1. 23
P A R T B
The Digestive System

2. From the mouth, the oro- and laryngopharynx allow passage of:
Food and fluids to the esophagus
Air to the trachea
Lined with stratified squamous epithelium and mucus glands
Has two skeletal muscle layers
Inner longitudinal
Outer pharyngeal constrictors

3. Esophagus
Muscular tube going from the laryngopharynx to the stomach
Travels through the mediastinum and pierces the diaphragm
Joins the stomach at the cardiac orifice

4. Esophagus
Figure 23.12

5. Esophageal Characteristics
Esophageal mucosa – nonkeratinized stratified squamous epithelium
The empty esophagus is folded longitudinally and flattens when food is present
Glands secrete mucus as a bolus moves through the esophagus
Muscularis changes from skeletal (superiorly) to smooth muscle (inferiorly)
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6. Digestive Processes in the Mouth
Food is ingested
Mechanical digestion begins (chewing)
Propulsion is initiated by swallowing
Salivary amylase begins chemical breakdown of starch
The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach

7. Deglutition (Swallowing)
Coordinated activity of the tongue, soft palate, pharynx, esophagus, and 22 separate muscle groups
Buccal phase – bolus is forced into the oropharynx

8. Deglutition (Swallowing)
Pharyngeal-esophageal phase – controlled by the medulla and lower pons
All routes except into the digestive tract are sealed off
Peristalsis moves food through the pharynx to the esophagus
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9. Figure 23.13 Bolus of food Tongue Uvula Pharynx Bolus Epiglottis
Trachea
Esophagus
Bolus
(a)
Upper esophageal
sphincter contracted
(b)
Upper esophageal
sphincter relaxed
(c)
Upper esophageal
sphincter contracted
Relaxed
muscles
Relaxed
muscles
Circular muscles
contract,
constricting
passageway
and pushing
bolus down
Bolus of
food
Gastroesophageal
sphincter open
Longitudinal
muscles
contract,
shortening
passageway
ahead of bolus
Stomach
Gastroesophageal
sphincter closed
(d)
(e)
Figure 23.13

10. Deglutition (Swallowing)
Bolus of food
Tongue
Pharynx
Epiglottis
Glottis
Trachea
(a)
Upper esophagea
sphincter contracted
Figure 23.13

11. Deglutition (Swallowing)
Uvula
Bolus
Epiglottis
Esophagus
(b)
Upper esophageal
sphincter relaxed
Figure 23.13

12. Deglutition (Swallowing)
Bolus
(c)
Upper esophageal
sphincter contracted
Figure 23.13

13. Deglutition (Swallowing)
Relaxed
muscles
Circular muscles
contract,
constricting
passageway
and pushing
bolus down
Bolus of
food
Longitudinal
muscles
contract,
shortening
passageway
ahead of bolus
Stomach
Gastroesophageal
sphincter closed
(d)
Figure 23.13

14. Deglutition (Swallowing)
Relaxed
muscles
Gastroesophageal
sphincter open
(e)
Figure 23.13

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

16. Stomach
Chemical breakdown of proteins begins and food is converted to chyme
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 which terminates at the pylorus
The pylorus is continuous with the duodenum through the pyloric sphincter

17. Stomach
Greater curvature – entire extent of the convex lateral surface
Lesser curvature – concave medial surface
Lesser omentum – runs from the liver to the lesser curvature
Greater omentum – drapes inferiorly from the greater curvature to the small intestine

18. Stomach
Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system
Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal system)
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19. Figure 23.14a

20. Microscopic Anatomy of the Stomach
Muscularis – has an additional oblique layer that:
Allows the stomach to churn, mix, and pummel food physically
Breaks down food into smaller fragments

21. Microscopic Anatomy of the Stomach
Epithelial lining is composed of:
Goblet cells that produce a coat of alkaline mucus
The mucous surface layer traps a bicarbonate-rich fluid beneath it
Gastric pits contain gastric glands that secrete gastric juice, mucus, and gastrin

22. Microscopic Anatomy of the Stomach
Figure 23.15a

23. Microscopic Anatomy of the Stomach
Figure 23.15b

24. Microscopic Anatomy of the Stomach
Figure 23.15c

25. Glands of the Stomach Fundus and Body
Gastric glands of the fundus and body have a variety of secretory cells
Mucous neck cells – secrete acid mucus
Parietal cells – secrete HCl and intrinsic factor

26. Glands of the Stomach Fundus and Body
Chief cells – produce pepsinogen
Pepsinogen is activated to pepsin by:
HCl in the stomach
Pepsin itself via a positive feedback mechanism
Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria
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27. To keep from digesting itself, the stomach has a mucosal barrier with:
Stomach Lining
The stomach is exposed to the harshest conditions in the digestive tract
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 glands that have cells impermeable to HCl
Damaged epithelial cells are quickly replaced

28. Digestion in the Stomach
Holds ingested food
Degrades this food both physically and chemically
Delivers chyme to the small intestine
Enzymatically digests proteins with pepsin
Secretes intrinsic factor required for absorption of vitamin B12

29. Regulation of Gastric Secretion
Neural and hormonal mechanisms regulate the release of gastric juice
Stimulatory and inhibitory events occur in three phases
Cephalic (reflex) phase: prior to food entry
Gastric phase: once food enters the stomach
Intestinal phase: as partially digested food enters the duodenum

30. Excitatory events include:
Cephalic Phase
Excitatory events include:
Sight or thought of food
Stimulation of taste or smell receptors
Inhibitory events include:
Loss of appetite or depression
Decrease in stimulation of the parasympathetic division

31. Excitatory events include:
Gastric Phase
Excitatory events include:
Stomach distension
Activation of stretch receptors (neural activation)
Activation of chemoreceptors by peptides, caffeine, and rising pH
Release of gastrin to the blood

32. Inhibitory events include:
Gastric Phase
Inhibitory events include:
A pH lower than 2
Emotional upset that overrides the parasympathetic division

33. Intestinal Phase
Excitatory phase – low pH; partially digested food enters the duodenum and encourages gastric gland activity
Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum
Initiates inhibition of local reflexes and vagal nuclei
Closes the pyloric sphincter
Releases enterogastrones that inhibit gastric secretion

34. Release of Gastric Juice: Stimulatory Events
Figure

35. Release of Gastric Juice: Inhibitory Events
Figure

36. Regulation and Mechanism of HCl Secretion
HCl secretion is stimulated by ACh, histamine, and gastrin through second-messenger systems
Release of hydrochloric acid:
Is low if only one ligand binds to parietal cells
Is high if all three ligands bind to parietal cells
Antihistamines block H2 receptors and decrease HCl release

37. Regulation and Mechanism of HCl Secretion
Figure 23.17

38. Response of the Stomach to Filling
Stomach pressure remains constant until about 1L of food is ingested
Relative unchanging pressure results from reflex-mediated relaxation and plasticity

39. Response of the Stomach to Filling
Reflex-mediated events include:
Receptive relaxation – as food travels in the esophagus, stomach muscles relax
Adaptive relaxation – the stomach dilates in response to gastric filling
Plasticity – intrinsic ability of smooth muscle to exhibit the stress-relaxation response

40. Gastric Contractile Activity
Peristaltic waves move toward the pylorus at the rate of 3 per minute
This basic electrical rhythm (BER) is initiated by pacemaker cells (cells of Cajal)

41. Gastric Contractile Activity
Most vigorous peristalsis and mixing occurs near the pylorus
Chyme is either:
Delivered in small amounts to the duodenum or
Forced backward into the stomach for further mixing
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42. Gastric Contractile Activity
Figure 23.18

43. Regulation of Gastric Emptying
Gastric emptying is regulated by:
The neural enterogastric reflex
Hormonal (enterogastrone) mechanisms
These mechanisms inhibit gastric secretion and duodenal filling

44. Regulation of Gastric Emptying
Carbohydrate-rich chyme quickly moves through the duodenum
Fat-laden chyme is digested more slowly causing food to remain in the stomach longer

45. Regulation of Gastric Emptying
Figure 23.19

46. Small Intestine: Gross Anatomy
Runs from pyloric sphincter to the ileocecal valve
Has three subdivisions: duodenum, jejunum, and ileum

47. Small Intestine: Gross Anatomy
The bile duct and main pancreatic duct:
Join the duodenum at the hepatopancreatic ampulla
Are controlled by the sphincter of Oddi
The jejunum extends from the duodenum to the ileum
The ileum joins the large intestine at the ileocecal valve

48. Small Intestine: Microscopic Anatomy
Structural modifications of the small intestine wall increase surface area
Plicae circulares: deep circular folds of the mucosa and submucosa
Villi – fingerlike extensions of the mucosa
Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes
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49. Duodenum and Related Organs
Figure 23.20

50. Small Intestine: Microscopic Anatomy
Figure 23.21

51. Small Intestine: Histology of the Wall
The epithelium of the mucosa is made up of:
Absorptive cells and goblet cells
Enteroendocrine cells
Interspersed T cells called intraepithelial lymphocytes (IELs)
IELs immediately release cytokines upon encountering Ag

52. Small Intestine: Histology of the Wall
Cells of intestinal crypts secrete intestinal juice
Peyer’s patches are found in the submucosa
Brunner’s glands in the duodenum secrete alkaline mucus

53. Slightly alkaline and isotonic with blood plasma
Intestinal Juice
Secreted by intestinal glands in response to distension or irritation of the mucosa
Slightly alkaline and isotonic with blood plasma
Largely water, enzyme-poor, but contains mucus
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54. The largest gland in the body
Liver
The largest gland in the body
Superficially has four lobes – right, left, caudate, and quadrate
The falciform ligament:
Separates the right and left lobes anteriorly
Suspends the liver from the diaphragm and anterior abdominal wall

55. Liver The ligamentum teres: Is a remnant of the fetal umbilical vein
Runs along the free edge of the falciform ligament

56. Liver: Associated Structures
The lesser omentum anchors the liver to the stomach
The hepatic blood vessels enter the liver at the porta hepatis
The gallbladder rests in a recess on the inferior surface of the right lobe

57. Liver: Associated Structures
Bile leaves the liver via:
Bile ducts, which fuse into the common hepatic duct
The common hepatic duct, which fuses with the cystic duct
These two ducts form the bile duct
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Secretion, pages 11-12

58. Gallbladder and Associated Ducts
Figure 23.20

59. Liver: Microscopic Anatomy
Hexagonal-shaped liver lobules are the structural and functional units of the liver
Composed of hepatocyte (liver cell) plates radiating outward from a central vein
Portal triads are found at each of the six corners of each liver lobule
Figure 23.24c

60. Liver: Microscopic Anatomy
Portal triads consist of a bile duct and
Hepatic artery – supplies oxygen-rich blood to the liver
Hepatic portal vein – carries venous blood with nutrients from digestive viscera
Figure 23.24d

61. Liver: Microscopic Anatomy
Liver sinusoids – enlarged, leaky capillaries located between hepatic plates
Kupffer cells – hepatic macrophages found in liver sinusoids

62. Liver: Microscopic Anatomy
Hepatocytes’ functions include:
Production of bile
Processing bloodborne nutrients
Storage of fat-soluble vitamins
Detoxification
Secreted bile flows between hepatocytes toward the bile ducts in the portal triads

63. Microscopic Anatomy of the Liver
Figure 23.24c, d

64. Bile salts are cholesterol derivatives that:
Composition of Bile
A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
Bile salts are cholesterol derivatives that:
Emulsify fat
Facilitate fat and cholesterol absorption
Help solubilize cholesterol
Enterohepatic circulation recycles bile salts
The chief bile pigment is bilirubin, a waste product of heme

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

66. Regulation of Bile Release
Acidic, fatty chyme causes the duodenum to release:
Cholecystokinin (CCK) and secretin into the bloodstream
Bile salts and secretin transported in blood stimulate the liver to produce bile
Vagal stimulation causes weak contractions of the gallbladder

67. Regulation of Bile Release
Cholecystokinin causes:
The gallbladder to contract
The hepatopancreatic sphincter to relax
As a result, bile enters the duodenum

68. Regulation of Bile Release4
Vagal stimulation causes
weak contractions of
gallbladder 3
Bile salts
and secretin
transported via
bloodstream
stimulate liver
to produce bile
more rapidly 5
Cholecystokinin
(via bloodstream)
causes gallbladder
to contract and
hepatopancreatic
sphincter to relax;
bile enters
duodenum 1
Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
Cholecystokinin
and secretin enter the
bloodstream 2 6
Bile salts reabsorbed into blood
Figure 23.25

69. Regulation of Bile Release1
Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
Figure 23.25

70. Regulation of Bile Release1
Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
Cholecystokinin
and secretin enter the
bloodstream 2
Figure 23.25

71. Regulation of Bile Release3
Bile salts
and secretin
transported via
bloodstream
stimulate liver
to produce bile
more rapidly 1
Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
Cholecystokinin
and secretin enter the
bloodstream 2
Figure 23.25

72. Regulation of Bile Release4
Vagal stimulation causes
weak contractions of
gallbladder 3
Bile salts
and secretin
transported via
bloodstream
stimulate liver
to produce bile
more rapidly 1
Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
Cholecystokinin
and secretin enter the
bloodstream 2
Figure 23.25

73. Regulation of Bile Release4
Vagal stimulation causes
weak contractions of
gallbladder 3
Bile salts
and secretin
transported via
bloodstream
stimulate liver
to produce bile
more rapidly 5
Cholecystokinin
(via bloodstream)
causes gallbladder
to contract and
hepatopancreatic
sphincter to relax;
bile enters
duodenum 1
Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
Cholecystokinin
and secretin enter the
bloodstream 2
Figure 23.25

74. Regulation of Bile Release4
Vagal stimulation causes
weak contractions of
gallbladder 3
Bile salts
and secretin
transported via
bloodstream
stimulate liver
to produce bile
more rapidly 5
Cholecystokinin
(via bloodstream)
causes gallbladder
to contract and
hepatopancreatic
sphincter to relax;
bile enters
duodenum 1
Acidic, fatty chyme
entering duodenum causes
release of cholecystokinin
and secretin from
duodenal wall
enteroendocrine cells
Cholecystokinin
and secretin enter the
bloodstream 2 6
Bile salts reabsorbed into blood
Figure 23.25