Digestive System Overview

Overview of the Digestive System

• Digestive system organs specialize in food digestion and absorption
• This system comprises a gastrointestinal tract and accessory organs
• The module focuses on raw material ingestion, breakdown (physical and chemical), nutrient absorption, and waste elimination

General Objectives

• Describe anatomical structures and physiological processes in physiological concepts
• Use anatomical facts, physiological concepts, and principles in nursing care
• Apply understanding of human organ systems for holistic health approaches
• Connect anatomy and physiology knowledge to real-world situations for healthy lifestyle choices and balance

Learning Objectives

• Locate and describe the functions of the main and accessory digestive system organs
• Differentiate between ingestion and digestion, and between chemical and mechanical food processing
• Trace the bolus's path from the mouth to the anus
• Describe the enzymes and chemicals needed for digestion

Discussion of the Digestive Tract

• The digestive tract, also known as the alimentary tract/canal, is a muscular tube
• The tube starts at the mouth and ends at the anus
• The pharynx, esophagus, stomach, small intestine, and large intestine lie between the mouth and anus
• Accessory organs like teeth, salivary glands, liver, pancreas, and gallbladder aid in processing materials

Principal Functions of the Digestive System

• Ingestion: Taking food into the stomach
• Mastication: Chewing food mixed with saliva
• Deglutition: Swallowing of food
• Peristalsis: Rhythmic contractions propelling food through the GIT
• Digestion: Mechanical and chemical breakdown for absorption
• Absorption: Food molecule passage into blood/lymph for cell distribution
• Defecation: Discharge of indigestible wastes (feces)

Divisions of the Digestive System

• The digestive tract (GIT) spans from mouth to anus, approximately 9 m (30ft) long

Organs of the GIT

• Oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestines
• Large intestines

Accessory Digestive Organs

• Teeth
• Tongue
• Salivary glands
• Liver
• Gallbladder
• Pancreas

Additional Definitions

• Gut: Developing stomach and intestines in the embryo
• Polymers: Large, organic food molecules
• Monomers: Subunits of polymers
• Hydrolysis reactions: Polymer digestion to monomer for absorption in the small intestine into blood/lymph

Functions of the Regions of the GIT

Oral Cavity (Mouth)

• Ingests food and receives saliva
• Grinds food and mixes with saliva (mastication)
• Initiates carbohydrate digestion
• Forms and swallows a bolus of chewed food (deglutition)

Pharynx

• Receives bolus from the oral cavity
• Autonomically continues deglutition of bolus to esophagus

Esophagus

• Transports bolus to the stomach via peristalsis
• The lower esophageal sphincter restricts food backflow

Stomach

• Receives bolus from esophagus
• Churns it with gastric juice
• Initiates protein digestion
• Carries out limited absorption
• Moves chyme (partly digested food and secretions) into the duodenum, and prohibits backflow
• Regurgitates when necessary

Small Intestines

• Receives from stomach and secretions from liver and pancreas
• Chemically and mechanically breaks down chyme
• Absorbs nutrients
• Transports wastes via peristalsis to large intestines
• Prohibits backflow of intestinal wastes

Large Intestines

• Receives undigested food from small intestines
• Absorbs water and electrolytes
• Forms, stores, and expels feces through defecation reflex

Histology of the Alimentary Canal

• Four tunics compose the alimentary canal

Tunica Mucosa

• Innermost lining of canal consists of a mucous membrane

Layers of the Mucous Membrane:

• Epithelial layer: Direct contact with canal contents
• Lamina propria: Connective tissue with lymph nodules, supporting epithelium by providing lymph and blood
• Muscularis mucosa: Muscle fibers increase digestive and absorptive area in the small intestine

Tunica Submucosa

• Loose connective tissue binding the tunica mucosa to the next layer

Tunica Muscularis

• Contains the Auerbach plexus, the major nerve supply for the alimentary canal
• The mouth, pharynx, and the first part of the esophagus consist of skeletal muscle for voluntary swallowing
• The Lower canal consists of smooth muscle that breaks down foods physically, mix it with digestive secretions chemically, and propel the food through the canal

Tunica Serosa

• Outermost layer, also known as the visceral peritoneum
• It is composed of connective and epithelial tissues
• Contains blood vessels, lymph vessels, and nerves
• One extension of the visceral peritoneum forms the mesentery.

Mouth or Oral/Buccal Cavity Structures

• Cheeks form the sides of the mouth
• The hard and soft palate form the roof of the mouth
• Tongue forms of the floor and consists of skeletal muscle and a mucous membrane divided by a structure called lingual frenulum.
• Papillae are small elevations covering the upper surface and sides of the tongue.
• Filiform papillae are found at the front and aid in licking
• Fungiform papillae are located toward the back
• Circumvalate papillae are arranged in a V shape at the back
• Lips are fleshy folds surrounding the mouth's opening

Uvula

• It is a cone-shaped muscular structure suspended from the posterior border of the oral cavity which aides swallowing, and prevents food from backing up into the nasal area

Salivary Glands

• It secretes saliva (99.5% water) for dissolving food
• 0. 5% of saliva consists of solutes:
  • Chlorides: Activate amylase
  • Amylase or Ptyalin: Breaks down complex carbohydrates (starch and glycogen)
  • Bicarbonates and phosphates: Buffers maintaining saliva pH (6.35 – 6.85)
  • Urea and uric acid: Waste products
  • Mucin: Forms mucus to lubricate food
  • Lysozyme: Destroys bacteria
  • Ptyalin: Breaks down starches to maltose

Three Pairs of Salivary Glands in the Mouth:

• Parotid gland: largest, found below & in front of ear
• Submandibular/submaxillary: Inferior to mandible body
• Sublingual: Under the tongue

Teeth

• Also known as dentes
• It is located in the sockets of the alveolar processes of the mandible and maxillae bones
• The alveolar processes are covered with gingivae

Three Principal Portions of the Teeth:

• Crown: Above the gums, covered with enamel
• Cervix/neck: Junction of crown and root
• Root: Projections embedded in the socket

Types of Teeth

• Deciduous/temporary/milk teeth: 20 teeth in infants (6 months – 2 years) - 5 per quadrant (2 incisors, 1 canine, 2 molars)
• Permanent teeth: 32 teeth, 8 per quadrant
  • 2 incisors - cutting food
  • 1 canine/cuspid - tearing food
  • 2 premolars/bicuspid - crushing food
  • 3 molars/tricuspids - grinding food

Pharynx

• The pharynx is a funnel-shaped tube for digestive and respiratory functions

Divisions of the Pharynx

• Oropharynx: At the back of the mouth for voluntary swallowing
• Nasopharynx
• Laryngopharynx

Esophagus

• A c ollapsible muscular tube behind the trachea, 10 inches (23-25 cm) long beginning at the laryngopharynx.
• The Esophagus secretes mucus and transports food to the stomach.
• The Esophageal/gastroesophageal sphincter prevents stomach content regurgitation into the esophagus

Stomach

• Most distensible part of the GIT lying under the diaphragm (J-shaped)
• Empty stomach is about the size of a large sausage

Parts of the Stomach:

• Cardia: Surrounds the gastroesophageal sphincter
• Fundus: Rounded portion above/left of cardia
• Body
• Pylorus/antrum: Narrow, inferior portion connecting to duodenum

Stomach Mucosa

• Mucosa contains gastric glands for secreting gastric juice.
  • Zymogenic/chief cells secrete pepsinogen
  • Parietal cells secrete HCl to activate pepsinogen into pepsin. Secretes polypeptides/intrinsic factor for Vitamin B12 absorption
  • Mucous/goblet cells secrete mucus
  • Argentaffin cells secrete seratonin and histamine
  • G Cells secrete gastrin hormone

Stomach Emptying

• The stomach empties into the duodenum around 2-6 hours post-ingestion
• Carbohydrates pass through first due to salivary enzyme digestion
• Protein foods pass through more slowly
• Stomach absorbs water, salts, drugs (aspirin, alcohol)

Small Intestines

• Approximately 21 feet long and 1 inch wide
• Major site for food digestion and absorption

Three Portions of the Small Intestines:

• Duodenum: Shortest (~10 inches), receives bile and pancreatic secretions via common ducts
  • Ampulla of Vater/Hepatopancreatic Ampulla: Merged common bile and pancreatic duct draining secretions
  • Sphincter of Ampulla/Ampulla of Odi: Controls secretion drainage to the duodenal papilla
  • Duodenal/Brunner’s gland: Mucus-secreting gland in the submucosa
• Jejunum: Second portion (~8 feet long)
• Ileum: Third portion (~12 feet), connects to the large intestine at the cecum via ileocecal valve (which prevents reflux)

Additional features of the Small Intestines

• Peyer's Patches/Mesenteric Patches are abundant lymph nodules
• Villi are finger-like projections that absorb nutrients and fats (via lacteals)
• Microvilli increase surface area for absorption
• Enterokinase activates trypsin
• Lactase digests milk sugar and lactose.

Large Intestine (Bowel)

• Begins at ileum's end, terminates at anus, ~5 feet long, 2.5 inches in diameter, supported by the mesocolon

Divisions of the Large Intestines

• Cecum: Pouch-like first part below the ileocecal valve. The appendix is attached
• Colon: The largest part, divided into sections:
  • Ascending colon: Rises on right side, turns left at the right colic (hepatic) flexure
  • Transverse colon: Crosses upper peritoneal cavity curving beneath spleen to the left colic (splenic) flexure
  • Descending colon: Descends to the sigmoid colon (joins rectum)

Remaining Divisions of the Large Intestines

• Rectum: Last 7-8 inches of GIT until the anal canal
• Anus: Anal canal opening to the exterior

Mechanical Movements Occurring in the Large Intestine

• Haustral churning
• Peristalsis: 3-12 contractions per min
• Mass peristalsis

Additional terms

• Internal and external anal sphincters are composed of smooth muscle guarding the anal canal
• The Anal Column/Column of Morgagni includes an artery and vein
• Hemorrhoids are varicose veins in the anal area

Functions of the Large Intestine

• Water absorption (mainly in ascending colon/cecum)
• Bacteria synthesizes vitamin K, biotin, vitamin B5
• Glands produce mucus

Feces

• Feces is a chime that remained, and is composed of water, inorganic salts, undigested food, and epithelial cells.
• Hydrogen sulfide (H2S) causes odor, together with bacterial decomposition

Pancreas

• A soft, pinkish, oblong gland (~6 inches long, 1 inch thick) under the stomach, attached to the duodenum
• The Pancreas has exocrine and endocrine functions

Clusters of Cells of the Pancreas

• Acini are its secretory and exocrine units for digestive enzymes (pancreatic juice) and sodium bicarbonate
  • Lipases digest triglycerides
  • Carbohydrases/Amylase digests starch
  • Protease/trypsin digests protein
• Ampulla of Vater/Hepatopancreatic Ampulla/common bile duct is where Wirsung units enter duodenum
• Pancreatic Islets / Islets of Langerhan is where endocrine function occurs
  • Alpha cells: Secretes glucagon which accelerates liver glycogenolysis
  • Beta cells: Secretes insulin which controls the metabolism of carbohydrates, protein, and fats

Functions of the Pancreas:

• As an exocrine gland: secretes pancreatic juice/digestive enzymes (acini) to continue food digestion in the small intestine
• As an endocrine gland: secretes insulin, glucagon to control blood sugar levels
• Secretin and cholecystokinin-pancreozymin (cck-pz) controls secretion
• Trypsinogen: Inactive trypsin, activated by enterokinase
• Pancreatic trypsin inhibitors prevent trypsin activation
• Acute pancreatitis: Reflux of pancreatic juice and bile from duodenum

Liver

• The liver is one of the largest organs of digestion and the largest gland
• The liver is in the right hypochondriac and epigastric regions, about 1. 7 kgs (3.5-4 lbs), divided into lobes separated by the falciform ligament (attaches to abdominal wall/diaphragm)

Functions of the Liver:

• Manufactures heparin and most plasma proteins
• Kupffer cells phagocytose bacteria and remove damaged cells
• Liver contains various enzymes to breaks down and transforms them into less harmful substances
• Excessive newly absorbed nutrients accumulate in the liver. Excess glucose and other monosaccharides can be stored as glycogen or converted to fat
  • The liver can transform stored glycogen back into glucose when needed
• Stores glycogen, copper, iron, and vitamins A, D, E, and K
• Produces bile salts which are sent to the duodenum of the small intestine for the emulsification and absorption of fats.

Gallbladder

• A sac-like, pear-shaped organ (3-4 inches) under the liver

Function of the Gallbladder

• Stores and concentrates bile made liver lobules until needed in the small intestine, where bile enters via the common bile duct

Components of Bile:

• Bile salts
• Bilirubin
• Cholesterol
• Other compounds

Sphincter of Odi/Sphincter of Ampulla

• It closes when the small intestine is empty, allowing bile to back up into the gallbladder
• It permits bile outflow when fats stimulate cholecystokinin release

Additional conditions

• Cholelithiasis: Gallstones, formed from precipitation
• Cholecystitis: Gallbladder inflammation
• Lithotripsy: Ultrasound fragmentation of gallstones

Disorders of the Digestive System:

• Hepatitis
• Hepatitis is caused by excessive alcohol or viral infection

Types of Hepatitis:

• Hepatitis A: Transmitted through contaminated food/liquids
• Hepatitis B: Transmitted through contaminated blood, needles, sexual contact

More Disorders of the Digestive System:

• Cirrhosis is a degenerative liver disease
• Appendicitis is inflammation of the appendix due to obstruction
• Crohn’s disease is a chronic, inflammatory bowel disease
• Diverticulosis is the presence of pouch-like herniations

Common symptoms of Digestive Disorders

• Diarrhea
• Severe abdominal pain
• Fever
• Chills
• Nausea
• Weakness
• Anorexia
• Weight loss

Other Common Digestive Disorders

• Colorectal cancer: Cancer of the intestine and rectum
• Peptic ulcer: Erosion of stomach/duodenum by HCl
  • Causes include: alcohol, aspirin, stress, H. pylori infections
• Enteritis: Inflammation of the small intestine (intestinal flu)
  • Causes include: Infection, irritation, stress
  • Symptoms include: Abdominal pain, nausea, diarrhea.
• Peritonitis: Inflammation of the peritoneum
  • Causes include: Bacterial contamination
• Dysentery: Inflammation of intestinal mucosa

More conditions

• Amoebic dysentery: caused by Entamoeba histolytica
• Food poisoning: Caused by bacterial toxins
• Botulism: A serious food poisoning caused by Clostridium botulinum
• Dental caries: Tooth enamel erosion due to bacterial acids
• Plaque: Debris formed from accumulated food particles between teeth
• Periodontal disease: Gum inflammation and deterioration.
  • Causes include: plaque, smoking, crooked teeth
  • Symptoms include: loosening of teeth, bad breath, bleeding gums, edema