Physio Chapter -The Digestive System by AmnaKhan

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									The Digestive System
      Chapter 16
The main function of the digestive
system is to breakdown and absorb
nutrients, water, and electrolytes
 This system has four functions:
   Motility is the muscular contractions that mix and
    move the contents of the digestive tract.
   Secretion of digestive juices by exocrine glands
    into the digestive tract.
   Digestion via chemical change (hydrolysis) of
    large molecules (e.g., carbohydrates, proteins, and
    fats) into their smaller subunits
   Absorption of nutrients, along with water,
    vitamins, and electrolytes, into the blood and
    lymph.
The digestive system                        Mouth
consists of a tract plus          Salivary glands
the accessory organs.
                                     Pharynx
 The tract is a continuous
  tube that consists of the    Esophagus
  mouth, pharynx, esophagus,                        Stomach
  stomach, small intestine,
  large intestine, and anus.     Liver

 The lumen of this tube Gallbladder
  is continuous with the      Pancreas
  external environment.         Large
 The accessory organs          Intestine
  are the salivary glands,     Small
                               Intestine
  exocrine glands, & biliary
  system (liver & gallbladder). Rectum
                                     Anus
Other facts of the four main
digestive processes are:
 The smooth muscle in the walls of the digestive tract
  maintains a low level of contraction called tone.
 There are two types of digestive                  Lumen
  motility: propulsive movements
  & mixing movements.                     Duct
                                          cells
 Digestive secretions
  consist of water,              Exocrine
                                 gland
  electrolytes, & organic        cells          Secretory
  constituents such as                           product
  enzymes, bile, salts, & mucus.
 They are released by hormonal
  or neural stimulation.        Capillary
 Most absorption occurs in
  the small intestine where it is completed.
 Polysaccharides (e.g., starch and glycogen) are
  chemically changed into disaccharides, which are
  changed into monosaccharides.
                                     Intermediate     End Products
Category of foodstuffs                 Products       of Digestion
Carbohydrates
                                                     Monosaccharides
 Polysaccharides (starch and glycogens)   Maltose
                                                    (glucose, galactose,
                                                         fructose)

              Amylase                     Maltase
 Disaccharides
      Sucrose


                Sucrase
                                                         Absorbable
      Lactose                                            Units
                                                          = Glucose
                                                         = Fructose
                Lactase
                                                         = Galactose
 Proteins are chemically changed into polypeptides,
 which are changed into amino acids.
Proteins                     Peptide
                             Fragments          Amino Acids



Carboxypeptidase Pepsin &   Aminopeptidase
& Chymotrypsin   Trypsin

 The end products of fat digestion are
 monoglycerides and free fatty acids.
Fats
                                                        Free
    Triglyceride                             Glycerol   fatty
                                                        acids

                   Lipase

                                              Monoglyceride
Macromolecules are broken down via
hydrolysis.




Maltose                Glucose   Glucose
The wall of the digestive tract consists
of four layers.                  Body wall

 The mucosa lines the     Peritoneum            Muscularis
  luminal surface.                               externa
                          Mesentery
    It has an inner      Serosa              Outer longitudinal muscle
     epithelial layer.                        Inner circular muscle
    The lamina propria                              Mucosa
     is a middle layer of                            Mucous membrane
     connective tissue.                              Lamina propria
                                                     Muscularis mucosa
    The muscularis
     mucosa is a sparse              Lumen            Submucosa
     layer of smooth
     muscle.
                        Duct of
 The submucosa is accessory                        Myenteric plexus
  made of connective digestive gland            Submucous plexus
  tissue
 The muscularis externa is the          The serosa is the outer
  main smooth layer of the tract.         connective tissue layer.
Digestive motility and secretion are
regulated by four factors.
1) Smooth muscle cells display rhythmic, spontaneous
   variations in membrane potentials induced by
   pacesetter cells.
    Peristalsis and segmentation depend on pacesetters
2) The myenteric plexus and submucous plexus are the
   enteric nervous system
    Acetylcholine release promotes muscle contraction
3) Extrinsic nerves of the autonomic nervous system
   innervate digestive structures from the outside.
    Parasympathetic signals induce digestive activity.
4) Endocrine glands within the mucosa release hormones
   that signal digestive responses.
 Sensory information comes from chemoreceptors,
  mechanoreceptors, and osmoreceptors
The oral cavity is the entrance to the
digestive tract.
 Its opening is formed by the muscular lips.
 The palate separates the mouth & nasal passages.
 The tongue consists of voluntary skeletal muscles.
 The pharynx is the cavity at the rear of the throat.
 Mastication by the teeth starts digestion.
   This grinds and breaks as well as mixing it with saliva
    and stimulating the taste buds.
 Salivary secretion begins chemical digestion of
  starch via amylase
 Saliva also keeps the mouth and teeth clean.
     Lysozyme in the saliva lyses bacteria.
 Salivary secretion is continuous but can be reflexly
  increased.
Multiple factors control salivation
                     Cerebral cortex            Other inputs



                      Salivary center   Conditioned
                      in medulla        reflex




Pressure receptors     Simple reflex    Autonomic nerves
& chemoreceptors
in mouth

                                         Salivary glands



                                           Salivary
                                           secretions
External                 Local changes in
influence                digestive tract

                   Receptors in digestive tract




Intrinsic                   Extrinsic              Gastro-
  nerve                     automatic             intestinal
plexuses                     nerves               hormones



     Self-             Smooth muscle
                       contraction
     excitable
                       Exocrine gland cells
                       Producing digestive
  = Short reflex       fluids
  = Long reflex        Endocrine gland cells
                       digestive hormones)
   = Hormonal
     pathway
Swallowing                       Nasal passages
mechanisms                       Soft palate             Uvula
                                 Hard palate
force food into                       Bolus
the esophagus.                        Tongue
                                       Pharynx
                                                         Glottis

                                      Epiglottis
 Swallowing is initiated when                           Esophagus
                                     Trachea
  bolus contacts the pharynx.
                                             Swallowing center
 Afferent impulses are sent to the          inhabits respiratory
  swallowing center in the medulla.          center in brain stem
 Elevation of uvula prevents food
  from entering nasal passages
 Position of tongue prevents food
  from re-entering mouth
 Food is prevented from entering the
  respiratory tract by closing the
  glottis and tilting of the epiglottis.
Motility associated with the pharynx and
esophagus is swallowing.
 The bolus passes the
                                   Bolus
  pharyngoesophageal
  sphincter which prevents
  air from entering the
  digestive tract during
  breathing & also prevents
  eructation (burping).
 A swallowing center initiates
  a primary peristaltic wave.
 Peristalic waves push food through the esophagus.
 The gastroesophageal sphincter prevents reflux of
  gastric contents.
 Esophageal secretion is entirely protective.
The stomach stores food & begins protein
 This J-shaped chamber has a digestion.
  fundus, body, and antrum.
 Its terminal part has a pyloric       Esophagus Smooth muscle
  sphincter.
 The most important function                         Fundus
  of the stomach is to store          Gastro-
  food.                               esophageal        Body
    The body of the stomach          sphincter
   acts a holding area                Stomach
                           Pyloric
                                      folds
 The stomach also        sphincter
 secretes pepsinogen
 & HCl to begin                        Antrum
 protein digestion                                             Oxyntic
   Occurs mostly in                                           mucosa
    the antrum.                           Pyloric gland area
                                  Esophagus
 The stomach accommodate
  a twenty-fold increase in          Gastro-
                                     esophageal
  volume by relaxation.              sphincter
 Peristalic waves pass        Pyloric sphincter
  down the stomach
  about 3 times/min
 The mixing movements
  of the stomach                       Peristaltic contraction
  produces chyme which
  is passed on in small
  amounts to the doudenum.
 Most of this mixing occurs
  in the antrum as food
  pushes against the pyloric
  sphincter.
Gastric emptying is largely controlled by
factors in the duodenum.
 Peristaltic action pushes stomach chyme into the
  duodenum.
 Distension of the stomach increases gastric motility.
   Signaling by the vagus nerve and the hormone
    gastrin also increases motility.
 Gastric emptying is inhibited by the neural response
 and hormonal responses.
   Secretin and CCK are enterogastrones that inhibit
    this process.
   Duodenal factors that limit gastric emptying
           1) undigested fat       2)  pH
           3)  osmolarity         4)  distension
Vomiting relfex.
 The stomach does not actively participate in
  vomiting.
 It arises from contraction of the respiratory
  muscles.
 The stomach sphincters are relaxed & the stomach
  is squeezed to evacuate the food during vomiting.
 This activity is coordinated by the vomiting center
  in the medulla.
 Causes of vomiting include tactile stimulation,
  irritation/distension of the stomach and duodenum,
  elevated intracranial pressure, chemical agents, &
  psychogenic factors.
 Severe vomiting can lead to dehydration and
  circulatory problems.
Gastric digestive juice is               Gastric
                                         pit
secreted by glands




                                                                Mucosa
located a the base of
gastric pits.




                                                                Submucosa
 In oxyntic mucosa
               Surface
               epithelial
               cells
 Gastric                                           In pyloric
 pit           Mucosa cells
                mucus
                                                   gland area

               Chief cells  HCl
Gastric                                              G cells
gland          Parietal cells
                Pepsinogen                           gastrin
               Enterochomaffin-like                D cells
               (ECL) cells  Histamine              somatostatin
Production of HCl by parietal cells
    Plasma

               Parietal cell                     Gastric
                                                 lumen
             Cellular
             metabolism




                       Chief cell
       = Active cell           ca = Carbonic anhydrase
 Functions of HCl in               Autocatalysis
 the stomach include:         Pepsinogen         Pepsin
                                                          Digestion

   Pepsin formation,
                                                     Protein
   The breakdown of      Gastric
   connective tissue &                     HCI
                           lumen
                                                 Peptide fragments
   muscle fibers,
   The denaturation
   of proteins,
   Killing of most
   microorganisms.
 A mucus lining on the
 surface of the
 gastric mucosa. is
 protective.
Control of stomach parietal & chief cell activity
 Acetylcholine from the intrinsic nerve plexuses &
  the vagus nerve stimulate secretion.
 Histamine increases on parietal cells for HCl release
 Gastrin activates parietal and ECL cells
 Somatostatin inhibits secretions from parietal cells
  G & ECL cells.
 The control of gastric secretion involves 3 phases.
    During the cephalic phase stimuli from the head
    (cheweing and thinking about food) increase the
    secretion of HCl & pepsinogen.
   By the gastric phase stimuli (proteins & distension) in
    the stomach initiate gastric secretions through
    efferent pathways.
   The intestinal phase is inhibitory, shutting off the
    flow of gastric juices.
Other facts about stomach activity include:
 Salivary amylase continues carbohydrate digestion
  in the body of the stomach.
 Proteins digestion begins in the antrum where food
  is mixed with gastric secretions.
 The stomach lining is protected from gastric
  secretions by the gastric mucosal barrier.
 Gastric secretion gradually decreases as food
  empties from the stomach into the intestine.


        Mucus coating
                                               Impermeable
                                               to HCI


      Cells lining gastric mucosa   Tight
                                    junction
The pancreas contains exocrine &
endocrine cells.
 Endocrine cells of the islets of Langerhans secrete
  hormones—insulin & glucagon.
 The exocrine pancreas secretes digestive enzymes
  and an aqueous alkaline fluid.
    The alkaline fluid has sodium carbonate.
    The enzymes are proteolytic enzymes, pancreatic
     amylase, & pancreatic lipase.
       Trypsin, Chymotrypsin and carboxypeptidase
        are the proteolytic enzymes.
 Pancreatic exocrine secretion is regulated by
  secretin and CCK, hormones from the small intestine
    Secretin signals the secretion of NaHCO3.
    CCK induces the secretion of digestive enzymes.
    Bile duct
    from liver    Stomach

                               Hormones
                               (insulin,
                               glucagon)
                                           Blood




                  Duct cells    Endocrine portion
Acinar cells      secrete       of pancreas
secrete digestive NaHCO3        (Islets of
enzymes           solution      Langerhans)

Exocrine portion of panaceas
    Acid in         Neutralizes   Fat and protein        Digests
    duodenal                      products in
    lumen                         duodenal lumen



Secretion release                  CCK release
from duodenal                      from duodenal
mucosa                             mucosa
         (via bloodstream)               (via bloodstream)


  Pancreatic duct                  Pancreatic
  cells                            acinar cells




Secretion of aqueous              Secretion of
NaHCO3 solution into              pancreatic digestive
duodenal lumen                    enzymes into
                                  duodenal lumen
                                                Heart
Blood from the
digestive tract carries
it contents to the liver.                Inferior
                                         vena cava
                                                             Aorta
                                                             Hepatic
                                         Hepatic vein
 Blood enters the liver from                                vein
  the digestive tract by the                     Liver
                                                 sinusoids
  hepatic portal system.
 The portal vein of this                Liver      Arteries to
  system breaks into a                            digestive tract
  capillary network, the liver     Hepatic
  sinusoids.                      portal vein

                                 Digestive
                                 capillaries
                             Digestive
                             tract
The liver carries out numerous metabolic
functions.
 It carries out the metabolic processing of
    nutrients.
   It stores substances such as glycogen and fats.
   It detoxifies or degrades body wastes.
   It synthesizes plasma proteins.
   It activates vitamin D.
   It removes bacteria and worn-out RBCs.
   It excretes cholesterol and bilirubin.

 The liver also produces bile, which is essential for
    the digestion of fats
Anatomy and flow through the liver.
 The liver lobules are delineated by vascular
  sinusoids.
 Hepatocytes secrete bile into these canniculli which
  converge to form bile ducts
                 Central      Sinusoids                 Branch of
    Bile          vein                                  hepatic portal vein
    canaliculi                                                            Bile
                                                      Branch of           duct
                                                      hepatic artery
                                                 Kupffer cell
                                                 (Macrophage)
                                                   Bile
                                             canaliculi
                                          Sinusoids



 Bile                                            Central
 duct                           Hepatic           vein
        Hepatic portal vein     artery                          Hepatic plate
 Bile is stored in the
  gallbladder                     Bile salts    Cholesterol
  between meals.
 After a meal
  the liver and
  gallbladder                      Liver
  secrete bile into
  the small intestine
  for fat digestion.
 Its secretion        Portal       Gallbladder
                       circulation
  is induced by                     Sphincter of Oddi
  chemical (bile salts
  being returned
  to the liver),        Colon
                                                            Duodenum
  hormonal
  (secretin), & neural
                                            Terminal ileum
  (cephalic-vagal)
  mechanisms.                         Bilirubin is a waste product
                                              excreted in the bile.
 The detergent action of bile emulsifies fats.
 Fat globules are broken into smaller droplets,
  increasing surface area to facilitate enzymatic
  attack (pancreatic lipase).
 Pancreatic lipase is anchored to a fat droplet by the
  polypeptide colipase.
Negativity charged
H2O-soluble portion
(a carboxyl group at
the end of a glycine
or taurine chain)




                                           Small lipid (fat)
                                           droplet with bile
   Lipid-soluble portion
                                           salt molecules
(derived from cholesterol)
                                           absorbed on
                                           its surface
          Large fat droplet

Through action
of bile salts




                              Lipid
                              emulsion
Micelle formation.                                        Tiny micelles
                                                          are water
                                                          soluable and
Hydrophobic core                                          can be
                                                          transported
Hydrophilic shell
                                                          through
                                                          aqeous
                                                          pathways
        Lipids
      including
     Cholesterol




      Water-soluble portion       Water-soluble portion

      Lipid-soluble portion      Lipid-soluble portion

  Bile salt                   Lecithin
The small intestine is where most
digestion and absorption occur.
 Its three segments are the duodenum, jejunum, &
  ileum.
 It does not secrete digestive enzymes. The
  pancreas secretes enzymes into the tract.
 The small intestine enzymes complete digestion
  intracellularly.
    These include the disaccharidases and
     aminopeptidases.
 The process of segmentation mixes and slowly
 propels the food.
   Segmentation contractions are initiated by BER cells.
   The circular smooth muscle responsiveness is
   influenced by the distension of the intestine, gastrin,
   and extrinsic nerve activity.
 Segmentation mixes
 chyme with secretions
 and slowly moves the
 contents through the
 tract.
 The migrating motility
 complex is an internal
 housekeeper sweeping
 the intestine clean
 between meals.
The small intestine has adaptations
to maximize absorption:
 The mucosal lining has a large surface area due to
    its circular folds and fingerlike projections called
    villi.
   The epithelial cells also have microvilli.
   A villus has a cover of epithelial cells, a connective
    tissue core, a capillary network, and the terminal
    lymphatic vessel.
   During absorption molecules produced by digestive
    enter the capillary or lymphatic vessel.
   The mucosal lining has a rapid turnover.
   The crypts of Liberkuhn have stem cells for cell
    regeneration.
           Epithelial cell
               Capillaries


           Central lacteal
           Mucous cell
Circular
fold           Crypt of Lieberkühn
               Arteriole
               Venule
  Villus       Lymphatic vessels


              Microvilli
The epithelial cells in the small intestine
have a variety of transport mechanisms.
 Na+ is pumped from the tract lumen into the
    interstitial fluid.
      From their it enters capillaries by diffusion.
   The transport of sodium creates an osmotic
    pressure drawing in water as N+ it is absorbed.
   Glucose and galactose are moved by secondary
    active transport.
      They are cotransported with sodium.
   Fructose is absorbed by passive facilitated
    diffusion.
   Amino and small peptides are are also absorbed
    across intestinal cells by secondary active
    transport.
Lumen                                                 cotransport
                                                      carrier




             Na+ &
Epithelial   energy
cell         dependent
                                        Facilitated
                                        diffusion
of villus       secondary
                active transport




                                   Capillary
Lumen




                     Na+- and
                     energy-
                     dependent      Energy
                     absorption     required




   Epithelial cell
        of villus
                        Capillary
Digested fat is absorbed passively
and enters the lymph.
 Triglycerides are converted to monoglycerides &
    free fatty acids are produced by hydrolysis.
   These water-insoluble products are carried to the
    inside of water-soluble micelles.
   On the mucosal surface these molecules leave the
    micelle and passively diffuse through the lipid
    bilayer of the luminal membrane.
   They are resynthesized into triglycerides inside
    the epithelial cells.
   There they associate with carrier protiens to
    create water-soluble chylomicrons which leave the
    cells by exocytosis.
   They enter the central lacteals, lymphatic vessels.
Lipid emulsion

                                                              Micelles           Lumen
                                                              diffusion

Lumen                                                               Micelle
Micelles
                                                                                 Microvillus
                                                                 Fatty acids,
                                                                monoglycerides




                     Aggregate and                                         Passive
                     coated with     Short or
                     lipoprotien     medium                                absorption
                                     chain      Basement
 Epithelial                                     membrane
 cell of villus    (Exocytosis)
                  Central lacteal                 Capillary
Other facts on adsorption include:
 Vitamin adsorption is mainly passive.
    Water-soluble vitamins are absorbed with water.
    Fat-soluble vitamins are absorbed in micelles.
 Iron and calcium absorption is regulated.
   Transferrin carries some iron to the bone marrow.
    While the rest is stored in the ferritin pool.
   Most Ca++ is absorption is increased by parathyroid
    hormone mediated activation of vitamin D
 Most absorbed nutrients immediately pass through
  the liver for processing.
 The liver monitors nutrient molecules and controls
  their concentration in the blood or lymph leaving
  the liver.
 Communication between the stomach, pancreas, and
  small intestine balance absorption secretion.
                              Iron not absorbed by cells

                 Dietary                                    Iron lost
                 iron                        Iron lost      in feces
                               Absorbable
                               iron          as cell is
                                             sloughed
Lumen




                                  Iron
                               absorbed
                                into cell
                                            Ferritin –
                                            pool of iron
                                            not
                                            absorbed
                                            into blood

         Epithelial
         cell of villus


                           Iron absorbed into              Excess
Plasma                     blood; bound to                 iron in
                           transferrin                     blood
Digestive tract
lumen
                  Stomach
                  parietal cell
                                            Blood
                                                    Maintance
                                                    of pH
                                                    balance

                            Pancreatic
                            duct cell




                          Intestinal
                          epithelial cell
                                                      New
  Ascending                                           meal
  colon
                                Gastrin



Ileocecal valve    Ileocecal sphincter
                                          The ileocecal
                                          juncture, between
                  Pushes valve open
                  and relaxes sphincter   the small and
 Pushes
 valve closed
                                          large intestine,
 and contracts             Ileum          prevents
 sphincter                                contamination of
         Cecum
                                          the small intestine
                          Appendix
                                          by colonic
                                          bacteria.
The large intestine
                                            Transverse colon
is a drying &
storage organ.



                       Ascending colon
 Most digestion &                       Taeniae coli
  absorption has                             Haustra
  been accomplished
  in the small                   Ileocecal
  intestine.                     valve

 The colon extracts
  water and salt and    Cecum
  eliminates the
  feces.                  Appendix
                                       Rectum
 The colon contains
  many beneficial                                 External anal
  bacteria including Internal anal                sphincter
  some that generate sphincter                    (skeletal muscle)
                     (smooth muscle)
  vitamin K                                    Anal canal
Movement through the large intestine.
 The large-intestine secretion is entirely protective
    and consists of an alkaline mucus solution
   Contractions of the haustrae slowly shuffle the
    colonic contents back and forth.
   Mass movements propel colonic contents long
    distances.
   They drive the feces into the distal part of the
    large intestine.
   Material is stored here until eliminated by
    defecation.
   Muscle contractions and the relaxation of two
    sphincter muscles eliminate the feces.
   Intestinal gases are absorbed or expelled.
There are several important gastrointestinal
hormones.
1) Gastrin is produced by G cells in the stomach
   increases the secretion of hydrochloric acid and
   pepsinogen and enhances gastric motility.
2) Secretin is produced in the duodenum
    inhibits gastric emptying and gastric secretion,
    stimulates the pancreas to produce NaHCO3,
    stimulates the liver to produce bile.
3) CCK is produced in the duodenum
    inhibits gastric emptying and gastric secretion,
    stimulates the pancreas to produce NaHCO3,
    signals the gallbladder to secrete bile.
4) GIP promotes metabolic processing of nutrients
   once they are absorbed.

								
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