Hormonal Control in Gastrointestinal Tract

Hormonal Control of the GI Tract

• Endocrine cells in gut walls secrete hormones for medium- and long-distance signaling.
• G cells in the stomach antrum release gastrin, stimulating gastric acid secretion by parietal cells.
• Cholecystokinin (CCK) from the small intestine signals fat digestion products, inducing gall bladder contraction and bile secretion.
• Glucagon-like peptide 1 (GLP-1) secreted from the ileum decreases stomach motility and promotes insulin secretion from the pancreas.

GI Control by Peptides and Amines

• The enteric nervous system contains opioid peptides that reduce motility, aiding in diarrhea treatment.
• Opioid analgesics can lead to constipation due to their impact on gut motility.
• Enterochromaffin (EC) cells release histamine in the stomach and serotonin in the small intestine, stimulating gastric acid secretion and increasing gastric motility, respectively.

Immune Components of the Small Intestine

• The small intestine is equipped with significant immune protections against ingested pathogens.
• Peyer’s patches serve as central hubs for gut immune activity, monitoring intestinal contents.

Immune Components of the Colon

• The colon hosts the majority of gut microbiota, which aid digestion, immune function, and tolerance.
• It contains lymphocytes and macrophages that provide immune protection.

Clinical Aspects of Acute Pancreatitis

• Common causes include gallstones, alcohol consumption, and drug reactions.
• It can lead to critical illness, systemic inflammatory response syndrome, and multiorgan dysfunction.
• Repeated acute episodes can result in chronic pancreatitis.

Basic Principles of Gut Digestion and Absorption

• Nutrients are predominantly absorbed in the proximal small intestine (duodenum, jejunum).
• The vascularized gut supports nutrient uptake and circulation via the portal vein to the liver.
• End products of digestion include:
  • Starches: glucose, galactose, fructose
  • Proteins: amino acids
  • Triglycerides: free fatty acids and glycerol
  • Phospholipids: free fatty acids and lysophospholipids
  • Cholesterol esters: cholesterol and free fatty acids

Common Mechanisms of Diarrhea

• Secretory: Triggered by bacterial exotoxins (e.g., cholera) causing secretion overload.
• Malabsorptive: Loss of gut surface area, seen in conditions like celiac disease.
• Osmotic: Occurs with lactase deficiency, where undigested lactose causes osmotic pull.
• Inflammatory: Associated with inflammatory bowel disease, leading to surface area loss and inflammation.

Immune-Based Gut Disorders: Celiac Disease

• Celiac disease arises from the immune response to gluten in sensitive individuals.
• Ingestion of gluten leads to the production of gliadin peptides, which become antigenic, causing pathological immune activation and impaired gut function.

Stomach Disorders: Peptic Ulcers

• Common sites for peptic ulcers are the duodenum and stomach.
• Risk factors include hyperacidity, smoking, bile reflux, and NSAID use.
• Helicobacter pylori infection increases the risk of ulcer recurrence and stomach cancer.
• Management of peptic ulcers involves reducing acid secretion through:
  • Histamine H2-blockers
  • Proton pump inhibitors
  • Short-term antacids

Stomach Motility and Gastroparesis

• Normal stomach motility involves receptive relaxation to add food and strong contractions to mix and move chyme.
• Gastroparesis results from reduced motility due to altered neural modulation, often seen in diabetic autonomic neuropathy or after surgery.

Small Intestine Structure and Absorption

• Crypts and villi amplify the absorptive surface area of the small intestine.
• Intestinal epithelial cells possess microvilli, enhancing absorption.
• Tight junctions between cells prevent the entry of toxins and pathogens into circulation.

Importance of Pancreatic Enzymes in Digestion

• Pancreatic cells secrete enzyme inhibitors to prevent autodigestion.
• Pancreatic enzymes break down dietary proteins, carbohydrates, and lipids.
• Pancreatic fluid, rich in bicarbonate, neutralizes acidic chyme entering the duodenum from the stomach.