GI Lecture 01: GI Overview & Physiology Introduction

The myenteric (Auerbach's) plexus lies between the circular and longitudinal smooth muscle layers of the muscularis externa. It is the primary controller of GI motility, including peristalsis and segmentation. The submucosal (Meissner's) plexus lies between the circular muscle and the submucosa and controls secretion and local blood flow. The muscularis mucosae is a thin layer within the mucosa itself (not in the muscularis externa).

Infantile hypertrophic pyloric stenosis involves hypertrophy of the smooth muscle of the muscularis externa (primarily the circular muscle layer) at the pylorus. This creates gastric outlet obstruction. The vomiting is non-bilious because obstruction is proximal to the ampulla of Vater (no bile has mixed with the content). Treatment is Ramstedt pyloromyotomy — cutting through the hypertrophied muscle while preserving the mucosa.

Parietal cells produce both HCl (via H⁺/K⁺ ATPase) and intrinsic factor. Autoimmune destruction of parietal cells → simultaneous loss of HCl (achlorhydria) and intrinsic factor. Achlorhydria predisposes to bacterial overgrowth and impairs protein digestion (pepsinogen activation requires low pH). Pepsinogen is produced by chief cells (not parietal cells), so pepsinogen secretion itself is not impaired, but the enzyme cannot be activated without acid.

The terminal ileum is the sole site of bile salt reabsorption (enterohepatic circulation). When the terminal ileum is diseased or resected, bile salts are not reabsorbed → lost in stool → bile salt pool depleted → inadequate micelle formation → fat malabsorption → fat-soluble vitamins (ADEK) are not absorbed. This is distinct from option D, which implies direct absorption failure rather than the indirect mechanism through bile salts. Additionally, the terminal ileum is the only site for B12-intrinsic factor complex absorption — so B12 deficiency also occurs in terminal ileal disease (macrocytic anemia).

Gastrin is produced by G cells in the gastric antrum and duodenum. It is released in response to protein (amino acids and peptides), stomach distension, and vagal stimulation. Gastrin is secreted into the blood and travels to parietal cells in the gastric body/fundus, where it activates CCK-B receptors → stimulates HCl secretion. This is the "Endocrine" function (E in DEEP MASS) — GI hormones are secreted into the bloodstream to act at distant targets.

Even fasting patients secrete ~7–8 L of GI fluid daily from salivary glands, stomach, pancreas, bile, and intestinal crypts. Normally this fluid is almost entirely reabsorbed. With a complete small bowel obstruction, secretions continue to accumulate above the obstruction, but absorption is impaired. The result: massive third-spacing of fluid into the obstructed bowel → vomiting → dehydration → electrolyte abnormalities → shock. Without IV fluid replacement, this can be rapidly fatal — demonstrating how critical the Secretion + Absorption balance (two of the S/A components of DEEP MASS) is even without oral intake.

Pancreatic lipase is essential for hydrolysis of triglycerides into monoglycerides and free fatty acids (the absorbable forms). Without it, fat passes undigested into the colon → bacterial fermentation of fat → steatorrhea (bulky, oily, malodorous stools). This also impairs absorption of fat-soluble vitamins (A, D, E, K). Treatment: oral pancreatic enzyme replacement therapy (PERT) — enzymes given with meals to replace the missing exocrine function. Note: salivary lipase and gastric lipase provide some (~10–30%) residual fat digestion.

Gastric acid (pH 1.5–2.0) is a critical first line of immune defense. Most bacteria, viruses, and parasites cannot survive exposure to this extremely low pH. Clinical evidence: patients on proton pump inhibitors (PPIs) have a significantly higher risk of Clostridioides difficile, Salmonella, Campylobacter, and Vibrio infections — because the protective acid barrier has been reduced. Similarly, patients with achlorhydria (e.g., pernicious anemia) are more susceptible to GI infections. This is why the GI tract — despite being in contact with the outside world through food — maintains such a hostile environment.

The GI tract secretes 7–8 L per day in addition to the ~2 L ingested, totaling ~9–10 L. The small intestine reabsorbs ~8.5 L (driven largely by Na⁺ co-transporters); the colon reabsorbs most of the remaining ~400 mL, leaving only 100–200 mL in stool. In cholera, the ↑cAMP → massive Cl⁻ secretion overwhelms this reabsorptive capacity, flipping the intestine from net absorber to net secretor. Oral rehydration solution (ORS) works by providing glucose + Na⁺ to drive Na⁺-glucose cotransport (SGLT1), which remains functional even during cholera, pulling water back across.

After gastric bypass, the gastric remnant is small and cannot buffer or dilute food properly. Hyperosmolar food enters the small intestine rapidly without the normal pyloric regulation. Small intestinal osmoreceptors detect extreme hyperosmolarity and fluid pours from the blood into the intestinal lumen to dilute the content → intravascular hypovolemia → tachycardia, lightheadedness, sweating. This is "early" dumping (15–30 min post-meal). "Late" dumping occurs 1–3 hours later due to rebound hypoglycemia from exaggerated insulin response to the absorbed glucose load.

Gastric juice is highly acidic (pH 1.5) — it contains HCl, K⁺, and some Na⁺. Continuous NG suction removes HCl → extracellular [H⁺] falls → metabolic alkalosis (↑pH, ↑HCO₃⁻). Simultaneously, K⁺ is lost in gastric juice AND the kidneys respond to volume depletion by activating the RAAS → aldosterone → exchanges K⁺ for Na⁺ in the kidney → further hypokalemia. The alkalosis drives K⁺ into cells (K⁺/H⁺ exchange) → worsening hypokalemia. Cl⁻ is directly lost in HCl → hypochloremia. This is "contraction alkalosis" or "hypokalemic hypochloremic metabolic alkalosis."

Intestinal and pancreatic secretions are bicarbonate-rich. Diarrhea causes loss of HCO₃⁻-rich fluid → non-anion gap metabolic acidosis (↓pH, ↓HCO₃⁻). Potassium is also lost (colon secretes K⁺, and aldosterone response worsens this) → hypokalemia. Vomiting causes loss of HCl → metabolic alkalosis (↑pH, ↑HCO₃⁻) + hypokalemia + hypochloremia. This is one of the most commonly tested acid-base patterns on Step 1: know the fluid composition at each level of the GI tract and you can predict the electrolyte/acid-base consequence of losing it.

In Hirschsprung disease, the aganglionic segment lacks both excitatory and inhibitory ENS neurons. Without inhibitory neurons (VIP/NO), smooth muscle remains in tonic contraction — the aganglionic segment cannot relax to allow stool to pass. The proximal segment, which has normal ganglia, dilates from the backed-up stool. This is counterintuitive: the narrow segment is the diseased one; the dilated segment is normal. Down syndrome (trisomy 21) has an increased association with Hirschsprung disease. Treatment is surgical resection of the aganglionic segment (Swenson pull-through).

The ENS contains ~500 million neurons organized into complete local reflex arcs (sensory, interneurons, motor). It can initiate and coordinate peristalsis, secretion, and local blood flow without any extrinsic input — demonstrated by experiments in which denervated bowel segments still contract rhythmically. Vagotomy (cutting the vagus nerve) does not stop peristalsis; it reduces the efficiency of coordination and decreases gastric acid secretion (used therapeutically). Sympathetic nerves generally inhibit GI motility (not stimulate). The ENS extends throughout the entire GI tract from esophagus to anus.

The "Braak hypothesis" (Braak et al., 2003) proposes that Parkinson's disease may originate in the gut ENS or olfactory bulb and then spread to the brain via vagal afferents. This is supported by the finding of Lewy bodies in ENS neurons before substantia nigra involvement, and by the observation that vagotomy is associated with reduced Parkinson's risk in some studies. Constipation from ENS neurodegeneration is a common early non-motor symptom of Parkinson's. This is an active and exciting area of gut-brain axis research.

The peristaltic reflex has two simultaneous limbs: (1) Ascending excitation — oral (proximal) to the bolus: excitatory ENS neurons release ACh and Substance P → circular muscle contracts → propels bolus forward. (2) Descending inhibition — aboral (distal) to the bolus: inhibitory ENS neurons release VIP and NO → circular muscle relaxes → creates space for the bolus to move into. This coordination creates the propagating peristaltic wave.

Enterochromaffin cells release 5-HT in response to luminal stimuli. 5-HT acts on 5-HT3 receptors located on intrinsic primary afferent neurons (IPANs) in the ENS and on vagal afferent fiber terminals in the gut wall. Activation of vagal afferents relays signals to the nucleus tractus solitarius and area postrema (chemoreceptor trigger zone) → nausea and vomiting. Ondansetron blocks 5-HT3 on these vagal afferents → breaks the signal → antiemetic. It also has central action at the area postrema (which lacks a blood-brain barrier). 5-HT4 agonists (metoclopramide) enhance ENS excitatory neuron ACh release → prokinetic effect.

VIP acts on VPAC1/VPAC2 receptors, which are Gs-coupled. Activation → ↑adenylyl cyclase → ↑cAMP → activation of protein kinase A → opens CFTR Cl⁻ channels in intestinal crypt cells → Cl⁻ secretion → water follows osmotically → massive watery diarrhea. This is the same downstream mechanism as cholera toxin (which also ↑cAMP). VIPoma (Verner-Morrison syndrome, WDHA = Watery Diarrhea-Hypokalemia-Achlorhydria): diarrhea causes K⁺ loss → hypokalemia; high VIP also suppresses gastric acid → achlorhydria. Treatment: octreotide (somatostatin analogue) suppresses VIP secretion.

True achalasia (idiopathic or secondary to Chagas disease) is caused by loss of inhibitory ENS neurons (VIP/NO) at the LES → LES fails to relax. Manometrically, it shows aperistalsis in the distal esophagus and impaired LES relaxation. Pseudoachalasia from malignancy at the GEJ (e.g., adenocarcinoma, gastric cancer infiltrating the cardia) can appear identical on manometry. Key differentiating features: (1) Rapid weight loss — achalasia typically shows gradual weight loss; malignancy is rapid; (2) Short duration of symptoms (<1 year); (3) Older age at presentation; (4) EGD + CT scan to exclude mass lesion. Any achalasia presentation should prompt EGD to rule out malignancy at the GEJ.

Normally, serotonin released by enterochromaffin cells is almost entirely cleared by MAO in hepatocytes during portal first-pass metabolism. Liver metastases overwhelm this capacity → excess 5-HT enters systemic circulation → reaches the right heart (right atrium and ventricle) → fibrotic plaques on tricuspid and pulmonary valves → tricuspid regurgitation + pulmonary stenosis. The pulmonary circulation contains MAO in vascular endothelial cells → inactivates 5-HT before blood reaches the left atrium → left heart is spared. Exception: if there is a right-to-left shunt (PFO) or bronchial carcinoid releasing 5-HT directly into pulmonary veins, left-sided valvular disease can occur.

The IMA supplies the hindgut: distal 1/3 of transverse colon, descending colon, sigmoid colon, and upper rectum. During AAA repair, the IMA is often sacrificed or may go into spasm after aortic clamping. If collateral flow from the SMA (through the marginal artery of Drummond) is insufficient, the left colon develops ischemia → bloody diarrhea ("ischemic colitis"). This most commonly affects the watershed area at the splenic flexure (junction of SMA and IMA territories) — the "Griffith's point." The rectum is usually spared because of its collateral blood supply from middle/inferior rectal arteries (internal iliac).

The gastroduodenal artery (a branch of the common hepatic artery from the celiac trunk) runs posterior to the first part of the duodenum. A posterior duodenal ulcer (most commonly H. pylori-related) can erode through the posterior duodenal wall into the GDA — a large, high-pressure artery — causing massive arterial hemorrhage. In contrast, anterior duodenal ulcers tend to perforate (free air under the diaphragm) rather than bleed, because the anterior wall is not in contact with major vessels. The GDA is also closely related to the head of the pancreas — severe pancreatitis can also erode it.

Long-chain fatty acids (C12+) are re-esterified into triglycerides in enterocytes, assembled with apolipoproteins B-48, cholesterol, and phospholipids into chylomicrons. Chylomicrons are too large (~100–1000 nm) to enter blood capillaries, so they enter lacteals (lymphatic capillaries in each intestinal villus) → mesenteric lymphatics → thoracic duct → left subclavian vein → systemic circulation. This entirely bypasses portal circulation and hepatic first-pass metabolism. Note: short-chain (C2–C6) and medium-chain fatty acids (C8–C12) are more water-soluble and can enter portal blood directly without packaging into chylomicrons.

In portal hypertension, portal blood seeks alternative routes to the systemic circulation via portosystemic anastomoses. At the lower esophagus, the left gastric (coronary) vein (a portal tributary) connects to the esophageal venous plexus, which drains into the azygos vein (systemic). Increased portal pressure → blood backs up through the left gastric vein → esophageal submucosal veins dilate (varices) → fragile submucosal vessels at risk for massive hemorrhage (30–50% mortality per bleed). Treatment: band ligation, sclerotherapy; prevention: non-selective β-blockers (propranolol/nadolol) to reduce portal pressure by decreasing cardiac output and splanchnic vasoconstriction.

Vagotomy cuts vagal efferent fibers to the stomach. Effects: (1) Cephalic phase is completely abolished — the entire cephalic response depends on vagal efferents to parietal cells and G cells. (2) Gastric phase is significantly reduced — vagal stimulation amplifies gastrin release from G cells during the gastric phase; without this, gastrin release is reduced (~50% of gastric phase preserved via local mechanisms). (3) Intestinal phase is largely unaffected (mediated by CCK/secretin). Side effects of truncal vagotomy: gastroparesis (requires pyloroplasty to ensure gastric emptying), diarrhea (from disrupted intestinal motility regulation).

Sympathetic activation (T5–L2 → splanchnic nerves → celiac/SMA/IMA ganglia → NE release) produces coordinated suppression of GI function: ↓peristalsis, ↑sphincter tone, ↓secretion, and most importantly, splanchnic vasoconstriction. This diverts blood from the GI vasculature (which normally receives 25–30% of cardiac output) to skeletal muscle, heart, and brain for the "fight-or-flight" response. This is the physiologic basis of the classic advice "don't swim right after eating" — vigorous exercise activates the SNS, diverting blood away from the GI tract just when it needs blood for post-prandial digestion, causing cramping.

The cephalic phase involves: sensory stimuli (sight, smell, taste, thought of food) → activation of cerebral cortex and hypothalamus → activation of the dorsal motor nucleus of the vagus nerve (CN X) in the medulla → preganglionic vagal fibers travel to the stomach → synapse on ENS postganglionic neurons → ACh release → stimulates both parietal cells (HCl via muscarinic M3) and G cells (gastrin release). This accounts for ~20–30% of total gastric acid secretion in response to a meal. Pavlov demonstrated this with his conditioned reflex experiments, for which he received the Nobel Prize in 1904.

This is the mechanism of metoclopramide (and the newer prucalopride, which is more selective for 5-HT4). 5-HT4 agonism → increases ACh release from ENS excitatory motor neurons → ↑peristalsis + ↑gastric emptying. Dopamine D2 antagonism → removes dopaminergic inhibition of ENS → further increases motility. Metoclopramide's CNS side effects (extrapyramidal symptoms, tardive dyskinesia) occur because it crosses the blood-brain barrier and blocks D2 receptors in the basal ganglia. Peripheral D2 antagonists (domperidone) don't cross the BBB — reduced CNS side effects. Prucalopride is a highly selective 5-HT4 agonist approved for chronic idiopathic constipation.