Digestive System

DIGESTIVE SYSTEM

Overview

  • The digestive system consists of two groups of organs:

    • Alimentary Canal (Gastrointestinal or GI tract):

    • Responsible for the digestion and absorption of food.

    • Comprises the following organs:

      • Mouth

      • Pharynx

      • Esophagus

      • Stomach

      • Small intestine

      • Large intestine

    • Accessory Digestive Organs:

    • Assist in the mechanical and chemical breakdown of ingested food.

    • Includes:

      • Teeth

      • Tongue

      • Gallbladder

      • Liver

      • Pancreas

      • Salivary glands

Essential Activities

  • The two groups of digestive organs manage the following essential activities:

    • Ingestion and Mastication

    • Propulsion and Mixing

    • Secretion

    • Digestion (both mechanical and chemical)

    • Absorption

    • Defecation

Major Digestive Organs

  • List of organs:

    • Oral Cavity (Mouth)

    • Pharynx (Throat)

    • Esophagus

    • Stomach

    • Small Intestine

    • Large Intestine

    • Rectum

    • Anus

Layers of the Digestive System

  • The digestive system has four basic layers (tunics):

    • Mucosa:

    • Contains, in order from the innermost layer:

      • Epithelium

      • Lamina propria

      • Muscularis mucosa

    • Functions:

      • Secretion

      • Protection against infections

      • Absorption of products

    • Submucosa:

    • Composed of dense connective tissue filled with blood and lymphatic vessels.

    • Contains embedded submucosal nerve plexuses.

    • Muscularis Externa:

    • Responsible for segmentation and peristalsis.

    • Contains two sub layers made of smooth muscle:

      • Inner circular layer

      • Outer longitudinal layer

    • Forms sphincters that can control movement along the digestive tract.

    • Serous Layer:

    • Equivalent to the visceral peritoneum, encasing organs.

Neural Control of the Digestive System

  • The digestive system is regulated by:

    • Enteric Nervous System (often referred to as the "Brain of the gut")

    • Autonomic Nervous System

    • Digestive Canal Reflexes

Enteric Nervous System

  • It has two major neural plexuses:

    • Myenteric Plexus:

    • Found in the muscular layer, controls motility.

    • Submucosal Plexus:

    • Controls secretions into the digestive tract.

  • Contains two types of sensory receptors initiating digestive canal reflexes:

    • Chemoreceptors

    • Mechanoreceptors

Autonomic Nervous System

  • Parasympathetic Control:

    • Known as “Rest and Digest” or “Wine and Dine.”

    • Mostly operates through the Vagus nerve (CN X).

  • Sympathetic Control:

    • Primarily functions through the thoracic and lumbar regions of the spinal cord to slow down digestion.

Peritoneum and Peritoneal Cavity

  • Visceral Peritoneum:

    • Covers the external surface of most digestive organs.

  • Parietal Peritoneum:

    • Lines the body walls.

  • Peritoneal Cavity:

    • Space between the two layers.

    • Provides lubrication for mobile parts of the digestive tract and reduces friction during organ movement.

Intraperitoneal vs Retroperitoneal

  • Some organs like the kidneys, ascending colon, and descending colon are retroperitoneal, lying on the posterior abdominal wall and covered by the peritoneum only on their anterior surface.

  • The peritoneum stabilizes organs and stores fat, forming a double-layered structure called the mesentery that facilitates routes for blood vessels, lymphatics, and nerves.

Functional Anatomy

Mouth

  • Functions:

    • Ingestion

    • Propulsion (into the pharynx)

    • Mechanical Digestion

    • Chemical Digestion:

    • Salivary amylase starts the breakdown of carbohydrates.

  • Oral Cavity (Buccal):

    • Surrounded by lips, cheeks, palate, and tongue.

  • Hard Palate:

    • Aids with chewing and formation of the bolus.

  • Soft Palate:

    • Closes off the nasopharynx during swallowing.

  • Uvula:

    • Projects from the edge of the soft palate.

Salivary Glands

  • Intrinsic Salivary Glands (Minor):

    • Keep the mouth moist.

  • Extrinsic Salivary Glands (Major):

    • Produce secretions when chemoreceptors and mechanoreceptors are stimulated, include:

    • Parotid

    • Submandibular

    • Sublingual

  • Saliva Composition:

    • 99% water and 1% solutes.

    • Slightly acidic, contains electrolytes, enzymes, and defense compounds.

  • Regulation:

    • Parasympathetic: Increases salivary activity.

    • Sympathetic: Decreases activity, resulting in dry mouth.

Tongue

  • Functions:

    • Repositions and mixes food during chewing.

  • Attachment:

    • Attached to the floor via the lingual frenulum.

  • Intrinsic Muscles:

    • Change the shape of the tongue.

  • Extrinsic Muscles:

    • Alter the tongue’s position (Genioglossus, styloglossus, hypoglossus).

  • Terminal Sulcus:

    • Divided into:

    • Anterior 2/3 (body) in oral cavity

    • Posterior 1/3 (root) in oropharynx

  • Papillae Types:

    • Filiform: Mechanical digestion

    • Fungiform

    • Circumvallate

    • Foliate: Contains taste buds.

Teeth

  • Types:

    • 20 baby (deciduous) teeth, 32 permanent teeth (all except third molars erupt by end of adolescence).

    • Types of teeth:

    • Incisors: Cutting

    • Canines: Tearing

    • Molars: Grinding

Mechanical and Chemical Digestion in the Mouth

  • Mechanical Digestion:

    • Mastication (chewing), results in food becoming a bolus.

  • Chemical Digestion:

    • Enzymes initiate nutrient breakdown.

    • Salivary Amylase starts to break down carbohydrates.

Pharynx

  • Contains oropharynx and laryngopharynx, allowing passage of food, fluids, and air.

Esophagus

  • Description:

    • Flat, muscular tube connecting the laryngopharynx to the stomach.

  • Structure:

    • 1st one third consists of voluntary skeletal muscle.

    • Remaining two thirds consist of involuntary smooth muscle.

  • Sphincters:

    • Upper Esophageal Sphincter: Skeletal muscle

    • Lower Esophageal Sphincter (gastroesophageal): Smooth muscle

    • Penetrates the diaphragm at esophageal hiatus and joins the stomach at the cardiac orifice.

  • Function:

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

Deglutition (Swallowing)

  • Defined as the movement of food from the mouth into the stomach.

  • The control center is located in the medulla and lower pons.

  • Stages of Deglutition:

    • Buccal Stage: (voluntary) moves food from the mouth to oropharynx.

    • Pharyngeal Stage: moves the bolus from the pharynx to the esophagus.

    • Esophageal Stage: moves the bolus through the esophagus into the stomach.

Stomach

Regions of the Stomach

  • Cardiac Region (Cardia):

    • Surrounds the cardiac orifice.

  • Fundus:

    • Dome-shaped region beneath the diaphragm.

  • Body:

    • Midportion of the stomach.

  • Pyloric Region:

    • Comprises the antrum, pyloric canal, and pylorus.

Adaptations of the Stomach

  • The Muscularis Externa has an additional inner oblique layer, which increases power for mixing.

  • The Mucosa contains gastric pits leading to gastric glands:

    • Glands in the fundus secrete gastric juices.

    • Glands in the pyloric region function endocrinally.

Gastric Glands

  • Mucous Neck Cells:

    • Secrete thin, acidic mucus.

  • Enteroendocrine Cells (G Cells):

    • Secrete chemical messengers (paracrine).

    • Hormones:

    • Serotonin and histamine stimulate chief cells when proteins are present.

    • Gastrin is released into the blood stream to increase gastric motility and relax the pyloric sphincter.

  • Chief Cells (Peptic Cells):

    • Secrete pepsinogen onto the stomach surface; it converts to pepsin.

  • Parietal Cells:

    • Secrete hydrochloric acid (HCl) that:

    • Lowers pH to 1.5 – 3.5 (denaturing proteins in food).

    • Activates pepsin from pepsinogen.

    • Secrete Intrinsic Factor (the only essential substance from the stomach) necessary for Vitamin B12 absorption; lack of it leads to pernicious anemia.

Gastric Secretion Control

  • Gastric secretions are regulated by neural and hormonal mechanisms in three phases:

    • Cephalic Phase: Triggered by the sight, thoughts, and smell of food (via CN X).

    • Gastric Phase: Happens once food/fluid enters the stomach and lasts 3-4 hours.

    • Intestinal Phase: Occurs once food enters the duodenum.

  • Upon combining with gastric secretions, the bolus becomes chyme.

Accessory Glands

Liver

  • Size: Largest gland in the body.

  • Function: Most important for processing absorbed nutrients (where metabolism takes place).

    • Produces and releases bile as its only direct contribution to digestion.

  • Receives blood from two circulations:

    • Hepatic Artery (oxygenated blood)

    • Hepatic Portal Vein (nutrient-rich blood).

Liver Components

  • Hepatocytes:

    • Liver cells that secrete bile.

  • Bile Canaliculi:

    • Collect bile into larger ducts forming right and left hepatic ducts, which merge to form the hepatic duct.

  • Hepatic Sinusoids:

    • Highly permeable capillaries that receive oxygenated and nutrient-rich blood, draining into the hepatic vein and then into the inferior vena cava (IVC).

Gallbladder

  • Description: Thin-walled muscular sac on the ventral surface of the liver.

  • Function:

    • Receives and stores bile from the liver (via the cystic duct).

    • Releases bile through the cystic duct into the bile duct.

Regulation of Bile Secretion

  • Gallbladder contraction is stimulated by:

    • Cholecystokinin (CCK): Released by intestinal cells when chyme exposes to proteins and fats.

    • Vagal Stimulation: Minor stimulus.

    • CCK also causes the hepatopancreatic sphincter to relax allowing bile to enter the duodenum.

Pancreas

  • Exocrine Functions:

    • Secretes pancreatic juice, which is alkaline and contains:

    • Electrolytes such as HCO3- to neutralize acids.

    • Enzymes:

      • Amylase (starch)

      • Lipase (fats)

      • Nucleases (nucleic acids).

    • Secretions regulated by:

    • Cholecystokinin (CCK): Stimulates enzyme-rich juice release.

    • Secretin: Stimulates the release of bicarbonate-rich juice.

  • Endocrine Functions:

    • Contains endocrine cells that produce hormones:

    • Insulin (from Beta cells)

    • Glucagon (from Alpha cells).

Drainage of Pancreas

  • The pancreatic duct drains the pancreas, meeting the bile duct from the liver and gallbladder.

  • All secretions enter the small intestine (duodenum) at the hepatopancreatic ampulla.

SMALL INTESTINE

Overview

  • Major organ of digestion and absorption:

    • Measures 7-14 ft, extends from the pyloric sphincter to the ileocecal valve.

  • Subdivisions:

    • Duodenum

    • Jejunum

    • Ileum

Duodenum

  • Has chemoreceptors analyzing chyme and releasing chemical messengers for regulating additional digestion and absorption.

  • Site where secretions from the liver and pancreas combine with food.

Small Intestine Cell Types

  • Absorptive Cells (Enterocytes)

  • Goblet Cells:

    • Secrete mucus.

  • Paneth Cells:

    • Secrete lysosomes for bacterial protection.

  • Enteroendocrine Cells: Secrete hormones such as:

    • Secretin

    • Cholecystokinin

Main Functions

  • Absorption of Nutrients:

    • Surface area greatly increased through:

    • Circular folds: Forces chyme to spiral through the lumen.

    • Villi: Finger-like extensions promoting absorption.

    • Microvilli: Epithelial extensions form a "brush border" containing digestive enzymes.

Small Intestine Motility

  • Intrinsic Pacemaker Cells: Initiate smooth muscle contractions mixing contents toward the ileocecal valve.

  • Once nearing emptiness, hormone motilin induces peristaltic waves pushing contents forward.

  • Ileocecal Sphincter: Relaxes to permit contents into the large intestine.

Chemical Digestion & Absorption of Carbohydrates

  • Begins with salivary amylase in the mouth, continues with pancreatic amylase in the small intestine, and ultimately finishes with brush border enzymes (e.g., dextrinase, lactase).

  • Mechanism of Absorption:

    • Secondary Active Transport (cotransport with Na+) or facilitated diffusion (fructose).

  • All carbohydrates are transported to the liver via the hepatic portal vein.

Path of Carbohydrate Digestion

  • Initial breakdown of starch and disaccharides occurs in the mouth, as well as in the small intestine.

  • Enzymes and sources include:

    • Salivary amylase (mouth)

    • Pancreatic amylase (small intestine)

    • Brush border enzymes: dextrinase, glucoamylase, lactase, maltase, and sucrase (small intestine)

Chemical Digestion & Absorption of Lipids

  • Lipid absorption is complicated due to their poor solubility in water.

  • Emulsification: Essential pre-treatment for lipid digestion.

  • Lipids Classified:

    • Monoglycerides

    • Long-chain or short-chain fatty acids.

    • Short-chain: Water-soluble.

    • Long-chain: Not water-soluble, requiring additional steps.

Lipid Absorption Mechanism

  • Long-chain fatty acids are surrounded by bile salts forming micelles that transport them to absorptive cells.

  • Once inside, they combine with monoglycerides to form triglycerides, which then aggregate into chylomicrons.

  • Chylomicrons:

    • Too large to enter the bloodstream, hence enter the lymphatic system via lacteals.

Fat Digestion Summary

  • Starts with unemulsified fats, undergoing emulsification by bile salts from the liver.

  • Pancreatic lipases degrade fats into monoglycerides and fatty acids.

  • Fatty acids and monoglycerides diffuse into intestinal cells, recombined into triglycerides and combined with lipids/proteins to form chylomicrons.

  • Chylomicrons are extruded through exocytosis and enter the lacteals for transportation to the systemic circulation via the thoracic duct.

Chemical Digestion & Absorption of Proteins

  • Protein digestion starts in the stomach (pepsin) and continues in the small intestine via:

    • Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase)

    • Brush border enzymes (aminopeptidases, carboxypeptidases, dipeptidases).

  • Absorption of amino acids:

    • via secondary active transport (cotransport with Na+).

  • Transported to the liver via hepatic portal vein.

Path of Protein Digestion

  • Breakdown sequence:

    • Pepsin acts in the stomach (presence of HCl).

    • Further breakdown by pancreatic enzymes in the small intestine.

    • Absorption of amino acids occurs through facilitated diffusion into capillary blood in villi, then routed to the liver.

Vitamin Absorption

  • In the small intestine:

    • Fat-soluble vitamins (A, D, E, K) are absorbed alongside fats.

    • Water-soluble vitamins (C, B) are absorbed by diffusion or active transporters.

    • Vitamin B12 absorption requires intrinsic factor and is internalized via endocytosis.

Electrolyte Absorption

  • Occurs mainly throughout the length of the small intestine.

  • Notably:

    • Iron and Calcium: Absorbed in the duodenum.

    • Sodium (Na+): Absorbed via the Na+/K+ pump.

    • Calcium (Ca2+): Absorption regulated by Vitamin D and parathyroid hormone (PTH).

Water Absorption

  • Approximately 95% is absorbed in the small intestine via osmosis.

  • Remaining water is lost during transit in the large intestine.

LARGE INTESTINE

Functions of the Large Intestine

  • Major function: Propulsion of feces toward the anus.

  • Reabsorption of vitamins, water, and electrolytes.

  • The colon is not essential for life.

Regions of the Large Intestine

  • Cecum:

    • Connects with the vermiform appendix.

  • Colon Regions:

    • Ascending Colon

    • Transverse Colon

    • Descending Colon

    • Sigmoid Colon

  • Rectum and Anal Canal.

Unique Features of the Large Intestine

  • Teniae Coli:

    • Three bands of longitudinal smooth muscle that run the length of the colon.

  • Haustra:

    • Pouch-like structures caused by contractions of the teniae coli.

Rectum and Anus

  • Rectum:

    • Contains three rectal valves to prevent fecal passage with gas.

  • Anal Canal:

    • Final segment of the large intestine containing two sphincters:

    • Internal anal sphincter (smooth muscle).

    • External anal sphincter (skeletal muscle).

Large Intestine Motility

  • Movement is achieved via Haustral Contractions:

    • Induced by chyme distension.

    • Typically includes 3-4 peristaltic waves (mass movements) per day, facilitated by the gastrocolic reflex.

Defecation

  • Mass movements force feces into the rectum.

  • Distension of the rectum triggers the spinal defecation reflex.

  • Parasympathetic Signals:

    • Stimulate contraction of the sigmoid colon and rectum, relax the internal anal sphincter.

  • Conscious control allows relaxation of the external anal sphincter.

HOMEOSTATIC IMBALANCES

Peptic Ulcers

  • The stomach protects itself from low pH and pepsin through a mucosal barrier consisting of:

    • HCO3- rich mucus

    • Tight junctions.

  • Anything breaching this barrier may lead to inflammation (gastritis) or an ulcer.

  • Most caused by Helicobacter pylori, treatable with antibiotics.

Diverticulitis

  • Sac-like pouches in the colon weaken and become inflamed if obstructed.

  • Symptoms: pain, constipation, or increased defecation.

  • High fiber diets are recommended to avoid complications.

Colorectal Cancer

  • Among the more deadly cancers (second to lung cancer).

  • Increased risk factors include alcohol intake, high-fat diets, genetics, and the presence of polyps (precancerous growths).

Hepatitis

  • Defined as inflammation of the liver.

  • Types:

    • Hepatitis A: Infectious (fecal contamination)

    • Hepatitis B: Spread via sexual contact and contaminated syringes

    • Hepatitis C: Similar to B, may cause cirrhosis and liver cancer

    • Hepatitis D

    • Hepatitis E