Chapter 14 Digestive System

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Chapter 14 Digestive System

• Biology 112
• Tri-County Technical College
• Pendleton, SC

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Functions of Digestive System

• Digestion is mechanical and chemical breakdown of
  foods into nutrients that cell membranes can
  absorb
• System consists of alimentary canal (GI tract)
• Extends from mouth to anus
• Includes several accessory organs that release
  secretions into the canal

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Functions, cont.

• Alimentary canal digests food and absorbs
  fragments through its lining into the blood
• Accessory organs (teeth, tongue, and several
  large digestive glands) assist the digestive
  breakdown in several ways
• Digestive system makes nutrition possible
• One could accurately state that a human is a
  digestive tube with a body built around it

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Organ Sequence

• Bolus defined as rounded mass of food prepared by
  the mouth for swallowing
• Mouth?pharynx?esophagus?stomach? small
  intestine?large intestine?anal canal
• Accessory organs include the salivary glands,
  liver, gallbladder, and pancreas

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Digestive System Visual
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Layers of the Tract

• Wall of tract consists of 4 distinct layers
  (tunics)
• Innermost mucosa (mucous membrane) layer composed
  of surface epithelium, underlying connective
  tissue, and small amt. of smooth muscle
• Develops folds/tiny projections in some regions
  that extend into passageway (LUMEN)
• Increase mucosas absorptive surface area
• May contain glands that secrete mucus/digestive
  enzymes
• Mucosa secretes/absorbs protects tissues
  beneath it (really exterior to it)

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Layers, cont.

• SUBMUCOSAloose connective tissue, glands, blood
  vessels, lymphatic vessels, nerves (organized
  into a plexus)
• Vessels nourish surrounding tissue carry away
  absorbed material
• MUSCULAR layer moves tube and has 2 coats of
  smooth muscle tissue and some nerves (plexus)
• Fibers of inner coat encircle tube (circular)
• when these contract, tube diameter gets smaller

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Layers, cont.

• Fibers of outer coat run lengthwise
  (longitudinal)
• fibers contract, tube length gets shorter
• SEROUS layer comprised of visceral peritoneum
• Outermost layer
• Cells protect underlying tissues secrete serous
  fluid-moistens and lubricates outer surface
• Organs within abdominal cavity slide freely
  against one another

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Layers Visual
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Function(s) of the Stomach

• Located under diaphragm in upper left portion of
  abdominal cavity
• Thick folds (rugae) of mucosal and submucosal
  layers mark inner lining
• Disappear when stomach distends
• Stomach receives food from esophagus, mixes it
  with gastric juice, initiates protein digestion,
  does limited absorption, and moves food into
  small intestine

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Regions of the Stomach

• Stomach divided into cardiac, fundic, body, and
  pyloric regions
• Cardiac-small area near esophageal opening
• Fundic-balloons above cardiac portion is
  temporary storage area
• Body-main part of stomach that lies between
  fundic and pyloric portions
• Pyloric-narrows and becomes pyloric canal as
  approaches small intestine

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Stomach Functions, cont.

• At end of pyloric canal, wall thickens to form
  pyloric sphincter
• muscular valve controlling stomach emptying
• After meal, mixing movements of stomach help
  produce semifluid paste called chyme
• Peristalic waves push chyme toward pyloric region
• Rate stomach empties depends on chymes fluidity
  and type of food present
• Liquids?carbs?proteins?fatty foods

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Stomach Digestive Secretions

• Stomach mucosa studded with gastric pits
• gastric pits are ends of tubular gastric glands
• Gastric glands contain 3 types of secretory cells
• Mucous (goblet cells) occur in necks near
  openings of gastric pits
• Chief cell occur in deeper parts of pits and
  secrete digestive enzymes
• Parietal cells occur in deeper parts and secrete
  hydrochloric acid

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Gastric Pits Visual
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Stomach Functions, cont.

• Products of mucous, chief, and parietal cells
  form gastric juice
• PEPSIN most important enzyme in juice
• Made in inactive form (pepsinogen) which is
  converted to active form (pepsin) by contact with
  HClbegins protein digestion
• Intrinsic factor (parietal cells) important for
  vitamin B12 absorption (small intestine)
• Mucous membrane produces mucus (viscous and
  alkaline secretion to coat stomach walls

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Digestive Secretions into Duodenum

• Duodenum is first part of small intestine
• As chyme enters, pancreas, liver, and gallbladder
  add digestive secretions
• Pancreas secretes pancreatic juice
• Liver secretes bile which is stored in
  gallbladder
• Bile salts important in lipid digestion
• Chyme moving into duodenum contains array of
  enzymes/substances

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Digestive Secretions, cont.

• Amylase from salivary glands
• Pepsin from gastric glands
• Amylase, lipase, proteolytic enzymes (trypsin,
  chymotrypsin, carboxypeptidase), and nucleases
  from the pancreas
• Peptidase, sucrase, maltase, lactase, lipase, and
  enterokinase from the duodenum

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Small Intestine Surface Area

• Three structures gt surface area
• Microvilli (brush border)-tiny projections of
  plasma membrane
• Inside each villus is rich capillary bed and
  modified lymphatic capillary called a lacteal
• Circular Folds (plicae circulares) are deep folds
  of both mucosa submucosa layer
• Peyers Patches-lymphatic tissue found in patches
  near end of small intestine help prevent
  bacteria from entering bloodstream

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Surface Area Visual
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The Appendix

• Is a subdivision of large intestine
• Hangs from saclike first part of large intestine
• Is wormlike and potential trouble spot
• Usually twisted and ideal location for bacteria
  to accumulate multiply
• Inflammation of appendix appendicitis
• Remnant from grass-digestion days????

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The Pancreas

• Pancreatic enzymes secreted into duodenum in
  fluid rich in bicarbonate (ph 9) which helps
  neutralize acidic chyme
• Secretes pancreatic amylase (starch), trypsin,
  chymotrypsin, carboxylpeptidase, others
  (proteins), lipases (fat), and nucleases (nucleic
  acids)
• Also has endocrine functionsproduces the
  hormones insulin and glucagon

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Hold the liver..pass the onions

• Liver largest gland in body with 4 lobes
• Suspended from diaphragm and abdominal wall by
  mesentery cord (falciform ligament)
• Livers digestive function is to produce bile
• Bile contains bile salts, bile pigments
  (bilirubin from hemoglobin), cholesterol,
  phospholipids, and variety of electrolytes
• Leaves liver via common hepatic duct and enters
  duodenum through bile duct

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Liver, cont.

• ONLY bile salts and phospholipids aid digestive
  process
• Bile salts emulsify fats by breaking down large
  fat globules into smaller ones gting surface area
  for fat digesting enzymes
• Gallbladder is small green sac in shallow fossa
  in inferior surface of liver
• Food digesting not occurring, bile backs up
  cystic duct

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Enough liveralready

• Bile enters gallbladder via cystic duct
• While being stored bile ed by removal of
  water
• Later, when fatty foods enter duodenum, hormonal
  stimulus prompts gallbladder to release bile into
  duodenum
• Bile stored too long or too ed gallstones
• Jaundice occurs if blockage of common hepatic
  bile ducts prevent bile from entering small
  intestine (also from hepatitis/cirrhosis)

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The Salivary Glands

• Three pairs of salivary glands
• Parotids are large/lie anterior to the ears
• Mumps is an inflammation of the parotids
• Submandibulars and small sublinguals empty
  secretions into floor of mouth
• All produce saliva (mixture of mucus and serous
  fluids
• Mucus moistens/binds food into bolus

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Salivary Glands, cont.

• Clear serous portion contains salivary amylase
  (enzyme) which begins process of starch digestion
  in the mouth
• Saliva also contains lysozyme and IgA antibodies
  that inhibit bacteria
• Also serves to dissolve food chemicals so they
  can be tasted

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Human Dentition

• Are 32 permanent teeth in full set
• Same number and arrangement of teeth exist in
  both upper and lower jaw
• 2 incisors 1 canine 2 premolars (precuspids) 3
  molars (3rd molar wisdom tooth)
• Incisors for cutting canines for
  tearing/piercing premolar and molars for grinding

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Human Dentition Visual
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Anatomy of a Tooth

• Tooth consists of 2 major regions crown and root
• Enamel-covered crown is exposed part of tooth
  above the gingiva (gum)
• Portion of tooth embedded in jawboneroot
• Root and crown connected by neck
• Outer surface of root covered by cementum which
  attaches tooth to periodontal membrane (ligament)
• this ligament holds tooth in place in bony jaw

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Tooth Anatomy, cont.

• Dentin is bonelike material underlying enamel
  forms bulk of tooth
• Dentin surround pulp cavity containing
  connective tissue, blood vessels, and nerve
  fibers (collectively called pulp
• Where pulp extends into root, it becomes the
  root canal which provides route for blood
  vessels, nerves, and other pulp structures to
  enter pulp cavity of tooth

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Tooth Anatomy Visual
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Digesting Digestion

• Mechanical digestion prepares food for
  degradation by enzymes
• biting, chewing, mixing of food in mouth by
  tongue, churning of food in stomach, and
  segmentation in small intestine
• Chemical digestion large food molecules broken
  down to their building blocks by enzymes
  (hydrolytic reactions)
• Ingestion?propulsion?mechanical digestion?
  chemical digestion?absorption?defecation

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To Digest or to Absorb??

• Food consumed generally a polymer
• Must be taken apart into monomers small enough to
  cross PMs of cells lining small intestine
• Materials NOT broken apart cannot be absorbed
  (cellulose)
• Carbohydrates (glucose, fructose, galactose)
• Sucrose, lactose, maltose, and starch
• Proteins and Fats

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Digestion in the Mouth

• Mouth begins mechanical/chemical process
• Saliva contains salivary amylase that begins
  starch digestionmaltose
• Essentially NO food absorption in mouth
• nitroglycerin and alcohol are exceptions
• Pharynx and esophagus have NO digestive functions
• Simply provide passageways to carry food to
  stomach

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Digestion in the Stomach

• Food must be swallowed (deglutition)
• Involves coordinated activity of tongue, soft
  palate, pharynx, and esophagus
• BUCCAL phase occurs in mouth when food has been
  chewed mixed with saliva
• BOLUS is forced into pharynx by tongue
• Enters pharynx passes from conscious control
• Now in realm of reflex activity

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Stomach Digestion, cont.

• PHARYNGEAL-ESOPHAGEAL phase transport bolus
  through
• Parasympathetic controls events (Vagus)
• Promotes motility of digestive organs
• Bolus reaches end of esophagus, presses against
  cardioesophageal sphincter
• Open and allows food to enter stomach
• Secretion of gastric juice regulated by neural
  and hormonal factors

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Stomach Digestion, III

• Presence of food in stomach and falling pH
  ?stomach cells release gastrin
• causes release of protein-digesting enzymes
  (pepsinogens), mucus, and HCl
• If CE sphincter fails to close tightly? gastric
  juices back up into esophagus ? heartburn (hiatal
  hernia)
• Acid environment activates pepsinogen into PEPSIN
• Rennin also produced in stomach of young

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Stomach Digestion, IV

• Stomach will compress, pummel, break apart, and
  mix food with enzymes into chyme (semisolid
  fluid)
• Protein digestion begins in stomach but very
  little chemical digestion occurs
• Alcohol and aspirin ONLY absorbed through stomach
  walls
• Pylorus?pyloric valve?duodenum

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Digestion in Small Intestine

• Carbs and protein digestion begun by time food
  reaches small intestine but virtually NO fat
  digestion
• Digestion completed and nearly all food
  absorption by time reaches end of SI (3-6 hrs)
• Microvilli of SI cells bear few important enzymes
  called brush border enzymes

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SI Digestion, cont.

• Break down disaccs and complete protein digestion
• Dextrinase, glucoamylase, lactase, maltase,
  sucrase
• Aminopeptidase, carboxypeptidase, dipeptidase
• Foods entering SI deluged with enzyme-rick
  pancreatic juice

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SI digestion, cont.

• Mucosa cells produce secretin and CCK
  (cholecystokinin) which influence release of
  pancreatic juice and bile
• Absorption of water and end products of digestion
  occurs all along length of SI
• Most absorbed by active transport
• Enter capillary bed in villus to be transported
  in blood to liver via hepatic portal vein

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Digestion, Visual
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SI digestion, cont.

• Lipids absorbed by diffusion
• Enter capillary bed and lacteal in villus for
  transport via blood and lymphatic fluids
• At end of ileum, all that remains is water,
  indigestible food materials, and large amounts of
  bacteria
• Debris enters large intestine through the
  ILEOCECAL VALVE

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The Large Intestine

• Colon produces NO digestive enzymes
• Spends 12-24 hours in LI
• Normal flora metabolize some remaining
  nutrients?methane/hydrogen sulfide gas
• Vitamin K and some B vitamin synthesis
• Absorption in LI limited to these vitamins, some
  ions, and water
• Feces delivered to rectum for elimation
• Undigested food, mucus, bacteria, and enough
  water for smooth passage

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Important Terms

• Metabolism, anabolism, and catabolism
• Glycogenesis combining glucose to make glycogen
  (occurs in liver) for storage in liver and muscle
  cells
• Glycogenolysis glycogen splitting blood
  glucose levels fall liver breaks apart glycogen
• Gluconeogenesis formation of new sugar
• Liver can make glucose from fats/proteins

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Ole Glyand more
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Metabolism and more

• Carbohydrate ( fig. 14.9a on p. 461)
• Fat (fig. 14.9b)
• Protein (fig. 14.9c)
• ATP formation (14.9d)
• Hypoglycemia low blood glucose level
• Hyperglycemia excessive high level of blood
  glucose?glycogen?still too much fat

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You ate yet, Brutus?
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Basal Metabolic Rate

• BMR amount of heat produced by body per unit time
  when under basal (rest) conditions
• Reflects energy needed to perform essential life
  activities (breathing, heartbeat, kidney
  function)
• 70-80 kg adult 60 to 72 kcal/hour
• Amount of thyroxine most important factor in
  determining BMR
• Dubbed metabolic hormone
• More thyroxine produced, gt use of oxygen and more
  ATP produced

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Body Temperature

• 60 percent of energy of food heat
• Warms tissues and blood?homeostatic
  conditions?efficient metabolism
• Body temp balance between production and loss
• Bodys thermostat is hypothalamus
• Heat promotingvasoconstriction/shivering
• Frostbite and hypothermia

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Body Temperature, cont.

• Heat lossradiation/evaporation
• heat exhaustion, heat stroke, fever
• Fever is controlled hyperthermia
• Pyrogens produced by macrophages, WBCs, and
  injured tissue reset thermosat in hypothalamus
• Time for an EstesismYeah!!!

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To riseor not to rise???