Lecture Notes on Digestion

Chapter 24 - Digestion

General Overview and Course Information

  • Professor: Dr. K. LaConte, ND

  • Institutional Information: Housatonic Community College, 900 Lafayette Blvd, Bpt CT 06604

  • Important Note: There are no makeups for final exams (lab or lecture); grades are submitted a few days later.

Introduction to the Digestive System

Learning Outcomes

  • Identify the organs of the digestive system, list their major functions, describe the functional histology of the digestive tract, and outline the mechanisms that regulate digestion.

  • Discuss the anatomy of the oral cavity and list the functions of its major structures and regions.

  • Describe the structure and functions of the pharynx and esophagus.

  • Describe the anatomy of the stomach, including its histological features, discuss its roles in digestion and absorption.

  • Describe the anatomical and histological characteristics, functions and regulation of intestinal secretions, of the small intestine.

  • Describe the structure, functions, and regulation of the accessory digestive organs.

The Digestive System

Basic Functions

  • The main objective is to acquire nutrients from the environment through two chemical processes:
      - Anabolism: Utilizes raw materials to synthesize essential compounds.
      - Catabolism: Breaks down substances to provide energy that cells need to function.

Catabolic Reactions

  • Require two essential ingredients:
      - Oxygen
      - Organic molecules that are broken down by intracellular enzymes (e.g., carbohydrates, fats, proteins).

Digestive Tract Anatomy

Overview

  • Also called the gastrointestinal (GI) tract or alimentary canal.

  • It is a muscular tube extending from the oral cavity to the anus.

  • It includes the following structures in sequence:
      - Pharynx
      - Esophagus
      - Stomach
      - Small intestine
      - Large intestine

Six Functions of the Digestive System

  1. Ingestion: Entry of materials into the digestive tract via the mouth.

  2. Mechanical Processing: Involves crushing and shearing, making materials easier to propel along the digestive tract.

  3. Digestion: The chemical breakdown of food into smaller organic fragments for absorption by digestive epithelium.

  4. Secretion: Release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by glandular organs.

  5. Absorption: Movement of organic substrates, electrolytes, vitamins, and water across the digestive epithelium and into the interstitial fluids of the digestive tract.

  6. Excretion: Removal of waste products from body fluids, primarily accomplished through defecation to remove feces.

Lining of the Digestive Tract

  • Safeguards surrounding tissues against:
      - Corrosive effects of digestive acids and enzymes.
      - Mechanical stresses, such as abrasion.
      - Bacteria ingested with food or residing in the digestive tract.

Peritoneal Fluid

  • Produced by the serous membrane lining, it provides lubrication and separates parietal and visceral surfaces, allowing sliding without friction or irritation.

  • Ascites: Condition of excess peritoneal fluid causing abdominal swelling.

Mesenteries

  • Dorsal Mesentery: Enlarges to form the greater omentum, extending inferiorly between the body wall and anterior surface of the small intestine.

  • Adipose Tissue in Greater Omentum: Conforms to the shapes of surrounding organs, pads, and protects abdominal surfaces, provides insulation, and stores lipid energy reserves.

  • Lesser Omentum: Stabilizes the position of the stomach and provides access for blood vessels and structures entering or leaving the liver.

  • Mesentery Proper: A thick mesenterial sheet providing stability, permitting some independent movement, and suspending all but the first 25 cm (10 in.) of small intestine.

  • Mesocolon: A mesentery associated with portions of the large intestine, supporting the transverse and sigmoid colon.

  • During development, the mesocolon of ascending colon, descending colon, and rectum fuse to the dorsal body wall, locking regions in place.

Histological Organization of the Digestive Tract

  • The digestive tract is composed of four major layers:
      1. Mucosa
      2. Submucosa
      3. Muscularis Externa
      4. Serosa

Digestive Epithelium

  • The oral pharynx, esophagus, vagina, anus, and lips are lined by non-keratinized stratified squamous epithelium for protection against mechanical stresses. This type of tissue needs to remain moist.

  • The stomach, small intestine, and most of the large intestine are lined by simple columnar epithelium with mucous cells.

Lamina Propria and Muscularis Mucosae

  • Lamina Propria: Contains a narrow band of smooth muscle and elastic fibers.
      - Muscularis Mucosae: Arranged in two concentric layers; the inner layer encircles the lumen (circular muscle) and the outer layer contains muscle cells parallel to the tract (longitudinal layer).

Submucosa

  • A layer of dense, irregular connective tissue surrounding the muscularis mucosae which contains large blood vessels, lymphatic vessels, and may have exocrine glands that secrete buffers and enzymes into the digestive tract.

Submucosal Plexus

  • Also known as the plexus of Meissner.

  • Innervates the mucosa and submucosa, consisting of:
      - Sensory neurons
      - Parasympathetic ganglionic neurons
      - Sympathetic postganglionic fibers.

Muscularis Externa

  • Composed of smooth muscle cells arranged in:
      - Inner circular layer
      - Outer longitudinal layer

  • Involved in mechanical processing and movement of materials along the digestive tract, coordinated by the enteric nervous system (ENS).

Serosa

  • A serous membrane covering the muscularis externa, except in the oral cavity, pharynx, esophagus, and rectum, where dense collagen fibers firmly attach the digestive tract to adjacent structures.

Movement Through the Alimentary Tract

  • The movement of digestive materials is conducted by muscular layers of the digestive tract, consisting of visceral smooth muscle tissue which has rhythmic cycles of activity facilitating contraction waves throughout the entire muscular sheet.

Peristaltic Motion

  • Defined as the contraction of circular muscles behind the bolus while those ahead of it relax; longitudinal muscles ahead of the bolus contract, shortening adjacent segments, thus forcing the bolus forward.

Control of Digestive Functions

  • Local Factors: Involve chemicals such as prostaglandins and histamine released into interstitial fluid affecting cells within a small segment of the digestive tract.

The Oral Cavity

Lining

  • Lined with stratified squamous epithelium, the cheeks, lips, and inferior surface of the tongue are relatively thin, non-keratinized, and delicate with muscles supported by fat pads.

Major Structures and Their Functions

  • Labia (Lips): Continuous with mucosa of the cheeks.

  • Vestibule: Space between the cheeks (or lips) and the teeth.

  • Gingivae (Gums): Ridges of oral mucosa surrounding the base of each tooth.

The Tongue

  • Important for manipulating materials inside the mouth, performing several functions:
      - Mechanical processing via compression, abrasion, and distortion.
      - Manipulation of food to aid in chewing/preparation for swallowing.
      - Sensory analysis via touch, temperature, and taste receptors.
      - Secretion of mucins and lingual lipase (an enzyme).

Salivary Glands

  • Three pairs secrete saliva into the oral cavity:
      1. Parotid Salivary Glands: Produces serous secretion containing salivary amylase (enzyme to break down starches). Drains into vestibule near second molar.
      2. Sublingual Salivary Glands: Located under the tongue; produce mucous secretion as buffer and lubricant.
      3. Submandibular Salivary Glands: Secretes buffers, glycoproteins, and salivary amylase. Ducts open behind the teeth on each side of the lingual frenulum.

Composition of Saliva

  • Volume: Glands produce 1.0–1.5 liters of saliva daily:
      - 70% from submandibular glands
      - 25% from parotid glands
      - 5% from sublingual glands

  • Composition:
      - 99.4% water
      - 0.6% includes:
        - Electrolytes (Na+, Cl-, HCO3-)
        - Buffers
        - Glycoproteins (mucins)
        - Antibodies
        - Enzymes
        - Waste products.

Teeth

  • Functions of Tongue Movements: Pass food across the occlusal surfaces of teeth; associated with chewing (mastication).
      - Dentin: A mineralized matrix similar to bone but without cells.
      - Pulp Cavity: Contains blood vessels and nerves via the root canal.

Tooth Anatomy

  • Root: Sits in a bony socket (alveolus) and is covered by cementum, which protects the dentin and anchors the periodontal ligament.

  • Crown: The exposed portion of the tooth beyond the soft tissue of the gingiva, with the dentin covered by enamel.

  • Types of Teeth:
      - Incisors: Blade-shaped, used for cutting, with a single root.
      - Cuspids (Canines): Conical, sharp points for tearing and slashing, possessing a single root.
      - Bicuspids (Premolars): Flattened crowns with prominent ridges for crushing, having one or two roots.
      - Molars: Large, flat crowns for crushing and grinding with three or more roots.

Dental Succession

  • Primary Dentition: 20 temporary teeth (deciduous teeth) that include:
      - 5 on each side of upper and lower jaws; 2 incisors, 1 cuspid, and 2 deciduous molars.

  • Secondary Dentition: 32 permanent teeth that replace primary teeth, consisting of:
      - 8 on each side of upper and lower jaws; 2 incisors, 1 cuspid, and 5 molars.

The Pharynx

  • The pharynx (throat) serves as a common passageway for food, liquids, and air with three regions:
      1. Nasopharynx
      2. Oropharynx
      3. Laryngopharynx

  • Food passes through the oropharynx and laryngopharynx to reach the esophagus.

The Esophagus

Overview

  • A hollow muscular tube approximately 10 inches wide that conveys food and liquids to the stomach.

  • Begins posterior to the cricoid cartilage and enters the abdominopelvic cavity through the esophageal hiatus.

Resting Muscle Tone

  • The circular muscle layer in the superior 3 cm (1.2 in.) of the esophagus prevents air from entering it.

Histology of the Esophagus

  • Composed of three layers:
      1. Mucosal Layer: Contains non-keratinized, stratified squamous epithelium.
      2. Submucosal Layer: Forms large folds extending along the esophagus.
      3. Muscularis Layer: Comprised of irregular smooth muscle.

Swallowing (Deglutition)

  • Can be initiated voluntarily, proceeding automatically, divided into three phases:
      1. Buccal Phase
      2. Pharyngeal Phase
      3. Esophageal Phase

The Stomach

Functions of the Stomach

  • Major functions include:
      - Storage of ingested food.
      - Mechanical and chemical breakdown of food materials through acids and enzymes.
      - Production of intrinsic factor required for vitamin B12 absorption in the small intestine.

Anatomy of the Stomach

  • Shape: Expanded J, with:
      - Short lesser curvature on the medial surface.
      - Long greater curvature on the lateral surface.
      - Anterior and posterior surfaces are rounded; shape varies by individual and meal.

  • Typically expands between vertebrae T7 and L3.

Structure

  • Parts of the stomach include:
      - Cardia
      - Fundus
      - Body
      - Pylorus

Muscular Layers

  • Muscularis mucosae and muscularis externa contain an additional oblique layer of smooth muscle cells in addition to circular and longitudinal layers, enhancing mechanical processing.

Histology of the Stomach

  • Epithelium acts as a secretory sheet producing mucus covering the stomach's interior surface.

  • Gastric pits that open onto the gastric surface are lined with:
      - Mucous Cells at the base actively dividing to replace superficial cells.
      - Associated gland types: Parietal cells (secrete hydrochloric acid and intrinsic factor) and Chief cells (secrete pepsinogen, converted by HCl to pepsin).

Gastric Glands

  • Pyloric Glands: Found in the pylorus; produce mucous secretion and contain G cells (gastrin producers) and D cells (somatostatin, which inhibits gastrin release).

Digestion and Absorption in the Stomach

  • Performs preliminary digestion of:
      - Proteins (by pepsin).
      - Carbohydrates (by salivary amylase).
      - Lipids (by lingual lipase).

  • Stomach contents become more fluid with a pH approaching 2.0, leading to increased pepsin activity and initiation of protein breakdown.

The Small Intestine

  • Plays a pivotal role in digestion and nutrient absorption; 90% of nutrient absorption occurs in the small intestine.

Sections of the Small Intestine

  1. Duodenum: First section, approximately 25 cm (10 in) long, receiving chyme and digestive secretions; neutralizes gastric acid.

  2. Jejunum: Middle segment, about 2.5 m (8.2 ft) long, primarily responsible for chemical digestion and nutrient absorption with few plicae circulares.

  3. Ileum: Last segment, about 11.48 ft long; regulates flow into the cecum of the large intestine.

Histology of the Small Intestine

  • Plicae Circulares: Permanent transverse folds in intestinal lining that do not disappear when the small intestine fills.

  • Intestinal Villi: Finger-like projections covered by simple columnar epithelium and microvilli to increase absorptive surface area.

Intestinal Glands

  • Duodenal Glands: Produce large amounts of mucus when chyme arrives from the stomach, aiding digestion.

Intestinal Secretions

  • Approximately 1.8 liters of watery intestinal juice enters the intestinal lumen daily to moisten chyme, buffer acids, and keep digestive enzymes in solution.

The Pancreas

Anatomy

  • Lies posterior to the stomach from duodenum towards spleen and bound to the posterior wall of the abdominal cavity.

  • Regions include:
      - Head: Looped around the duodenum.
      - Body: Extends toward the spleen.
      - Tail: Short and rounded.

Histological Organization

  • Pancreatic Acini: Blind pockets lined with simple cuboidal epithelium containing scattered pancreatic islets (endocrine tissue).

Functions

  1. Endocrine Function: Regulation of blood glucose levels via insulin (lowers levels post-meal) and glucagon (released when levels are low).

  2. Exocrine Function: Processes primarily release pancreatic juice that includes buffers to neutralize stomach acid entering the duodenum.

Pancreatic Enzymes

  • Key enzymes include:
      - Pancreatic Alpha-Amylase: Carbohydrase that breaks down starches.
      - Pancreatic Lipase: Breaks down complex lipids into absorbable products, like fatty acids.
      - Proteolytic Enzymes: Includes proteases and peptidases, responsible for breaking down proteins and small peptides to amino acids, accounting for approximately 70% of pancreatic enzyme production.

The Liver

General Information

  • The largest visceral organ, weighing approximately 3.3 lbs.

  • Located in the right hypochondriac and epigastric regions, extending to the left hypochondriac and umbilical regions.

Hepatic Blood Supply

  • Receives 1/3 of blood supply from the hepatic artery and 2/3 from the hepatic portal vein, which drains blood from the:
      - Esophagus
      - Stomach
      - Small intestine and most of the large intestine.

Histological Organization

  • Consists of liver lobules, which are hexagonal in cross-section and include:
      - Six portal areas or triads at each corner.

Hepatic Portal Triad

  • Contains:
      1. A branch of the hepatic portal vein
      2. A branch of the hepatic artery proper
      3. A small branch of a bile duct.

Functions of Hepatocytes

  • Hepatocytes, alongside Kupffer cells (located in sinusoidal linings), absorb solutes from plasma and secrete plasma proteins.

Functions of Bile

  • Produced in the liver and secreted into bile canaliculi, it aids in emulsifying dietary lipids, increasing surface area for enzymatic action.

Bile Duct System

  • Bile flows from right and left hepatic ducts, forming the common hepatic duct, which leads to either the:
      - Common bile duct (empties into the duodenal ampulla)
      - Cystic duct (connects to gallbladder).

Metabolic Regulation

  • The liver regulates:
      - Composition of circulating blood
      - Nutrient metabolism
      - Storage of nutrients
      - Drug inactivation.

The Gallbladder

Overview

  • A hollow, pear-shaped muscular sac that stores and concentrates bile prior to secretion into the small intestine.

  • Regions include:
      - Fundus
      - Body
      - Neck

Cystic Duct

  • Connects the gallbladder to the common hepatic duct, forming the common bile duct.

Physiology

  • Stores bile but only releases it into the duodenum via stimulation from the intestinal hormone cholecystokinin (CCK). When the gallbladder is full, it contains approximately 40–70 mL of bile, which becomes concentrated as water is absorbed.

Coordination of Secretion and Absorption

Hormones of Duodenal Enteroendocrine Cells

  • These hormones coordinate digestive functions and include:
      - Gastrin: Secreted by G cells when exposed to incompletely digested proteins; promotes stomach motility and acid/enzyme production.
      - Secretin: Released when chyme arrives in duodenum; increases secretion of bile and buffers by the liver and pancreas.
      - Gastric Inhibitory Peptide (GIP): Secreted when fats and carbohydrates enter the small intestine.
      - Cholecystokinin (CCK): Secreted when chyme contains lipids and partially digested proteins; accelerates pancreatic enzyme secretion and relaxes the hepatopancreatic sphincter and gallbladder, ejecting bile and pancreatic juice into the duodenum.

Intestinal Absorption

  • Takes about 5 hours for materials to pass from the duodenum to the end of the ileum, with mucosal movements enhancing absorptive effectiveness by stirring and mixing intestinal contents, constantly changing the environment around the epithelial cells.

The Large Intestine

Overview

  • The large intestine is horseshoe-shaped, extending from the end of the ileum to the anus, situated inferiorly to the stomach and liver, framing the small intestine. It measures about 4.9 ft long and 3 in wide.

Functions

  • Key functions include:
      - Reabsorption of water.
      - Compaction of intestinal contents into feces.
      - Absorption of important vitamins produced by bacteria.
      - Temporary storage of fecal materials before defecation.

Parts of the Large Intestine

  1. Cecum: The expanded pouch receiving material from the ileum, storing material and beginning compaction.

  2. Appendix: A lymphoid organ also known as the vermiform appendix, about 3.6 in long, connected to the cecum via mesoappendix.

  3. Colon: The largest portion, with a larger diameter and thinner wall than the small intestine, forming a series of pouches (haustra) and possessing three longitudinal bands of smooth muscle (taeniae coli).

  4. Rectum: Last 15 cm (6 in) of digestive tract; expandable for temporary storage of feces. The movement of feces triggers the urge to defecate.

Appendages and Structure of the Colon

  • The colon consists of the following regions:
      1. Ascending Colon: Begins at the superior border of the cecum, ascending alongside the right side of the abdomen to the right colic flexure.
      2. Transverse Colon: Crosses the abdomen from right to left, turns at the left colic flexure.
      3. Descending Colon: Proceeds inferiorly along the left side; retroperitoneal and firmly attached to the abdominal wall.
      4. Sigmoid Colon: An S-shaped section about 15 cm (6 in) long, starting at the sigmoid flexure and leading into the rectum.

Blood Supply of the Large Intestine

  • Receives blood from tributaries of the:
      - Superior mesenteric artery
      - Inferior mesenteric artery

  • Venous blood is collected in tributaries of the:
      - Superior mesenteric vein
      - Inferior mesenteric vein.

Rectal and Anal Anatomy

  • The rectum forms the last 6 in. of the digestive tract with:
      - The Anal Canal: Final section containing longitudinal folds called anal columns.
      - Anus: Exit point of the anal canal with keratinized epidermis similar to skin.

  • Anal Sphincters:
      - Internal Anal Sphincter: Circular muscle layer of muscularis externa; smooth muscle, not volitional.
      - External Anal Sphincter: Encircles the distal portion of the anal canal; skeletal muscle fibers under voluntary control.

Vitamins Produced in the Large Intestine

  • Important vitamins synthesized include:
      - Vitamin K (fat soluble): Essential for synthesizing clotting factors like prothrombin.
      - Biotin (water soluble): Important for glucose metabolism.
      - Vitamin B5 (pantothenic acid, water soluble): Required for steroid hormone and neurotransmitter manufacture.