The Digestive System

NURS 1012 A & P II: The Digestive System

Learning Objectives

  • Discuss the structure and function of the digestive system.

  • Identify the organs of the digestive system.

  • List their major functions.

  • Describe the functional histology of the digestive tract.

  • Outline the mechanisms that regulate digestion.

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

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

  • List the digestive enzymes and the nutrients digested by each enzyme.

  • List the nutrients required by the body.

  • Describe the chemical events responsible for the digestion of organic nutrients.

  • Describe the mechanisms involved in the absorption of organic and inorganic nutrients.

  • Discuss the process of defecation.

Main Functions of the Digestive System

The primary functions of the digestive system include:

  1. Ingestion: The process of taking in food.

  2. Breakdown: The breakdown of food into nutrient molecules.

  3. Absorption: The uptake of these nutrient molecules into the bloodstream.

  4. Excretion: The elimination of any indigestible substances from the body in the form of feces.

Essential Activities of Digestion

The processing of food involves six essential activities:

  1. Ingestion: The act of eating.

  2. Propulsion: Movement of food through the alimentary canal, which includes:

    • Swallowing

    • Peristalsis: This is the primary means of propulsion in the digestive tract, involving alternating waves of contraction and relaxation to push food through the GI tract.

  3. Mechanical Breakdown: A process that includes:

    • Chewing: The physical grinding of food.

    • Mixing food with saliva.

    • Churning food in the stomach.

    • Segmentation: Local constriction of the intestine that mixes food with digestive juices.

  4. Digestion: A series of catabolic steps that involve enzymes breaking down complex food molecules into simpler chemical building blocks.

  5. Absorption: This is the passage of digested fragments from the GI tract lumen into the blood or lymph.

  6. Defecation: The elimination of indigestible substances from the body via the anus.

Anatomy of the Digestive System

The organs of the digestive system can be categorized into two groups:

Alimentary Canal (Gastrointestinal Tract)

  • A continuous muscular tube running from the mouth to the anus, which is responsible for:

    • Digesting food by breaking it down into smaller fragments.

    • Absorbing these fragments through its lining into the blood.

  • Organs involved:

    • Mouth

    • Pharynx

    • Esophagus

    • Stomach

    • Small intestine

    • Large intestine

    • Anus

Accessory Digestive Organs

  • Include structures that assist in the digestion process but are not part of the alimentary canal:

    • Teeth: Aid in mechanical breakdown through chewing.

    • Tongue: Aids in mixing food with saliva and forming the bolus for swallowing.

    • Gallbladder: Stores bile produced by the liver.

    • Digestive glands: Produce secretions that aid in breaking down foodstuffs.

    • Salivary glands: Produce saliva to initiate digestion.

    • Liver: Produces bile.

    • Pancreas: Produces digestive enzymes and bicarbonate.

Peritoneum

The peritoneum consists of serous membranes that line the abdominal cavity:

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

  • Parietal Peritoneum: Lines the wall of the body cavity.

  • Peritoneal Cavity: The fluid-filled space between the two layers of peritoneum, which lubricates mobile organs.

  • Mesentery: A double layer of peritoneum that extends from the body wall to digestive organs, providing routes for blood vessels, lymphatics, and nerves; it also holds organs in place and stores fat.

Types of Organs

  • Intraperitoneal (Peritoneal) Organs: Located entirely within the peritoneum.

  • Retroperitoneal Organs: Organs located outside or posterior to the peritoneum; includes parts of the pancreas, duodenum, and parts of the large intestine.

Peritonitis

  • Inflammation of the peritoneum caused by factors such as:

    • Piercing abdominal wound

    • Perforating ulcer

    • Ruptured appendix

  • Treatment: Typically involves debris removal and megadoses of antibiotics.

Histological Layers of Digestive Organs

All digestive organs are lined with the same four basic layers, or tunics:

  1. Mucosa: The innermost layer, which lines the lumen of the GI tract, performing functions such as secretion, absorption, and protection against diseases.

    • Made up of three sublayers:

      • Epithelium

      • Lamina Propria

      • Muscularis Mucosae

  2. Submucosa: Contains blood and lymphatic vessels, lymphoid follicles, and the submucosal nerve plexus; also includes elastic tissues to help the organ revert to its shape after a meal.

  3. Muscularis Externa: Responsible for segmentation and peristalsis; includes an inner circular layer and an outer longitudinal layer, with sphincters formed where circular muscle layer thickens.

  4. Serosa: The outermost layer of the digestive organs, which is a protective layer.

Nervous System Interactions with the GI Tract

Enteric Nervous System (ENS)

  • Also referred to as the gut brain, containing more neurons than the spinal cord. The gut brain is made up of enteric neurons that communicate extensively with each other, representing the major nerve supply to the GI tract, controlling motility.

  • The ENS consists of two main interconnecting intrinsic nerve plexuses:

    • Submucosal Nerve Plexus: Regulates glands and smooth muscle in the mucosa.

    • Myenteric Nerve Plexus: Controls the GI tract motility.

Reflex Arcs

  • Short reflexes: Mediated by the enteric nerve plexuses, responding to stimuli within the GI tract.

  • Long reflexes: Respond to stimuli arising inside or outside of the gut, primarily related to the autonomic nervous system.

    • Parasympathetic System: Enhances digestive processes.

    • Sympathetic System: Inhibits digestion.

Regulation of GI Activity

Three key concepts regulate GI activity:

  1. Stimuli: Mechanical and chemical factors provoke digestive activity:

    • Receptors in the walls of GI tract organs respond to stretch, changes in osmolarity and pH, and the presence of substrates and end products of digestion.

  2. Effectors: Smooth muscles and glands produce movement and secretions:

    • stimulated receptors initiate reflexes that promote mixing and movement of contents within the lumen.

    • Reflexes may also activate or inhibit digestive glands to secrete juices and hormones.

  3. Nervous and Hormonal Control: Includes both short (intrinsic) reflexes and long (extrinsic) reflexes, in addition to hormonal controls from cells in the stomach and small intestine that prompt target cells to secrete or contract.

Anatomy and Functions of the Oral Cavity

  • The mouth (oral cavity) is where food is chewed, mixed with enzyme-containing saliva, which starts the digestion process and initiates swallowing.

  • The oral cavity is bounded by:

    • Lips (anteriorly)

    • Cheeks (laterally)

    • Palate (superiorly)

    • Tongue (inferiorly)

Major Structures and Their Functions

  • Lips and Cheeks: Provide structural support and protection.

  • Palate: Forms the roof of the mouth, consisting of:

    • Hard Palate: The bony anterior portion that assists with friction during manipulation of food.

    • Soft Palate: Composed mostly of skeletal muscle, assists in swallowing.

  • Tongue: A muscular organ that occupies the floor of the mouth with multiple functions:

    • Formation of bolus (mixing food and saliva).

    • Initiation of swallowing, speech, and taste.

  • Saliva Functions: Cleanses the mouth, dissolves food chemicals for taste, moistens food into a bolus, and begins the breakdown of starch with the enzyme amylase.

Major Salivary Glands

  1. Parotid Glands

  2. Submandibular Glands

  3. Sublingual Glands

Teeth and Dentition

  • Teeth lie in sockets in the gum-covered margins of the mandible and maxilla, facilitating mastication (chewing) that tears and grinds food into smaller fragments.

  • Dentition:

    • Primary dentition: 20 deciduous (milk) teeth erupt at 6-24 months of age.

    • 32 permanent teeth (except for wisdom teeth) develop by the end of adolescence.

Structure and Functions of the Pharynx and Esophagus

  • Food passes from the mouth into the oropharynx and then into the laryngopharynx, allowing passage of food, fluids, and air.

  • The pharynx is lined with stratified squamous epithelium with mucus-producing glands for lubrication and protection.

  • Esophagus:

    • Composed of a flat muscular tube running from the laryngopharynx to the stomach.

    • Collapses when not involved in food propulsion and pierces the diaphragm at the esophageal hiatus, joining the stomach at the cardial orifice.

    • Gastroesophageal (cardiac) sphincter: Surrounds the cardial orifice, keeping it closed when food is not being swallowed; mucus cells here help protect the esophagus from acid reflux.

Functions of the Stomach

  • The stomach serves as a temporary storage tank, initiating the chemical breakdown of proteins and converting the bolus to a semi-liquid substance known as chyme.

  • Empty stomach has a volume of approximately 50 mL but can expand to about 4 L.

  • Major Regions of the Stomach:

    • Cardial part: Surrounds the cardial orifice.

    • Fundus: The dome-shaped region beneath the diaphragm.

    • Body: The mid-portion of the stomach.

    • Pyloric part: The pyloric canal terminates in the pylorus, connecting to the duodenum through the pyloric valve.

  • Greater Curvature: The convex surface of the stomach.

  • Lesser Curvature: The concave surface of the stomach.

  • Mesenteries: The greater omentum extends from the greater curvature, draping over the intestines, spleen, and transverse colon, with fat deposits and lymph nodes.

Layers of the Stomach Wall

  • The stomach is made up of the same four tunics as other digestive organs, with modifications:

    • The muscularis externa includes an additional oblique layer, enhancing the churning and mixing of chyme.

    • The mucosa secretes a two-layer coat of alkaline mucus for protection, dotted with gastric pits leading to gastric glands, which produce gastric juice.

Accessory Organs: Liver, Gallbladder, and Pancreas

  • Liver: Major digestive function is the production of bile, which emulsifies fats.

    • The hepatic artery and vein enter the liver at the porta hepatis. Bile ducts include the common hepatic duct, and the cystic duct connects to the gallbladder.

  • Gallbladder: Functions primarily to store bile.

  • Pancreas:

    • Exocrine Function: Produces pancreatic juice that neutralizes stomach acid and contains digestive enzymes.

    • Endocrine Function: Secretion of insulin and glucagon by pancreatic islet cells.

  • The bile duct and pancreatic duct unite in the wall of the duodenum, controlled by the hepatopancreatic sphincter that regulates the entry of bile and pancreatic juice.

Small Intestine

The small intestine is the major organ of digestion and absorption, measuring 2-4 m (7-13 ft) in length and approximately 2.5-4 cm (1.0-1.6 inches) in diameter. Its subdivisions include:

  1. Duodenum: Mostly retroperitoneal and curves around the pancreas.

  2. Jejunum: Approximately 2.5 m (8 ft) long.

  3. Ileum: Approximately 3.6 m (12 ft) long, joins the large intestine at the ileocecal valve.

  • Blood Supply: Supplied by the superior mesenteric artery, with veins draining into the hepatic portal vein for processing in the liver.

  • Nerve Supply: Receives parasympathetic innervation via the vagus nerve and sympathetic innervation from the thoracic splanchnic nerves.

Structural Modifications for Absorption

  • The length and surface area of the small intestine is optimized for nutrient absorption and includes:

    • Circular Folds: Permanent folds that force chyme to spiral through the lumen, maximizing nutrient absorption.

    • Villi: Fingerlike projections containing capillary beds and lymphatic capillaries (lacteals) for nutrient transport.

    • Microvilli: Small cytoplasmic extensions that give the brush border effect, containing membrane-bound enzymes for the final stages of carbohydrate and protein digestion.

Immunological Functions

  • Mucosa-associated lymphoid tissue (MALT) protects the intestine against microorganisms, including:

    • Individual lymphoid follicles.

    • Peyer’s Patches: Found in large numbers in the distal ileum where bacterial counts increase, secreting IgA from plasma cells.

Digestion and Absorption Process

  • Chyme entering the small intestine contains undigested fats and partially digested carbohydrates and proteins. The process takes about 3-6 hours to absorb nutrients and water.

  • Regulation of Chyme Entry: Chyme delivery must be slow to prevent osmotic loss of water from blood; also, it needs to be mixed with bile and pancreatic juice for continued digestion.

  • Motility of the Small Intestine: Segmentation is most common after meals, while peristalsis increases between meals, moving contents toward the large intestine.

Large Intestine

The large intestine has distinct structural features not found in the small intestine, including:

  • Teniae Coli: Three bands of longitudinal smooth muscle in the muscularis layer.

  • Haustra: Pocketlike pouches formed from the tension of the teniae coli.

  • Epiploic Appendages: Fat-filled pouches of visceral peritoneum.

  • Divisions:

    • Cecum: First part of the large intestine containing the appendix, a lymphoid tissue mass; it acts as a bacterial reservoir.

    • Colon: Several regions including ascending, transverse, descending, and sigmoid portions.

    • Rectum: Contains three valves that prevent simultaneous passage of feces and gas.

    • Anal Canal and Anus: The last segments of the large intestine, with two sphincters controlling fecal expulsion.

The Digestive Process Overview

  • Digestion is a catabolic process where macromolecules are broken down into monomers for absorption, beginning in the mouth, with the pharynx and esophagus facilitating food passage.

  • Swallowing involves the coordination of 22 muscle groups and is broken down into:

    • Buccal Phase: Voluntary contraction of the tongue.

    • Pharyngeal-Esophageal Phase: An involuntary phase primarily governed by the vagus nerve.

Stomach Functionality

  • The stomach serves multiple functions, including:

    • Breakdown of food

    • Storage of food

    • Delivery of chyme to small intestine

    • Denaturing proteins with HCl and absorbing lipid-soluble substances.

  • The secretion of intrinsic factor is vital for vitamin B12 absorption; a lack of this factor can lead to pernicious anemia.

Small Intestine: Role of Digestion

  • Small intestine absorbs nutrients and water, facilitated by bile, bicarbonate, and digestive enzymes.

  • Motility: Segmentation facilitates mixing; peristalsis moves remnants toward the large intestine.

Residue in Large Intestine

  • Residue remains for 12-24 hours with minimal breakdown by enteric bacteria. Nutrients and water are reclaimed, and the major functions include the propulsion of feces and defecation.

  • The large intestine features:

    • Haustral contractions: Primary contractions, responding to distension.

    • Mass Movements: Strong peristaltic waves initiated by food presence in the stomach, occurring several times daily.

The Process of Defecation

  • Mass movements push feces towards the rectum, initiating the spinal defecation reflex and stimulating contraction of the sigmoid colon and rectum while relaxing the internal anal sphincter for discharge. Conscious control over the external anal sphincter allows for defecation to occur, often assisted by the Valsalva maneuver.

Bacterial Flora and Its Functions

The large intestine hosts over 1000 types of bacteria that outnumber human cells 10 to 1, engaging in various metabolic functions:

  • Fermentation: Fermenting indigestible carbohydrates and releasing gases.

  • Vitamin Synthesis: Producing B vitamins and vitamin K for liver use in clotting.

  • Pathogen Control: Beneficial bacteria prevent pathogenic bacteria from thriving, supported by an immune response against breaching bacteria.

Process of Absorption

  • Absorption transfers substances from the gut lumen into the body, facilitated by tight junctions. Molecules pass through epithelial cells via:

    • Active transport for polar molecules.

    • Passive diffusion for lipid molecules.

  • Vitamins:

    • Fat-soluble vitamins (A, D, E, K) are absorbed by micelles.

    • Water-soluble vitamins (C, B) through diffusion or transport.

    • Vitamin B12 requires intrinsic factor for absorption through endocytosis.

  • Water Absorption: About 9 L enters the small intestine, with 95% absorbed by osmosis. Most remaining water is absorbed in the large intestine.

Digestive Enzymes and Their Actions

  1. Starch Digestion: Begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase and brush border enzymes to produce monosaccharides like glucose and galactose.

  2. Protein Digestion: Initiated in the stomach with pepsin and continued in the small intestine with pancreatic enzymes, concluding with brush border enzymes.

  3. Fat Digestion: Primary action in the small intestine involving emulsification by bile salts, with pancreatic lipases playing a major role in converting triglycerides into fatty acids and monoglycerides for absorption.

  • Path of Nutrient Absorption:

    • Monosaccharides and amino acids enter blood capillaries; fatty acids enter lymphatic capillaries (lacteals) forming chylomicrons for transport.