Digestive

Digestive System

Overview

  • Structure: Composed of a long muscular tube labeled as the gastrointestinal (GI) tract and a set of accessory organs.

  • GI Tract: Includes oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anal canal.

  • Accessory Organs: Include tongue, teeth, salivary glands, pancreas, liver, and gallbladder.

  • Function:

    • Ingest, digest, absorb water, nutrients, and electrolytes needed to sustain life.

    • Eliminate waste products.

Basic Divisions of the Digestive System

Alimentary Canal Organs (Gastrointestinal Tract)

  • Definition: Organs through which food and food waste will pass; creates a continuous passage for food movement.

  • Components: Oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine.

Accessory Digestive Organs

  • Definition: Organs that contribute to the processes of digestion and absorption but no food or waste passes through them.

  • Components: Teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Schematic Diagram of the Digestive System

  • Illustrates the anatomy of the alimentary canal and accessory organs:

    • Alimentary Canal: Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine.

    • Accessory Organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Organ Structure and Functional Roles

Alimentary Canal Organs

  • Oral Cavity (Mouth)

    • Properties: Region posterior to teeth, lined with stratified squamous epithelium, contains tongue and teeth, surrounded by salivary glands.

    • Functions: Ingestion, mechanical digestion (mastication), secretion.

  • Pharynx

    • Properties: Region posterior to the oral cavity and larynx, lined with stratified squamous epithelium; includes oropharynx and laryngopharynx.

    • Functions: Propulsion (swallowing).

  • Esophagus

    • Properties: Muscular tube extending from the pharynx to the stomach, lined with stratified squamous epithelium; composed of both skeletal and smooth muscle in different thirds.

    • Functions: Propulsion (swallowing), limited secretion.

  • Stomach

    • Properties: J-shaped organ from esophagus to duodenum, lined with simple columnar epithelium; folds into gastric pits with gastric glands; contains an additional muscle layer.

    • Functions: Propulsion, chemical digestion, mechanical digestion, limited absorption.

  • Small Intestine

    • Properties: Long tube from stomach to large intestine, lined with simple columnar epithelium; features circular folds, villi, and microvilli for absorption.

    • Functions: Chemical digestion, mechanical digestion, absorption, secretion, and propulsion.

  • Large Intestine

    • Properties: Terminal portion of the alimentary canal from small intestine to anal canal, lined with simple columnar epithelium; contains taeniae coli.

    • Functions: Absorption of water, electrolytes, and vitamins; propulsion, limited secretion, defecation.

Accessory Digestive Organs

  • Teeth

    • Properties: Two sets—primary and secondary dentition; consist of crown above gum, root in bone; inner pulp cavity surrounded by dentin, enamel, or cementum.

    • Functions: Mechanical digestion (mastication).

  • Tongue

    • Properties: Composed of skeletal muscle, covered by stratified squamous epithelium with papillae.

    • Functions: Mechanical digestion, propulsion (swallowing), taste sensation.

  • Salivary Glands

    • Properties: Three sets—parotid, submandibular, sublingual; composed of mucous and serous cells.

    • Functions: Secrete saliva which assists in chemical digestion, deters harmful microorganisms, and moistens food for swallowing.

  • Pancreas

    • Properties: Composed of pancreatic acini surrounding a duct.

    • Functions: Secretes enzymes for digestion of lipids, carbohydrates, proteins, and nucleic acids; secretes bicarbonate to neutralize acidic chyme.

  • Liver

    • Properties: Comprised of hexagonal lobules surrounding a central vein containing hepatocytes.

    • Functions: Mechanical digestion (via bile production), excretion of wastes through bile.

  • Gallbladder

    • Properties: Muscular sac on inferior surface of liver.

    • Functions: Stores, concentrates, and releases bile.

Basic Digestive Processes

  • Main Processes:

    • Ingestion: Taking food into the mouth.

    • Secretion: Transfer of water and ions from extracellular fluid to lumen, release of bile and enzymes.

    • Propulsion: Movement of food through the alimentary canal, includes deglutition (voluntary) and peristalsis (involuntary).

    • Digestion: Comprises both mechanical (breaking down food physically) and chemical (breaking food down with enzymes) processes.

    • Absorption: Passage of digested macromolecules, vitamins, minerals, and water from GI tract across mucosa into blood or lymph, primarily occurring in the small intestine.

    • Defecation: Elimination of indigestible substances via the anus as feces.

Detailed Digestive Processes

  • Ingestion:

    • Definition: The act of taking food into the mouth.

  • Secretion:

    • Definition: The process that involves the transfer of water and ions from the extracellular fluid into the lumen of the GI tract and the release of bile and digestive enzymes.

  • Propulsion:

    • Definition: The process of moving food through the alimentary canal.

    • Methods: Includes:

    • Deglutition: also known as swallowing which is a voluntary action.

    • Peristalsis: Involuntary waves of muscle contraction in the GI tract responsible for moving food along.

  • Digestion:

    • Mechanical Digestion: Initial physical breakdown of food. Involves:

    • Chewing.

    • Mixing of food with saliva (tongue action).

    • Churning of food in the stomach.

    • Segmentation in the small intestine, rhythmic contractions mixing food without pushing it down the tract.

    • Chemical Digestion: The hydrolytic breakdown of food molecules into their respective building blocks (e.g., proteins into amino acids). It begins in the mouth and completes in the small intestine.

  • Absorption:

    • Definition: The process by which digested end products, along with vitamins and minerals, pass from the lumen of the GI tract across the mucosa into blood or lymph.

    • Primary site: Small intestine.

  • Defecation:

    • Definition: The act of eliminating indigestible substances from the body via the anus as feces.

Oral Cavity and its Components

Palate

  • Definition: The roof of the mouth, which also forms the floor of the nasal cavity.

  • Structure:

    • Hard Palate: Anterior part, contains bone, and is rigid; covered with stratified squamous keratinized epithelium; contains glands.

    • Soft Palate: Posterior part, movable, consisting of strong fibroconnective tissue. Inner side covered with stratified squamous non-keratinized epithelium.

Salivary Glands

  • Functions of Saliva:

    • Cleanses the mouth.

    • Dissolves food particles.

    • Moistens food to form a bolus.

    • Begins the chemical digestion of starch through enzyme action (salivary amylase).

  • Types of Salivary Glands:

    • Parotid Gland: Anterior to the ear, between the masseter muscle and skin.

    • Submandibular Gland: Located along the medial aspect of the body of the mandible.

    • Sublingual Gland: Anterior to the submandibular gland, under the tongue.

  • Composition of Saliva:

    • 97-99% water, electrolytes, salivary amylase, immunoglobulin A (IgA), lysozyme (immune defense), and mucin (forms mucus when dissolved in water).

Digestive Processes in the Mouth

  • Mechanism:

    • The presence of food activates salivation.

    • Mastication occurs through teeth and tongue, increasing the surface area for digestive enzymes.

    • Mixed food compacted into a bolus, ready for swallowing.

Pharynx

  • Structure: Muscular, fibrous, cone-shaped tube forming a passage for air and food.

  • Dimensions: Approximately 13 cm long, varies from 3.5 cm (width) at the top to 1.5 cm at the bottom.

Esophagus

  • Structure: A straight tube approximately 25 cm long, continuous with the pharynx, perforates the diaphragm, and connects to the stomach.

  • Layers: Consists of four layers (mucosa, submucosa, muscularis externa, adventitia).

Histology of the GI Tract

  • Four Layers: Mats:

    • Mucosa: Lined with different types of epithelium; contains lamina propria and muscularis mucosa.

    • Submucosa: Composed of connective tissue with blood vessels and glands.

    • Muscularis: Contains smooth muscle layers responsible for peristaltic action.

    • Serosa/Adventitia: Outermost layer; serosa in abdominal cavity organs, adventitia connects to surrounding structures.

Physiology of Mastication and Deglutition
Mastication (Chewing)

  • Innervation: Controlled by the motor branch of CN V (Trigeminal Nerve).

  • Mechanism:

    • The chewing reflex and presence of food in the mouth initiates the drop-and-rebound contraction of jaw muscles, leading to continuous chewing until bolus is formed.

Deglutition (Swallowing)

  • Stages of Deglutition:

    • Voluntary Stage (Buccal): Initiation - Tongue pushes the bolus posteriorly.

    • Pharyngeal Stage: Involuntary stage where bolus moves through the pharynx to the esophagus.

    • Esophageal Stage: Involuntary peristaltic waves propel the bolus to the stomach.

Gastric Digestive Processes

Stomach Functions

  • Mixes bolus with gastric juice to form chyme.

  • Gastric Juice Composition: Includes:

    • Hydrochloric Acid (HCl): Destroys ingested bacteria, aids in pepsin formation.

    • Pepsin: Active enzyme catalyzing dietary protein breakdown.

Phases of Gastric Juice Release

  • Cephalic Phase: Involves receptor inputs (sight, smell, taste, thought) sending signals to hypothalamus; vagus nerve activation.

  • Gastric Phase: Triggered by food presence stretches the stomach; gastrin released, increasing gastric secretions.

  • Intestinal Phase: Chyme reaching the duodenum stimulates the release of hormones (CCK, secretin) that regulate gastric secretion.

Gastric Structure

  • Regions: Fundus, body, pyloric region, greater curvature, lesser curvature, and rugae.

  • Muscularis External: Comprising outer longitudinal, middle circular, and inner oblique muscles.

Regulation of Gastric Emptying

  • Excitatory reflexes (increase gastric motility) are induced by antrum expansion; inhibitory reflexes (via the enterogastric reflex) slow down gastric emptying.

Gastric Hormones

  • Gastrin: Increases acid secretion, stimulates gastric contractions.

  • Enterogastrones (CCK, Secretin): Inhibit gastric contractions and slow gastric motility.

  • Somatostatin: Inhibits gastrin secretion and decreases gastric secretory activity.

Small Intestine

Structure and Function

  • Composed of duodenum, jejunum, and ileum; major site for digestion and absorption of carbohydrates, proteins, and fats.

  • Secretions from Accessory Organs: Essential for digestion and absorption.

Motility

  • Propel chyme from the duodenum to the large intestine; includes segmentation and peristalsis.

Intestinal Secretions

  • Mucus: Secreted by Brunner’s glands and goblet cells for protection of intestinal walls.

  • Enzymes (Brush Border Enzymes): Breakdown of small peptides and disaccharides; includes peptidases, sucrase, maltase, lactase, lipase, and enterokinase.

  • Electrolytes and Water: Provide a solvent for digestion; secreted by all epithelial cells.

Liver Functions

  • Involved in metabolism (CHO, lipid, protein), storage (glycogen, vitamins), detoxification, blood filtering, and bile secretion.

Bile Composition and Function

  • Yellow-green liquid consisting of bile salts, pigments, cholesterol, and electrolytes; assists in emulsifying fats and absorbing fat-soluble vitamins.

Gallbladder Function

  • Stores and concentrates bile; stimulated by CCK release to contract and release bile into the duodenum.

Pancreas Structure and Function

  • Dual function: Exocrine (producing digestive juices) and endocrine (hormone production). Hormones include glucagon, insulin, somatostatin, and pancreatic polypeptide.

Absorption in the Large Intestine

Components and Motility

  • Involved in water and electrolyte absorption; facilitates fecal matter excretion via rectal contractions and sphincter relaxation.

Mechanisms Involved

  • Local, neural, and hormonal mechanisms regulate digestive activities across the GI tract, influencing absorption and motility.

Vitamins

Water-Soluble Vitamins

  • B1 (Thiamine): Cofactor in CHO metabolism; deficiency causes Beriberi.

  • B2 (Riboflavin): Involved in oxidation-reduction reactions; deficiency leads to angular stomatitis.

  • B3 (Niacin): Coenzyme in redox reactions; deficiency causes Pellagra.

  • B5 (Pantothenate): Precursor for CoA; deficiency may lead to dermatitis.

  • B6 (Pyridoxine): Essential for amino acid metabolism; deficiency may have drug-induced causes.

  • B12 (Cobalamin): Involved in amino acid metabolism; deficiency can lead to pernicious anemia.

  • Folic Acid: Involved in nucleic acid synthesis; deficiency can cause neural tube defects.

Fat-Soluble Vitamins

  • Vitamin A: Important for vision and skin health; deficiency causes night blindness.

  • Vitamin D: Regulates calcium metabolism; deficiency results in Rickets.

  • Vitamin E: Antioxidant; deficiencies can increase RBC fragility.

  • Vitamin K: Critical for blood clotting; often synthesized by intestinal flora.

Toxicity and Absorption of Vitamins

  • Fat-soluble vitamins can accumulate leading to toxicity, while the absorption depends on pancreatic and liver health along with the ileum's functionality.