1. Digestive System: Overview
The alimentary canal or gastrointestinal (GI) tract digests and absorbs food
Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine
Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas

2. Figure 23.1

3. The GI tract is a “disassembly” line
Digestive Process
The GI tract is a “disassembly” line
Nutrients become more available to the body in each step
There are six essential activities:
Ingestion, propulsion, and mechanical digestion
Chemical digestion, absorption, and defecation

4. Figure 23.2

5. Gastrointestinal Tract Activities
Ingestion – taking food into the digestive tract
Propulsion – swallowing and peristalsis
Peristalsis – waves of contraction and relaxation of muscles in the organ walls
Mechanical digestion
Physically prepares food for chemical digestion. Includes chewing, mixing of food w/ saliva by the tongue, churning food in the stomach, and segmentation of the intestine (mixing food w/digestive enzymes)

6. Peristalsis and Segmentation
Figure 23.3

7. Gastrointestinal Tract Activities
Chemical digestion
Series of catabolic steps in which food molecules are broken down to amino acids, simple sugars, free fatty acids by enzymes (mouth & s. intestine)
Absorption – movement of nutrients from the GI tract to the blood or lymph (mainly s. intestine)
Defecation – elimination of indigestible solid wastes

8. External environment for the digestive process
GI Tract
External environment for the digestive process
Regulation of digestion involves:
Mechanical and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in the lumen
Extrinsic control by CNS centers
Intrinsic control by local centers

9. Receptors of the GI Tract
Mechano- and chemoreceptors respond to:
Stretch, osmolarity, and pH
Presence of substrate, and end products of digestion
They initiate reflexes that:
Activate or inhibit digestive glands that secrete digestive enzymes into the lumen or hormones into the blood
Mix lumen contents and move them along by stimulating smooth muscles of the GI tract walls

10. Nervous Control of the GI Tract
Intrinsic controls (short reflexes)
Nerve plexuses near the GI tract initiate short reflexes
Short reflexes are mediated by local enteric plexuses (gut brain)
Extrinsic controls (long reflexes)
Long reflexes arising within or outside the GI tract
Involve CNS centers and extrinsic autonomic nerves

11. Nervous Control of the GI Tract
Figure 23.4

12. Peritoneum and Peritoneal Cavity
Peritoneum – serous membrane of the abdominal cavity
Visceral – covers external surface of most digestive organs & is continuous w/ the parietal peritoneum
Parietal – lines the body wall
Peritoneal cavity
Lies between the visceral & parietal peritonea
Contains serous fluid
Allows the visceral & parietal peritonea to slide across one another

13. Peritoneum and Peritoneal Cavity
Figure 23.5a

14. Peritoneum and Peritoneal Cavity
Mesentery – double layer of peritoneum that extends to the digestive organs from the body wall and provides:
Vascular, lymphatic and nerve supplies to the digestive viscera
Hold digestive organs in place and store fat
Retroperitoneal organs – organs outside the peritoneum
Adhere to the dorsal abdominal wall and lack mesentary
E.g. pancreas and parts of the l. intestine
Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum & remain in the peritoneal cavity
E.g. stomach

15. Peritoneum and Peritoneal Cavity
Figure 23.5b

16. Blood Supply: Splanchnic Circulation
Involves those arteries that branch off the abdominal aorta to serve the digestive organs & the hepatic portal circulation
Hepatic, splenic and left gastric branches of the celiac trunk serving the spleen, liver and stomach
Hepatic portal circulation:
Collects nutrient-rich venous blood from the digestive viscera
Delivers this blood to the liver for metabolic processing and storage
Mesenteric arteries serving the s. & l. intestine

17. Histology of the Alimentary Canal
From esophagus to the anal canal the walls of the GI tract have the same four tunics (layers)
From the lumen outward they are the:
mucosa, submucosa, muscularis externa, and serosa
Each tunic has a predominant tissue type and a specific digestive function

18. Histology of the Alimentary Canal
Figure 23.6

19. Mucosa (Mucous Membrane)
Moist epithelial layer that lines the lumen of the alimentary canal
Three major functions:
Secretion of mucus, enzymes & hormones
Absorption of end products of digestion into the blood
Protection against infectious disease
Consists of three sublayers: a lining epithelium, lamina propria, and muscularis mucosae

20. Mucosa: Epithelial Lining
Simple columnar epithelium and mucus-secreting goblet cells
Mucus secretions:
Protect digestive organs from digesting themselves
Ease food along the tract
Stomach and small intestine mucosa contain:
Enzyme-secreting cells
Hormone-secreting cells (making them endocrine and digestive organs)

21. Mucosa: Lamina Propria and Muscularis Mucosae
Loose areolar and reticular connective tissue
Nourishes the epithelium and absorbs nutrients
Contains lymph nodes (part of MALT) important in defense against bacteria
Muscularis mucosae – smooth muscle cells that produce local movements of mucosa

22. Mucosa: Other Sublayers
Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves
Muscularis externa
responsible for segmentation and peristalsis
Inner circular layer that may thicken to form sphincters
Outer longitudinal layer
Serosa – the protective visceral peritoneum
Replaced by the fibrous adventitia in the esophagus
Retroperitoneal organs have both an adventitia and serosa

23. Enteric Nervous System
Composed of two major intrinsic nerve plexuses:
Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa
Myenteric nerve plexus – Located between the circular & longitudinal muscle layers.
Major nerve supply that controls GI tract mobility
Segmentation and peristalsis are largely automatic involving local reflex arcs

24. Enteric Nervous System
Linked to the CNS via long autonomic reflex arc
Parasympathetic inputs enhance secretory activity and motility
Sympathetic impulses inhibit digestive activities

25. Saliva: Source and Composition
Secreted from serous and mucous cells of salivary glands
% water, hyposmotic, slightly acidic solution containing
Electrolytes – Na+, K+, Cl–, PO42–, HCO3–
Digestive enzyme – salivary amylase
Proteins – mucin, lysozyme, defensins, and IgA
Metabolic wastes – urea and uric acid
Lingual lipase (enzyme) digests fat
Protection agaisnt microrganisms: e.g. IgA & lysozyme

26. Control of Salivation
Intrinsic salivary glands secrete saliva continuously to keep mouth moist
Extrinsic salivary glands: submandibular (submax.), sublingual & parotid are activated when food enters the mouth
Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to:
Ingested food which stimulates chemoreceptors and mechanoreceptors
The thought of food
Controlled by parasympathetic nervous system
Sympathetic N.S. releases mucin rich saliva
Strong activation of the sympathetic N.S. inhibits saliva release (dry mouth)

27. Control of Salivation
Chemo- & mechanoreceptors send signals to the salivary nuclei in the brain stem (pons & medulla)
Chemoreceptors: activated strongly by acidic substances
Mechanoreceptors: “ “ any mechanical stimulus
Efferent impulses are sent via motor fibers in the facial & glossopharyngeal nerves
Strong sympathetic stimulation inhibits salivation and results in dry mouth

28. 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

29. 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

30. Digestive Processes in the Mouth
Mouth & salivary glands are involved in the digestive process
Food is ingested
Mechanical digestion begins (chewing)
Propulsion is initiated by swallowing
Salivary amylase begins chemical breakdown of polysaccharides (starch & glycogen)
Lingual lipase secreted in the mouth acts in the acidic condition of the stomach
The pharynx and esophagus serve only as conduits to pass food from the mouth to the stomach

31. Mastication (Chewing)
Partly voluntary, partly reflexive
The pattern & rhythm of continued jaw movements are controlled mainly by stretch reflexes and in response to pressure inputs from receptors in the cheeks, gums, tongue

32. Deglutition (Swallowing)
Food is first compacted by the tongue
Deglutination involves the coordinated activity of the tongue, soft palate, pharynx, esophagus, and 22 separate muscle groups

33. Deglutition (Swallowing)
Deglutination involves 2 main phases:
1) Buccal phase (mouth)
Voluntary
Bolus is forced into the oropharynx
Bolus stimulates mechanoreceptors in the pharynx generating reflex activity
2) Pharyngeal-esophageal phase
Involuntary
Motor impulses from medulla & pons via the vagus nerve cause contraction of muscles of the pharynx & esophagus
All routes except into the digestive tract are sealed off
Peristalsis moves food through the pharynx to the esophagus
The gastroesophogeal sphincter relaxes as the peristaltic wave reaches the end of the esophagus allowing food to enter the stomach

34. 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

35. Stomach
Temporary storage area
Chemical breakdown of proteins begins and food is converted to chyme
Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system
Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal system)

36. Figure 23.14a

37. 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
Protein digestion is the only significant type of enzymatic digestion that occurs in the stomach
Secretes intrinsic factor required for absorption of vitamin B12

38. Digestion in the Stomach
Proteins are denatured by HCl produced by stomach glands to prepare proteins for enzymatic cleavage
Major enzymes in stomach:
Pepsinogen (inactive) is released by chief cells and is cleaved by HCl forming pepsin (active) in the stomach lumen (protective mechanism)
Rennin: cleaves casein & produced by infants
Lingual lipase (intrinsic salivary gland): triglyceride digestion
Intrinsic factor: required for intestinal absorption of vitamin B12 needed to produce mature RBCs

39. Regulation of Gastric Secretion
Neural and hormonal mechanisms regulate the release of gastric juice
Nervous control is provided by long (vagus) and short (enteric) nerve reflexes
When the vagus nerves stimulate the stomach, secretory activity in all its glands increases
The hormone gastrin stimulates secretion of enzymes and HCl

40. Regulation of Gastric Secretion
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
The effector site in all 3 phases is the stomach

41. Phase 1: Cephalic Phase
Reflex phase that occurs before food enters stomach
Several minutes in duration
Excitatory events include:
Sight, smell, taste or thought of food
Stimulation of taste or smell receptors relays signals to the hypothalamus which stimulates the vagal nuclei of the medulla oblongata causing motor impulses to be transmitted via the vagus nerve to parasympathetic enteric ganglion which stimulate stomach glands
Inhibitory events include:
Loss of appetite or depression
Decrease in stimulation of the parasympathetic division

42. Phase 2: Gastric Phase Once food reaches the stomach
Several hours in duration
Excitatory events include:
Stomach distension
Activation of stretch receptors (neural activation)
Activation of chemoreceptors by peptides, caffeine, and rising pH
All of these initiate both local reflexes and long vagal reflexes resulting in ACh release causing…
Release of gastrin to the blood and increase in HCl production in the stomach

43. Phase 2: Gastric Phase (cont.)
Inhibitory events include:
A pH lower than 2
Emotional upset that overrides the parasympathetic division

44. Phase 3: Intestinal Phase
Excitatory phase – low pH; partially digested food enters the duodenum of the s. intestine stimulating intestinal mucosal cells to release hormones that mimic gastrin (intestinal gastrin)
Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum initiates the enterogastric reflex:
1) Initiates inhibition of local reflexes
2) Inhibition of vagal nuclei in the medulla
3) Activate sympathetic fibers constricting the pyloric sphincter preventing entry into the s. intestine
Thus, gastric secretory activity stops protecting the s. intestine from excess acid

45. Release of Gastric Juice: Stimulatory Events
Figure

46. Release of Gastric Juice: Inhibitory Events
Figure

47. The Hormone Gastrin
Chemical stimuli provided by partially digested proteins, caffeine, and rising pH activate gastrin secreting cells (G cells) in the stomach stimulating them to produce more HCl
As pH falls, G cells are inhibited
G cells are also activated by neural reflexes (fear, anxiety, etc…) and the sympathetic N.S. overrides the parasympathetic N.S.

48. Regulation and Mechanism of HCl Secretion
HCl secretion is stimulated by ACh, histamine, and gastrin through second-messenger systems (e.g Ca++ for ACh & gastrin; cAMP for histamine)
Release of HCl:
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

49. Regulation and Mechanism of HCl Secretion
Bicarbonate
Alkaline Tide: blood draining
from the stomach is more
alkaline than the blood serving it
Figure 23.17

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

51. Gastric Contractile Activity
Peristaltic waves begin at the gastroesophogeal sphincter and move toward the pylorus at the rate of 3 per minute
Contractions become more powerful approaching the pylorus region (not much going on at the fundus or body)
Pyloric valve acts as a filter letting only liquid and small particles to pass. Retropulsion: back and forth mixing

52. Gastric Contractile Activity
Basic electrical rhythm (BER) is initiated by pacemaker cells in the longitudinal smooth muscle layer
Muscle-like noncontractile cells depolarize and repolarize spontaneously 3x/min.
Coupled to smooth muscle cells via gap junctions
Depolarization is brought to threshold by neural and hormonal factors, the same factors that enhance gastric secretory activity
E.g. stretch receptors & gastrin secreting cells

53. Regulation of Gastric Emptying
Stomach empties w/i 4hrs after a meal
Gastric emptying is regulated by:
The neural enterogastric reflex
Hormonal (enterogastrone) mechanisms
These mechanisms inhibit gastric secretion and duodenal filling

54. Regulation of Gastric Emptying
The rate of gastric emptying depends more on the contents of the duodenum than those of the stomach
Carbohydrate-rich chyme quickly moves through the duodenum
Fat-laden chyme is digested more slowly causing food to remain in the stomach longer

55. Small Intestine: Modifications for Absorption
Large surface area due to length (13 ft. in a living person, 30 ft. in a cadaver)
Microscopic structures like circular folds, villi and microvilli that further increase the surface area
Most absorption occurs
in the proximal region
of the s. intestine

56. Small Intestine: Microscopic Anatomy
Structural modifications of the small intestine wall increase surface area
Circular folds: 1 cm. Deep circular folds of the mucosa and submucosa
Villi – 1 mm. Fingerlike extensions of the mucosa
Capillary bed is found in the core of each villus as well as a lacteal (lymph capillary bed)
Smooth muscle is also found in the core of each villus allowing it to shorten & lengthen as needed
Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes
Brush border: projections of the microvilli
Contain enzymes that complete the breakdown of proteins & carbohydrates

57. Duodenum and Related Organs
Figure 23.20

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

59. Small Intestine: Histology of the Wall
Cells of intestinal crypts secrete intestinal juice
Paneth cells release antimicrobial agents and protect the s. intestine from breaching bacteria
Peyer’s patches: lymphoid follicles
Duodenal (Brunner’s) glands in the duodenum secrete alkaline mucus that neutrilizes acidic chyme

60. Intestinal Juice
Intestinal juice is alkaline: pH
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

61. Liver and Gallbladder
Accessory organs associated w/ the s. intestine
Livers digestive function is to produce bile for export to the duodenum
Bile is a fat emulsifier (breaks fat globules into smaller fat globules)

62. Gallbladder and Associated Ducts
Figure 23.20

63. 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

64. 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

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

66. Liver: Microscopic Anatomy
Hepatocytes’ functions include:
Production of bile
Store glucose & glycogen
Processing bloodborne nutrients
Storage of fat-soluble vitamins
Detoxification: convert ammonia to urea
Secreted bile flows in bile canaliculi towards bile ducts
Bile in the bile ducts flows in the opposite direction of blood flow

67. Only bile salts and phospholipids contribute to digestive process
Composition of Bile
A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
Only bile salts and phospholipids contribute to digestive process
Bile salts are cholesterol derivatives that:
Emulsify fat (the way dish soap works)
Facilitate fat and cholesterol absorption
Help solubilize cholesterol

68. Composition of Bile Enterohepatic circulation recycles bile salts
1) bile salts are reabsorbed into the blood by the ileum
2) returned to the liver via hepatic portal blood
3) resecreted in newly formed bile
Bilirubin: The chief bile pigment, a waste product of heme
Bile salts are the major stimulus for enhanced bile secretion

69. 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

70. The Gallbladder
Thin-walled, green muscular sac on the ventral surface of the liver
Stores bile that is not immediately needed for digestion and concentrates it by absorbing water and ions
Upon muscular contraction (vagal stimulation), bile is secreted thru the cystic duct than the bile duct and into the duodenum

71. Regulation of Bile Release
During periods of fasting, the hepatopancreatic sphincter is closed, backing up bile into the gall bladder
Acidic, fatty chyme causes the duodenum to release:
Cholecystokinin (CCK) is the major stimulus for gallbladder contraction.
CCK also stimulates secretion of pancreatic juice
CCK also relaxes the hepatopancreatic sphincter
RESULT: Bile enters the duodenum

72. Accessory digestive organ Location
Pancreas
Accessory digestive organ
Location
Lies deep to the greater curvature of the stomach
The head is encircled by the duodenum and the tail abuts the spleen

73. Pancreas Exocrine function
Secretes pancreatic juice which breaks down all categories of foodstuff
Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes

74. Pancreas
Pancreatic juice drains into the main pancreatic duct which in turn fuses w/ the bile duct
The pancreatic accessory duct drains directly into the duodenum
Acini manufacture the digestive enzymes

75. Pancreatic Juice pH of 8.0: helps neutralize acidic chyme
Proteases made by the pancreas become active
in the duodenum (prevents self-digestion)
Figure 23.27

76. Composition and Function of Pancreatic Juice
Water solution of enzymes and electrolytes (primarily HCO3–)
Neutralizes acid chyme
Provides optimal environment for pancreatic enzymes
Enzymes are released in inactive form and activated in the duodenum

77. Composition and Function of Pancreatic Juice
Examples include
Trypsinogen is activated to trypsin
Procarboxypeptidase is activated to carboxypeptidase
Active enzymes secreted
Amylase, lipases, and nucleases
These enzymes require ions or bile for optimal activity

78. Regulation of Pancreatic Secretion
Secretin and CCK are released when fatty or acidic chyme enters the duodenum
CCK and secretin enter the bloodstream
Upon reaching the pancreas:
CCK induces the secretion of enzyme-rich pancreatic juice
Secretin causes secretion of bicarbonate-rich pancreatic juice
Vagal stimulation also causes release of pancreatic juice

79. Regulation of Pancreatic Secretion
During cephalic and gastric
phases, stimulation by
vagal nerve fibers causes
release of pancreatic juice
and weak contractions of
the gallbladder. 1
Acidic chyme entering
duodenum causes the
enteroendocrine cells of
the duodenal wall to release
secretin, whereas fatty,
protein-rich chyme induces
release of cholecystokinin. 2
Cholecystokinin
and secretin enter
bloodstream. 3
Upon reaching the
pancreas, cholecystokinin
induces the secretion of
enzyme-rich pancreatic juice;
secretin causes copious
secretion of bicarbonate-rich
pancreatic juice.
Figure 23.28

80. Digestion in the Small Intestine
As chyme enters the duodenum:
Carbohydrates and proteins are only partially digested
No fat digestion has taken place

81. Digestion in the Small Intestine
Digestion continues in the small intestine
Chyme is released slowly into the duodenum
Because it is hypertonic and has low pH, mixing is required for proper digestion
Required substances needed are supplied by the liver
Virtually all nutrient absorption takes place in the small intestine
Most H2O absorption also occurs in the s. intestine

82. Requirements for Optimal Intestinal Digestive Activity
Most substances required for digestion are imported from the liver and pancreas
Chyme must be released into the intestine slowly
Chyme is hypertonic
If chyme is released to fast, low blood volume will result
Low pH of chyme must be adjusted by bile and pancreatic juice

83. Motility in the Small Intestine
The most common motion of the small intestine is segmentation
It is initiated by intrinsic pacemaker cells (Cajal cells) in the longitudinal smooth muscle layer
Depolarize frequently (12-14x/min) vs. ileum (8-9x/min)
Intensity of segmentation is altered by long and short reflexes (Parasymp. enhances; Symp. Decreses)
The hormone motilin is released by duodeneal mucosa and initiates the peristaltic waves in the proximal duodenum
Migrating Motility Complex: sweeping waves of contraction along the s. intestine
Local enteric neurons coordinate these intestinal motility patterns
Moves contents steadily toward the ileocecal valve

84. Impulses sent proximally by cholinergic effector neurons cause:
Control of Motility
Impulses sent proximally by cholinergic effector neurons cause:
Contraction and shortening of the circular muscle layer
Impulses sent distally by cholinergic effector neurons cause:
Shortening of longitudinal muscle
Distension of the intestine

85. Control of Motility
Two mechanisms (hormonal & neural) cause ileocecal sphincter to relax and allow food residues to enter the cecum
1) Gastroileal reflex: enhanced activity of stomach forces segmentation in the ileum
2) Gastrin: released by the stomach, gastrin increases motility in the ileum and relaxes the iliocecal sphincter
Back-pressure of chyme in the cecum forces the sphincter closed

86. Has three unique features:
Large Intestine
Has three unique features:
Teniae coli – three bands of longitudinal smooth muscle in its muscularis
Haustra – pocketlike sacs caused by the tone of the teniae coli
Epiploic appendages – fat-filled pouches of visceral peritoneum

87. Large Intestine
Is subdivided into the cecum, appendix, colon, rectum, and anal canal
1.5 m in length
Primary function is to absorb H2O from indigestable food residues, temporarily store them, and finally excrete them
No breakdown of foodstuff occurs in the L. intestine
The saclike cecum:
Lies below the ileocecal valve in the right iliac fossa
Contains a wormlike vermiform appendix

88. Large Intestine
Figure 23.29a

89. Large Intestine: Microscopic Anatomy
Contains no villi and no cells that secrete digestive enzymes
Colon mucosa is simple columnar epithelium except in the anal canal
Has numerous deep crypts lined with goblet cells
Abundant supply of mucus eases the passage of feces and protects the colon against acids and gases released by resident bacteria

90. Large Intestine: Microscopic Anatomy
Anal canal mucosa is stratified squamous epithelium
Anal sinuses exude mucus and compress feces
Superficial venous plexuses are associated with the anal canal
Inflammation of these veins results in itchy varicosities called hemorrhoids

91. Bacterial Flora
The bacterial flora (over 700 species!) of the large intestine consist of:
Bacteria surviving the small intestine that enter the cecum and
Those entering via the anus
These bacteria:
Colonize the colon
Metabolize some host derived proteins
Ferment indigestible carbohydrates
Release irritating acids and gases (flatus)
Synthesize B complex vitamins and vitamin K used by the liver to make clotting proteins

92. Containment of Bacterial Flora
The gut mucosa release chemicals that recruit dendritic cells
Dendritic cells sample the lumen with cytoplasmic extensions and present antigens to T cells
T cells initiate an IgA antibody response restricted to the gut lumen and keeps bacteria in check if they were to breach the mucosa

93. Functions of the Large Intestine
Other than digestion of enteric bacteria, no further digestion takes place
Vitamins, water, and electrolytes are reclaimed
Its major function is propulsion of fecal material toward the anus
Though essential for comfort, the colon is not essential for life

94. Motility of the Large Intestine
Generally poor contractile ability
Haustral contractions
Slow segmenting movements of the transverse and descending colon that move the contents of the colon (last about 1 min. and occur every 30 sec.)
Haustra sequentially contract as they are stimulated by distension
Presence of food in the stomach:
Activates the gastrocolic reflex
Initiates peristalsis that forces contents toward the rectum

95. Distension of rectal walls caused by feces:
Defecation
Distension of rectal walls caused by feces:
Stimulates contraction of the rectal walls
Relaxes the internal anal sphincter
Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs
Valsalva’s maneuver helps force fecal material out

96. Chemical Digestion
Large food products are broken down to monomers small enough to be absorbed by the GI tract lining
Accomplished by enzymes secreted into the lumen
Breakdown is called hydrolysis and involves the addition of H2O

97. Chemical Digestion: Carbohydrates
Simple sugars (monosaccharides) are absorbed immediately, e.g. glucose, fructose, galactose
Breakdown must occur w/ disaccharides, e.g. sucrose, lactose, maltose & w/ polysaccharides, e.g. starch and glycogen

98. Chemical Digestion: Carbohydrates
Digestion begins in the mouth
Salivary amylase: splits starch into oligosaccharides (2-8 linked glucose molecules) and is completed in the stomach until salivary amylase is denatured and digested by enzymes
Pancreatic amylase: breaks down starch completely to oligosaccharides in the S. intestine
Intestinal brush border enzymes further digest products to monosaccharides
Chemical digestion ends in the S. Intestine

99. Carbohydrate Absorption
Glucose and galactose are transported by secondary active transport into epithelial cells by protein carriers

100. Chemical Digestion: Proteins
Source of digested proteins: food, enzymes released into the GI tract, sloughed cells into the lumen of the GI tract
Protein digestion begins in the stomach when pepsinogen is activated to pepsin which is functionally active between pH
Pepsin is then inactivated at high pH in the duodenum
Protein is further broken down by trypsin and chymotrypsin secreted by the pancreas into the S. Intestine
Brush border enzymes carboxypepsidase & aminopepsidase split off 1 amino acid at a time from the carboxy and amino ends

101. Protein Absorption
Carriers (active transport) are used to transport amino acids into epithelial cells and enter the capillary blood by diffusion

102. Figure 23.34

103. Chemical Digestion: Fats
S. Intestine is the sole site for lipid digestion
Pancreas is the only significant source of lipases, fat digesting enzymes
Triglycerides are treated with bile salts before being digested (in duodenum)
Bile salts allow triglycerides to interact with H2O forming emulsions.
Does not break chemical bonds, but increases the number of triglycerides exposed to pancreatic lipases
Lipases break down fats by cleaving off 2 fatty acid chains leaving free fatty acids and monoglycerides

104. Chemical Digestion: Fats
Glycerol and short chain fatty acids are:
Absorbed into the capillary blood in villi
Transported via the hepatic portal vein

105. Fatty Acid Absorption
Fatty acids and monoglycerides associate with bile salts forming micelles and diffuse between the microvilli
Lipids leave the micelles and enter intestinal cells via diffusion
Free fatty acids and monoglycerides are resynthesized into triglycerides in the epithelial cells
Triglycerides are then combined with proteins forming chylomicrons that enter lacteals and are transported to the circulation via lymph
Once back in the blood, triglycerides are hydrolyzed to free fatty acids and glycerol and used by tissue cells

106. Fatty Acid Absorption Figure 23.36 Fatty acids and monoglycerides
associated with
micelles in
lumen of intestine
Lumen of
intestine 1
Fatty acids and
monoglycerides
resulting from fat
digestion leave
micelles and enter
epithelial cell by
diffusion.
Absorptive
epithelial cell
cytoplasm 2
Fatty acids are
used to synthesize
triglycerides in
smooth endo-
plasmic reticulum. ER
Golgi
apparatus 3
Fatty globules are
combined with
proteins to form
chylomicrons
(within Golgi
apparatus). 4
Vesicles containing
chylomicrons
migrate to the
basal membrane,
are extruded from
the epithelial cell,
and enter a lacteal
(lymphatic capillary). 5
Lymph in the
lacteal transports
chylomicrons away
from intestine.
Chylomicron
Lacteal
Figure 23.36

107. Electrolyte Absorption
Most ions are actively absorbed along the length of small intestine
Na+ is coupled with absorption of glucose and amino acids
Ionic iron is transported into mucosal cells where it binds to ferritin
Anions passively follow the electrical potential established by Na+

108. Electrolyte Absorption
K+ diffuses across the intestinal mucosa in response to osmotic gradients
Ca2+ absorption:
Is related to blood levels of ionic calcium
Is regulated by vitamin D and parathyroid hormone (PTH)

109. Water Absorption
95% of water is absorbed in the small intestines by osmosis
Water moves in both directions across intestinal mucosa
Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells
Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients