Digestive System I Notes

• Functions:

  • Ingestion: The process of taking in food through the mouth, which begins the digestive process.

  • Mechanical Digestion: This involves the physical breakdown of food into smaller particles. For example, chewing in the mouth breaks down food, making it easier for enzymes to work.

  • Chemical Digestion: Enzymatic breakdown of food occurs in various parts of the digestive system, such as the saliva in the mouth, gastric juices in the stomach, and bile in the small intestine. This step is crucial for nutrient absorption.

  • Secretion: The release of enzymes, acids, and buffers into the digestive tract is key to digestion. The stomach secretes hydrochloric acid, while the pancreas secretes digestive enzymes and bicarbonate.

  • Absorption: This phase involves the uptake of nutrients through the intestinal walls into the bloodstream. The small intestine is primarily responsible for nutrient absorption.

  • Excretion: The removal of waste products from the body, primarily through the large intestine, where water is reabsorbed, and the remaining waste is formed into stool.

  • Compaction: The dehydration of indigestible materials occurs primarily in the large intestine, where remaining waste is prepared for excretion.

• Major Portions:

  • Alimentary Tract: This is a continuous tube extending from the mouth to the anus and comprises various structures that contribute to the digestive process, including the esophagus, stomach, and intestines.

  • Accessory Organs: These include the salivary glands, liver, gallbladder, and pancreas, which produce essential digestive juices and substances but are not part of the alimentary tract. Their functions significantly enhance digestion and nutrient absorption.

Major Subdivisions of the Digestive Tract

• Oral Cavity:

  • Involves mechanical processing through mastication, moistening food, and mixing it with salivary secretions which contain enzymes like salivary amylase for starch digestion. The oral cavity is the first site of both mechanical and chemical digestion.

• Pharynx:

  • A muscular tube that plays a critical role in swallowing by propelling food from the mouth into the esophagus, ensuring the food bolus is directed appropriately into the digestive system.

• Esophagus:

  • A muscular tube responsible for transporting the food bolus to the stomach through peristalsis, a series of wave-like muscle contractions. It connects the pharynx to the stomach and can be the site of various disorders like GERD (Gastroesophageal Reflux Disease).

• Stomach:

  • The stomach's functions include the mechanical mixing of food and the chemical breakdown through gastric juices, which contains hydrochloric acid (HCl) and digestive enzymes. The stomach has four main regions (cardia, fundus, body, and pyloric part) that facilitate its diverse functions.

• Small Intestine:

  • Divided into three sections—duodenum, jejunum, and ileum—this segment plays a significant role in enzymatic digestion and nutrient absorption. It features structures like circular folds and villi, which increase surface area for efficiency.

• Large Intestine:

  • Responsible for the dehydration and compaction of indigestible materials, leading to waste formation. It includes the cecum (with appendix), colon, and rectum. The large intestine also serves as a site for bacterial fermentation and plays a role in absorbing vitamins.

• Anus:

  • The final part of the digestive system, vital for excretion. It consists of two anal sphincters (internal and external), which regulate the expulsion of feces from the body.

Accessory Organs

• Salivary Glands:

  • Three main pairs of glands (parotid, submandibular, and sublingual) produce saliva, which is essential for lubricating food, aiding in taste, and beginning the digestion of carbohydrates.

• Liver:

  • Produces bile, which is essential for fat digestion and absorption, and has roles in metabolism, detoxification, and nutrient storage, including vitamins and minerals.

• Gallbladder:

  • Stores and concentrates bile produced by the liver until it is needed in the small intestine, particularly during the digestion of fats.

• Pancreas:

  • Has both exocrine and endocrine functions. The exocrine component secretes digestive enzymes and buffers to regulate pH in the small intestine, while the endocrine portion releases hormones like insulin and glucagon into the bloodstream.

Mechanical Contraction

• Peristalsis:

  • A coordinated muscular contraction that propels the bolus of food through the digestive tract, involving both circular and longitudinal muscles to ensure unidirectional movement.

• Segmentation:

  • A contraction type specific to the small intestine that mixes the contents without propelling them in one direction. It enhances digestion and absorption by increasing contact between the food and the intestinal walls.

Oral Cavity

• Primary Function:

  • The oral cavity is primarily responsible for mastication (chewing) and the initiation of the digestive process through mechanical and enzymatic actions.

• Food Mixes with Saliva:

  • Saliva contains enzymes (like salivary amylase for starch digestion), mucus for lubrication, and antimicrobial agents (like lysozyme) to protect against pathogens.

Oral Cavity Anatomy

• Key anatomical structures include the hard palate, soft palate, frenulum of the upper lip, fauces, and the various arches (palatoglossal and palatopharyngeal) that facilitate swallowing and contribute to oral function.

• The anatomy also includes the tongue (dorsum and root), oral vestibule, and openings of submandibular ducts, which play vital roles in food manipulation and taste.

Deglutination (Swallowing)

• Oral Phase:

  • A voluntary phase where food is formed into a bolus and pushed into the oropharynx in preparation for swallowing.

• Pharyngeal/Esophageal Phase:

  • An involuntary phase where the bolus moves through the pharynx into the esophagus. The epiglottis covers the larynx to prevent aspiration of food and support proper passage into the digestive tract.

Esophagus

• A muscular tube that facilitates the transport of the bolus from the mouth to the stomach. It extends from the cervical spine at C6 to the thoracic area at T7 and features the lower esophageal sphincter, which prevents reflux and manages the passage of food into the stomach.

Clinical Conditions

• GERD (Gastroesophageal Reflux Disease):

  • A chronic condition characterized by the reflux of stomach acid into the esophagus, leading to inflammation, discomfort, and potential esophageal damage due to a malfunctioning lower esophageal sphincter (LES).

• Hiatal Hernia:

  • A condition wherein a portion of the stomach protrudes through the esophageal hiatus of the diaphragm, potentially resulting in GERD symptoms and complications due to pressure changes in the abdominal cavity.

Esophagus Muscular Layer

• Comprises inner circular and outer longitudinal layers that aid in the peristaltic movement of the bolus. Muscle types vary along the length:

  • Upper 1/3: Composed entirely of skeletal muscle for voluntary swallowing.

  • Middle 1/3: A combination of skeletal and smooth muscle.

  • Lower 1/3: Composed entirely of smooth muscle for involuntary propulsion.

Stomach

• Four Anatomical Regions:

  • Cardia: The entry point where the esophagus connects to the stomach.

  • Fundus: A dome-shaped region above the cardia.

  • Body: The largest part of the stomach, where the main digestive activity occurs.

  • Pyloric Part/Pylorus: The funnel-shaped region leading to the duodenum, which regulates the passage of chyme through the pyloric sphincter.

Stomach Anatomy

• Notable structures include the pyloric sphincter, pyloric orifice, and complex musculature comprised of three layers of muscle (longitudinal, circular, and oblique), which allow for both mixing and churning of food.

Stomach - Internal Anatomy

• Gastric Rugae are folds that allow for expansion as the stomach fills with food. The stomach features robust muscular layers facilitating mechanical digestion via churning action. The pyloric sphincter marks the border between the stomach and small intestine and controls the release of chyme.

Functions of Stomach

• Food Storage: Temporarily holds food, allowing for time to process and digest it.

• Protein Digestion: Begins with the activation of pepsinogen to pepsin under acidic pH, aiding in protein breakdown.

• Bacterial Control: The acidic environment kills many ingested pathogens.

• Chyme Movement: Regulates the rate at which chyme enters the small intestine.

• Secretion of Gastric Juices: Includes enzymes and HCl necessary for digestion.

• Limited Absorption: Primarily absorbs alcohol and some medications.

Gastric Secretions

• Gastric pits are indentations in the stomach lining containing different types of cells, including:

  • Parietal Cells: Secrete HCl for acidity and intrinsic factor for vitamin B12 absorption.

  • Chief Cells: Secrete pepsinogen, which is activated to pepsin in acidic conditions for protein digestion.

  • Enteroendocrine Cells: Include G cells that secrete gastrin to promote more acid and enzyme secretion, and D cells that produce somatostatin to inhibit these actions.

Functions of HCl

• Key to maintaining a low pH (pH = 1-2) in the stomach, denaturing proteins, and activating pepsinogen to pepsin, essential for protein digestion.

Gastric Secretion Mechanism

• The enzyme carbonic anhydrase facilitates the formation of bicarbonate, with chloride ions entering cells against their gradient to exchange for bicarbonate. Bicarbonate then exits into the bloodstream, creating an alkaline tide. Chloride subsequently moves into the stomach lumen through facilitated diffusion, contributing to stomach acidity.

Gastric Secretion - Parietal Cells

• Parietal cells on the gastric epithelium secrete H+ ions and chloride ions to produce gastric acid. The H^+/K^+ ATPase actively pumps H+ ions out against their concentration gradient, with chloride ions diffusing passively to form HCl in the gastric lumen.

Stomach Protection

• The gastric epithelium is protected by a thick layer of mucus secreted by goblet cells, which contains bicarbonate that neutralizes HCl and serves as a barrier against pepsin. Tight junctions between epithelial cells prevent acid and enzymes from leaking into the tissue, while epithelial cells are replaced every 3 days to maintain a healthy lining.

Clinical: Peptic Ulcer

• A peptic ulcer is an erosion in the stomach or duodenal lining caused by various factors, including H. pylori infection and excessive acid production. Symptoms may include pain, bleeding, and discomfort.

Gastric Secretion Regulation

• Gastric secretion is regulated by neural and hormonal factors.

  • Stimuli: Hormone gastrin and the vagus nerve (via acetylcholine) stimulate the release of histamine from ECL cells, enhancing acid secretion.

Gastric Secretion Regulation - Cellular Level

• Synchronization between the vagus nerve and gastric cells ensures effective digestion, with G cells secreting gastrin, histamine production from ECL cells, and consequent acid secretion from parietal cells.

Small Intestine

• The small intestine, measuring approximately 6 meters (20 feet), is structured to maximize nutrient absorption through circular folds (plicae circulares) and villi. Each of the three sections—duodenum, jejunum, and ileum—plays distinct roles in digestion and absorption.

Intestinal Villi

• These fingerlike projections lined with microvilli significantly increase the surface area for absorption. Each villus contains a lacteal, which is a lymphatic vessel responsible for the absorption of long-chain fatty acids and fat-soluble vitamins.

Layers of the Small Intestine

• The small intestine has a four-layer structure:

  • Mucosa: Contains intestinal villi, goblet cells for mucus secretion, and epithelial cells for nutrient absorption.

  • Submucosa: Contains blood vessels, lymphatic vessels, nerves, and connective tissue that supports the intestinal structure and function.

  • Muscular Layer: Composed of circular and longitudinal smooth muscle layers, working in coordination for peristalsis and segmentation.

  • Serosa: The outermost layer providing protection and structural support.

Brush Border Enzymes

• Brush border enzymes located on the microvilli enhance digestion by breaking down carbohydrates and proteins. An example is enterokinase, which activates the pancreatic enzyme trypsinogen, critical for protein digestion.

Clinical: Lactose Intolerance

• A condition resulting from a deficiency in lactase, the enzyme responsible for lactose digestion. Symptoms include diarrhea, gas, and cramps after consuming dairy products. Yogurt is often better tolerated due to its lower lactose content and the presence of live bacteria that can assist in digestion.

Intestine Motility & Action Potential

• The movement of food through the intestine is regulated by electrical signaling. Depolarization occurs when voltage-gated Ca^{2+} channels open, leading to muscle contraction. Repolarization involves the outward flow of K^+ ions, returning the membrane potential to its resting state. This coordinated electrical activity is essential for digestive motility.

Large Intestine

• About 1.5 meters (5 feet) long, the large intestine serves as the final site for water, electrolyte, and vitamin absorption, facilitating the formation of solid waste. It harbors a vast microbiota, which aids in fermenting undigested materials and producing essential vitamins like vitamin K. The large intestine includes structures such as the cecum (with appendix), colon, and rectum.

Clinical: Appendicitis

• Appendicitis is characterized by inflammation of the appendix, usually presenting with pain at McBurney's Point, which is located two-thirds of the way from the umbilicus to the anterior superior iliac spine (ASIS). It often requires surgical intervention.

Defecation

• The process of elimination involves increased pressure in the rectum due to accumulated waste material, inducing contraction of abdominal and pelvic muscles, which raises intra-abdominal pressure. Coordination of internal (involuntary) and external (voluntary) anal sphincter relaxation leads to the expulsion of feces from the body, completing the digestive process.