Digestive System

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Digestive System

• Chapter 23

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Two major parts

• Alimentary canal
• GI tract- long hollow canal from mouth to anal
  canal continuous with outside of body
• Oral cavity, pharynx, esophagus, stomach, small
  intestine, large intestine, anal canal

• Accessory organs
• Teeth, tongue, salivary glands, liver, pancreas

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Functions of Digestive System

• Mechanical and chemical breakdown of food
• absorption of nutrients
• elimination of waste products

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QUIZ PICTURE
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Mechanical Breakdown of Food

• Physical preparation of food for breakdown by
  enzymes
• Includes chewing, mixing of food with saliva,
  food churning in stomach, segmentation of food in
  small intestine

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Chemical Breakdown of Food

• Series of steps which breaks down large food
  molecules with enzymes secreted into the
  alimentary canal
• Begins in the mouth and is completed in the small
  intestine

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Absorption of Nutrients

• The passage of digested molecules of food, water,
  minerals, and vitamins from the lumen of the GI
  tract into the mucosal cells by active or passive
  transport into blood or lymph

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Elimination of Waste

• Ridding the body of indigestible by products of
  digestion

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Upper esophageal
Tonic contractions are shown by sphincters most
are smooth muscle, some are skeletal.
Lower esophageal
Pyloric
Oddi
Ileocecal
Internal anal
Fig. 22-9, pg 691
External anal
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Histology of GI Tract

• 4 Basic Layers
• Mucosa
• Submucosa
• Muscularis Externa
• Serosa

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Quiz Picture
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Mucosa

• Functions
• Secretion of mucous, hormones, enzymes
• Absorption of nutrients
• Protection against infection
• Simple columnar epithelium atop loose areolar
  connective tissue and lymphatic tissue with a
  base of smooth muscle that controls movement of
  this layer

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Submucosa

• Dense CT layer with blood and lymph vessels,
  lymph follicles, nerve fibers

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Muscularis Externa

• Layer responsible for segmentation and
  peristalsis
• Inner circular and outer longitudinal layers of
  smooth muscle thickened areas can form
  sphincters that act as valves

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Serosa

• This is also the visceral peritoneum formed of
  areolar CT and covered by simple squamous
  epithelium
• The esophagus doesnt have a serosa because it is
  in the thorax it has an adventitia (fibrous CT)
  that connects it to surrounding structures in
  chest

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Homeostatic Imbalance

• Peritonitis
• Inflammation of peritoneum
• Puncture wound of abdomen

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Salivary Glands

• Parotid Glands- located anterior to ear
• Sublingual Glands- located in floor of mouth
• Submandibular Glands- located under jaw
• Function produce saliva- mostly water, salivary
  amylase, antimicrobials
• Saliva moistens food, starts starch breakdown,
  dissolves food chemicals for taste

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Homeostatic Imbalance

• Mumps
• Inflammation of parotid gland
• Passed by saliva
• In males can lead to testes infection

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Homeostatic Imbalance

• Halitosis
• Inhibited saliva production
• No salivano washing of mouth
• Cavities, anaerobic bacteria make hydrgen
  sulfide, methyl mercaptan and cadaverine which
  smell like rotten eggs, feces and corpes

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Mouth, Pharynx, Esophagus

• Mouth- place where mechanical breakdown of food
  begins with chewing
• Pharynx- has 2 skeletal muscle layers whose
  contractions propel food into esophagus
• Esophagus- long tube to stomach posterior to
  trachea conduit for food peristalsis (alternate
  waves of contraction/relaxation of muscle that
  propels food to stomach)

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Swallowing uses contraction of tongue, upper
pharynx constrictors, and laryngeal muscles. Is
voluntarily initiated, but medullary swallowing
center coordinates muscles.
Fig. 22-10, pg 692
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Stomach

• Chemical breakdown of proteins begins here
• food becomes chyme
• upper end to esophagus is cardiac portion fundus
  is top
• middle is body and end is pylorus

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Quiz Picture
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Stomach Histology

• Epithelium- simple columnar
• Gastric pits- primarily goblet cells
• Gastric glands- below gastric pits
• Neck Cells- produce acidic mucous
• Parietal Cells- secrete HCl and Intrinsic Factor
• Chief Cells- produce pepsinogen to digest
  protein
• Enteroendocrine Cells- release hormones
  gastrin, histamine, endorphins, serotonin,
  cholecystekinin, somatostatin

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Stomach Mucosal Barrier

• Bicarbonate rich mucous on stomach wall
• Epithelial cells connected by tight junctions
• Gastric Gland cells impermeable to HCl
• Damaged epithelial cells replaced quickly lining
  shed every 3-6 days

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Functions of Stomach

• Protein digestion- only enzymatic type that
  occurs here
• Secretion of IF- essential for intestinal
  absorption of B12 B12 deficiency causes
  Pernicious Anemia

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Control of Gastric Secretion

• Neural Control- CN X (Vagus)- stimulation causes
  increase of all gland secretion
• Gut Brain local reflexes
• Hormonal Control- Gastrin- stimulates
  secretion of enzymes and HCl and hormones of
  small intestine

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Homeostatic Imbalances

• Gastroesophageal reflux disease (GERD)
• Heartburn
• Stomach acid travels back up esophagus
• Hiatal hernia
• Damage to esophagus if prolonged

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Homeostatic Imbalances

• Gastric Ulcers
• Erosion of stomach wall
• Breach mucosal barrier
• Stress, medicines, bacteria (helicobacter pylori

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Small Intestine

• Three Sections
• 1. Duodenum
• 2. Jejunem
• 3. Ilium

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Small Intestine

• 1. Duodenum- 10 inches receives bile from the
  bile duct and pancreatic juice from the
  pancreatic duct 2 ducts join together and
  secretion regulated by sphincter of Oddi

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Small Intestine (cont)

• 2. Jejunum- about 8 feet long
• 3. Ilium- about 12 feet long joins the large
  intestine at the ileocecal valve

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Peritoneum

• Serous membrane- double membrane
• Mesentery is a sheet of 2 serous membranes fused
  back to back holds part of the GI tract
  together extends from body wall to digestive
  organs provides route for blood/lymphatic
  vessels and nerves

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Modifications for Absorption

• Plicae Circularis- deep permanent folds of
  mucosa/submucosa
• Villi- fingerlike projections of mucosa act like
  the loops of terry cloth to increase absorptive
  surface area contains the lacteals of the lymph
  system for fat absorption
• Microvilli- fingerlike projections of plasma
  membrane of each individual cell of the simple
  columnar epithelium containing brush border
  enzymes

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Quiz Picture
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Sm Int
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• 2) villi
• fingerlike projections on mucosa
• Surface cells for absorption
• Contains capillary bed and lacteal (lymph
  capillary)
• Made of cells

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• 3) microvilli
• tiny plasma membrane projections of absorptive
  cells of mucosa
• Fuzzy appearance called brush border
• Brush border enzymes located here
• On a single cell

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Histology of Small Intestine

• Epithelium- simple columnar with goblet cells
  enteroendocrine cells that secrete intestinal
  Gastrin, Secretin, Cholecystikinin (CCK), GIP,
  VIP
• Intestinal Pits located above intestinal glands
  that are called Crypts of Leiberkuhn- secrete
  intestinal juice, lysozyme
• Brunners Glands- duodenum only alkaline mucous

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Liver

• Digestive function- produce bile for transport to
  small intestine
• Other functions include storage of Vit. A, D, E,
  and K and production of proteins including
  albumin, clotting factors and production of
  cholesterol

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Gross anatomy of Liver

• About 3 pounds
• Divided into right and left lobes
• Bile produced in the liver leaves through the
  common hepatic duct which joins the Cystic Duct
  of the gall bladder to form the Common Bile Duct
• Under R costal margin from 5th to 10th rib

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Histology of Liver

• Functional Unit- Liver Lobule consisting of
  hepatocytes (liver cells) and vessels hexagonal
  with each corner being a portal triad (artery,
  vein, bile duct) center is the central vein
• Sinusoids- leaky capillaries between liver cells
  also Kupffer cells (macrophages
• Bile Canaliculi- canals between liver cells that
  take bile to bile ducts

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Homeostatic Imbalances

• Cirrhosis
• Chronic inflammation of the liver
• Obstructed blood flow through liver, portal
  hypertension
• Chronic alcoholism or hepatitis

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Bile

• Bile Salts- cholesterol derivatives emulsify
  fats
• Phospholipids
• Fats
• Electrolytes
• Bile Pigments

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Gall Bladder

• Green muscular sac about 4 inches long stores
  bile until needed during digestion
• Major Stimulus for release- Cholecystekinin (CCK)

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Pancreas

• Retroperitoneal organ under stomach
• Endocrine and exocrine functions
• Digestive function is exocrine in nature
  secretes pancreatic juice containing water,
  bicarbonate ions and enzymes
• Neural and hormonal control
• Secretin- release bicarb CCK- release enzymes
  pancreatic juice - on list

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Large Intestine

• About 4-5 feet long
• Function absorb water from undigested food
  products and deliver waste to be eliminated

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Gross Anatomy

• Teniae Coli- Longitudinal muscle of muscularis is
  tightened up to form a line through the outside
  of the LI
• Haustra- pocket-like sacs formed due to tone of
  teniae
• Epiploic Appendages- fat filled pouches of
  visceral peritoneum on surface

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Quiz Picture
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Subdivisions of Large Intestine

• Cecum
• Appendix
• Colon
• Rectum
• Anal Canal

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Microscopic Anatomy

• Simple columnar Epithelium up to anal canal which
  is stratified squamous epithelium
• Deep crypts containing an enormous number of
  goblet cells no digestive enzymes

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Homeostatic Imbalances

• Appendicitis
• Infection of appendix
• Blocked by feces, swells and cut of venous flow
  so appendix dies
• adolescence

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Homeostatic Imbalances

• Diarrhea/Constipation
• Watery stools/Hard stools
• Food rushed through large intestine
• Irritation of colon
• Bad diet

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Chemical Digestion/Absorption
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Chemical Digestion/Absorption

• Carbohydrates
• broken down into sugars
• Starts with salivary amylase in mouth and
  progresses to pancreatic amylase in small
  intestine and brush border enzymes in the
  microvilli

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Chemical Digestion/Absorption

• Proteins
• starts in stomach when pepsinogen is converted to
  pepsin by HCl environment of stomach
• Brush border enzymes and pancreatic enzymes
  (Trypsin/Chymotrypsin) digest proteins all the
  way to amino acids

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Chemical Digestion/Absorption

• Nucleic Acids
• broken down by pancreatic nucleases and by brush
  border enzymes in the small intestine

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Chemical Digestion/Absorption

• Lipids
• Small intestine is the only place that this
  occurs
• needs to be emulsified by bile into fat droplets
• some lipases secreted by pancreas
• as emulsifications get smaller they become
  micelles

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Chemical Digestion/Absorption

• Vitamins- A, D, E, and K are fat soluble and
  follow the absorption of fat water soluble
  vitamins are absorbed by diffusion B12 can only
  be absorbed if attached to IF absorbed by
  endocytosis
• Electrolytes- absorbed by active and passive
  transport along entire small intestine

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Chemical Digestion/Absorption

• Water- directly coupled to solute intake water
  chases particles and is absorbed by osmosis along
  the intestines
• Carbohydrates- facilitated diffusion and
  secondary active transport
• Proteins- coupled to the active transport of
  sodium ion
• Lipids- micelles by simple diffusion (into Lymph)

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Nutrition

• Process by which an organism obtains substances
  to promote growth, maintenance and repair

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Major Nutrients

• Carbohydrates
• Lipids
• Proteins

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Vitamins

• Most used as coenzymes to accomplish some
  particular task

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Minerals

• Seven required in large amounts calcium,
  phosphorus, potassium, sulfur, sodium, chlorine,
  magnesium
• Trace amounts of several others F, Co, Cr, Cu,
  I, Fe, Mn, Se, Zn
• Function added to other molecules for strength
  and better function eg. Fe in hemoglobin

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Adipose Tissue Metabolism

• Stored fat in adipose tissue
• Most calories per gram compared to CHO and
  proteins
• Body will metabolize fat if glucose is not
  available

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Energy Balance

• Energy Intake Energy Output
• If we consume more calories than we need to
  perform essential functions we store the excess
  as adipose
• If we consume fewer calories than we need to
  perform essential functions we lose adipose fat
  stores and can starve