Gastrointestinal (GI) System Anatomy and Physiology

• The GI tract is a complex multi-organ system that includes the alimentary canal and accessory organs like salivary glands, liver, gallbladder, and pancreas.
• The alimentary canal, a hollow tube from the mouth to the anus, spans 7-9 meters in an adult human.
• Organs within the GI tract: mouth, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon, cecum), rectum, and anus.

Digestion Process

• Food is mechanically chewed and mixed with saliva in the mouth.
• Salivary glands release amylase to break down carbohydrates and lipase to break down fats.
• The food bolus passes through the upper esophageal sphincter, down the esophagus, past the lower esophageal sphincter, and into the stomach.
• The stomach mixes food through mechanical contractions.
• Parietal cells secrete hydrochloric acid in the stomach.
• Chief cells secrete pepsin for protein breakdown and lipase for fat breakdown.
• The stomach transforms food into a liquid solution called chyme.
• Chyme moves through the pyloric sphincter into the duodenum.
• Digestion occurs in the duodenum via contractions and enzymes released from epithelial cells, along with bile from the gallbladder and liver.
• The pancreas secretes amylase, trypsin, chymotrypsin, and lipase into the duodenum.
• Bulk absorption and reabsorption of nutrients and water occurs in the jejunum and ileum
• The ileum absorbs more fats while the jejunum absorbs more carbohydrates and amino acids.
• The food bolus then passes through the ileocecal sphincter and rests briefly in the cecum.
• The colon absorbs remaining water, and its microbiota bacteria break down fibers.
• Food waste is stored in the rectum before passing through the anus.

Macronutrient Breakdown

• Carbohydrates are broken down by amylase in the mouth, duodenum (via pancreas), and jejunum.
• Proteins are broken down by pepsin, trypsin, and chymotrypsin in the stomach and duodenum (via pancreas).
• Fats are broken down by lipase in the mouth, stomach, duodenum (via gallbladder/liver/pancreas), and ileum.

Functions of the GI Tract

• Digestion of food.
• Secretion of hormones (endocrine) and enzymes (exocrine).
• Endocrine secretion helps coordinate digestion, regulate glucose, and stimulate hunger; an example is the release of cholecystokinin (CCK) by fat in the duodenum, triggering bile release and inhibiting the pyloric sphincter.
• Absorption of nutrients and water from the GI tract lumen into the body.
• Motility, which is the mixing and movement of food throughout the GI tract.
• Defense, where the gut acts as the body's largest immune organ and a barrier against harmful components.
• Gut microbiome helps break down fibers, aiding digestion and energy expenditure, and is linked to diseases like hypertension, mental health issues, schizophrenia, and depression.

GI Anatomy - Tissue Layers

• Tunica Mucosa consists of:
  • Epithelium for lining the lumen.
  • Lamina Propria: Connective tissue with immune cells.
  • Muscularis Mucosa: A thin muscle layer providing structure and aiding in secretion.
• Submucosa Layer consists of dense connective tissue connecting glands, blood, and lymphatic tissue.
• Muscularis Layer consists of circular and longitudinal smooth muscle layers and the Myenteric Plexus, which contains enteric neurons between smooth muscle layers (the "gut brain").
• Serosa Layer consists of mesothelium that lines the organ.

Histology of GI Tract

• Esophagus: Stratified squamous epithelium with multiple layers of square, flat cells that can slough off when damaged.
• Stomach: Simple columnar epithelium forming deep pits, specialized epithelial cells secrete acid and enzymes; foveolar cells produce protective mucus.
• Small Intestine: Single columnar epithelium forming villi, enterocytes form a brush border of microvilli for reabsorption, goblet cells produce mucus; the brush border also acts as a barrier against germs and viruses.
• Colon: Single columnar epithelium organized in a flat surface, predominantly goblet cells produce mucus, absorptive enterocytes reabsorb water.

Overview of GI Disorders

• Categorized as motility, secretion, digestion, or absorption disorders.
• Motility Disorders: Affect major GI tract regions; examples being damage to GI smooth muscle (achalasia) or neural/hormonal issues (Hirschsprung's disease) prevent proper muscle contraction.
• Secretion Disorders: Inappropriate production of acid or mucus in the stomach, digestive enzymes or bicarbonate from the pancreas, or too much/little bile from the liver; an example is peptic ulcers caused by excessive hydrochloric acid.
• Digestion Disorders: Incomplete breakdown of nutrients; an example is lactose intolerance.
• Absorption Disorders: Decreased ability to absorb nutrients; an example is Crohn's disease.

GI Disease Symptoms

• Pain
• Altered Ingestion including nausea, vomiting, dysphagia (difficulty swallowing), odynophagia (painful swallowing), and anorexia (lack of appetite).
• Altered Bowel Movements like diarrhea or constipation.
• GI Tract Bleeding

Pain Pathways

• Ascending Pathway:
• Damage sends a signal to the dorsal root ganglia (DRG) outside the spinal cord.
• Transmits to neurons in the spinal cord.
• Neurons travel up the spinal cord to the thalamus.
• The thalamus connects with the somatosensory cortex, signaling pain.
• Descending Pathway:
• Inhibits the signal at the level of the spinal cord.
• Reduces pain signals.
• Opioids:
• Inhibit the ascending pathway at the dorsal root ganglion and spinal cord.
• Activate the descending pathway to increase inhibition.
• Non-Steroidal Anti-Inflammatory Drugs (NSAIDs):
• Act at the site of damage to reduce inflammation.
• Reduce signals sent to the brain.
• Visceral Pain:
• Poorly localized pain often referred to different structures.
• According to the referred pain theory, first-order neurons from different organs synapse on the same second-order neuron in the spinal cord, so the brain cannot differentiate the origin of the pain.

Nausea and Vomiting

• Nausea: The feeling of sickness in the stomach with an inclination to vomit.
• Vomiting: Forcible ejection of undigested GI content from the mouth and a defense mechanism.
• Physiological aspects of vomiting:
  • Relaxation of the lower esophageal sphincter.
  • Increased intra-abdominal pressure.
  • Contraction of diaphragm and abdominal muscles.
  • Closing of the epiglottis.

Vomiting Reflex Mechanism

• Brainstem
• Vomiting center (muscarinic receptors trigger the vomiting reflex).
• Chemoreceptor trigger zone (located outside the blood-brain barrier).
• Chemoreceptor Trigger Zone
• Detects toxins from chemotherapy, radiation, or ingested toxins.
• Triggers vomiting reflex.
• Higher Brain Centers
• Sensory, pain, smell, sight, memory, fear, or anticipation can trigger the vomiting center.
• Gut Signals
• Food that is not digested properly sends a signal to the vagus nerve.
• The vagus nerve signals the vomiting center.

Medications to Treat Nausea and Vomiting

• 5-HT3 Receptor Antagonists (Ondansetron/Zofran): Commonly prescribed, relatively safe anti-vomiting medication.
• Prochlorperazine: Anti-vomiting and antipsychotic drug (D2 receptor antagonist).
• Scopolamine: Muscarinic receptor antagonist used for motion sickness (prescribed as a patch).

Dysphagia and Odynophagia

• Dysphagia: Difficulty swallowing.
• Odynophagia: Painful swallowing.

Phases of Swallowing

• Voluntary Phase: The bolus of food is pushed by the tongue past the hard palate toward the back of the throat.
• Pharyngeal Phase: Involuntary movement of the bolus into the oropharynx and soft palate of the nasopharynx, eliciting closure of the epiglottis.
• Esophageal Phase: Involuntary contraction of the esophagus muscles pushes the bolus down to the stomach.

Problems with Swallowing

• Neurological disorders may affect the voluntary phase.
• Stroke may affect the pharyngeal phase.
• GERD, achalasia, or esophageal cancer may affect the esophageal phase.

Anorexia

• Loss of desire to eat.
• Primary Anorexia (Anorexia Nervosa): A neural disorder where the brain signals to stop eating.
• Secondary Anorexia: A consequence of a disease such as GERD, peptic ulcer, Crohn's disease, ulcerative colitis, or cholecystitis.
• Potential treatment for anorexia: CGRP receptor antagonist may target hunger signals in the parabrachial nucleus.

Gastrointestinal Tract Bleeding

• The location and color of blood help in diagnosis.
• Bright Red Blood Coating the Stool: Bleeding at the rectum or anus, possibly from hemorrhoids or polyps (especially in children).
• Dark Blood Mixed in the Stool: Colon problem.
• Black or Tarry Stool: Bleeding in the esophagus, stomach, or duodenum.
• Bright Red Blood in Vomit: Bleeding in the esophagus or stomach.
• Coffee Grounds Appearance in Vomit: Bleeding in the stomach or duodenum.
• Signs of Fatigue or Anemia: May indicate GI tract bleeding even without visible blood.

Test your knowledge of the gastrointestinal system. Questions cover accessory organs, digestion of carbohydrates, ileum function, parietal cells, and swallowing. The quiz also touches on conditions like GERD and the roles of enzymes and hormones like CCK.