Saladin Ch. 25

1
Saladin Ch. 25

• The Digestive System

2
Digestion

• Digestion
• breakdown of larger food molecules into
  particles small enough for absorption
• Digestive System
• organs that collectively perform digestion

3
Digestive System Overview
Figure 23.1
4
Digestive Process

• The GI tract is a disassembly line
• Nutrients become more available to the body in
  each step
• There are six essential activities
• Ingestion, propulsion, and mechanical digestion
• Chemical digestion, absorption, and defecation

5
Digestive Process
Figure 23.2
6
Gastrointestinal Tract Activities

• Ingestion eating
• Propulsion swallowing peristalsis
• Peristalsis waves of contraction relaxation
  of muscles in organ walls
• Mechanical digestion chewing, mixing,
  churning food

7
Peristalsis and Segmentation
Figure 23.3
8
Gastrointestinal Tract Activities

• Chemical Digestion Secretion of water, acid,
  buffers, enzymes. Large molecules are split into
  smaller pieces.
• Absorption material enters epithelial cells
  lining GI tract and is passed on to the lymph or
  blood.
• Defecation elimination of indigestible solid
  wastes.

9
Digestive Accessory Organs

• Digestive
• Gastrointestinal (GI) Tract Alimentary Canal -
  continuous tube running from mouth to anus.
  Digests and absorbs. Mouth, pharynx, esophagus,
  stomach, small intestine, large intestine
• Accessory Digestive Organs
• Aid in processing - food doesn't go into these
  organs. Teeth, tongue, salivary glands, liver,
  gallbladder, pancreas.

10
Histology of the Alimentary Canal

• All the walls of the GI tract have the same four
  tunics
• From the lumen outward they are the mucosa,
  submucosa, muscularis externa, serosa.

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Histology of the Alimentary Canal
Figure 23.6
12
Mucosa

• Moist epithelial layer that lines the lumen of
  the alimentary canal.
• Three layers a lining epithelium, lamina
  propria, muscularis mucosae.

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Mucosa Epithelial Lining

• Mouth, pharynx, esophagus, anal canal
  non-keratinized stratified squamous.
• Stomach, intestines simple columnar epithelium.
  
• Exocrine glands Put mucus fluid in
• Enteroendocrine glands - secrete hormones into
  blood stream.

14
Mucosa Lamina Propria and Muscularis Mucosae

• Lamina Propria layer of areolar tissue deep to
  lining - Connects to
• muscularis mucosae.
• Muscularis mucosae - thin smooth muscle fibers -
  helps increase surface area by moving folds of
  mucosa so they are fully exposed to nutrients.

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Submucosa

• Submucosa - binds mucosa to muscularis externa
• Areolar CT
• Contains submucosal plexus - part of the enteric
  nervous system
• Also contains - glands, blood, lymph

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Muscularis Externa Serosa

• Muscularis Externa Smooth muscle layer.
• Contains the myenteric plexus - the other enteric
  nervous system component.
• Serosa - superficial layer of GI organs in the
  abdominal cavity CT and epithelium
• Visceral layer of peritoneum - serous membrane.

17
Enteric Nervous System

• Composed of two major intrinsic nerve plexuses
• Submucosal nerve plexus regulates glands and
  smooth muscle in the mucosa.
• Myenteric nerve plexus Major nerve supply that
  controls GI tract mobility.

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Enteric Nervous System

• Segmentation peristalsis are largely automatic
  involving local reflex arcs.
• Linked to the CNS via long autonomic reflex arc.

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Peritoneum and Peritoneal Cavity

• Peritoneum serous membrane of the abdominal
  cavity.
• Parietal lines the body wall.
• Visceral covers external surface of most
  digestive organs.

20
Peritoneum and Peritoneal Cavity

• Peritoneal cavity
• Narrow space between parietal and visceral
  layers.
• Lubricates digestive organs.
• Allows them to slide across one another.

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Peritoneum and Peritoneal Cavity
Figure 23.5a
22
Peritoneum and Peritoneal Cavity

• Lesser Omentum - from stomach and duodenum -
  attaches these to the liver.
• Greater Omentum - fatty apron hanging down over
  colon and small intestine on the anterior aspect.
  
• Mesocolon - fold that binds the large intestine
  to the posterior abdominal wall

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Peritoneum and Peritoneal Cavity

• Retroperitoneal organs organs outside the
  peritoneum.
• Only have peritoneum on anterior surface.
• Peritoneal organs (intraperitoneal) organs
  surrounded by peritoneum.

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Regulation of GI Tract

• Regulation of digestion involves
• Mechanical chemical stimuli stretch
  receptors, osmolarity, presence of substrate in
  the lumen
• Extrinsic control
• Intrinsic control

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Nervous Control of the GI Tract

• Intrinsic controls
• Nerve plexuses near the GI tract initiate short
  reflexes.
• Short reflexes are mediated by local enteric
  plexuses (gut brain).

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Nervous Control of the GI Tract

• Extrinsic controls
• Long reflexes arising within or outside the GI
  tract.
• Involve CNS centers and extrinsic autonomic
  nerves.

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Regulation of GI Tract

• Hormones
• Gastrin, secretin, CCK, histamine, paracrines
  see hormone chart

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Mouth

• Oral or buccal cavity
• Is bounded by lips, cheeks, palate, tongue.
• Vestibule bounded by the lips cheeks
  externally, teeth and gums internally.

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Anatomy of the Oral Cavity Mouth
Figure 23.7a
31
Tongue

• Skeletal muscle covered with mucous membrane.
• Intrinsic muscles originate and insert in
  tongue - For swallowing and speech
• Extrinsic muscles originate outside the tongue
  insert in it. Move side to side in out. Aid
  in movement of food into bolus.

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Palate

• Palate
• Hard palatine bones palatine processes of
  maxilla
• Soft skeletal muscle

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Oral Cavity and Pharynx Anterior View
Figure 23.7b
34
Teeth

• Primary and permanent dentitions have formed by
  age 21.
• Primary 20 deciduous milk teeth.
• Permanent 32 secondary teeth,

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Classification of Teeth

• Teeth are classified according to their shape and
  function.
• Incisors chisel-shaped - adapted for cutting or
  nipping.
• Canines conical or fanglike - tear or pierce
• Premolars (bicuspids) molars broad crowns
  with rounded tips - for grinding or crushing

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Deciduous Teeth
Figure 23.10.1
37
Permanent Teeth
Figure 23.10.2
38
Tooth Structure

• Two main regions crown root.
• Crown exposed part above the gingiva (gum).
• Root portion embedded in the jawbone.

39
Tooth Structure

• Enamel acellular, brittle material composed
  mostly of calcium phosphate.
• Enamel is the hardest substance produced by the
  body.
• Encapsulates the crown of the tooth.

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Tooth Structure

• Anatomy of a tooth
• Crown, neck, root, apical foramen.
• Enamel hardest substance produced, covers
  crown Mostly calcium phosphate
• Dentin bone-like, secreted by cells of pulp

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Tooth Structure

• Cementum covering of root, similar to bone
• Periodontal ligament anchors tooth to alveolar
  socket, which is surrounded by gingiva (gums)

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Tooth Structure
Figure 23.11
43
Salivary Glands

• Parotid anterior to ear over the masseter.
• Parotid duct opens into vestibule next to the
  second upper molar.
• Secretes salivary amylase.
• Swollen when infected by mumps virus

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Salivary Glands

• Submandibular beneath posterior tongue.
• Secretes saliva with salivary amylase.
• Sublingual anterior to submandibular gland
  under the tongue.
• Opens into the floor of the mouth.

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Salivary Glands
Figure 23.9a
46
Saliva

• Saliva
• 1-1.5 L/day
• Blood filtrate salivary amylase, mucin
  lysozyme

47
Saliva

• Control ANS food stimulates tactile, pressure
  and taste sensor ? salivatory nuclei in medulla
• Parasympathetic normal salivation facial and
  glossopharyngeal nerves
• Sympathetic decreased salivation

48
Pharynx

• From the mouth, the oro- laryngopharynx allow
  passage of
• Food and fluids to the esophagus.
• Air to the trachea.
• See Ch. 22

49
Esophagus

• Muscular skeletal to smooth a collapsible
  tube. All 4 layers, modified.
• Mucosa stratified squamous epithelium
  esophageal glands in submucosa musc. Ext. has
  both smooth and skeletal muscle serosa replaced
  with adventitia

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Esophagus

• Passes through mediastinum and diaphragm
  esophageal hiatushiatal hernias
• Functions secretes mucus, transports bolus to
  stomach cardiac orifice

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Swallowing

• Deglutition swallowing controlled by medulla
  pons swallow center involves trigeminal,
  facial, glossopharyngeal, hypoglossal nerves.
• 2 phases
• Buccal phase bolus formation,
  pharyngealesophageal phase moves to stomach
• Peristalsis waves of muscular contraction
  move bolus

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Deglutition (Swallowing)
Bolus of food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Esophagus
Trachea
Bolus
(c) Upper esophageal sphincter contracted
(a) Upper esophageal sphincter contracted
(b) Upper esophageal sphincter relaxed
Relaxed muscles
Relaxed muscles
Circular muscles contract, constricting
passageway and pushing bolus down
Gastroesophageal sphincter open
Bolus of food
Longitudinal muscles contract, shortening
passageway ahead of bolus
Gastroesophageal sphincter closed
Stomach
(d)
(e)
Figure 23.13
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Stomach

• Functions
• Storage
• Mechanical breakdown of food
• Enzymatic digestion of food
• Absorption of simple compounds
• Produces gastrin
• Bacterial protection
• Produces intrinsic factor

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Stomach

• Cardiac region surrounds the cardiac orifice.
• Fundus dome-shaped region beneath the
  diaphragm.
• Body midportion of the stomach.
• Pyloric region made up of the antrum and canal,
  through the pyloric sphincter.

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Stomach
Figure 23.14a
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Stomach

• Innervation circulation
• Para vagus, sym celiac ganglion
• Arterial celiac artery, venous hepatic portal
  system

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Stomach Wall

• Muscularis 3 layers instead of two
  longitudinal, circular and oblique smooth layers
• Submucosa mucosa - Ruggae large folds in
  mucosa
• Mucosa simple columnar epithelium

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Glands of the Stomach Fundus and Body

• Gastric Pits of the fundus body have a variety
  of secretory cells
• Mucous cells gt mucus
• Parietal cells gt HCl intrinsic factor
• Chief cells gt pepsinogen
• Pepsinogen is activated to pepsin by

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Glands of the Stomach Fundus and Body

• Enteroendocrine cells gt gastrin, histamine,
  serotonin, somatostatin into the lamina propria

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Microscopic Anatomy of the Stomach
Figure 23.15
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Gastric Secretions

• Produces 2-3L of gastric juice/day
• HCl get H from CO2 carbonic anhydrase
  reaction parietal cells pumped out by active
  transport.
• Pepsin digests protein also get chymosin
  gastric lipase in infants

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Gastric Secretions

• Intrinsic factor - also from parietal cells
  needed to absorb B12 only indispensable function
  of stomach
• Chemical messengers enteroendocrine cells make up
  to 20 include gut-brain peptides like substance
  P, vasoactive intestinal peptide, secretin, etc.

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Stomach Lining

• To keep from digesting itself, the stomach has a
  mucosal barrier with
• A thick coat of bicarbonate-rich mucus on the
  stomach wall.
• Epithelial cells that are joined by tight
  junctions.
• Gastric juices released only in presence of food
  which dilutes its effects

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Gastric Motility

• Receptive-relaxation response stomach relaxes
  to receive food stim. by medulla
• Peristaltic contractions increasing in
  strength.
• Squirts 3 mL of chime into duodenum/ wave

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Vomiting

• Emetic center of medulla
• Usually preceded by nausea retching
• Projectile vomiting no prior nausea, etc.
  common in infants.
• Bulimia eating disorder erodes tooth enamel,
  aspiration of acid injures respiratory tree, acid
  reflux damages esophagus

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Regulation of Gastric Secretion

• Neural hormonal mechanisms regulate release of
  gastric juice.
• 3 phases of events
• Cephalic (reflex) phase prior to food entry.
• Gastric phase once food enters the stomach.
• Intestinal phase as partially digested food
  enters the duodenum.

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Cephalic Phase

• Events include
• Sight or thought of food Stimulation of taste or
  smell receptors.
• Stimulate secretions from parietal cells, Chief
  cells, and mucus cells.

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Gastric Phase

• Events include
• Release of gastrin to the blood.
• Stomach distension increase in pH due to food.

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Intestinal Phase

• Mostly inhibitory small intestine receptors are
  activated by influx of chyme enterogastric
  reflex.
• Slows exit of chyme and prevents duodenal
  overload.

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Intestinal Phase

• Chyme also stimulates enteroendocrine cells to
  release secretin, CCK and gastric inhibitory
  protein.
• Secretin CCK stim. pancreas and gallbladder.
  All 3 suppress gastric secretion motility
•  

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Release of Gastric Juice
Figure 23.16
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Liver

• The largest gland in the body
• Functions
• Synthesis of bile salts
• Excretion of bile

73
Liver

• 2 Lobes R,L separated by falciform running from
  diaphragm between lobes.
• The right lobe is further subdivided into two
  lobes - the quadrate and caudate.
• Falciform Ligament - binds liver to anterior
  wall.
• The round ligament exits the falciform ligament
  from the liver - former umbilical vein.

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Liver Microscopic Anatomy

• Lobes are divided into working units called
  lobules
• Plates of hepatocytes specialized epithelium
  that secrete bile
• Central vein

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Liver Microscopic Anatomy

• Liver sinusoids enlarged, leaky capillaries
  located between hepatic plates.
• Blood from stomach SI is filtered and glucose,
  amino acids, iron, vitamins, etc are removed, as
  are toxins, drugs, etc.
• Kupffer cells hepatic macrophages found in
  liver sinusoids.

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Microscopic Anatomy of the Liver
Figure 23.24c, d
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Liver Bile

• Produced by hepatocytes.
• Bile leaves the liver via
• Bile canalculi to bile ducts
• Bile ducts fuse into the common hepatic duct

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Liver Bile

• The common hepatic duct fuses with the cystic
  duct? the bile duct
• Bile duct joins pancreatic duct ?
  hepatopancreatic ampulla ? SI

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Gallbladder

• Thin-walled, green muscular sac on the ventral
  surface of the liver.
• Releases bile via the cystic duct, which flows
  into the bile duct.
• Stores and concentrates bile by absorbing its
  water and ions.

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Composition of Bile

• Bile - Yellow, brownish to olive green -
  500-1000mL/day
• Water, bile acids, bile salts, cholesterol, ions,
  phospholipids lecithin bile pigments bilirubin
  etc.
• Bile Salts acids steroids made from
  cholesterolemulsify fats

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Composition of Bile

• Recycled 80 reabsorbed by SI and re-secreted
  by liver
• Wastes include from RBC breakdown ? bilirubin
  from heme -- intestine ? broken into stercobilin,

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Gallbladder and Associated Ducts
Figure 23.20
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Pancreas

• Size 5 inches long and 1 inch thick
• Regions head, body, tail
• Ducts
• 1 pancreatic duct fuses with common bile duct
  forming the hepatopancreatic ampulla - enters
  duodenum about 4 inches below pylorus
• 2 accessory duct enters 1 inch above ampulla

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Pancreas

• Exocrine function - Secretes pancreatic juice
  which breaks down all categories of foodstuff.
• Endocrine function release of insulin
  glucagon.

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Acinus of the Pancreas
Figure 23.26a
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Pancreatic Juice

• Pancreatic Juice clear, colorless,
  1.2-1.5L/day, pH about 8
• Mostly water
• Sodium bicarbonate increase pH, buffers acidic
  chyme, stops pepsin action
• Salts

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Composition Function of Pancreatic Juice

• Enzymes secreted
• Trypsinogen is activated to trypsin
• Carboxypeptidase
• Chymotrypsinogen
• Amylase, lipases, nucleases
• These enzymes require ions or bile for optimal
  activity

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Regulation of Secretion

• Increased with parasympathetic, decreased with
  sympathetic.
• Bile pancreatic juice stim. By CCK, gastrin
  secretin
• CCK released from duodenum response to acid fat
  ? contraction of gallbladder, secretion of
  pancreatic enzymes, relaxation of
  hepatopancreatic sphincter

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Regulation of Secretion

• Secretin stimulates secretion of bicarbonate to
  neutralize stomach acid in duodenum.

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Small Intestine Gross Anatomy

• Runs from pyloric sphincter to the ileocecal
  valve.
• 3 subdivisions duodenum, jejunum, ileum.
• Ileocecal valve sphincter to the large
  intestines.
• Mesentery peritoneum supporting the small
  intestines.

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Small Intestine Microscopic Anatomy

• Structural modifications of the small intestine
  wall increase surface area.
• Circular folds of the mucosa
• Villus structure with a lamina propria core
  containing arterioles, venules, capillary nets
  and lymphatic lacteals, covered with simple
  columnar epithelium with microvilli , goblet
  cells and absorptive cells

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Small Intestine Microscopic Anatomy

• Lacteals-absorption of fats
• Goblet cells - secrete mucus
• Absorptive cells simple columnar epithelium -
  absorb and make brush border enzymes
• Intestinal Crypts - tubular glands in floor of
  SI between villi bases secrete intestinal juice
  enzymes

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Small Intestine Microscopic Anatomy

• Paneth Cells - phagocytic - produce lysozyme
• Enteroendocrine glands - secrete secretin and
  CCK
• Duodenal Brunner's in submucosa -secrete
  alkaline mucus neutralizes stomach acid

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Small Intestine Microscopic Anatomy
Figure 23.21
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Intestinal Juice

• Clear yellow - 1-2 L/day, pH 7.6
• Contains water, mucus, enzymes - aid absorption
  of nutrients from chyme
• Resorbed by villi

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Motility in the Small Intestine

• Segmentation-
• Localized mixing motions bring contents in
  contact with mucosa.
• Regulated by pacemaker cells 12 contractions/
  min duod. 8-9 ileum.
• After nutrients have been absorbed
• Peristalsis begins.
• Meal remnants, bacteria, mucosal cells, debris
  are moved into large intestine.

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Control of Motility and Secretion

• Local enteric neurons of the GI tract coordinate
  intestinal motility
• Distension
• Presence of Chyme
• Vasoactive intestinal polypeptide VIP increases
  secretions
• Gastrin

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Control of Motility and Secretion

• ANS
• Parasympathetic increases motility.
• Sympathetic-decreases motility.

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Chemical Digestion Carbohydrates

• Enzymes salivary amylase, pancreatic amylase,
  brush border enzymes
• Absorption - glucose and galactose enter
  epithelial cells via symporters, accompanied by
  Na ions. Fructose enters by facilitated
  diffusion.
• All exit to the blood by facilitated diffusion.

103

• Lactose intolerance lack enzyme lactase large
  proportion of world population

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Chemical Digestion Proteins

• Enzymes used pepsin in the stomach.
• Enzymes acting in the small intestine
• Pancreatic enzymes trypsin, chymotrypsin
• Brush border enzymes peptidases.

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Chemical Digestion Proteins

• Absorption
• Amino acids, di and tripeptides - aa - by simple
  active transport or in symporters with Na
• Di and tripeptides - by symporters with H. All
  exit to blood by diffusion.

106
Chemical Digestion Proteins
Figure 23.34
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Chemical Digestion Fats

• Lipids - digested by lipases
• Fats emulsified by bile salts first
• Products are fatty acidsFFAsand
  monoglycerides
• Micelles in bile take up phospholipids
  cholesterol, fat soluble vitamins, FFAs and
  monoglycerides and transport to intestinal cells

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Chemical Digestion Fats
Figure 23.35
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Chemical Digestion Fats

• FFAs and monoglycerides enter epithelial cells
  and are re-assembled into triglycerides, combine
  with cholesterol, phospholipids and protein to
  form chylomicrons and exit by exocytosis into
  lacteals

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Fatty Acid Absorption
Figure 23.36
111
Chemical Digestion Nucleic Acids

• Enzymes used pancreatic nucleases in the small
  intestines.
• Brush Border enzymes nucleosidases,
  phosphatases.
• Sugars, bases and phosphate ions absorbed by
  active transport ? across epithelium and into
  blood

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Vitamins Minerals

• Vitamins - water soluble diffuse, lipid soluble
  enter with lipids A,D,E,K
• Minerals Electrolytes - absorbed through entire
  length of SI by diffusion or active transport -
  Na/K and chloride/bicarbonate pumps.
• Fe Ca absorbed as needed, rest absorbed at
  constant rate regardless of need.

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Water

• Water - absorbed by osmosis - 9L into system -
  about 8L absorbed - into blood
• Diarrhea occurs when the large intestine absorbs
  too little
• Constipation occurs when passage through the LI
  is too slow and too much water is absorbed

114
Large Intestine

• Functions
• Absorption, manufacture of some vitamins.
• Formation expulsion of feces.

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LI Anatomy

• 1.5 M long, runs from cecum to rectum
• Ileocecal sphincter - joins SI to LI.
• Cecum - region of LI inferior to the ileocecal
  sphincter.
• Appendix - next to the cecum.

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LI Anatomy

• Colon - above sphincter - ascending, transverse,
  descending and Sigmoid regions
• Rectum valves- region inferior to sigmoid
  colon
• Anal Canal - below rectum

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LI Anatomy

• Taenia coli - longitudinal bands of muscles
  running the length of the LI
• Haustra - pouches produced in the LI by tonal
  contraction of taenia coli.

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Large Intestine
Figure 23.29a
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Large Intestine Microscopic Anatomy

• Mucosa Simple columnar epithelium no villi or
  enzyme secreting cells goblet cells

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Bacterial Flora

• These bacteria
• Colonize the colon
• Ferment indigestible carbohydrates
• Release irritating acids and gases (flatus)
• Synthesize B complex vitamins and vitamin K

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Motility of the Large Intestine

• Haustral contractions
• Slow segmenting movements that move the contents
  of the colon.
• Haustra sequentially contract as they are
  stimulated by distension.
• Presence of food in the stomach
• Activates the gastrocolic reflex.
• Initiates peristalsis that forces contents toward
  the rectum.

122
Defecation

• Defecation Reflexes initiated by rectal
  stretching
• Intrinsic entirely within enteric system weak
  peristalsis
• Sympathetic Spinal cord reflex - Sends message
  to CNS to decide whether to relax external
  sphincter.

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Defecation
Figure 23.32