Digestive System Functions and Histology

Digestive System Functions

• The digestive system breaks down food into absorbable organic molecules (carbohydrates, fats, proteins), absorbs nutrients, and eliminates wastes.
• The digestive tract lining protects the body from corrosive substances and ingested pathogens.
• Ingestion: Food intake into the oral cavity.
• Digestion and propulsion involve mechanical churning (mechanical digestion aiding chemical digestion) and chemical breakdown by enzymes and digestive juices.
• Smaller molecules like glucose absorb easily, but larger molecules (proteins) need enzyme breakdown.
• Propulsion moves material through the digestive tract.
• Secretions provide enzymes and substances for chemical digestion.
• Absorption takes up organic/inorganic nutrients and water.
• Defecation eliminates waste products.

Histology of the Digestive Tract

• The digestive tract has four major layers:
  • Mucosa: Innermost layer with epithelium (simple or stratified), lamina propria (connective tissue with vessels/nerves), and muscularis mucosae (smooth muscle).
  • Submucosa: Dense irregular connective tissue binding mucosa to muscular layer; contains submucosal nerve plexus. Rich in blood and lymphatic vessels.
  • Muscular layer: Smooth muscles (circular and longitudinal) for mechanical digestion; contains myenteric nerve plexus.
  • Serosa: Serous membrane (visceral peritoneum) covering most parts; adventitia supports areas outside the peritoneum.

Peritoneum and Mesenteries

• The peritoneal cavity is lined by the peritoneum (serous membrane).
• Two parts: Visceral peritoneum (covers organs), parietal peritoneum (lines abdominal/pelvic cavity walls).
• Peritoneal fluid reduces friction.
• Retroperitoneal structures lie behind the peritoneum.
• Mesenteries are double-layered serous membranes stabilizing organs and associated vessels.
  • Lesser omentum: Connects stomach to liver.
  • Greater omentum: Connects stomach to intestines, functions as a protective sheet.
  • Mesentery proper & Mesocolon.

Control of Gastric Motility

• Gastric motility (movement) is controlled by:
  • Local factors (stretch, pH change).
  • Neural control (enteric nervous system, parasympathetic nervous system).
  • Hormonal influences (e.g., ghrelin, cholecystokinin).
• Pacesetting cells initiate rhythmic contractions (peristalsis) moving food through the tract.
• Segmentation churns food as it moves.

Oral Cavity

• The oral cavity (mouth) is lined with oral mucosa (stratified squamous epithelium).
• Functions:
  • Sensation, mechanical digestion (mastication), lubrication, and chemical digestion (salivary amylase).
• The uvula helps prevent food from entering the nasal passages.
• Some lipid-soluble drugs (e.g., nitroglycerin) can be absorbed through the tongue's mucosa.

Tongue

• Functions: Mechanical digestion, food manipulation, sensory analysis (taste, temperature), secretion of mucins and lingual lipase.
• Lingual papillae are projections on the tongue's surface.
• Lingual frenulum can affect tongue movement.

Teeth

• Teeth (or dentition) consist primarily of dentin.
• Enamel protects biting surfaces.
• Two sets: Deciduous (baby teeth) and permanent (adult teeth).
• Dental health affects overall health.

Saliva and Salivary Glands

• Saliva lubricates, moistens, dissolves chemicals for taste, and initiates digestion (salivary amylase).
• Saliva cleanses the mouth.
• Three major salivary glands: Parotid (salivary amylase), sublingual (mucus), and submandibular (mixture of mucus, buffers and salivary amylase).

From Oral Cavity to Stomach

• Food forms a bolus in the oral cavity.
• Pharyngeal muscles propel food to the esophagus.
• Epiglottis prevents food from entering the trachea.

Esophagus

• Approx. 25 cm long, tube carrying food.
• Passes through the esophageal hiatus in the diaphragm.
• Lined with stratified squamous epithelium.
• Folded mucosa/submucosa for expansion during swallowing.

Swallowing (Deglutition)

• Three phases: Buccal, pharyngeal, and esophageal.
• Swallowing reflex is initiated by sensory receptors in the mouth and pharynx.

Stomach

• Located in the left upper quadrant.
• J-shaped organ for food storage & digestion.
• Rugae allow for expansion.
• Mechanical and chemical digestion occur.
• Chyme (mixed food and gastric secretions) is formed.
• Four regions: Cardia, fundus, body, pylorus.

Gastric Secretions

• Gastric glands secrete products into gastric pits
  • Chief cells: Pepsinogen (activated to pepsin).
  • Parietal cells: HCl (activates pepsin, kills microbes), Intrinsic factor (Vitamin B12 absorption).
  • Enteroendocrine cells: Gastrin, histamine, somatostatin, and ghrelin (regulate stomach function).

Chemical Digestion in Stomach

• Salivary amylase and lingual lipase continue until pH falls below 4.5.
• Protein digestion begins when pH reaches ~2.0.
• Little nutrient absorption in stomach; some drugs (alcohol, aspirin) can be absorbed.

Regulation of Digestion

• Three phases of gastric control overlap:
  • Cephalic phase (brain/sensory input).
  • Gastric phase (distention, increased pH, protein).
  • Intestinal phase (controls emptying rate, inhibits gastric motility).

Small Intestine

• Extends from pyloric sphincter to ileocecal valve.
• Divided into duodenum, jejunum, and ileum.
• Circular folds, villi, and microvilli increase absorptive surface area.
• Secretions include mucus and brush border enzymes (maltase, sucrase, lactase, etc).
• Peyer's patches (lymphoid tissue) are abundant in the ileum, part of the digestive system’s first line of defence against pathogens entering through the digestive tract.

Small Intestine (Continued)

• Secretion from intestinal glands include pancreatic and brush border enzymes. Lacteals absorb lipids.
• Interstitial hormones (gastrin, secretin, GIP, CCK, VIP, enterocrinin) regulate small intestine function and other digestive organs.

The Pancreas

• Endocrine (insulin, glucagon) and exocrine (pancreatic juices) functions.
• Pancreatic juices are an alkaline mix of enzymes, water, and ions.
• Enzymes break down food; fluids dilute and neutralize chyme.
• Pancreatic duct delivers secretions to the duodenum.

The Liver

• Largest visceral organ.
• Divided into lobules with hepatocytes.
• Blood supply from hepatic artery and hepatic portal vein.
• Function:
• Metabolic regulation, drug metabolism, hematological regulation (plasma protein, hormone/antibody removal), bile regulation.

Bile Duct System

• Bile flows from liver canaliculi to larger ducts, then leaves the liver by the right and left hepatic ducts which unit at the common hepatic duct, and through the cystic duct to the gallbladder for storage.
• Bile salts aid lipid digestion and absorption.

Gallbladder

• Stores and concentrates bile.
• CCK stimulates gallbladder contraction to release bile into the duodenum for lipid digestion and absorption.

Large Intestine

• Absorbs water; forms and expels feces.
• Divided into cecum, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal.
• Lacks villi; contains abundant goblet cells for lubrication.
• Microbiome (bacteria) aids in vitamin synthesis.
• Defecation reflex controls elimination.

General Digestive Reminders

• Hydrolysis breaks down food molecules into smaller units (e.g., carbohydrates to monosaccharides).
• Lipids absorb as triglycerides in chylomicrons.

Immunity & the Digestive Tract

• The digestive tract has layers that have a protective function (mouth, rectum); saliva contains antibodies and lysosomes; lymphatic tissue (tonsils, Peyer's patch, appendix and Paneth cells) play a role in immunity.

Age-Related Changes

• Epithelial cell division declines; decreased smooth muscle tone weakens peristalsis and increases risk for injury and can lead to increased risk for cancer and dehydration.

Explore the key functions of the digestive system, including ingestion, digestion, absorption, and elimination of wastes. Learn about the histological layers of the digestive tract and their roles in protecting the body and aiding in digestion.