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The Digestive System - EarthLink

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The Digestive System - EarthLinkThe,the

    The Digestive System

    th Tortora (11 ed.): Chapter 24, pages 895 to 940 Martini, Timmons & Tallitsch, 6th edition: Chapter 25 page 655

    (version of 04-16-10)

     Digestive System Overview

     Alimentary Canal (gastrointestinal [GI]) tract:

     Accessory Digestive Organs:

    Digestive Processes

     Ingestionfood is taken into the mouth

     Propulsionincludes deglutition (voluntary) and peristalsis (involuntary) +

    segmentations

     Masticationchewing (teeth) and

     Churning (stomach)

     Chemical Digestionbreakdown of carbohydrates, proteins, lipids, nucleic acids

     Absorptiontransport of material from the lumen of the alimentary canal to blood

    and lymph capillaries

     Defecationremoval of indigestible material as feces

    Histology of the Digestive System The Four Layers of the Alimentary Canal:

____________________________

    ____________________________

    ____________________________

    ____________________________

    Three Layers of the Mucosa Epithelial layer

    Function:

     Lamina propria

     Tissue type: Areolar CT

     Function:

     Contains lymphatic tissue: MALT

     1

Muscularis mucosa

     A thin layer of smooth muscle

     Forms the folds

Submucosa

     Tissue type:

Contains

     Major blood vessels

     Mucin-secreting glands

     Lacteals: lymphatic vessels

     Nerve fibers

     Submucosal nerve plexus (plexus of Meissner)

     Is part of the enteric nervous system (ENS)

     Function: Its motor neurons supply the secretory cells of the mucosal

    epithelium

Muscularis Externa

     Contains:

     Inner circular layer; muscle fibers orient around the circumference of the canal

    Function: Squeeze‖ the gut

     Outer longitudinal layer muscle fibers are oriented along the length of the

    alimentary canal;

    Function: These muscles shorten the tube

     2

     The myenteric plexus lies between the layers of muscle

    * Supplies the longitudinal and circular muscle

    * Controls the frequency and strength of contractions of the muscularis

Serosa

     Composed of the visceral peritoneum

    * Adventitia superior to the diaphragm; composed of areolar CT

    * Serosa inferior to the diaphragm; same as adventitia but covered by visceral

    peritoneum

    Organs of the Digestive System

    The Oral Cavity

     Oral orificeanterior opening

     Vestibulespace between the teeth and cheeks, lips

     Labialips; formed by the orbicularis oris

     Frenula

     Inferior labial frenulum

     Superior labial frenulum

     Lingual frenulum

     Cheeksformed by the buccinator muscles

     Palateroof of the mouth

     Hard palate + soft palate

     Uvula + palatine tonsils

     Arches

    Glossopalatine

     Pharyngopalatine

     Fauces

     Palatine tonsils

     Associated organs of the mouth

     Tonguea muscle; contains lingual tonsil and papillae

     Dentitionteeth are in the alveolar sockets of mandible and maxillary bones

     Deciduous (primary) teeth (20)incisors, canines (cuspids, eyeteeth), molars

     Permanent teeth (32)incisors, canines, (cuspids, eyeteeth), premolars

    (bicuspids), molars (including wisdom teeth)

     Tooth structure

     Gingivaegums

     Crown(above the gums) vs. Root (below the gums)

     Enamel99% mineralized (hydroxyapatite); Hardest substance in the body

     3

     Dentinlike bone but harder, and lacking blood vessels

     Pulp cavitycontains pulp plus tooth’s blood vessels and nerves

     * The root canal is the portion of the cavity that is below the gum line. The

    pulp can become infected and die, requiring root canal therapy

    Comparison of number of teeth per quadrant in adults and children

     Incisors Canines Premolars Molars

    Child (deciduous) 2 1 o 2

    Adult (permanent) 2 1 2 3

Associated organs of the mouth

     Salivary glandsproduce salivaa combination of water, ions, mucus and

    enzymes

     Enzymes:

     Amylasestarch digestion

     Lysozymebreaks down bacterial cell walls

     Two types:

     Intrinsic salivary glands

     Extrinsic salivary glands

Intrinsic Salivary Glands

     Many small glands

     Keep mouth moist at all times

     Found in the mucous membrane of the tongue, palate, lips and cheeks (lingual,

    palatal, labial, buccal)

Extrinsic Salivary Glands

     Are external to the mouth

     Secrete saliva during eating or in anticipation of a meal Include the parotid (par = near, to = ear);

     Located between skin and masseter muscle;

    nd Parotid ducts open into the vestibule opposite the 2 maxillary molar

     Secrete serous (watery) liquid containing salivary amylase

Submandibular glands

     Located in the floor of the mouth

     Contain both mucus and serous cells

     Therefore, secrete saliva containing both mucus and salivary amylase

     Sublingual glands

     4

     Beneath tongue and superior to the submandibular glands

     Contain mostly mucous cells

     Secrete a thicker mucus

Saliva Composition

     99.5% water, 0.5% solutes

     Solutes:

     Ionssodium, potassium, chloride,

     Buffersbicarbonate and phosphate; Neutralize acidic foods

     Mucuslubricates food, helps form bolus

     Immunoglobulin aprevents microbes from attaching to epithelium

     Lysozymeantibacterial

     Salivary amylasebreaks starch down

Oropharynx and Laryngopharynx

     Three sections of the pharynx:

     Nasopharynx

     Oropharynx

     Laryngopharynx

     Three sets of musclespharyngeal constrictor muscles

     Three stages of deglutition (swallowing)

Deglutition

     Stage 1: tongue forms a bolus voluntary; breathing is suspended Stage 2: begins as a voluntary movement, becomes involuntary; bolus is pressed by

    the tongue into the pharynx; hyoid is lifted, the glottis is also lifted against the

    epiglottis

     Stage 3: the bolus is moved deeper into the pharynx; peristaltic waves begin

     Time:

     Fluids: about one second

     Solids: 5 to 8 seconds

    The Esophagus

     A muscular tube that propels food to the stomach; About 25 cm (10 inches) long

     Lies directly posterior to the trachea

     Passes through the diaphragm through esophageal hiatus, connects to the

    stomach in the cardiac region

     Gastroesophageal (Cardiac) ―sphincter‖—helps prevent regurgitation of stomach

    acid

Histology of the Esophagus

     Mucosanonkeratinized stratified squamous epithelium

     5

Submucosanear the stomach, the submucosa has mucous glands embedded in

    areolar CT

     Muscularis:

    rd Superior 3 is skeletal muscle

     Intermediate: skeletal + smooth

    rd Inferior 3: smooth muscle

     Muscularis is slightly more pronounced at either end to form two sphincters:

     Upper esophageal sphincterconsists of skeletal muscle

     Lower esophageal sphinctersmooth muscle

    The Stomach

     Functions

     Food storage for about 4 hours; In the fundus

     Churning of food to convert it into a paste called chyme

     Protein digestion begins here

     Secretions:

     Pepsinogenthat is converted into pepsin (a digestive enzyme that breaks

    down proteins) in an acidic environment

     Hydrochloric aciddestroys harmful bacteria in food; Provides pH appropriate

    for pepsin activity

Gross Anatomy of the Stomach

     Greater and lesser curves

     Muscle layers

     Longitudinal

     Circular

     Oblique

     Regions

     Cardiac region (cardia)

     Fundic region (fundus)

     Pyloric region (pylorus)

     Body

     Rugaenon-permanent folds that allow the stomach to stretch Pyloric sphinctercontrols the leakage of chyme into the duodenum; it is an

    thickening of the circular of the muscularis externa

Histology of the Stomach

     Epithelium

     Surface mucous cells

     Gastric pits

     Gastric glands: extensions of epithelium down into the lamina propria; These line

    many gastric pits

     6

Cellshave endocrine and exocrine functions

     Mucous cells

     Surface mucous cells

     Neckfound in the gastric glands

     Chief cellssecrete pepsinogen + gastric lipase

     Parietal cellsHCl + intrinsic factor (needed for absorption of vitamin B12

Gastric juiceconsists of the exocrine secretions of the cells of the gastric pits:

    mucus, HCl, pepsin, gastric lipase and intrinsic factor Gastric juice mixes with food (bolus) to form acid chyme

Churning

     Gentle mixing wavesseveral minutes after food enters stomach More vigorous mixing wavesat the body of the stomach to the pylorus Each wave forces about 3 ml of chyme into duodenum (gastric emptying)

     Chyme not pushed out returns to the body of the stomach

    The Small Intestine

     Site of most chemical digestion and absorption Averages 2.5 cm in diameter; 15 m long

     7

Structures:

     Villiincrease surface area

     Microvilligreatly increase the surface area

     Segments: duodenum, jejunum and ileum

Histology of the Small Intestine

     The epithelium of the mucosa: simple columnar cells including the following:

     Absorptive cells

     Goblet cellssecrete mucus

     Paneth cellssecrete lysozyme; Are capable of phagocytosis

     Three types of enteroendocrine cells

     Crypts of Lieberkuhnintestinal glands lining deep crevices; Contain enteroendocrine glands

     Peyer’s patches—aggregated lymphatic follicles

Special Structures of the Small Intestine

     Circular foldsplicae circularis;

     Folds of the mucosa and submucosa;

     Are permanent ridges;

     Begin near the proximal portion of duodenum and extend to middle of ileum Villifinger-like projections of the mucosa (0.5 to 1 mm long); 20 to 40 per mm2

     Microvilli

     Crypts of Lieberkuhn

     Where mitosis takes place

    The Duodenum

     Receives chyme from the stomach

     Receives bile from the gall bladder and liver via the hepatopancreatic ampulla

     Receives digestive enzymes from the pancreas also via the hepatopancreatic ampulla

     Amylases, proteases, nucleases, lipases

     Receives NaHCO from pancreas 3

     Neutralizes pH

     Protects the small intestine

     Creates an environment that is optimal for enzyme function

    The Ileum

     Last ten feet of the small intestine

     The iliocecal valve prevents feces from backing up into the small intestine Clusters of lymphatic nodules (Peyers patches)

     8

Motility of Small Intestine

     Segmentations

     Localized contractions

     Move contents back and forth

     Peristalsis

     Occurs 15 to 20 times per minute

     Both layers of smooth muscle are involved Pendular movement

    The Large Intestine

     Functions

     Small amount of digestion by bacteria

     Absorption of some water

     Formation of feces

     Elimination of feces

     Vitamin synthesis

Gross Anatomy of Large Intestine

     Cecumfirst portion of the large intestine; Connects to the ileum at the iliocecal valve

     Vermiform appendixworm shaped; Lymphatic function similar to the tonsils

     Colon:

     ascending colon

     ______________ flexure

     transverse colon

     splenic flexure

     descending colon

     sigmoid colon

     rectum

     Rectum

     Anal canal

     Special features

     Internal anal sphinctersmooth muscle

     External anal sphincterskeletal muscle

     Anal columnslengthwise folds on the luminal surface of anal canal; permit

    changes in diameter

    Specializations of the wall of the Large Intestine Teniae coli

     Thickened portions of the longitudinal muscle forming 3 longitudinal bands

     Maintain constant muscle tone to constrict the large intestine and form pouches

    called haustrae

     Epiploic appendagessmall pouches of visceral peritoneum filled with fat; Attached

    to teniae coli

     9

Histology of Large Intestine

     Mucosal epitheliummostly absorptive and goblet cells Solitary lymphatic nodules found in the lamina propria Microvilli

     No circular folds, no villi

Motility of the Large Intestine

     Peristalsis

     Haustral churning

     Mass movements

     Reflexes located in the LI

     Gastroileal reflex

     Gastrocolic reflex

     Defecation reflex

    Liver and Gall Bladder Functions of the liver

     Largest gland in the body

     Performs over 500 functions

     Performs many metabolic functions

     Converts glucose into glycogen

     Detoxifies poisons and drugs (e.g., alcohol)

     Liver cells, called hepatocytes, produce bile

Anatomy of the Liver

     Lobes:

     Major lobes: right and left,

     Minor lobes: caudate and quadrate

     Ligamentum venosum, ligamentum teres, and falciform ―ligament‖

     Hepatic portal vein carries blood from the digestive tract

    The hepatic portal system: the hepatic portal vein is formed from the merging of the

    splenic vein and the superior mesenteric vein. At the liver it branches into the right and

    left hepatic veins.

Gall bladder

     Bile is stored in and concentrated in the gall bladder

     Bile is released episodically

     Ducts

     Right and left hepatic ducts

     Common hepatic duct

     10

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