Digestive System

1
Chapter 24

• Digestive System

2
Digestive System Anatomy

• Digestive tract also called alimentary tract or
  canal
• GI tract technically refers to stomach and
  intestines
• Accessory organs
• Primarily glands, secrete fluids into tract
• Regions
• Mouth or oral cavity with salivary glands and
  tonsils
• Pharynx (throat) with tubular mucous glands
• Esophagus with tubular mucous glands
• Stomach with many different kinds of glands that
  are tubular
• Small intestine (duodenum, ileum, jejunum) with
  liver, gallbladder and pancreas as major
  accessory organs
• Large intestine including cecum, colon, rectum
  and anal canal with mucous glands
• Anus

3
Functions

1. Ingestion introduction of food into stomach
2. Mastication chewing. Chemical digestion requires
   large surface area so breaking down large
   particles mechanically facilitates chemical
   digestion.

• 3. Propulsion (movement of food-----24-36 hours
  oral end to anal end)
• Deglutition swallowing (oral cavity -gt
  esophagus) (bolus mass of food or liquid)
• Peristalsis moves material through digestive
  tract . A wave of circular smooth muscle
  relaxation moves ahead of the bolus of food or
  chyme allowing the digestive tract to expand.
  Then a wave of contraction of the circular smooth
  muscles behind the bolus of food or chyme
  (ingested food stomach secretions) propels it
  through the digestive tract.
• Mass movements in large intestine (contractions
  that extend for larger parts of digestive tract)

4
4. Mixing Segmental contractions ( mixing
contractions that occur in small intestine.
-Some contractions do not propel food from one
end of digestive tract to the other but, rather,
move it back forth within digestive tract to
mix it with digestive secretions help break it
into smaller pieces)
5
Functions, cont.

• 5. Secretion lubricate, liquefy, digest
• Mucus secreted along entire digestive tract,
  lubricates food and lining, coats lining and
  protects from mechanical digestion, from acid and
  from digestive enzymes.
• Water liquefaction makes food easier to digest
  and absorb
• Bile emulsifies fats
• Enzymes chemical digestion
• 6. Digestion Mechanical and chemical
• 7. Absorption Movement from tract into
  circulation or lymph
• 8. Elimination Waste products removed from body
  feces. Defecation

6
Digestive Tract Histology The Tunics

• Mucosa. Innermost layer, consisting of mucous
  epithelium (stratified squamous in mouth,
  oropharynx, esophagus and anal canal), simple
  columnar epithelium in the rest of the tract.
• Loose connective tissue lamina propria
• Muscularis mucosae smooth muscle
• Submucosa. Thick C.T. layer with nerves, blood
  vessels, small glands. Parasympathetic submucosal
  plexus.

7
Digestive Tract Histology The Tunics

• Muscularis 2 or 3 layers of smooth muscle, two
  of which are circular and longitudinal.
  Exception esophagus where the upper 1/3 is
  striated stomach. This layer also contains the
  myenteric plexus. The myenteric and submucosal
  plexi together are called the enteric or
  intramural plexus. Important in control of
  movement and secretion
• Serosa or adventitia Connective tissue. Where
  serosa is present, called visceral peritoneum.
  Where adventitia is present, connective tissue
  blends with connective tissue of surrounding
  structures

8
Digestive System Regulation

• Nervous regulation
• Local enteric nervous system
• Types of neurons sensory, motor, interneurons
• Coordinates peristalsis and regulates local
  reflexes
• General coordination with the CNS. May initiate
  reflexes because of sight, smell, or taste of
  food. Parasympathetic primarily (through vagas
  nerve). Sympathetic input inhibits muscle
  contraction, secretion, and decrease of blood
  flow to the digestive tract.

• Chemical regulation
• Production of hormones to be discussed later
• Gastrin, secretin
• Production of paracrine chemicals like histamine
• Help local reflexes in ENS control the conditions
  of the internal environment of the digestive
  tract such as pH levels

9
Peritoneum and Mesenteries

• Peritoneum
• Visceral Covers organs
• Parietal Covers interior surface of body wall
• Retroperitoneal Certain organs covered by
  peritoneum on only one surface and are considered
  behind the peritoneum (lie against abdominal
  wall) e.g., kidneys, pancreas, duodenum
• Mesenteries two layers of peritoneum with thin
  layer of loose C.T. between
• Routes by which vessels and nerves pass from body
  wall to organs

• Greater omentum connects greater curvature of
  the stomach to the transverse colon (extends
  inferiorly from stomach over surface of small
  intestine).
• Lesser omentum connects lesser curvature of the
  stomach and the proximal part of the duodenum to
  the liver and diaphragm.
• Transverse mesocolon, sigmoid mesocolon,
  mesoappendix (mesentery refers to serous
  membranes attached to abdominal organs).
• Ligaments
• Coronary between liver and diaphragm
• Falciform between liver and anterior abdominal
  wall

10
Oral Cavity

• Bounded by lips anteriorly, fauces (opening into
  pharynx) posteriorly
• Vestibule space between lip/cheeks and alveolar
  processes with teeth
• Oral cavity proper medial to alveolar processes
• Lined with moist stratified squamous epithelium

11
Lips and Cheeks

• Both structures important in mastication and
  speech
• Lips (labia) orbicularis oris muscle within.
  Keratinized stratified squamous exterior is thin
  and color of blood in dermis gives a red/pink
  color.
• Labial frenula (mucous folds) extend from
  alveolar processes of maxilla and mandible to the
  upper and lower lips, respectively.
• Many facial muscles act to move lips
• Cheeks lateral walls of oral cavity
• Buccinator muscle
• Buccal fat pad

12
Palate and Palatine Tonsils

• Palate
• Hard palate anterior, supported by maxilla and
  palatine bone
• Soft palate posterior, consists of skeletal
  muscle and connective tissue
• Uvula projects from posterior of soft palate
• Palatine tonsils lateral walls of fauces

13
Tongue

• Muscular with free anterior surface and attached
  posterior surface. Covered with moist stratified
  squamous epithelium.
• Intrinsic muscles change shape
• Extrinsic muscles protrude or retract tongue,
  move side to side
• Lingual frenulum attaches tongue inferiorly to
  floor of oral cavity
• Terminal sulcus groove divides tongue into
  anterior 2/3 posterior 1/3
• Anterior part papillae, some of which have taste
  buds
• Posterior part no papillae and a few scattered
  taste buds. Lymphoid tissue embedded in posterior
  surface lingual tonsil
• Moves food in mouth, participates in speech and
  swallowing

14
Teeth

• Two sets
• Primary, deciduous, milk Lost during childhood
• Permanent or secondary Adult (32)
• Types
• Incisors, canines, premolars and molars

15
Teeth

• Involved in mastication and speech
• Anatomic crown enamel-covered part of tooth
  clinical crown is section of tooth above gum line
• Neck enameled part of tooth below gum line
• Enamel outermost layer of anatomical crown.
  Non-living acellular. Protective.

• Dentin living, cellular, calcified tissue. In
  the root, dentin is covered by cellular bone-like
  structure that helps hold tooth in the socket.
• Pulp cavity filled with blood vessels, nerves,
  and connective tissue
• Periodontal ligaments hold tooth in socket.
• Gingiva dense, fibrous C.T. covered by
  stratified squamous epithelium.

16
Mastication

• Chewing incisors and canines bite or cut off
  food molar-type teeth grind food
• Muscles involved masseter, temporalis, medial
  and lateral pterygoids.
• Elevate mandible (close jaw) temporalis,
  masseter, medial pterygoids
• Depress mandible (open jaw) lateral pterygoids
• Protraction (moving in anterior direction) and
  lateral and medial excursion (lateral moves
  mandible to either right or left of
  midline--------medial returns mandible to
  neutral position) pterygoids and masseter
• Retraction (moves structure back to anatomical
  position)- temporalis
• Mastication reflex medulla oblongata, but
  descending pathways from cerebrum provide
  conscious control. Controls basic movements
  involved in chewing

17
Salivary Glands

• Three pairs of multicellular glands
• Parotid largest. Serous. Just anterior to the
  ear. Parotid duct crosses over masseter,
  penetrates buccinator, and enters the oral cavity
  adjacent to the 2nd upper molar

• Submandibular mixed, but more serous than
  mucous. Posterior half of inferior border of
  mandible. Duct enters oral cavity on either side
  of lingual frenulum
• Sublingual smallest. Mixed, but primarily
  mucous. Each has 10-12 ducts that enter the floor
  of the oral cavity.
• Lingual glands. Small, coiled tubular glands on
  surface of tongue.

18
Saliva

• Compound alveolar salivary glands. Produce saliva
• Prevents bacterial infection
• Lubrication
• Contains salivary amylase that breaks down starch
  into disaccharides maltose and isomaltose (gives
  starch sweet taste in mouth).
• Helps to form bolus for swallowing
• Parasympathetic input causes salivary production

19
Pharynx and Esophagus

• Pharynx
• Posterior walls of oropharynx and laryngopharynx
  contains group of muscles called pharyngeal
  constrictors that contribute to swallowing

• Esophagus
• Transports food from pharynx to stomach
• Passes through esophageal hiatus (opening) of
  diaphragm and ends at stomach
• Hiatal hernia widening of hiatus (causes ulcers,
  acid reflux)
• Sphincters
• Upper. Striated
• Lower. Smooth
• Mucosa is moist stratified squamous epithelium.
  Produces thick layer of mucus.

20
Swallowing (Deglutition)

• Three phases
• Voluntary bolus of food moved by tongue from
  oral cavity to pharynx.
• Pharyngeal reflex. Controlled by swallowing
  center in medulla oblongata. Soft palate
  elevates, upper esophageal sphincter relaxes,
  elevated pharynx opens the esophagus, food pushed
  into esophagus by pharyngeal constrictors
  successive contraction from superior to inferior.
  Epiglottis is tipped posteriorly due to pressure
  of the bolus, larynx elevated to prevent food
  from passing into larynx.
• Esophageal reflex. Stretching of esophagus
  causes enteric NS to initiate peristalsis of
  muscles in the esophagus.

21
Three Phases of Swallowing
22
Stomach Anatomy

• Openings
• Gastroesophageal (cardiac) to esophagus
• Pyloric to duodenum
• Parts
• Cardiac
• Fundus
• Body
• Pyloric antrum and canal

• Greater and lesser curvatures attachment sites
  for omenta
• Sphincters
• Cardiac (lower esophageal)
• Pyloric

23
Stomach Histology

• Layers
• Serosa or visceral peritoneum
• Muscularis three layers
• Outer longitudinal
• Middle circular
• Inner oblique (Having a slanting or sloping
  direction)
• Submucosa
• Mucosa

• Rugae folds in stomach when empty. Mucosa and
  submucosa.

24
Stomach Histology

• Gastric pits openings for gastric glands. Lined
  with simple columnar epithelium
• Cells of gastric pits
• Surface mucus mucus that protects stomach lining
  from acid and digestive enzymes
• Mucous neck mucus
• Parietal hydrochloric acid and intrinsic factor
• Chief pepsinogen
• Endocrine regulatory hormones
• Enterochromaffin-like cells secretes histamine
  that stimulates acid secretion
• Gastrin-containing cells secrete gastrin (a
  hormone that stimulates acid secretion)
• Somatostatin-containing cells secrete
  somatostatin that inhibits gastrin and insulin
  secretion

25
Secretions of the Stomach

• Chyme ingested food plus stomach secretions
• Mucus surface and neck mucous cells
• Viscous and alkaline
• Protects from acidic chyme and enzyme pepsin
• Irritation of stomach mucosa causes greater mucus
  
• Intrinsic factor parietal cells. Binds with
  vitamin B12 and helps it to be absorbed in the
  ileum. B12 necessary for DNA synthesis and RBC
  production (lack of B12 absorption leads to
  pernicious anemia)
• HCl parietal cells
• Kills bacteria (found in ingested food)
• Stops carbohydrate digestion by inactivating
  salivary amylase
• Denatures proteins
• Helps convert pepsinogen to pepsin (optimal
  activity at pH 3 or less)
• Pepsinogen packaged in zymogen granules released
  by exocytosis. Pepsin catalyzes breaking of
  covalent bonds in proteins (breaks them into
  smaller peptide chains)

26
Hydrochloric Acid Production
27
Cephalic Phase

• The taste or smell of food, tactile sensations of
  food in the mouth, or even thoughts of food
  stimulate the medulla oblongata.
• Parasympathetic action potentials are carried by
  the vagus nerves to the stomach, where enteric
  plexusneurons are activated.

• Postganglionic neurons stimulate secretion by
  parietal and chief cells (HCl and pepsin) and
  stimulate the secretion of the hormone gastrin
  and histamine.
• Gastrin is carried through the circulation back
  to the stomach where it and histamine stimulate
  further secretion of HCl and pepsin.

28
Gastric Phase

• Distention of the stomach activates a
  parasympathetic reflex. Action potentials are
  carried by the vagus nerves tothe medulla
  oblongata.
• Medulla oblongata stimulates further secretions
  of the stomach.
• Distention also stimulates local reflexes that
  amplify stomach secretions.

29
Intestinal Phase

1. Chyme in the duodenum with a pH less than 2 or
   containing lipids inhibits gastric secretions by
   three mechanisms
2. Sensory input to the medulla from the duodenum
   inhibits the motor input from the medulla to the
   stomach. Stops secretion of pepsin and HCl.
3. Local reflexes inhibit gastric secretion
4. Secretin, and cholecystokinin produced by the
   duodenum decrease gastric secretions in the
   stomach.

30
Movements in Stomach

• Combination of mixing waves (80) and peristaltic
  waves (20)
• Both esophageal and pyloric sphincters are
  closed.

31
Small Intestine

• Site of greatest amount of digestion and
  absorption of nutrients and water
• Divisions
• Duodenum- first 25 cm beyond the pyloric
  sphincter.
• Jejunum- 2.5 m
• Ileum- 3.5 m. Peyers patches or lymph nodules

32
Duodenum

• Curves to the left head of pancreas in the curve
• Major and minor duodenal papillae openings to
  ducts from liver and/or pancreas.

33
Modifications to Increase Surface Area

• Increase surface area 600 fold
• Plicae circulares (circular folds)
• Villi that contain capillaries and lacteals.
  Folds of the mucosa
• Microvilli folds of cell membranes of absorptive
  cells

34
Mucosa and Submucosa of the Duodenum

• Cells and glands of the mucosa
• Absorptive cells cells with microvilli, produce
  digestive enzymes and absorb digested food
• Goblet cells produce protective mucus
• Endocrine cells produce regulatory hormones
  (Secretin, and cholecystokinin)
• Granular cells (paneth cells) may help protect
  from bacteria (contain lysozymes)
• Intestinal glands (crypts of Lieberkühn) tubular
  glands in mucosa at bases of villi secrete
  sucrase ,maltase, trypsin, chymotrypsin, and
  pepsin (endopeptidases and exopeptidases)
• Duodenal glands (Brunners glands) tubular
  mucous glands of the submucosa. Open into
  intestinal glands produce a mucus-rich alkaline
  secretion (containing bicarbonate)

35
Jejunum and Ileum

• Gradual decrease in diameter, thickness of
  intestinal wall, number of circular fold, and
  number of villi the farther away from the stomach
• Major site of nutrient absorption
• Peyers patches lymphatic nodules numerous in
  mucosa and submucosa
• Ileocecal junction where ileum meets large
  intestine. Ileocecal sphincter (ring of smooth
  muscle) and ileocecal valve (one-way valve)

36
Small Intestine Secretions

• Fluid primarily composed of water, electrolytes
  and mucus.
• Mucus
• Protects against digestive enzymes and stomach
  acids
• Digestive enzymes bound to the membranes of the
  absorptive cells
• Disaccharidases Break down disaccharides to
  monosaccharides
• Peptidases Hydrolyze peptide bonds
• Nucleases Break down nucleic acids
• Duodenal glands
• Stimulated by vagus nerve, secretin, chemical or
  tactile irritation of duodenal mucosa

37
Movement in Small Intestine

• Mixing and propulsion over short distances
• Segmental contractions mix
• Peristalsis propels
• Ileocecal sphincter remains slightly contracted
  until peristaltic waves reach it it relaxes,
  allowing chyme to move into cecum
• Cecal distention causes local reflex and
  ileocecal valve constricts
• Prevents more chyme from entering cecum
• Increases digestion and absorption in small
  intestine by slowing progress of chyme
• Prevents backflow

38
Liver

• Lobes
• Major Left and right
• Minor Caudate and quadrate
• Porta on inferior surface. Vessels, ducts,
  nerves, exit/enter liver
• Hepatic portal vein, hepatic artery, hepatic
  nerve plexus enter
• Lymphatic vessels, two hepatic ducts exit

• Ducts
• Right and left hepatics (which transport bile out
  of liver) unite to form
• Common hepatic
• Cystic from gallbladder
• Common bile union of cystic duct and common
  hepatic duct (common bile joins the pancreatic
  duct at the hepatopancreatic ampulla------ampulla
  empties into duodenum at major duodenum papilla)

39
Liver, Gallbladder, Pancreas and Ducts
40
Histology of the Liver

• Connective tissue septa branch from the porta
  into the interior
• Divides liver into lobules
• Nerves, vessels and ducts follow the septa
• Lobules portal triad at each corner
• Three vessels hepatic portal vein, hepatic
  artery, hepatic duct
• Central vein in center of lobule
• Central veins unite to form hepatic veins that
  exit liver and empty into inferior vena cava

41
Liver Histology

• Hepatic cords radiate out from central vein.
  Composed of hepatocytes
• Hepatic sinusoids between cords, lined with
  endothelial cells and hepatic phagocytic
  (Kupffer) cells
• Bile canaliculus between cells within cords
• Hepatocyte functions
• Bile production
• Storage
• Interconversion of nutrients
• Detoxification
• Phagocytosis
• Synthesis of blood components

42
Functions of the Liver

• Bile production 600-1000 mL/day. Bile salts,
  bilirubin (bile pigment that results from
  breakdown of hemoglobin), cholesterol, fats,
  fat-soluble hormones, lecithin
• Neutralizes and dilutes stomach acid (neutralizes
  chyme so that pancreatic enzymes can function)
• Bile salts emulsify fats. Most are reabsorbed in
  the ileum. (90 bile salts reabsorbed in the
  ileum carried back to liver)
• Secretin (from the duodenum) stimulates bile
  secretions, increasing water and bicarbonate ion
  content of the bile
• Storage
• Glycogen, fat, vitamins (A, B12, D, E, and K),
  copper and iron. Hepatic portal blood comes to
  liver from small intestine (nutrients are stored
  and secreted back into circulation when needed)
• Synthesis
• Blood proteins Albumins, fibrinogen, globulins,
  heparin, clotting factors (liver produces its own
  new compounds)

43
Functions of the Liver

• Nutrient interconversion
• Amino acids to energy producing compounds (ex
  person on a excessively high protein diet and low
  fat carb diet----------an oversupply of amino
  acids an undersupply of lipids carbs are
  delivered to the liver. The hepatocytes break
  down the amino acids and cycle them through
  metabolic pathways so they can be used to produce
  adenosine triphosphate, lipids, and glucose)
• Hydroxylation of vitamin D. Vitamin D then
  travels to kidney where it is hydroxylated again
  into its active form
• Hepatocytes also transform substances that cannot
  be used by most cells into usable sunstances.
  (ex ingested fats combined with choline
  nutrient in B vitamin family phosphorous in
  liver to produce phospholipids, which are imp.
  for cell membranes)
• Detoxification
• Hepatocytes remove ammonia (by-product of amino
  acid metabolism) which is toxic not readily
  removed by kidneys. Hepatocytes convert it to
  urea which is less toxic and easily eliminated by
  kidneys.
• Phagocytosis
• Kupffer cells phagocytize worn-out and dying red
  and white blood cells, some bacteria

44
Blood and Bile Flow Through the Liver
45
Gallbladder

• Sac lined with mucosa folded into rugae, inner
  muscularis, outer serosa
• Bile arrives constantly from liver is stored and
  concentrated
• Stimulated by cholecystokinin (from the
  intestine) and vagal stimulation
• Bile exits through cystic duct then into common
  bile duct
• Gallstones precipitated cholesterol (occurs when
  excess cholesterol in bile due to
  high-cholesterol diet and not enough bile salts
  to keep it in solution)
• Can block cystic duct
• If gallstone moves far down the duct, it can
  block pancreatic duct, resulting in pancreatitis.
• Can occur because of drastic dieting (as the body
  metabolizes fat during prolonged fasting and
  rapid weight losssuch as crash dietsthe liver
  secretes extra cholesterol into bile, which can
  cause gallstones.)

46
Control of Bile Secretion and Release
47
Pancreas

• Pancreas both endocrine and exocrine
• Head, body and tail
• Endocrine pancreatic islets. Produce insulin,
  glucagon, and somatostatin
• Exocrine groups acini (grape-like cluster) form
  lobules separated by septa.
• Intercalated ducts lead to intralobular ducts
  lead to interlobular ducts lead to the pancreatic
  duct.
• Pancreatic duct joins common bile duct and enters
  duodenum at the hepatopancreatic ampulla
  controlled by the hepatopancreatic ampullar
  sphincter

48
Pancreatic Secretions Pancreatic Juice

• Aqueous. Produced by columnar epithelium lining
  smaller ducts. Na, K, HCO3-, water. Bicarbonate
  lowers pH inhibiting pepsin and providing proper
  pH for enzymes
• Enzymatic portion (without the enzymes produced
  by pancreas, lipids, proteins, carbs not
  adequately digested)
• Trypsinogen- active form is trypsin--------proteol
  ytic enzyme
• Chymotrypsinogen- active form is
  chymotrypsin--------proteolytic enzyme
• Procarboxypeptidase- active form is
  carboxypeptidase-------proteolytic enzyme
• Pancreatic amylase- continues digestion of
  starch.
• Pancreatic lipases- lipid digesting enzyme
• Deoxyribonucleases and ribonucleases- reduce DNA
  RNA to their nucleotide
• Interaction of duodenal and pancreatic enzymes
• Enterokinase is a proteolytic enzyme from the
  duodenal mucosa and it activates trypsinogen to
  trypsin.
• Trypsin activates chymotrypsinogen to
  chymotrypsin.
• Trypsin activates procarboxypeptidase to
  carboxypeptidase.

49
Bicarbonate Ion Production in Pancreas
50
Control of Pancreatic Secretion
51
Large Intestine

• Extends from ileocecal junction to anus
• Consists of cecum, colon, rectum, anal canal
• Movements sluggish (18-24 hours) chyme converted
  to feces.
• Absorption of water and salts, secretion of
  mucus, extensive action of microorganisms are
  involved in the formation of feces.
• 1500 mL chyme enter the cecum, 90 of volume
  reabsorbed yielding 80-150 mL of feces

52
Anatomy of Large Intestine

• Cecum
• Blind sac, vermiform appendix attached. Appendix
  walls contain numerous lymph nodules
• Colon
• Ascending, transverse, descending, sigmoid
• Circular muscle layer complete longitudinal
  incomplete (three bands called teniae coli).
  Contractions of teniae form pouches called
  haustra.Small fat-filled pouches called epiploic
  appendages
• Mucosa has numerous straight tubular glands
  called crypts. Goblet cells predominate,
  butthere are also absorptive and granular cells
  as in the small intestine

53
Anatomy of Large Intestine

• Rectum
• Straight muscular tube, thick muscular tunic
• Anal canal- superior epithelium is simple
  columnar inferior epithelium is stratified
  squamous
• Internal anal sphincter (smooth muscle)
• External anal sphincter (skeletal muscle)
• Hemorrhoids Vein enlargement or inflammation

54
Secretions of Large Intestine

• Mucus provides protection
• Parasympathetic stimulation increases rate of
  goblet cell secretion
• Pumps bacteria produce acid and the following
  remove acid from the epithelial cells that line
  the large intestine
• Exchange of bicarbonate ions for chloride ions
• Exchange of sodium ions for hydrogen ions
• Bacterial actions produce gases (flatus) from
  particular kinds of carbohydrates found in
  legumes and in artificial sugars like sorbitol
• Bacteria produce vitamin K which is then absorbed
• Feces consists of water, undigested food
  (cellulose), microorganisms, sloughed-off
  epithelial cells

55
Movement in Large Intestine

• Mass movements (strong contractions)
• Common after meals
• Integrated by the enteric plexus
• Local reflexes instigated by the presence of food
  in the stomach and duodenum
• Gastrocolic initiated by stomach
• Duodenocolic initiated by duodenum
• Defecation
• Defecation reflex distension of the rectal wall
  by feces
• Parasympathetic stimulation
• Usually accompanied by voluntary movements to
  expel feces. Abdominal cavity pressure caused by
  inspiration and by contraction of muscles of
  abdominal wall.

56
Digestion, Absorption, Transport

• Digestion
• Breakdown of food molecules for absorption into
  circulation
• Mechanical breaks large food particles to small
• Chemical breaking of covalent bonds by digestive
  enzymes
• Absorption and transport
• Molecules are moved out of digestive tract and
  into circulation for distribution throughout body

57
(No Transcript)
58
Carbohydrates Hydrolyzed into Monosaccharides

• Glucose is transported to cells requiring energy
  insulin influences rate of transport

59
Transport of Lipids Across Intestinal Epithelium
60
Lipids

• Include triglycerides, phospholipids, steroids,
  fat-soluble vitamins
• Bile salts surround fatty acid and glycerol to
  form micelles
• Chylomicrons are 90 triglyceride, 5
  cholesterol, 4 phospholipid, 1 protein.
• Chylomicrons enter blood stream and travel to
  adipose tissue. In blood, triglycerides converted
  back into fatty acids and glycerol where they are
  transported into the adipose cells, then
  converted back into triglycerides.

61
Lipoproteins

• All lipids carried in the blood are done so in
  combination with protein to make them soluble in
  plasma.
• Cholesterol 15 ingested 85 manufactured in
  liver and intestinal mucosa
• Lipids are lower density than water proteins are
  higher density than water
• Chylomicrons 99 lipid and 1 protein (extremely
  low density) enter lymph
• VLDL 92 lipid, 8 protein
• Form in which lipids leave the liver
• Triglycerides removed from VLDL and stored in
  adipose cells. VLDL has been converted to LDL.
• LDL 75 lipid, 25 protein
• Transports cholesterol to cells
• Cells have LDL receptors
• of LDL receptors become less once cells
  lipid/cholesterol needs are met.
• HDL 55 lipid, 45 protein
• Transports excess cholesterol from cells to liver

62
Transport of LDL into Cells
63
Amino Acid Transport
64
Proteins

• Pepsin breaks proteins into smaller polypeptide
  chains
• Proteolytic enzymes produce small peptide chains
• Dipeptides, tripeptides, amino acids
• After absorption, amino acids are carried through
  the hepatic portal vein to the liver.

65
Water and Ions

• Water can move in either direction across wall
  of small intestine depending on osmotic gradients
• Ions sodium, potassium, calcium, magnesium,
  phosphate are actively transported

66
Effects of Aging

• Decrease in mucus layer, connective tissue,
  muscles and secretions
• Increased susceptibility to infections and toxic
  agents, increase in incidences of ulcerations and
  cancers