CHAPTER 23: LECTURE B

The Digestive System: Part B Study Notes

Pharynx

  • Structure and Function:

    • Connects food passage from mouth to oropharynx to laryngopharynx.

    • Allows passage for food, fluids, and air.

    • Lined with stratified squamous epithelium which is adapted for durability.

    • Contains mucus-producing glands for lubrication.

    • Composed of skeletal muscle in two layers:

    • Inner longitudinal layer.

    • Outer pharyngeal constrictors.

Esophagus

  • Structure and Function:

    • A flat muscular tube connecting laryngopharynx to the stomach.

    • Passes through the diaphragm at the esophageal hiatus.

    • Joins stomach at the cardial orifice.

    • Surrounded by gastroesophageal (cardiac) sphincter which controls passage into the stomach.

Esophageal Histology

  • Mucosa: Stratified squamous epithelium transitioning to simple columnar at the stomach junction.

  • Glands:

    • Esophageal glands in the submucosa secrete mucus to facilitate bolus movement.

  • Muscularis:

    • Consists of skeletal muscle in the upper part, mixed muscle in the middle, and smooth muscle in the lower part.

  • Adventitia: Instead of serosa, the esophagus contains an adventitia layer.

Homeostatic Imbalance: Heartburn

  • Description:

    • Occurs when stomach acid regurgitates into the esophagus.

    • Can occur due to excessive food/drink intake, obesity, pregnancy, or running.

    • Also associated with a hiatal hernia, a structural abnormality of the stomach portion above the diaphragm.

    • Potential consequences: esophagitis, esophageal ulcers, and esophageal cancer.

Digestive Processes in the Mouth

  • Major Functions:

    • Ingestion: Introduction of food into the body through the mouth.

    • Mechanical Breakdown: Done through chewing (mastication).

    • Propulsion: Deglutition (swallowing) moves food towards the esophagus.

    • Digestion: Enzymatic breakdown begins with salivary amylase and lingual lipase.

    • Absorption: Minimal absorption occurs here, largely limited to a few drugs.

Mastication (Chewing)

  • Function: Holds food between teeth using cheeks and closed lips.

  • Role of Tongue:

    • Mixes food with saliva;

    • Compacts the mixed food into a bolus for swallowing.

  • Teeth Function: Cut and grind food.

  • Control: Partly voluntary involving conscious effort and partly reflexive due to stretch reflexes and pressure receptors in cheeks, gums, and tongue.

Deglutition (Swallowing)

  • Components: Involves coordination of 22 muscle groups.

  • Phases:

    • Buccal Phase: This is voluntary. The tongue contracts to move food into the oropharynx.

    • Pharyngeal-Esophageal Phase: Involuntary phase mainly controlled by the vagus nerve.

    • Control Center: Located in the medulla and lower pons.

Mechanism of Swallowing

  1. Uvula and larynx rise to prevent food from entering the respiratory tract.

  2. The tongue blocks off the mouth.

  3. The upper esophageal sphincter relaxes, allowing food to enter the esophagus.

  4. The constrictor muscles of the pharynx contract to push food down into the esophagus.

  5. The gastroesophageal sphincter opens to allow food into the stomach.

Stomach Anatomy

  • Location: Upper left quadrant of the abdomen.

  • Functions: Temporary storage of food; digestion converts bolus to chyme.

  • Parts:

    • Cardial Part: Surrounds the cardial orifice.

    • Fundus: Dome-shaped region beneath the diaphragm.

    • Body: Midportion of the stomach.

    • Pyloric Part: Divided into antrum (superior) and pyloric canal leading to the pylorus, which is continuous with the duodenum through the pyloric valve controlling stomach emptying.

Exterior Structure

  • Curvatures:

    • Greater Curvature: Convex lateral surface.

    • Lesser Curvature: Concave medial surface.

  • Mesenteries: Include lesser omentum (from liver to lesser curvature) and greater omentum (contains fat deposits and lymph nodes)

Nerve and Blood Supply

  • ANS Supply:

    • Sympathetic: From thoracic splanchnic nerves via celiac plexus.

    • Parasympathetic: Through the vagus nerve.

  • Blood Supply:

    • From the celiac trunk (gastric and splenic branches).

    • Venous drainage through the hepatic portal system.

Microscopic Anatomy of the Stomach

  • Four Tunics:

    • Mucosa: Contains simple columnar epithelium consisting of mucous cells.

    • Function: Secretion of a two-layer coat of alkaline mucus to protect against stomach acid.

    • Muscularis Externa: Modified to have three layers of smooth muscle, including an inner oblique layer that enables the stomach to churn and mix food.

Gastric Glands and Secretions

  • Types of Cells in Gastric Glands:

    • Mucous Neck Cells: Unknown function; secrete thin, acidic mucus.

    • Parietal Cells:

    • Secretes hydrochloric acid (HCl) which denatures proteins, activates pepsin, and kills bacteria.

    • Produces intrinsic factor, crucial for vitamin B12 absorption.

    • Chief Cells: Secrete pepsinogen (inactive enzyme) and lipases that digest ~15% of lipids.

    • Enteroendocrine Cells: Secrete hormones such as serotonin and gastrin involved in digestive regulation.

Protective Mechanisms

  • Mucosal Barrier: Protects stomach lining from harsh acidic conditions.

    • Thick bicarbonate-rich mucus layer prevents acid from seeping into tissues.

    • Tight junctions between epithelial cells to maintain integrity.

    • Stem cells rapidly replace damaged epithelial cells every 3-6 days.

Homeostatic Imbalance: Gastritis and Ulcers

  • Gastritis: Inflammation of the gastric mucosa due to breach in mucosal barrier.

  • Peptic Ulcers: Erosions in the stomach wall, can result in peritonitis or hemorrhage and are often caused by Helicobacter pylori bacteria.

Digestive Processes in the Stomach

  • Mechanical Breakdown: Churning food and mixing with gastric secretions.

  • Denaturation: HCl denatures proteins facilitating digestion.

  • Enzymatic Digestion: Pepsin digests proteins; rins in infants (rennin) digests milk protein.

  • Alcohol and Aspirin Absorption: Some lipid-soluble substances directly absorbed into the bloodstream.

  • Essential Stomach Function: Secretion of intrinsic factor necessary for vitamin B12 absorption; lack causes pernicious anemia treated with B12 injections.

Regulation of Gastric Secretion

  • Daily Secretion Volumes: Gastric mucosa secretes up to 3 liters of gastric juice daily.

  • Nervous and Hormonal Controls:

    • Vagus nerve enhances secretion.

    • Sympathetic stimulation reduces secretion.

    • Hormones (specifically gastrin) stimulate enzyme and HCl secretion.

Phases of Gastric Secretion

  1. Cephalic Phase: Triggered by aroma, taste, sight, thought of food even before eating.

  2. Gastric Phase: Lasts 3-4 hours with ⅔ gastric juice released; stimulated by stomach distension, uction of peptides, and rising pH which triggers gastrin release.

  3. Intestinal Phase: Regulates gastric secretion in response to chyme entering small intestine; inhibitory effects from enterogastric reflex and secretions like secretin and CCK.

Factors Influencing Gastric Secretion

  • 4 Chemical Stimuli: Acetylcholine (ACh), histamine, and gastrin are necessary for maximum HCl secretion. Low pH inhibits gastrin release.

Mechanism of HCl Formation

  • H+ ions are pumped into the stomach lumen while K+ enters the parietal cell to balance charge.

  • HCO3- ions exchange into the blood leading to an "alkaline tide".

  • Chloride ions (Cl-) follow H+ ions to combine and form HCl in the stomach lumen.

Gastric Emptying Regulation

  • Upon chyme entry into the duodenum, stretch receptors and chemical signals trigger enterogastric reflex which inhibits gastric secretion and limits duodenal filling.

  • Carbohydrate-rich chyme is expelled faster than fatty chyme which may remain for hours.

Homeostatic Imbalance: Vomiting (Emesis)

  • Causes: Extreme stretching, irritants, toxins, etc. Lead to dehydration and electrolyte imbalances if excessive.

Small Intestine Anatomy

  • Function and Structure: Major organ for digestion and nutrient absorption, 2-4 m long, extends from pyloric sphincter to ileocecal valve with three subdivisions: duodenum, jejunum, and ileum.

Duodenum

  • Shortest segment (25 cm) curves around the pancreas. It receives bile and pancreatic juice at the major duodenal papilla controlled by the hepatopancreatic sphincter.

Jejunum and Ileum

  • Jejunum: About 2.5 m long. Ileum: Joins the large intestine at the ileocecal valve, roughly 3.6 m long.

Structural Modifications for Absorption

  • Circular Folds (Plicae Circulares): Permanent folds forcing chyme to swirl for enhanced absorption.

  • Villi: Finger-like projections containing capillaries and lacteals for nutrient absorption.

  • Microvilli: Extremely small projections also known as the brush border, containing enzymes for digestion.

Intestinal Crypts

  • Contain secretory cells, enteroendocrine cells, intraepithelial lymphocytes (IELs) to target infected cells, and Paneth cells for antimicrobial agents.

Mucosa Functionality

  • Peyer's patches protect against bacteria, specifically in distal segments. The duodenal glands help neutralize the acidic chyme entering from the stomach.

Intestinal Juice Secretion

  • Volume: Daily secretion of 1-2 L in response to mucosa distension.

  • Composition: Alkaline, isotonic, mainly water, but enzyme-poor, serving to facilitate absorption rather than direct enzymatic digestion.

The Liver and Gallbladder

Liver

  • Structure: Largest gland in the body; consists of four lobes: right, left, caudate, and quadrate.

  • Falciform Ligament: Separates left and right lobes and attaches to abdominal wall.

  • Round Ligament: Fetal remnant of the umbilical vein.

  • Functions: Bile production, detoxification, blood nutrient processing, vitamin storage, and more.

Liver Microscopy

  • Hepatocyte Function: Process, detoxify and produce bile (about 900 ml/day). Hepatic lobules structure with central veins and portal triads (bile duct, hepatic artery, and portal vein).

Homeostatic Imbalance

  • Hepatitis: Inflammation usually due to infection or toxins.

  • Cirrhosis: Chronic inflammation leading to liver damage and difficulties in blood flow.

Bile Production

  • Composition: Yellow-green solution containing bile salts for fat emulsification, bilirubin from hemoglobin breakdown, cholesterol, and electrolytes.

  • Recycling: Enterohepatic circulation reabsorbs bile salts from the intestine back to the liver.

Gallbladder

  • Stores and concentrates bile; releases bile into the bile duct upon muscular contraction.

  • Gallstone Formation: Results from excess cholesterol or insufficient bile salts, leads to pain and potential obstructive jaundice.