The Digestive System

The Digestive System

1. Organs that break down food into nutrient molecules hat are delivered by the bloodstream to cells n the body

   • Alimentary canal also known as gastrointestinal (GI) tract or digestive tract
     • Continuous tube through which food passes directly
       • Consist of oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and some large intestine
   • Accessory organs
     • Assist digestion in some way
       • Includes teeth, tongue, salivary glands, liver, gallbladder and pancreas

2. Digestive functions

   • Food and water are brought into the digestive system by ingestion and move through organs because of rhythmic contractions of smooth muscle called peristalsis
   • Digestive organs secrete a variety of substances including mucus, enzymes, acid, and hormones, which aid the digestive processes
   • Food breakdown occurs by digestion:
     • Mechanical digestion- physical breakdown of food into smaller pieces via chewing and mixing of food by muscular movements of alimentary canal
     • Chemical digestion- Acidic and enzymatic breaking of chemical bonds within food particles until only small molecules remain
   • Food particles move through the walls of the alimentary canal into blood or lymphatic vessels to complete absorption
   • Water, electrolytes, and vitamins are also absorbed into blood in the same way
   • Certain ingested materials are not digestible or usable by the body and such materials continue their transit through alimentary canal until they exit the body as feces through defecation
     • Includes microorganism (mostly bacteria), stereobilin (by-product of RBC breakdown) undigested food, sloughed-off epithelial cells

3. Regulation of Motility

   • Motility is regulated by the Nervous System
     • Autonomic nervous system or ANS- Sympathetic activity inhibits digestive processes and parasympathetic activity stimulates them
     • Enteric Nervous System or ENS- ENS is a self-contained branch of the ANS that extends from the esophagus to the anus
   • Molitity is also regulated by the Endocrine System
     • Digestive organs, particularly the stomach and small intestine secrete hormones

4. Alimentary Canal Histology

   • Mucosa-Innermost layer, made of epithelium, loose connective tissue (lamina propria) and smooth muscle (muscularis mucosae)
   • Submucosa- Thick layer of connective tissue with nerves (parasympathetic submucosal plexus), blood vessels, and glands)
   • Muscularis Externa- Two or 3 layers of smooth muscle with nervous tissue (myenteric plexus)
   • Serosa (Adventitia)- Connective tissue and epithelium (also called visceral peritoneum)

5. Oral Cavity

   • Digestion begins in the Oral cavity or mouth
     • The Oral cavity houses the teeth, tongue, and 3 pairs of salivary glands (located in and around the oral cavity)
     • The teeth chew the food (masication), saliva begins the chemical digestion of the food, and the tongue moves the Bolus (food mixed with saliva) the the back of the mouth
     • The posterior portion of the oral cavity is also involved in swallowing
   • Salivary glands secrete saliva, a fluid contains water, enzymes, mucus, and other solutes
     • Parotid, submandibular, sublingual
   • Other solutes include:
     • Salivary amylase-Begins carbohydrate digestion
     • Enzyme lysozyme- Catalyzes lysis (breaking) of bacterial cells
     • Secretory IgA-Antibody immunoglobulin A, or IgA, is found in body's secretions that binds antigens on pathogens and mediates their destruction
     • Bicarbonate Ions- Neutralizes any acid from stomach that regurgitates into the esophagus

6. Pharynx

   • Begins deglutition (swallowing) which passes bolus into esophagus
   • Consists of three divisions: Nasopharynx, oropharynx, and larryngopharynx
     • Oropharynx and laryngopharynx are part of the alimentary canal
     • Oropharynx houses two sets of tonsils -Palatine tonsils and Lingual tonsils which perform defensive functions

7. Esophagus

   • Muscular tube about 25cm (10in) long found posterior to trachea that transports a bolus from pharynx to stomach

8. Deglutition

   • Swallowing consists of following three phases:
     • Voluntary phase- Tongue pushes bolus posteriorly toward oropharynx
     • Pharyngeal phase- Bolus enters oropharynx and soft palate and epiglottis seal off nasopharynx and larynx
       • Controlled by Swallowing Reflex (medulla oblongata
     • Esophageal phase- Peristalltic waves move bolus down esophagus to stomach

9. Stomach

   • Divisions of the stomach
     • Cardiac sphincter, Fundus, Body, Pyloris, Pyloric sphincter (also contains folds called rugae)
   • The stomach is heavily indented to form deep gastric pits with gastric glands at the base
   • 4 Main types of cells in gastric galnds
     • Mucus cells-Secrete Muscus which protects the stomach from acid and enzymes
     • Parietal Cells- Secrete Hydrochloric Acid (HCI) which breaks down food and activates pepsinogen, and intrinsic factor (absorption of vitamin B12)
     • Chief cells- Secrete pepsinogen (inactive form of pepsin which digest proteins)
     • Hormone-secreting cells
       • Enterochromaffin-like cells- Secrete histamine that stimulates acid secretion
       • G-cells- Secrete Gastrin which stimulates parietal cells to secrete HCI and chief cells to secrete pepsiogen; contract cardiac sphincter and relax pyloric sphincter; increase stomach motility
       • Somatostatin cells- Secrete somatostatin which inhibits gastrin and insulin secretion
   • Cephalic phase- The sight, smell, taste, or thought of food activates centers in the brain which stimulates saliva and HCI secretion
   • Gastric phase- Begins when food enters the stomach which stretches the wall and triggers more motility and secretion
   • Intestinal phase- Begins when food enters the small intestine which inhibits the exit of chyme (food mixed with stomach secretions) from the stomach
   • Gastroesophageal Reflux Disease (GERD)
     • Gastroesophageal sphincter normally remains closed except during swallowing but when this mechanism fails, acid from stomach regurgitates into esophagus
     • If this occurs on a chronic basis it is called GERD and may lead to pain, difficulty swallowing, vocal cord damage, respiratory problems, and even esophageal cancer
   • The adult stomach has a volume of only about 50ml (0.2 cup), but it can expand to about 1500ml (6.3 cups) when filled with food and liquid
   • Emptying Functions- Different materials pass through pyloric sphincter at different rates
     • Liquids move rapidly from the stomach to the small intestine with essentially no delay
     • Solids must be converted to a nearly liquid state before they are able to enter the small intestine

10. Small Intestine

    • Is 6 meters long (almost 20 feet) and is the site of the greatest amount of specific digestion and absorption of nutrients and water
      • Includes duodenum, jejunum, ileum
        • Duodenum- About 25cm (10in.) long, site where secretions from gallbladder and pancreas enter small intestine
          • Duodenal (Brunner's) Glands produce alkaline mucus
          • Ileum-About 3.6 meters (10.8ft), terminates at the ileocecal valve
        • Jejunum-About 2.5 meters (7.5 feet), most active site for digestion and absorption
      • Cells that line the small intestine (enterocytes) produce multiple digestive enzymes, hormones, and mucus
      • Enzymes along with those from the pancreas are responsible for the bulk of chemical digestion
    • The internal surface area is increased 600 fold
      • Plicae circulares (circular folds)
      • Microvilli (folds of cell membranes)
        • Produce the Brush Border which secrete:
          • Disaccharidases-Break disaccharides into monosaccharides
          • Peptidases- Break peptide bonds of proteins
          • Nucleases- Break down nucleic acids
      • Villi (folds of mucosa)
    • Hormones:
      • Secretin- Decreases stomach motility
      • Cholecystokinin (CCK)- Decreases gastric motility (stimulated by fat and protein)
    • Appendicitis
      • The fact that the appendix is blind-ended may cause it to become blocked, usually by fecal matter
      • Bacteria within feces multiply in the appendix and cause infection which results in inflammation, a condition known as appendicitis
      • Symptoms of appendicitis include abdominal pain, particularly over the right lower quadrant, rebound tenderness with nausea and vomiting

11. Large Intestine

    • Is about 1.5 meters (5 feet) long but has a larger diameter than the small intestine
      • Includes cecum, appendix, colon (ascending, transverse, descending and sigmoid) rectum and anus
      • Absorbs water and electrolytes (but depends on osmotic gradients)
      • Passageway for feces or fecal matter to exit body
      • Houses numerous bacteria
      • Muscularis externa layer is not continuous through the length of the large intestine
        • This layer is gathered into three bands or ribbons of muscle called Taeniae Coli
        • Their constant tension bunches colon into pockets referred to as Haustra
        • Motility includes segmentation (claurning) and mass movement (mass peristalsis) by contractions of haustra
    • Mass Movements occur commonly after meals, integrated by the myenteric plexus
    • Local reflexes stimulated by food:
      • Gastrocolic-Food in the stomach
      • Duodenocolic-Food in the duodenum
    • Defecation includes:
      • Defecation reflex- Distension of the rectal wall
      • Regulated by parasympathetic stimulation
    • There are about 10 times more bacteria in the large intestine than cells in the human body
      • These normal gut flora coexist with humans in a symbiotic relationship
        • Produce Vitamin K which is necessary for blood clotting
      • Finish digestion of carbohydrates
      • Deter growth of harmful bacteria
      • Stimulate immune system (particularly during infancy)

12. Accessory Organs

    • The Salivary Glands, Pancreas, Liver and Gallbladder are accessory organs that secrete their products into the alimentary canal
      • The salivary glands secrete into the oral cavity
      • The pancreas, liver, and gallbladder secrete into the duodenum

13. Pancreas

    • Includes both endocrine and exocrine portions
      • Pancreas secrete insulin and glucagon which affect most cells in the body
      • The exocrine secretions are enzymes secreted primarily by clusters of acinar cells
      • Collectively, the enzymes are called "Pancreatic Juice" and are regulated by secretin and cholecystokinin (CCK)
      • Bicarbonate solution
        • Raises the pH from the stomach to neutralize HCI and inhibit pepsin
    • Enzymes in Pancreatic Juice
      • Trypsin (proteins), Chmotrypsin (proteins), carboxypeptidase (proteins), pancreatic amylase (starch), pancreatic lipase (lipids), DNA and RNA

14. Liver

    • Produces bile
      • Emulsifies fat
      • Contains bile salts (bilirubin, cholesterol, fats, lecithin)
    • Storage
      • Glycogen, fat, vitamins, minerals (i.e. copper and iron)
    • Nutrient interconversion
      • Amino acids are converted to energy producing compounds
    • Detoxification
      • Conversion of ammonia (from protein breakdown to urea)
      • Breakdown of drug metabolites
    • Phagocytosis
      • Blood cells and some bacteria
    • Synthesis
      • Albumins, fibrinogen, globulins, heparin, and clotting factors

15. Gallbladder

    • Stores bile from the liver (which is constantly secreted)
    • Stimulated by cholecystokinin and the parasympathetic nervous system
    • Gall stones:
      • Made of precipitated cholesterol
      • Can block cystic duct

16. Digestion

    • Process of digestion breaks food down into smaller and smaller pieces until only individual nutrient molecules remain
      • All nutrients go through the same mechanisms of mechanical digestion-mastication in mouth, churning in stomach, and segmentation in small intestine
      • Chemical digestion varies for each nutrient because digestive enzymes are specific for a single type of bond in a single type of nutrient
    • Most digestive enzymes catalyze hydrolysis reactions that use a water molecule to break a bond between two molecules
      • This explains why so much water is secreted with fluids like gastric and pancreatic juices because water molecules are key components of reactions that chemically break down food molecules
      • Digestive enzymes speed up these reactions by a process known as enzymatic hydrolysis
    • Most digestive enzymes catalyze hydrolysis reactions by using a water molecule to break a bond between two molecules

17. Absorption

    • Once nutrients are digested into molecules. they must be absorbed across the epithelial linig of the alimentary canal (enterocytes) and enter the bloodstream to be delivered to the body cells
    • The plasma membrane of enterocytes have different surfaces
      • Apical (feces lumen)
      • Lateral (attach to other enterocytes)
      • Basal (attaches to basement membrane)

18. Lactose intolerance

    • Many adults lack lactase and as a result cannot digest the milk sugar lactose, which is known as lactose intolerance
    • Most people produce lactase and infants, but up to 75% of adults show decreased lactase activity
    • The percentage of lactose intolerance varies with ethnic heritage- it is found in only about 5% of population of northern Europe, but about 90% of population in certain regions of Asia
    • Glucose and glactose make lactase