosan-Stearic acid (CS-SA), L-carnitine-Stearic acid (LC-SA), and paclitaxel molecules are shown self-assembling into a spherical micelle structure with a hydrophobic core and a functionalized outer shell. (B) Absorption mechanism: Depicts the intestinal transport of intact micelles from the intestinal lumen across the enterocyte membrane. The process is facilitated by carnitine/organic cation transporters (OCTN2), represented by oval docking sites on the brush border. Following transepithelial transport, the micelles enter the bloodstream where they undergo hydrolysis, releasing the encapsulated paclitaxel along with stearic acid, L-carnitine, and chitosan components. This illustration demonstrates a targeted drug delivery strategy to improve the bioavailability of lipophilic drugs like paclitaxel via the carnitine transporter pathway.

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GI Physiology - Detailed Notes

Primary sources: Guyton & Hall Textbook of Medical Physiology, 14th ed., Ganong's Review of Medical Physiology, 26th ed., Costanzo Physiology, 7th ed.

MODULE 1: GI WALL STRUCTURE & SMOOTH MUSCLE

Wall Layers (Outer → Inner)

Serosa
Longitudinal smooth muscle  ─┐
  [Myenteric / Auerbach's plexus]  ← between these two layers
Circular smooth muscle     ─┘
  [Submucosal / Meissner's plexus]
Submucosa
Mucosa (muscularis mucosae + lamina propria + epithelium)

GI Smooth Muscle - Key Properties

Fiber dimensions: 200-500 µm long, 2-10 µm diameter
Bundled in groups of up to 1000 parallel fibers
Connected by gap junctions → low-resistance ion movement between cells
Behaves as a functional syncytium: action potential anywhere spreads through entire muscle mass
Longitudinal layer: propulsion; Circular layer: narrows lumen, drives peristalsis

Electrical Activity

Two types of waves:

1. Slow waves (Basic Electrical Rhythm)

Not action potentials - undulating changes in resting membrane potential
Generated by Interstitial Cells of Cajal (ICC) - the "pacemaker cells" of the gut
Set the maximum frequency of contraction - do NOT cause contraction by themselves
Region-specific frequencies:
- Stomach: ~3/min
- Duodenum: ~12/min (fastest - generates duodenal-jejunal gradient)
- Ileum: ~8/min
- Colon: ~6/min

2. Spike potentials (Action Potentials)

Superimposed on slow waves when they reach threshold (~-40 mV)
Triggered by stretch, ACh, parasympathetic stimulation, hormones
Cause actual smooth muscle contraction
Inhibited by norepinephrine, epinephrine, secretin

Clinical pearl: The slow wave frequency sets the maximum peristaltic rate in each region. Ectopic pacemakers or slow wave disruptions can cause dysmotility. - Guyton & Hall, p. 776

MODULE 2: ENTERIC NERVOUS SYSTEM (ENS)

The ENS is the "second brain" - ~100 million neurons; can control GI function entirely independent of the CNS.

Two Plexuses

Plexus	Location	Primary Function
Myenteric (Auerbach's)	Between longitudinal & circular muscle	Controls motility (peristalsis, segmentation)
Submucosal (Meissner's)	Within submucosa	Controls secretion and local blood flow

Enteric Neurotransmitters

Function	Transmitter
Excitatory (contraction above bolus)	ACh, Substance P
Inhibitory (relaxation ahead of bolus)	NO, VIP (vasoactive intestinal peptide)

Autonomic Control

System	Origin	Mediator	Effect
Parasympathetic cranial	Vagus (CN X) → esophagus to mid-colon	ACh → ENS activation	↑ Motility, ↑ secretion
Parasympathetic sacral	S2-S4 → pelvic nerves → distal colon, rectum, anus	ACh	Defecation reflex
Sympathetic	T5-L2 → celiac + mesenteric ganglia	Norepinephrine (NE) → inhibits ENS	↓ Motility, ↓ secretion; vasoconstriction

Note: ~80% of vagal nerve fibers are afferent (gut-to-brain), not efferent. The gut constantly informs the brain. - Guyton & Hall, p. 779

Afferent sensory nerve stimuli:

Mucosal irritation
Excessive distension
Specific chemicals in the gut

MODULE 3: GASTROINTESTINAL MOTILITY

Types of Movements

Peristalsis

The law of the intestine: contraction behind the bolus + relaxation ahead
Initiated by gut wall distension → myenteric plexus activation
Travels aborally (mouth → anus) at 2-25 cm/sec
Depends on intact myenteric plexus; interrupted by nerve damage

Segmentation (Mixing Contractions)

Rings of circular muscle contract and relax rhythmically
No net propulsion - mixes chyme with digestive juices
Dominant movement in small intestine after a meal
Frequency set by slow waves

Migrating Motor Complex (MMC) - "Housekeeper Wave"

Occurs during fasting (between meals)
Cycle every 90-120 minutes
Phases: I (quiescence) → II (irregular contractions) → III (intense "activity front" sweeping from stomach to ileum)
Function: sweeps undigested debris, bacteria, and secretions aborally
Initiated by motilin from duodenum/jejunum
Interrupted by eating (→ switches to segmentation)

Gastric Motility

Receptive relaxation: fundus relaxes to accommodate food (mediated by vagus → NO/VIP)
Tonic contractions: fundus maintains pressure to push food toward antrum
Antral peristalsis: strong peristaltic waves grind food and push chyme through pylorus
Pyloric sphincter controls gastric emptying rate
Gastric emptying: accelerated by gastrin, motilin; slowed by CCK, secretin, GIP, fat/acid/hyperosmotic chyme in duodenum

Large Intestine Motility

Haustrations: segmental mixing contractions; slow (<1 cm/hr)
Mass movements: 1-3/day; triggered by gastrocolic reflex (food entering stomach → colonic propulsion)
Defecation reflex: rectal distension → involuntary relaxation of internal anal sphincter (IAS) + voluntary relaxation of external anal sphincter (EAS)

MODULE 4: GASTRIC SECRETION

Glands of the Stomach

Gastric (oxyntic) gland structure - Guyton & Hall

Gastric oxyntic gland showing mucous neck cells, oxyntic (parietal) cells, ECL cells, and peptic (chief) cells.

Cell Type	Location	Secretion	Function
Surface mucous cells	Entire gastric surface	Mucus + HCO₃⁻	Mucosal protection
Mucous neck cells	Upper gland (neck)	Mucus	Mucosal protection
Parietal (oxyntic) cells	Fundus/body glands	HCl + Intrinsic Factor	Acid environment; B12 absorption
Chief (peptic) cells	Base of fundus/body glands	Pepsinogen	Protein pre-digestion
ECL cells	Fundus/body glands	Histamine	Paracrine stimulation of parietal cells
G cells	Antral glands	Gastrin	Stimulates acid secretion
D cells	Antral/fundus glands	Somatostatin	Inhibits G cells (paracrine)

Oxyntic glands: proximal 80% (fundus + body). Pyloric glands: distal 20% (antrum).

HCl Secretion Mechanism (Parietal Cell)

The parietal cell secretes HCl at pH ~0.8 (H⁺ concentration 3 million times that of arterial blood). This requires >1500 calories/L of gastric juice.

Step-by-step mechanism:

CO₂ + H₂O → H₂CO₃ (carbonic anhydrase) → H⁺ + HCO₃⁻
H⁺ is actively pumped into the canaliculus in exchange for K⁺ via H⁺-K⁺ ATPase (proton pump) on the apical membrane
K⁺ recycled back into cell via the same pump; Na⁺ reabsorbed via Na⁺-K⁺ ATPase on basolateral membrane
HCO₃⁻ exits via basolateral Cl⁻/HCO₃⁻ exchanger → enters blood ("alkaline tide")
Cl⁻ enters cell and is secreted through apical Cl⁻ channels → combines with H⁺ in canaliculus → HCl
Water follows osmotically → final secretion: HCl 150-160 mEq/L + KCl 15 mEq/L

Drug target: PPIs (omeprazole, lansoprazole) irreversibly block H⁺-K⁺ ATPase. H2 blockers (ranitidine, famotidine) block histamine H2 receptors on parietal cells.

Stimulation of Parietal Cells

Three pathways stimulate HCl secretion:

Stimulus	Receptor on Parietal Cell	Second Messenger
Histamine (from ECL cells)	H2 receptor	cAMP ↑
Gastrin (from G cells)	CCK-B (gastrin) receptor	Ca²⁺ ↑
Acetylcholine (from vagus/ENS)	M3 receptor	Ca²⁺ ↑

All three converge on final activation of H⁺-K⁺ ATPase. Histamine acts as the final common amplifier - even when gastrin and ACh are present, they partly work by stimulating ECL cells to release histamine.

Pepsinogen

Secreted by chief cells; inactive zymogen
Activated by HCl → pepsin (molecular weight drops from 42,500 to ~35,000)
Pepsin itself can then autocatalytically activate more pepsinogen
Optimal activity: pH 1.5-3.5; inactivated above pH 5

Phases of Gastric Secretion

Phase	Trigger	Mediator	% of Total
Cephalic	Sight, smell, taste, thought of food	Vagus (ACh) → direct parietal + ECL cell stimulation	~30%
Gastric	Food in stomach (distension + protein products)	Local myenteric reflexes + gastrin + histamine	~60%
Intestinal	Chyme enters duodenum (acid + fat + protein)	Initially small stimulation by intestinal gastrin; then inhibited by secretin, CCK, GIP, GLP-1	~10% (net inhibitory)

Inhibition of gastric secretion:

Low antral pH (<2) → D cells release somatostatin → suppresses G cells (feedback loop)
Fat + acid in duodenum → secretin + CCK + GIP → "enterogastric reflex" → ↓ gastric motility + ↓ acid
Distension of duodenum → ↓ gastric emptying

MODULE 5: GI HORMONES SUMMARY TABLE

Hormone	Secreting Cell	Location	Stimulus	Actions
Gastrin	G cells	Gastric antrum, duodenum	Protein, distension, vagal/ACh; inhibited by pH<2	↑ HCl, ↑ mucosal growth, ↑ gastric motility
CCK	I cells	Duodenum, jejunum	Fat, protein, acid	↑ Pancreatic enzymes, ↑ gallbladder contraction, ↓ gastric emptying, ↑ sphincter of Oddi relaxation
Secretin	S cells	Duodenum, jejunum	Acid (pH <4.5), fat	↑ Pancreatic HCO₃⁻, ↑ biliary HCO₃⁻, ↓ gastric acid, ↓ gastric emptying
GIP	K cells	Duodenum, jejunum	Fat, protein, carbs	↑ Insulin (incretin), ↓ gastric acid
Motilin	M cells	Duodenum, jejunum	Fat, acid, nerve	↑ Gastric + intestinal motility, initiates MMC
GLP-1	L cells	Ileum, colon	Fat, carbs, protein	↑ Insulin (incretin), ↓ glucagon, ↓ gastric emptying, ↓ appetite
VIP	ENS neurons	Entire gut	Nerve stimulation	Relaxes smooth muscle, ↑ pancreatic/intestinal secretion
Somatostatin	D cells	Stomach, pancreas	Acid, fat, protein	↓ Gastrin, ↓ secretin, ↓ acid, ↓ pancreatic enzymes, ↓ motility (broad inhibitor)

MODULE 6: PANCREATIC SECRETION

The pancreas secretes ~1500 mL/day via the pancreatic duct → empties into duodenum at the ampulla of Vater (through sphincter of Oddi).

Two Functional Components

Component	Cell	Secretion	Stimulated by
Enzymatic	Acinar cells	Digestive enzymes (as zymogens)	ACh (vagus + ENS), CCK
Aqueous (bicarbonate)	Ductal cells	Large volume of NaHCO₃-rich fluid	Secretin

Pancreatic Digestive Enzymes

Proteolytic (secreted as inactive zymogens):

Trypsinogen → trypsin (activated by enterokinase from duodenal brush border; also autocatalytic)
Chymotrypsinogen → chymotrypsin (activated by trypsin)
Procarboxypeptidase → carboxypeptidase A and B (activated by trypsin)
Proelastase → elastase (activated by trypsin)

Safety mechanism: Trypsin inhibitor is co-secreted by acinar cells to prevent autodigestion. Failure = acute pancreatitis.

Amylolytic:

Pancreatic amylase (active, not a zymogen) → starch + glycogen → disaccharides + trisaccharides

Lipolytic:

Pancreatic lipase (+ colipase) → triglycerides → fatty acids + 2-monoglycerides
Cholesterol esterase → cholesterol esters → free cholesterol + FA
Phospholipase A2 → phospholipids → lysophospholipids + FA

Regulation of Pancreatic Secretion

Three main stimuli (multiplicative/potentiating effects):

ACh (vagal + ENS) → acinar cells → enzymes
CCK (I cells, duodenum/jejunum; stimulated by fat + protein) → acinar cells → large enzyme secretion
Secretin (S cells; stimulated by acid pH <4.5) → ductal cells → large volume of HCO₃⁻

Phases of Pancreatic Secretion

Phase	% of Total	Notes
Cephalic	~20%	Vagal ACh → enzyme secretion; little fluid
Gastric	~5-10%	Continued vagal stimulation; still little fluid
Intestinal	~70-80%	Secretin-driven HCO₃⁻ flood; CCK-driven enzyme release

Key point: The intestinal phase dominates. Secretin release is triggered when duodenal pH drops below 4.5-5.0, which triggers HCO₃⁻ secretion to neutralize the acid. This is a classic negative feedback loop. - Guyton & Hall, p. 807

MODULE 7: BILIARY SECRETION

Bile Production and Composition

Hepatocytes continuously produce ~600-1000 mL bile/day
Stored and concentrated (~10-fold) in the gallbladder
Released into duodenum via common bile duct through sphincter of Oddi in response to CCK

Bile composition:

Component	Function
Bile salts (bile acids + glycine/taurine)	Emulsification of fats; micelle formation
Lecithin (phosphatidylcholine)	Enhances fat emulsification
Cholesterol	Metabolic waste product
Bilirubin (conjugated)	Breakdown product of heme
Ions (Na⁺, Cl⁻, HCO₃⁻)	Buffering

Enterohepatic Circulation of Bile Salts

Bile salts secreted into duodenum
Aid fat digestion throughout small intestine
95% reabsorbed at terminal ileum (active transport)
Transported back to liver via portal vein
Re-secreted by hepatocytes
5% lost in feces → replaced by new hepatic synthesis from cholesterol

Clinical: Resection of terminal ileum (e.g., Crohn's disease) → loss of bile salt recycling → bile salt deficiency → fat malabsorption (steatorrhea) + ↑ colonic bile salts → secretory diarrhea.

MODULE 8: SALIVARY AND INTESTINAL SECRETION

Saliva

Volume: 1000-1500 mL/day
Secreted by: parotid (25%), submandibular (70%), sublingual (5%) glands
Composition: water, mucus, salivary amylase (ptyalin), lingual lipase, IgA, lysozyme
pH: 6.0-7.4 (bicarbonate-rich at high flow rates)
Functions: Lubrication, starch digestion (~30-40%), antimicrobial, taste, buffering dental plaque acid

Small Intestinal Secretion (Succus Entericus)

Crypts of Lieberkühn secrete 1800 mL/day of isotonic fluid
Mainly water + NaCl + HCO₃⁻ (no major enzymes)
The mucus and fluid dilute and neutralize chyme
Brush border enzymes (not secreted - bound to microvilli membrane):
- Lactase, sucrase, maltase, α-dextrinase
- Aminopeptidases, dipeptidases
- Enterokinase (trypsinogen → trypsin)

Large Intestinal Secretion

Goblet cells secrete mucus for lubrication
No digestive enzymes

MODULE 9: DIGESTION

All three macronutrients digested by hydrolysis (addition of H₂O breaks condensation bonds).

Carbohydrate Digestion

Complete carbohydrate digestion pathway showing ptyalin, pancreatic amylase, and brush-border enzymes.

Stage	Site	Enzyme	Product
Salivary	Mouth/fundus	Ptyalin (salivary α-amylase)	Starch → maltose + 3-9 glucose polymers (30-40% digested)
Pancreatic	Duodenum	Pancreatic amylase	Starch → maltose, maltotriose (nearly complete within 15-30 min)
Brush border	Small intestine	Maltase, sucrase, lactase, α-dextrinase	Disaccharides → monosaccharides

Final absorbed monosaccharides: Glucose (from starch + sucrose), Fructose (from sucrose), Galactose (from lactose)

Cellulose cannot be digested (no human cellulase). However, gut microbiota ferment it to short-chain fatty acids.

Protein Digestion

Stage	Site	Enzyme	Notes
Gastric	Stomach	Pepsin (from pepsinogen + HCl)	Endopeptidase; active at pH 1.5-3.5; cleaves at aromatic AA residues
Pancreatic endopeptidases	Small intestine	Trypsin, chymotrypsin, elastase	Cleave internal peptide bonds
Pancreatic exopeptidases	Small intestine	Carboxypeptidases A + B	Cleave from C-terminus
Brush border	Small intestine	Aminopeptidases, dipeptidases	→ Free amino acids + di/tripeptides

Activation cascade: Enterokinase activates trypsinogen → trypsin → activates all other zymogens (chymotrypsinogen, proelastase, procarboxypeptidase)

Fat Digestion

Step	Where	Mechanism
1. Lingual/gastric lipase	Mouth/stomach	Minor digestion of short-chain triglycerides
2. Emulsification	Duodenum	Bile salts + lecithin break large fat globules into droplets (↑ surface area for lipase)
3. Pancreatic lipase + colipase	Duodenum	Triglycerides → 2-monoglycerides + 2 free fatty acids (colipase anchors lipase to emulsion droplet surface)
4. Micelle formation	Duodenum/jejunum	Bile salts form cylindrical micelles (4-8 nm) carrying monoglycerides + FA → transport to brush border
5. Absorption	Jejunum brush border	Lipids diffuse from micelle into enterocyte (passive, concentration gradient)
6. Re-esterification	Enterocyte smooth ER	FAs + 2-MG → triglycerides; cholesterol esters re-formed
7. Chylomicron packaging	Golgi	TG + cholesterol + phospholipids + apoprotein B48 → chylomicron
8. Lymphatic entry	Enterocyte → lacteals	Chylomicrons too large for capillaries → enter lacteals → thoracic duct → systemic circulation

MODULE 10: ABSORPTION

Surface Area Amplification of Small Intestine

The small intestine achieves ~600-1000-fold amplification of surface area through three structures:

Small intestine villi and valvulae conniventes - Guyton & Hall

Longitudinal section of small intestine showing villi projecting from valvulae conniventes (folds of Kerckring).

Structure	Increase	Details
Valvulae conniventes (circular folds / Kerckring's folds)	3x	Protrude 8 mm into lumen; most prominent in duodenum + jejunum
Villi	10x	1 mm tall; capillary + lacteal in each villus core
Microvilli (brush border)	20x	1000 microvilli/cell; 1 µm long; 0.1 µm diameter

Total surface area of small intestine: ~250 m²

Daily fluid balance in GI tract:

Ingested fluid: ~1.5 L
Secreted: ~7 L (saliva 1.5L + gastric 2L + bile 1L + pancreatic 1.5L + intestinal 1L)
Total entering small intestine: ~8-9 L
Small intestine absorbs ~7.5 L
1.5 L passes to colon; colon absorbs 1.3-1.4 L
Feces: ~100-200 mL

Absorption of Specific Nutrients

Carbohydrates (monosaccharides):

Glucose + galactose: Na⁺-glucose cotransporter (SGLT-1) on apical membrane → secondary active transport (driven by Na⁺ gradient maintained by basolateral Na⁺-K⁺ ATPase) → exits via GLUT-2 on basolateral side
Fructose: GLUT-5 on apical (facilitated diffusion); GLUT-2 on basolateral

Amino acids and peptides:

Free amino acids: Na⁺-dependent cotransporters (multiple types for different AA classes)
Di/tripeptides: H⁺-peptide cotransporter (PepT1) → hydrolyzed intracellularly to free AAs
Most absorbed in jejunum (faster) and ileum

Fats (see Module 9 - via micelles → lacteals)

Fat-soluble vitamins (A, D, E, K):

Absorbed with lipid micelles; require bile; absorbed in small intestine
Vitamin K also produced by colonic bacteria

Vitamin B12 (Cobalamin):

Requires Intrinsic Factor (IF) from parietal cells
IF-B12 complex absorbed by specific receptors in terminal ileum only
Deficiency causes: pernicious anemia (lack of IF), terminal ileum disease (Crohn's), gastric bypass

Iron:

Absorbed in duodenum primarily
Fe³⁺ (ferric) reduced to Fe²⁺ (ferrous) by duodenal cytochrome b (DcytB) - enhanced by vitamin C
Fe²⁺ transported via DMT-1 (divalent metal transporter-1) on apical membrane
Stored as ferritin in enterocyte; exported via ferroportin on basolateral side (regulated by hepcidin)

Calcium:

Active absorption in duodenum (regulated): calcitriol (1,25-OH₂ Vit D₃) ↑ calbindin → active Ca²⁺ transport
Passive absorption throughout small intestine
PTH indirectly ↑ absorption by stimulating calcitriol synthesis

Water:

Absorbed osmotically following solute absorption
Small intestine: 7.5 L/day via both transcellular and paracellular routes

Electrolyte Absorption

Na⁺ absorption mechanisms (small intestine):

Na⁺-glucose/AA cotransport (jejunum)
Na⁺-H⁺ exchanger (NHE) (jejunum + ileum)
Na⁺-Cl⁻ cotransport (ileum)

Cl⁻: follows Na⁺ (paracellular) or Cl⁻-HCO₃⁻ exchange

K⁺: passive absorption in small intestine; active secretion in colon (aldosterone responsive)

Colon Na⁺ absorption:

ENaC (epithelial Na⁺ channel) on apical membrane
Aldosterone upregulates ENaC → ↑ Na⁺ and water absorption in colon
Basis for oral rehydration therapy: glucose + Na⁺ solution stimulates coupled absorption even in diarrhea

MODULE 11: LARGE INTESTINE PHYSIOLOGY

Functions

Absorb water + electrolytes (residual ~1.5 L → compacts to 100-200 mL feces)
Store feces until voluntary evacuation
Host gut microbiota
Produce mucus (no digestive enzymes)

Bacterial Fermentation

Gut bacteria ferment dietary fiber + undigested carbohydrates
Products: Short-chain fatty acids (SCFAs) - butyrate, propionate, acetate
Butyrate is the primary energy source for colonocytes (~70% of their energy)
Gases produced: CO₂, H₂, methane (CH₄ in some individuals - basis of hydrogen breath tests)

Defecation Reflex

Feces enter rectum → rectal distension
Stretch receptors → afferent signals via pelvic nerves
Rectosphincteric reflex: involuntary relaxation of internal anal sphincter (smooth muscle)
Sensation reaches consciousness → urge to defecate
Voluntary relaxation of external anal sphincter (skeletal; pudendal nerve, S2-S4) → defecation
If deferred: rectum accommodates; urge subsides temporarily
Valsalva maneuver + levator ani contraction facilitates evacuation

MODULE 12: GI BLOOD FLOW

Splanchnic Circulation

GI tract receives ~20-25% of resting cardiac output
Postprandially rises to ~30%

Artery	Supply
Celiac artery	Stomach, liver, spleen, pancreas head, duodenum
Superior mesenteric artery (SMA)	Small intestine, cecum, ascending + transverse colon
Inferior mesenteric artery (IMA)	Descending + sigmoid colon, upper rectum

Functional Hyperemia (Postprandial)

After eating: ↑ metabolic activity in absorptive epithelium → ↓ O₂, ↑ CO₂, ↑ adenosine, ↑ bradykinin
→ Local vasodilation → ↑ mucosal blood flow to carry absorbed nutrients
Nervous stimulation of secretion also stimulates local blood flow (secretion and blood flow always increase together)

Countercurrent Exchange in Villi

Arterial and venous capillaries run parallel in villi
O₂ can diffuse from arteriole to venule at villus tips → villi tips are relatively hypoxic
Risk: in low-flow states (shock, vasoconstrictors), villus tips most vulnerable to ischemia → bacterial translocation

MODULE 13: GI PHYSIOLOGY - QUICK EXAM REFERENCE

Key Numbers to Know

Parameter	Value
Total GI secretion/day	~7-8 L
Total absorption by small intestine	~7.5 L/day
Small intestine surface area	~250 m²
Gastric HCl concentration	150-160 mEq/L; pH ~0.8
Gastric emptying half-time (solid meal)	~2-4 hours
Small intestinal transit time	3-5 hours
Large intestinal transit time	12-72 hours
Slow wave frequency - stomach	3/min
Slow wave frequency - duodenum	12/min
MMC cycle	90-120 min (fasting)

Classic Hormone-Stimulant Pairs (High-Yield)

"Give this..."	"Get this..."
Fat in duodenum	CCK (gallbladder contracts) + Secretin (HCO₃⁻) + GIP (insulin)
Acid in duodenum (pH <4.5)	Secretin (↑ pancreatic HCO₃⁻)
Protein in stomach	Gastrin (↑ HCl)
Eating a meal (any)	GLP-1 (↑ insulin, ↓ glucagon)
Fasting	Motilin (→ MMC)

Key Enzyme Activation Cascade

Enterokinase (brush border)
    ↓
Trypsinogen → TRYPSIN
    ↓ (activates all below)
Chymotrypsinogen → Chymotrypsin
Proelastase → Elastase
Procarboxypeptidase → Carboxypeptidase A & B
Prophospholipase → Phospholipase

Mucosal Protection - Gastric Barrier

Mucus layer: viscous gel covering surface; traps HCO₃⁻
Bicarbonate secretion: surface mucous cells secrete HCO₃⁻ → local pH ~7 at cell surface despite luminal pH 2
Tight junctions: prevent backleak of H⁺
Prostaglandins (PGE₂): stimulate mucus + HCO₃⁻ production and mucosal blood flow
NSAIDs block COX-1 → ↓ prostaglandins → breach of barrier → peptic ulcer

Sources: Guyton and Hall Textbook of Medical Physiology (14e), p. 771-826 | Ganong's Review of Medical Physiology, 26e | Costanzo Physiology, 7e

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