Chp. 23

Overview of the Digestive System

  • Primary Function: The digestive system is designed to break down substances into absorbable fragments to support fetal development and cellular health.

  • Functional Divisions of Organs:

    • Alimentary Canal (Gastrointestinal or GI Tract): A continuous muscular tube that winds through the body from the mouth to the anus. Its primary role is to digest food (break it down into smaller fragments) and absorb those fragments through its lining into the blood. Organs include:

      • Mouth

      • Pharynx

      • Esophagus

      • Stomach

      • Small Intestine

      • Large Intestine

    • Accessory Digestive Organs: These produce various secretions that help break down food or assist in the mechanical processing of food. These include:

      • Tongue

      • Salivary Glands

      • Liver

      • Gallbladder

      • Pancreas

Major Processes of Digestive System Activity

There are six essential activities involved in the processing of food:

  1. Ingestion: The act of taking food into the digestive tract. This occurs only at the mouth.

  2. Propulsion: The movement of food through the alimentary canal. This includes swallowing (voluntary) and peristalsis (involuntary waves of contraction and relaxation of muscles in the organ walls).

  3. Mechanical Breakdown: Increases the surface area of ingested food, physically preparing it for digestion by enzymes. Examples include chewing (mastication), mixing food with saliva by the tongue, churning food in the stomach, and segmentation (rhythmic local constrictions of the small intestine).

  4. Digestion: A series of catabolic steps in which enzymes secreted into the lumen of the alimentary canal break down complex food molecules to their chemical building blocks.

  5. Absorption: The passage of digested end products (plus vitamins, minerals, and water) from the lumen of the GI tract through the mucosal cells by active or passive transport into the blood or lymph.

  6. Defecation: The elimination of indigestible substances from the body via the anus in the form of feces. This occurs only at the large intestine.

Histology and Circulation of the GI Tract

  • The Peritoneum:

    • Visceral Peritoneum: Covers the external surfaces of most digestive organs.

    • Parietal Peritoneum: Lines the body wall.

    • Peritoneal Cavity: The fluid-filled space between the two peritoneums. It contains a slippery serous fluid that allows digestive organs to slide across one another and the body wall without friction.

  • Organ Location:

    • Intraperitoneal: Organs located within the peritoneal cavity.

    • Retroperitoneal: Organs that lie posterior to the peritoneum. This includes most of the pancreas and parts of the duodenum.

  • Splanchnic Circulation:

    • The digestive system receives approximately 25%25\% of the cardiac output.

    • It includes arteries that branch off the abdominal aorta to serve the digestive organs.

    • Hepatic Portal Circulation: The venous portion that collects nutrient-rich blood from digestive viscera and delivers it to the liver for processing.

  • The Four Basic Layers of the Alimentary Canal (Lumen Outward):

    1. Mucosa: The innermost layer consisting of three sub-layers: the epithelium, lamina propria, and muscularis mucosae.

    2. Submucosa: External to the mucosa; consists of connective tissue containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers.

    3. Muscularis Externa: Responsible for segmentation and peristalsis. It typically has an inner circular layer and an outer longitudinal layer of smooth muscle.

    4. Serosa: The protective outer layer (visceral peritoneum). In the esophagus, it is replaced by an adventitia (dense connective tissue).

The Enteric Nervous System and Regulation

  • The Enteric Nervous System (ENS): The GI tract's own nervous system, often called the "gut brain," which regulates digestive activity.

  • Reflex Types:

    • Short Reflexes: Mediated entirely by enteric nervous system plexuses in response to stimuli within the GI tract (intrinsic control).

    • Long Reflexes: Involve CNS integration centers and extrinsic autonomic nerves. The parasympathetic system usually enhances digestive activity, while the sympathetic system inhibits it.

  • Stimuli for Digestive Activity: Mechanical and chemical stimuli such as GI tract stretch (detected by mechanoreceptors), osmolarity, pHpH, and the presence of substrates/end products of digestion.

The Upper GI Tract: Mouth, Pharynx, and Esophagus

  • The Mouth:

    • Epithelium: Lined with stratified squamous epithelium to protect against friction during mastication.

    • Soft Palate: Located behind the hard palate; its role is to rise and close off the nasopharynx during swallowing.

    • Mouth Processes: Ingestion, Mechanical breakdown (mastication/chewing), Digestion (start of carbohydrate/polysaccharide breakdown), and Propulsion (deglutition/swallowing).

  • Saliva and Salivary Glands:

    • Functions: Cleanses the mouth, dissolves food chemicals for taste, moistens food to help compact it into a bolus, and contains enzymes (salivary amylase and lingual lipase) to begin chemical breakdown.

    • Composition: 97−99%97-99\% water, slightly acidic (pHpH 6.75−7.006.75-7.00).

    • Regulation: Parasympathetic division stimulates secretion; Sympathetic division causes the release of thick, mucin-rich saliva and can inhibit salivation (dry mouth).

    • Pathology: Sjogren’s disease affects moisture-producing glands throughout the body.

  • Teeth:

    • Deciduous Dentition (Baby Teeth): Dental formula is 2I,1C,2M2I,1C,2M×2=20\frac{2I, 1C, 2M}{2I, 1C, 2M} \times 2 = 20 total teeth. (II = Incisor, CC = Canine, MM = Molar).

    • Permanent Dentition: Dental formula is 2I,1C,2PM,3M2I,1C,2PM,3M×2=32\frac{2I, 1C, 2PM, 3M}{2I, 1C, 2PM, 3M} \times 2 = 32 total teeth. (PMPM = Premolar).

    • Enamel: The hardest substance in the human body.

  • Pharynx and Esophagus:

    • Common Passageway: The oropharynx and laryngopharynx are shared by air and food.

    • Esophagus: Lined with stratified squamous epithelium. Primary process is propulsion.

    • Heartburn: Often the first symptom of GERD (Gastroesophageal Reflux Disease).

    • Esophageal Cancer: Mentioned as having a poor prognosis (3−53-5 years).

Swallowing (Deglutition)

Swallowing involves five major steps:

  1. Buccal Phase: The tongue forces the bolus into the oropharynx.

  2. Pharyngeal-esophageal Phase begins: The larynx rises, and the epiglottis tips over to block the trachea.

  3. Sphincter Action: The upper esophageal sphincter contracts after food enters the esophagus.

  4. Peristalsis: Waves of muscular contraction move the bolus through the esophagus.

  5. Entry to Stomach: The lower esophageal sphincter (gastroesophageal sphincter) opens to allow the bolus into the stomach.

The Stomach: Storage and Protein Digestion

  • Structural Modifications:

    • Muscularis Externa: Has a third, innermost layer called the oblique layer, which allows the stomach to churn, mix, and pummel food into chyme.

    • Mucosa: Features gastric pits that lead into gastric glands. The surface is lined with alkaline mucus to protect the wall from acid.

    • Rugae: Large, longitudinal folds of the mucosa and submucosa that allow the stomach to expand.

  • Cell Types and Secretions (Gastric Juice):

    • Chief Cells: Produce pepsinogen (the inactive form of pepsin).

    • Parietal Cells: Produce Hydrochloric acid (HClHCl) and Intrinsic Factor (needed for Vitamin B12B_{12} absorption).

    • Mucous Neck Cells: Produce acidic mucus.

    • Enteroendocrine Cells (G Cells): Produce gastrin and other hormones/paracrines.

    • Pepsin: The active enzyme that breaks down proteins; activated from pepsinogen by HClHCl.

    • Stomach pHpH: Typically ranges from 1.51.5 to 3.53.5, necessary for protein denaturation and enzyme activation.

  • Phases of Gastric Secretion:

    • Cephalic Phase: Triggered by the sight, thought, or smell of food; inhibited by depression.

    • Gastric Phase: Triggered by stomach distension and chemical stimuli; provides most of the gastric juice; involved GG cells releasing gastrin.

    • Intestinal Phase: Initially stimulates then inhibits gastric activity as chyme enters the duodenum; involves the enterogastric reflex and the release of secretin and cholecystokinin (CCKCCK).

  • Alkaline Tide: When parietal cells export H+H^+ into the stomach, they simultaneously export bicarbonate (HCO3−HCO_3^-) into the blood in exchange for interstitial Cl−Cl^-, causing the blood leaving the stomach to be more alkaline.

The Liver, Gallbladder, and Pancreas

  • Liver: Its primary digestive function is the production of bile. It is composed of liver lobules, which contain a central vein and a portal triad (bile duct, portal vein, hepatic artery).

    • Hepatocytes: Liver cells that process nutrients and produce bile.

    • Stellate Macrophages: Found in sinusoids to remove debris/bacteria.

    • Pathology: Cirrhosis is the last stage of chronic liver inflammation; NAFLD (Non-alcoholic Fatty Liver Disease) is the most common liver disease.

  • Gallbladder: A thin-walled muscular sac that stores and concentrates bile.

  • Pancreas: Produces pancreatic juice (exocrine function via acini cells).

    • Protease (Trypsin): Digests proteins.

    • Amylase: Digests carbohydrates (starch).

    • Lipase: Digests fats.

    • Nuclease: Digests nucleic acids.

  • Regulation of Secretion:

    • CCKCCK (Cholecystokinin): Stimulated by fats and proteins in the duodenum. It causes the gallbladder to contract, the hepatopancreatic sphincter to relax, and the pancreas to secrete enzyme-rich juice.

    • Secretin: Stimulated by acidic chyme. It causes the pancreas to release bicarbonate-rich juice to neutralize the acid.

    • Bile Salts: Returning bile salts from the enterohepatic circulation are the biggest stimulus for enhanced bile secretion.

The Small Intestine

  • Segments: Duodenum (proximal), Jejunum (middle), Ileum (distal).

  • Structural Modifications for Absorption:

    • Circular Folds: Force chyme to spiral through the lumen, slowing movement to allow time for absorption.

    • Villi: Fingerlike projections of the mucosa that increase surface area.

    • Microvilli: Tiny projections on the plasma membrane of enterocytes that form the brush border containing enzymes.

  • Cell Types:

    • Enterocytes: Primarily for nutrient absorption.

    • Goblet Cells: Mucus-secreting.

    • Paneth Cells: Release antimicrobial agents.

    • Stem Cells: Develop into all other intestinal cell types.

  • Motility:

    • Segmentation: The most common motion after a meal; mixes chyme with digestive juices.

    • Peristalsis: The Migrating Motor Complex (MMCMMC) occurs between meals (every 90−12090-120 minutes) to move food remnants and bacteria toward the large intestine, stimulated by the hormone motilin.

The Large Intestine and Defecation

  • Functions: Absorbs remaining water, electrolytes, and vitamins (synthesized by gut bacteria); eliminates feces. Food typically stays here for 12−2412-24 hours.

  • Bacterial Flora: These bacteria synthesize Vitamin BB and KK and can ferment indigestible carbohydrates.

  • Defecation Reflex:

    1. Feces move into and distend the rectum, stimulating stretch receptors.

    2. Spinal reflex (parasympathetic) stimulates contraction of the rectum and sigmoid colon.

    3. The internal anal sphincter (smooth muscle) relaxes involuntarily.

    4. The external anal sphincter (skeletal muscle) is under conscious control and is relaxed voluntarily to allow defecation.

Chemical Digestion and Absorption Mechanisms

  • Hydrolysis: The chemical breakdown of food molecules by adding an H2OH_2O molecule to each bond to be broken.

  • Absorption Pathways:

    • Nutrients enter the enterocyte through the apical membrane (lumen side) and exit through the basolateral membrane into the interstitial fluid.

    • Na+Na^+ Cotransport: Used for most monosaccharides (glucose, galactose) and amino acids.

    • Fats: Products of fat digestion (monoglycerides and fatty acids) are solubilized in micelles (by bile salts) for diffusion into cells. They are then resynthesized into triglycerides and combined with proteins to form chylomicrons, which enter the lacteals (lymphatic capillaries).

    • Nutrient Destination: Except for fats, all absorbed substances move from the blood capillaries to the liver via the hepatic portal vein.

    • Absorption Statistics: 80%80\% of electrolytes and most water are absorbed before reaching the large intestine.