The Digestive System

1
The Digestive System

• Chapter 25

2
The Digestive System

• General anatomy and digestive processes
• Mouth through esophagus
• Stomach
• Liver, gallbladder and pancreas
• Small intestine
• Chemical digestion and absorption
• Large intestine

3
Digestive Functions

• Ingestion
• ________________
• Digestion
• breakdown of molecules
• ___________
• uptake nutrients into blood/lymph
• ____________
• elimination of undigested material

4
Stages of Digestion

• Mechanical digestion
• ___________________________
• Teeth, churning action of stomach intestines
• _______________________
• hydrolysis rxns break macromolecules into
  monomers
• Proteins ? _____________
• Polysaccharides ? simple _________________________
  _
• Lipids ? glycerol and fatty acids
• enzymes in saliva, stomach, pancreas and
  intestines

5
Digestive Processes

• ________________
• Motility
• muscle contractions break up food, mix it with
  enzymes and move it along
• _______________________
• digestive enzymes and hormones
• Membrane transport
• ___________________________

6
Subdivisions of Digestive System

• Digestive tract (_________)
• 30 foot long tube
• from mouth to anus
• Accessory organs
• __________________________________________________
  ______

7
Tissue Layers

• _______________
• epithelium
• lamina propria
• muscularis mucosae
• Submucosa
• _________________________
• inner circular layer
• outer longitudinal layer
• Adventitia or Serosa
• areolar tissue or mesothelium

8
Enteric Nervous Control

• Able to function independently of CNS
• Composed of two nerve networks
• submucosal plexus
• controls glandular secretion of mucosa
• contractions of muscularis mucosae
• myenteric plexus
• controls peristalsis
• contractions of muscularis externa

9
Relationship to Peritoneum

• Duodenum, pancreas and parts of large intestine
  are __________________________
• Dorsal mesentery attaches GI tract and forms
  visceral peritoneum of stomach/ intestines
• Ventral mesentery forms lesser (stomach) and
  greater omentum
• CT contains lymph nodes vessels, blood vessels
• ___________________ colon to rear wall

10
Regulation of Digestive Tract

• Neural control
• Hormones
• Paracrine secretions
• messengers diffuse to nearby target cells

11
Oral Cavity

• Cheeks and lips
• __________________
• keeps food between teeth needed for speech and
  suckling in infants
• Tongue manipulates food
• papillae and taste buds on dorsal surface
• lingual glands secrete saliva, tonsils
• _______________________
• allow breathing and chewing at same time

12
Dentition

• Baby teeth (20) by 2 years Adult (32) between 6
  and 25
• Occlusal surfaces and cusp numbers differ

13
Tooth Structure

• __________________
• anchors into alveolus
• __________________
• living tissue
• __________________
• Root canal leads into pulp cavity
• nerves and blood vessels
• Gingiva or gums

14
Mastication or Chewing

• Breaks food into small pieces
• ? surface area ? digestive enzymes
• Contact of food triggers chewing reflex

15
Saliva

• Functions
• Moisten
• begin starch/fat digestion
• cleanse teeth, inhibit bacteria
• bind food together into bolus
• Hypotonic solution of 99.5 water and solutes
• salivary amylase? starch digestion
• lingual lipase, digests fat activated by stomach
  acid
• mucus, aids in swallowing
• lysozyme, enzyme kills bacteria
• immunoglobulin A, inhibits bacterial growth
• electrolytes Na, K, Cl-, phosphate and
  bicarbonate
• pH of 6.8 to 7.0

16
Salivary Glands

• Small intrinsic glands under mucous membrane of
  mouth, lips, cheeks and tongue - secrete at
  constant rate
• 3 pairs ducted extrinsic glands
• parotid, submandibular and sublingual
• Total of 1 to 1.5 L per day
• Food stimulates receptors

17
Pharynx and Esophagus

• Pharynx
• Skeletal muscle
• Straight muscular tube 25-30 cm long
• nonkeratinized stratified squamous epithelium
• glands in submucosa
• skeletal muscle in upper part and smooth in
  bottom
• Esophagus
• from pharynx to cardiac stomach
• Lower esophageal sphincter closes orifice to
  reflux

18
Swallowing (Deglutition)

• Series of muscle contractions coordinated by
  center in medulla and pons
• Buccal phase
• tongue collects food and pushes it into
  oropharynx
• Pharyngeal- esophageal phase
• soft palate rises and blocks nasopharynx
• hyoid muscles lift larynx epiglottis folded back
  
• pharyngeal constrictors push bolus down esophagus
• liquids in 2 seconds -- food bolus may take 8
  seconds
• lower esophageal sphincter relaxes

19
Stomach

• Mechanically breaks up and liquifies food and
  begins chemical digestion (protein and fat)
• resulting soupy mixture is called chyme
• Doesnt absorb significant amount of nutrients
• absorbs aspirin and some lipid-soluble drugs

20
Gross Anatomy of Stomach

• Muscular sac (volume 50ml - 4L)
• J - shaped organ with lesser and greater
  curvatures
• regions
• cardiac region
• fundus - domed portion superior to esophageal
  opening
• body - main portion of organ
• pyloric region - narrow inferior end
• Pylorus
• opening to duodenum
• thick ring of smooth muscle/sphincter

21
Circulation

• All blood from stomach enters hepatic portal
  circulation and is filtered through liver before
  returning to heart

22
Stomach Wall

• Mucosa
• simple columnar glandular epithelium
• lamina propria is filled with tubular glands
  (gastric pits)
• Muscularis externa (3 layers)
• outer longitudinal, middle circular and inner
  oblique layers

23
Cells of Gastric Glands

• Mucous cells secrete mucus
• Parietal cells
• secrete HCl acid and intrinsic factor
• Chief cells
• secrete pepsinogen
• chymosin and lipase in infancy
• Enteroendocrine cells
• secrete hormones and paracrine messengers

24
Gastric Secretions

• 2/3 L of gastric juice/day (H2O, HCl and pepsin)
• Parietal cells contain carbonic anhydrase (CAH)
• CO2 H2O ? H2CO3 ? HCO3- H
• H is pumped into stomach lumen
• ATP is used to pump H out and K in
• HCO3- exchanged for Cl- (chloride shift)
• Cl- pumped out to join H forming HCl
• ? HCO3- in blood causes alkaline tide (blood pH ?)

25
Functions of Hydrochloric Acid

• Activates pepsin and lingual lipase
• Converts ingested ferric ions (Fe3) to ferrous
  ions (Fe2)
• absorbed and used for hemoglobin synthesis
• Destroys ingested bacteria and pathogens

26
Gastric Enzymes and Intrinsic Factor

• Intrinsic factor
• essential for B12 absorption by small intestine
• RBC production (lack causes pernicious anemia)
• Pepsin - protein digestion
• secreted as pepsinogen (inactive)
• HCl converts it to pepsin (active)
• Gastric lipase and chymosin
• lipase digests butterfat of milk in infant
• chymosin curdles milk by coagulating proteins

27
Chemical Messengers

• Many produced by enteroendocrine cells
• hormones enter blood ? distant cells
• paracrine secretions ? neighboring cells
• Gut-brain peptides
• signaling molecules produced in digestive tract
  and CNS

28
Vomiting

• Induced by
• excessive stretching of stomach, psychological
  stimuli or chemical irritants (bacterial toxins)
• Emetic center in medulla causes
• retching
• lower esophageal sphincter to relax
• stomach and duodenum to contract spasmodically
• vomiting
• when abdominal contraction forces upper
  esophageal sphincter to open

29
Regulation of Gastric Function (Phases 1-2)

• Cephalic phase
• sight, smell, taste or thought of food vagus
  nerve stimulates gastric secretion and motility
• Gastric phase
• activated by presence of food or semidigested
  protein
• by stretch or ? in pH
• secretion stimulated by
• ACh (from parasympathetic fibers), histamine
  (from gastric enteroendocrine cells) and gastrin
  (from pyloric G cells)
• receptors on parietal and chief cells

30
Regulation of Gastric Function (Phase 3)

• Intestinal phase - duodenum regulates gastric
  activity (hormones and nerve reflexes)
• at first gastric activity increases (if duodenum
  is stretched or amino acids in chyme cause
  gastrin release)
• enterogastric reflex - duodenum inhibits stomach
• caused by acid and semi-digested fats in duodenum
• chyme stimulates duodenal cells to release
  secretin, cholecystokinin (CCK) and gastric
  inhibitory peptide
• all 3 suppress gastric secretion and motility

31
Liver, Gallbladder and Pancreas

• Release important secretions into small intestine
  to continue digestion

32
Gross Anatomy of Liver

• 3 lb. organ inferior to the diaphragm
• 4 lobes - right, left, quadrate and caudate
• falciform ligament separates left and right
• round ligament, remnant of umbilical vein
• Gallbladder adheres to ventral surface between
  right and quadrate lobes

33
Microscopic Anatomy of Liver

• Cylinders called hepatic lobules (2mm by 1mm)
• Central vein surrounded by hepatocytes separated
  by sinusoids lined with fenestrated epithelium
• Blood filtered by hepatocytes on way to central
  vein

34
Histology of Liver - Hepatic Triad

• Structures found at corners of lobules
• hepatic portal vein and hepatic artery bring
  blood to liver
• bile duct collects bile from bile canaliculi
  between sheets of hepatocytes to be secreted from
  liver in hepatic ducts

35
Ducts of Gallbladder, Liver, Pancreas

• Bile
• bile canaliculi ? bile ductules ?right left
  hepatic ducts
• Right and left ducts join outside liver to form
  common hepatic duct
• Cystic duct from gallbladder joins common hepatic
  duct to form bile duct
• Pancreatic and bile duct form hepatopancreatic
  ampulla emptying into duodenum at duodenal
  papilla
• sphincter of Oddi (hepatopancreatic sphincter)
  regulates release of bile and pancreatic juice

36
Gallbladder and Bile

• Sac on underside of liver -- 10 cm long
• 500 -1000 mL bile secreted daily
• Gallbladder stores and concentrates bile
• bile backs up into gallbladder from a filled bile
  duct
• between meals, bile is concentrated 20X
• Yellow-green fluid ?minerals, bile acids,
  cholesterol, bile pigments and phospholipids
• bilirubin pigment from hemoglobin breakdown
• intestinal bacteria convert to urobilinogen
  brown color
• bile acid (salts) emulsify fats and aid in their
  digestion

37
Gross Anatomy of Pancreas

• Retroperitoneal, posterior to stomach
• head, body and tail
• Endocrine and exocrine gland
• secretes insulin and glucagon into the blood
• secretes 1500 mL pancreatic juice into duodenum
• water, enzymes, zymogens, and sodium bicarbonate
• other pancreatic enzymes are activated by
  exposure to bile and ions in the intestine
• Pancreatic duct runs length of gland to open at
  sphincter of Oddi
• accessory duct opens independently on duodenum

38
Pancreatic Acinar Cells

• Zymogens proteases
• trypsinogen
• chymotrypsinogen
• procarboxypeptidase
• Other enzymes
• amylase
• lipase
• ribonuclease and deoxyribonuclease

39
Activation of Zymogens

• Trypsinogen converted to trypsin by intestinal
  epithelium
• Trypsin converts other 2 (also digests dietary
  protein)

40
Hormonal Control of Secretion

• Cholecystokinin released in response to acid and
  fat
• contraction of gallbladder, secretion of
  pancreatic enzymes, relaxation of
  hepatopancreatic sphincter
• Secretin released in response to acidic chyme
• stimulates secretion of more bicarbonate
• Gastrin from stomach and duodenum stimulates
  gallbladder contraction and pancreatic enzyme
  secretion

41
Small Intestine

• Nearly all chemical digestion and nutrient
  absorption occurs in small intestine

42
Small Intestine

• Duodenum curves around head of pancreas (10 in.)
• retroperitoneal along with pancreas
• receives stomach contents, pancreatic juice and
  bile
• neutralizes stomach acids, emulsifies fats,
  pepsin inactivated by pH increase, pancreatic
  enzymes
• Jejunum - next 8 ft. (in upper abdomen)
• has large tall circular folds walls are thick,
  muscular
• most digestion and nutrient absorption occur here
• Ileum - last 12 ft. (in lower abdomen)
• has peyers patches clusters of lymphatic
  nodules
• ends at ileocecal junction with large intestine

43
Small Intestine - Surface Area

• Circular folds (plicae circularis) up to 10 mm
  tall
• involve only mucosa and submucosa
• chyme flows in spiral path causing more contact

• Villi are fingerlike projections 1 mm tall
• contain blood vessels and lymphatics (lacteal)
• nutrient absorption
• Microvilli 1 micron tall
• cover surface
• brush border on cells
• brush border enzymes for final stages of
  digestion

44
Intestinal Crypts

• Pores opening between villi lead to intestinal
  crypts
• absorptive cells, goblet cells and at base,
  rapidly dividing cells
• life span of 3-6 days as migrate up to surface
  and get sloughed off and digested
• paneth cells antibacterial secretions
• Brunners glands in submucosa secrete
  bicarbonate mucus
• Peyer patches are populations of lymphocytes
• Secrete 1-2 L of intestinal juice/day
• water and mucus, pH 7.4-7.8

45
Intestinal Motility

• Mixes chyme with intestinal juice, bile and
  pancreatic juice
• Churns chyme to increase contact with mucosa for
  absorption and digestion
• Moves residue towards large intestine
• segmentation
• random ringlike constrictions mix and churn
  contents
• 12 times per minute in duodenum
• peristaltic waves begin in duodenum but each one
  moves further down
• push chyme along for 2 hours
• suppressed by refilling of stomach
• Food in stomach causes gastroileal reflex
  (relaxing of valve and filling of cecum)

46
Segmentation in Small Intestine

• Purpose of segmentation is to mix and churn not
  to move material along as in peristalsis

47
Peristalsis

• Gradual movement of contents towards colon

48
Carbohydrate Digestion - Small Intestine

• Salivary amylase stops working in stomach (pH lt
  4.5)
• 50 of dietary starch digested before it reaches
  small intestine
• Pancreatic amylase completes first step in 10
  minutes
• Brush border enzymes act upon oligosaccharides,
  maltose, sucrose, lactose and fructose
• lactose indigestible after age 4 in most humans
  (lactase declines)

49
Protein Digestion and Absorption

• Pepsin has optimal pH of 1.5 to 3.5 --
  inactivated when passes into duodenum and mixes
  with alkaline pancreatic juice (pH 8)

50
Protein Digestion and Absorption

• Pancreatic enzymes take over protein digestion by
  hydrolyzing polypeptides into shorter
  oligopeptides

51
Protein Digestion and Absorption

• Brush border enzymes finish task, producing amino
  acids that are absorbed into intestinal
  epithelial cells
• Infants absorb proteins by pinocytosis (maternal
  IgA)

52
Fat Digestion and Absorption
53
Fat Digestion and Absorption
54
Fat Digestion and Absorption
55
Nucleic Acids, Vitamins, and Minerals

• Nucleases hydrolyze DNA and RNA to nucleotides
• Vitamins are absorbed unchanged
• A, D, E and K with other lipids -- B complex and
  C by simple diffusion and B12 if bound to
  intrinsic factor
• Minerals are absorbed all along small intestine

56
Water Balance

• Digestive tract receives about 9 L of water/day
• .7 L in food, 1.6 L in drink, 6.7 L in secretions
• 8 L is absorbed by small intestine and 0.8 L by
  large intestine
• Water is absorbed by osmosis following the
  absorption of salts and organic nutrients
• Diarrhea occurs when too little water is absorbed
• feces pass through too quickly if irritated
• feces contains high concentrations of a solute
  (lactose)

57
Anatomy of Large Intestine
58
Gross Anatomy of Large Intestine

• 5 feet long and 2.5 inches in diameter in cadaver
• Begins as cecum and appendix in lower right
  corner
• Ascending, transverse and descending colon frame
  the small intestine
• Sigmoid colon is S-shaped portion leading down
  into pelvis
• Rectum - straight portion ending at anal canal

59
Microscopic Anatomy

• Mucosa - simple columnar epithelium
• anal canal has stratified squamous epithelium
• No circular folds or villi to increase surface
  area
• Intestinal crypts (glands sunken into lamina
  propria) produce mucus only
• Muscularis externa
• muscle tone in longitudinal muscle fibers
  (concentrated in taeniae coli) form pouches
  (haustra)
• Transverse and sigmoid have a serosa, rest
  retroperitoneal
• epiploic appendages are suspended fatty sacs

60
Bacterial Flora and Intestinal Gas

• Bacterial flora populate large intestine
• ferment cellulose and other undigested
  carbohydrates we absorb resulting sugars
• synthesize vitamins B and K
• Flatus (gas)
• average person produces 500 mL per day
• most is swallowed air but hydrogen sulfide,
  indole and skatole produce odor

61
Absorption and Motility

• Transit time is 12 to 24 hours
• reabsorbs water and electrolytes
• Feces consist of water and solids (bacteria,
  mucus, undigested fiber, fat and sloughed
  epithelial cells
• Haustral contractions occur every 30 minutes
• distension of a haustrum stimulates it to
  contract
• Mass movements occur 1 to 3 times a day
• triggered by gastrocolic and duodenocolic
  reflexes
• filling of the stomach and duodenum stimulates
  motility
• moves residue for several centimeters with each
  contraction

62
Anatomy of Anal Canal

• Anal canal 3 cm
• Anal columns are longitudinal ridges separated by
  mucus secreting anal sinuses
• Hemorrhoids are permanently distended veins

63
Defecation

• Stretching of the rectum stimulates defecation
• intrinsic defecation reflex via the myenteric
  plexus
• defecation occurs only if external anal sphincter
  is voluntarily relaxed
• parasympathetic defecation reflex involves spinal
  cord
• stretching of rectum sends sensory signals to
  spinal cord

64
Neural Control of Defecation

• 1. Filling of the rectum
• 2. Reflex contraction of rectum and relaxation of
  internal anal sphincter
• 3. Voluntary relaxation of external sphincter