Digestion

Chapter 18
The Digestive System

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
 Motility:
 Movement of of food through the GI tract.
 Ingestion:
 Taking food into the mouth.
 Mastication:
 Chewing the food and mixing it with saliva.
 Deglutition:
 Swallowing the food.
 Peristalsis:
 Rhythmic wave-like contractions that move food
through GI tract.
Functions of the GI Tract

 Secretion:
 Includes both exocrine and endocrine
secretions.
 Exocrine:
 HCl, H20, HC03
-, bile, lipase, pepsin, amylase, trypsin,
elastase, and histamine are secreted into the lumen of the
GI tract.
 Endocrine:
 Stomach and small intestine secrete hormones to help
regulate the GI system.
 Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and
somatostatin.
Functions of the GI Tract (continued)

 Digestion:
 Breakdown of food particles into subunits
(chemical structure change).
 Absorption:
 Process of the passage of digestion
(chemical subunits) into the blood or
lymph.
 Storage and elimination:
 Temporary storage and elimination of
indigestible food.
Functions of the GI Tract (continued)

Digestive System (GI)
 GI tract divided
into:
 Alimentary
canal.
 Accessory
digestive
organs.
 GI tract is 30 ft
long and
extends from
mouth to anus.
Insert fig. 18.2

Layers of GI Tract
 Composed of 4 tunics:
 Mucosa.
 Submucosa.
Muscularis.
Serosa.

 Lines the lumen of GI tract.
 Consists of simple columnar epithelium.
 Lamina propria:
 Thin layer of connective tissue containing lymph
nodules.
 Muscularis mucosae:
 Thin layer of smooth muscle responsible for the
folds.
 Folds increase surface area for absorption.
 Goblet cells:
 Secrete mucus.
Mucosa

 Thick, highly vascular layer of
connective tissue.
 Absorbed molecules enter the blood and
lymphatic vessels.
 Submucosal plexus (Meissner’s plexus):
 Provide autonomic nerve supply to the
muscularis mucosae.
Submucosa

 Responsible for segmental contractions and
peristaltic movement through the GI tract.
 Inner circular layer of smooth muscle.
 Outer longitudinal layer of smooth muscle.
 Contractions of these layers move food
through the tract; pulverize and mix the food.
 Myenteric plexus located between the 2
muscle layers.
 Major nerve supply to GI tract.
 Fibers and ganglia from both sympathetic and
parasympathetic nervous systems.
Muscularis

Serosa
 Binding and protective outer layer.
 Consists of areolar connective tissue covered
with simple squamous epithelium.

 Extrinsic innervation:
 Parasympathetic nervous system:
 Vagus and spinal nerves:
 Stimulate motility and GI secretions.
 Sympathetic nervous system:
 Postganglionic sympathetic fibers that pass
through submucosal and myenteric plexuses
and innervate GI tract:
 Reduce peristalsis and secretory activity.
Regulation of the GI Tract

 Enteric nervous system:
 Sites where parasympathetic fibers synapse with
postganglionic neurons that innervate smooth
muscle.
 Submucosal and myenteric plexuses:
 Local regulation of the GI tract.
 Paracrine secretion:
 Molecules acting locally.
 Hormonal secretion:
 Secreted by the mucosa.
Regulation of the GI Tract (continued)

 Mastication (chewing):
 Mixes food with saliva which contains salivary
amylase.
 Enzyme that can catalyze the partial digestion of starch.
 Deglutition (swallowing):
 Begins as a voluntary activity.
 Involves 3 phases:
 Oral phase is voluntary.
 Pharyngeal and esophageal phases are involuntary.
 Cannot be stopped.
 Larynx is raised.
 Epiglottis covers the entrance to respiratory tract.
From Mouth to Stomach

 Involuntary muscular contractions and
relaxations in the mouth, pharynx, larynx, and
esophagus are coordinated by the swallowing
center in the medulla.
 Esophagus:
 Connects pharynx to the stomach.
 Upper third contains skeletal muscle.
 Middle third contains a mixture of skeletal and smooth
muscle.
 Terminal portion contains only smooth muscle.
From Mouth to Stomach (continued)

Esophagus
 Peristalsis:
 Produced by a series of
localized reflexes in
response to distention of
wall by bolus.
 Wave-like muscular
contractions:
 Circular smooth muscle
contract behind, relaxes
in front of the bolus.
 Followed by longitudinal
contraction (shortening)
of smooth muscle.
 Rate of 2-4 cm/sec.
 After food passes into
stomach, LES constricts.
Insert 18.4a

 Most distensible part of GI tract.
 Empties into the duodenum.
 Functions of the stomach:
 Stores food.
 Initiates digestion of proteins.
 Kills bacteria.
 Moves food (chyme) into intestine.
Stomach

Stomach (continued)
 Contractions
of the
stomach
churn chyme.
 Mix chyme
with gastric
secretions.
 Push food
into
intestine.
Insert fig. 18.5

Stomach (continued)
 Gastric mucosa
has gastric pits
in the folds.
 Cells that line
the folds
deeper in the
mucosa, are
gastric glands.
Insert fig. 18.7

 Secrete gastric juice:
 Goblet cells: mucus.
 Parietal cells: HCl and intrinsic factor.
 Chief cells: pepsinogen.
 Enterochromaffin-like cells (ECL):
histamine and serotonin.
 G cells: gastrin.
 D cells: somatostatin.
 Stomach: ghrelin.
Gastric Glands

HCl Production
 Parietal cells
secrete H+ into
gastric lumen by
primary active
transport, through
H+/ K+ ATPase
pump.
 Parietal cell’s
basolateral
membrane takes
in Cl- against its
electrochemical
gradient, by
coupling its
transport with
HC03
-.
Insert fig. 18.8

 HCl production is stimulated:
 Indirectly by gastrin.
 Indirectly by ACh.
 ACh and gastrin stimulate release of
histamine.
 Histamine:
 Stimulates parietal cells to secrete HCl.
HCl Production (continued)

HCl Functions
 Makes gastric
juice very acidic.
 Denatures
ingested proteins
(alter tertiary
structure) so
become more
digestible.
 Activates
pepsinogen to
pepsin.
 Pepsin is more
active at pH of
2.0.
Insert fig. 18.9

Digestion and Absorption in the
Stomach
 Proteins partially digested by pepsin.
 Carbohydrate digestion by salivary
amylase is soon inactivated by acidity.
 Alcohol and aspirin are the only
commonly ingested substances
absorbed.

Gastric and Peptic Ulcers
 Peptic ulcers:
 Erosions of the mucous membranes of the stomach or
duodenum produced by action of HCl.
 Zollinger-Ellison syndrome:
 Ulcers of the duodenum are produced by excessive gastric
acid secretions.
 Helicobacter pylori:
 Bacterium that resides in GI tract that may produce ulcers.
 Acute gastritis:
 Histamine released by tissue damage and inflammation
stimulate further acid secretion.

 Parietal and chief cells impermeable to
HCl.
 Alkaline mucus contains HC03
-.
 Tight junctions between adjacent
epithelial cells.
 Rapid rate of cell division (entire
epithelium replaced in 3 days).
 Prostaglandins inhibit gastric secretions.
Protective Mechanisms of
Stomach

Small Intestine
 Each villus is a fold in
the mucosa.
 Covered with columnar
epithelial cells
interspersed with goblet
cells.
 Epithelial cells at the tips
of villi are exfoliated and
replaced by mitosis in
crypt of Lieberkuhn.
 Lamina propria contain
lymphocytes, capillaries,
and central lacteal.
Insert fig. 18.12

 Duodenum and jejunum:
 Carbohydrates, amino acids, lipids, iron,
and Ca2+.
 Ileum:
 Bile salts, vitamin B12, electrolytes, and
H20.
Absorption in Small Intestine

 Microvilli contain brush border enzymes that
are not secreted into the lumen.
 Brush border enzymes remain attached to the
cell membrane with their active sites exposed to
the chyme.
 Absorption requires both brush border
enzymes and pancreatic enzymes.
Intestinal Enzymes

Intestinal Contractions and
Motility
 2 major types of
contractions occur in the
small intestine:
 Peristalsis:
 Slow movement.
 Pressure at the pyloric end
of small intestine is greater
than at the distal end.
 Segmentation:
 Major contractile activity of
the small intestine.
 Contraction of circular
smooth muscle.
 Mix chyme.
Insert fig. 18.14

Contractions of Intestinal
Smooth Muscles
 Occur automatically in
response to
endogenous pacemaker
activity.
 Rhythm of contractions
is paced by graded
depolarizations called
slow waves.
 Slow waves produced by
interstitial cells of Cajal.
 Slow waves spread from
1 smooth muscle cell to
another through
nexuses.
Insert fig. 18.15

 When slow waves above threshold, it triggers
APs by opening of VG Ca2+ channels.
 Inward flow of Ca2+:
 Produces the upward depolarization phase.
 Stimulates contraction of smooth muscle.
 Repolarization:
 VG K+ channels open.
 Slow waves decrease in amplitude as they are
conducted.
 May stimulate contraction in proportion to
the magnitude of depolarization.
Contractions of Intestinal
Smooth Muscles

Cells and Electrical Events in the
Muscularis
Insert fig. 18.16

 Outer surface bulges outward to form haustra.
 Little absorptive function.
 Absorbs H20, electrolytes, several vitamin B complexes,
vitamin K, and folic acid.
 Intestinal microbiota produce significant amounts of folic acid and
vitamin K.
 Bacteria ferment indigestible molecules to produce short-chain
fatty acids.
 Does not contain villi.
 Secretes H20, via active transport of NaCl into intestinal
lumen.
 Guanylin stimulates secretion of Cl- and H20, and inhibits
absorption of Na+ (minor pathway).
 Membrane contains Na+/K+ pumps.
 Minor pathway.
Large Intestine

 Small intestine:
 Most of the fluid and electrolytes are absorbed by
small intestine.
 Absorbs about 90% of the remaining volume.
 Absorption of H20 occurs passively as a result of the
osmotic gradient created by active transport.
 Aldosterone stimulates NaCl and H20 absorption in the
ileum.
 Large intestine:
 Absorbs about 90% of the remaining volume.
 Absorption of H20 occurs passively as a result of the osmotic
gradient created by active transport of Na+ and Cl-.
Fluid and Electrolyte Absorption
in the Intestine

Defecation
 Waste material passes to the rectum.
 Occurs when rectal pressure rises and
external anal sphincter relaxes.
 Defecation reflex:
 Longitudinal rectal muscles contract to
increase rectal pressure.
 Relaxation of internal anal sphincter.
 Excretion is aided by contractions of
abdominal and pelvic skeletal muscles.
 Push feces from the rectum.

Structure of Liver
 Liver largest internal organ.
 Hepatocytes form hepatic plates that are 1–2 cells
thick.
 Arranged into functional units called lobules.
 Plates separated by sinusoids.
 More permeable than other capillaries.
 Contains phagocytic Kupffer cells.
 Secretes bile into bile canaliculi, which are
drained by bile ducts.

Structure of Liver(continued)
Insert fig. 18.20

 Products of digestion that are absorbed
are delivered to the liver.
 Capillaries drain into the hepatic portal
vein, which carries blood to liver.
 ¾ blood is deoxygenated.
 Hepatic vein drains liver.
Hepatic Portal System

Enterohepatic Circulation
 Compounds that
recirculate between
liver and intestine.
 Many compounds can
be absorbed through
small intestine and
enter hepatic portal
blood.
 Variety of exogenous
compounds are
secreted by the liver
into the bile ducts.
 Can excrete these
compounds into the
intestine with the bile.
Insert fig. 18.22

Major Categories of Liver Function

 The liver produces and secretes 250–1500 ml of
bile/day.
 Bile pigment (bilirubin) is produced in spleen, bone
marrow, and liver.
 Derivative of the heme groups (without iron) from
hemoglobin.
 Free bilirubin combines with glucuronic acid and
forms conjugated bilirubin.
 Secreted into bile.
 Converted by bacteria in intestine to urobilinogen.
 Urobilogen is absorbed by intestine and enters the hepatic
vein.
 Recycled, or filtered by kidneys and excreted in urine.
Bile Production and Secretion

Metabolism of Heme and
Bilirubin
Insert fig. 18.23

Bile Production and Secretion(continued)
 Bile acids are derivatives
of cholesterol.
 Major pathway of
cholesterol breakdown in
the body.
 Principal bile acids are:
 Cholic acid.
 Chenodeoxycholic acid.
 Combine with glycine or
taurine to form bile salts.
 Bile salts aggregate as
micelles.
 95% of bile acids are
absorbed by ileum.
Insert fig. 18.25

Detoxification of the Blood
 Liver can remove hormones, drugs, and other
biologically active molecules from the blood
by:
 Excretion into the bile.
 Phagocytosis by Kupffer cells.
 Chemical alteration of the molecules.
 Ammonia is produced by deamination of amino acids in
the liver.
 Liver converts it into urea.
 Excreted in urine.

Detoxification of the Blood (continued)
 Inactivation of steroid hormones and
drugs.
 Conjugation of steroid hormones and
xenobiotics make them anionic.
 Can be transported into bile by multispecific
organic anion transport carriers.
 Steroid and xenobiotic receptors stimulate
production of cytochrome P450 enzymes.

Secretion of Glucose,
Triglycerides and Ketones
 Liver helps regulate blood glucose
concentration by:
 Glycogenesis and lipogenesis.
 Glycogenolysis and gluconeogenesis.
 Contains enzymes required to convert
free fatty acids into ketone bodies.

Production of Plasma Proteins
 Albumin and most of the plasma globulins
(except immunoglobulins) are produced by
the liver.
 Albumin:
 Constitutes 70% of the total plasma protein.
 Contributes most to the colloid osmotic pressure in
the blood.
 Globulins:
 Transport cholesterol and hormones.
 Inhibit trypsin.
 Produce blood clotting factors I, II, III, V, VII,
IX, XI.

 Sac-like organ attached to the inferior surface
of the liver.
 Stores and concentrates bile.
 When gallbladder fills with bile, it expands.
 Contraction of the muscularis layer of the
gallbladder, ejects bile into the common bile duct
into duodenum.
 When small intestine is empty, sphincter of
Oddi closes.
 Bile is forced up to the cystic duct to gallbladder.
Gallbladder

Pancreas
 Exocrine:
 Acini:
 Secrete
pancreatic
juice.
 Endocrine:
 Islets of
Langerhans:
 Secrete
insulin and
glucagon.
Insert fig. 18.26

 Contains H20, HC03
- and digestive enzymes.
Pancreatic Juice

Pancreatic Juice
 Complete digestion of
food requires action of
both pancreatic and
brush border enzymes.
 Most pancreatic
enzymes are produced
as zymogens.
 Trypsin (when activated
by enterokinase)
triggers the activation
of other pancreatic
enzymes.
 Pancreatic trypsin
inhibitor attaches to
trypsin.
 Inhibits its activity in
the pancreas.
Fig. 18.29

 Neural and endocrine mechanisms
modify the activity of the GI system.
 GI tract is both an endocrine gland, and
a target for the action of hormones.
Neural and Endocrine Regulation

 Gastric motility and secretion are automatic.
 Waves of contraction are initiated
spontaneously by pacesetter cells.
 Extrinsic control of gastric function is divided
into 3 phases:
 Cephalic phase.
 Gastric phase.
 Intestinal phase.
Regulation of Gastric Function

Cephalic Phase
 Stimulated by sight, smell, and taste of food.
 Activation of vagus:
 Stimulates chief cells to secrete pepsinogen.
 Directly stimulates G cells to secrete gastrin.
 Directly stimulates ECL cells to secrete
histamine.
 Indirectly stimulates parietal cells to secrete
HCl.
 Continues into the 1st 30 min. of a meal.

 Arrival of food in stomach stimulates the gastric
phase.
 Gastric secretion stimulated by:
 Distension.
 Chemical nature of chyme (amino acids and short
polypeptides).
 Stimulates G cells to secrete gastrin.
 Stimulates chief cells to secrete pepsinogen.
 Stimulates ECL cells to secrete histamine.
 Histamine stimulates secretin of HCl.
 Positive feedback effect.
 As more HCl and pepsinogen are secreted, more polypeptides
and amino acids are released.
Gastric Phase

Gastric Phase (continued)
 Secretion of HCl is also
regulated by a
negative feedback
effect:
 HCl secretion
decreases if pH < 2.5.
 At pH of 1.0, gastrin
secretion ceases.
 D cells stimulate
secretion of
somatostatin.
 Paracrine
regulator to
inhibit
secretion of
gastrin.
Insert. Fig. 18.30

 Inhibits gastric activity when chyme enters
the small intestine.
 Arrival of chyme increases osmolality and
distension.
 Activates sensory neurons of vagus and produces
an inhibitory neural reflex:
 Inhibits gastric motility and secretion.
 In the presence of fat, enterogasterone inhibits gastric
motility and secretion.
 Hormone secretion:
 Inhibit gastric activity:
 Somatostatin, CCK, and GLP-1.
Intestinal Phase

 Submucosal and myenteric plexuses
contain 100 million neurons.
 Include preganglionic parasympathetic
axons, ganglion cell bodies,
postganglionic sympathetic axons; and
afferent intrinsic and extrinsic sensory
neurons.
Enteric Nervous System

Enteric Nervous System (continued)
 Peristalsis:
 ACh and
substance P
stimulate smooth
muscle
contraction
above the bolus.
 NO, VIP, and
ATP stimulate
smooth muscle
relaxation below
the bolus.
Insert fig. 18.31

Paracrine Regulators of the
Intestine
 Serotonin (5-HT):
 Stimulates intrinsic afferents, which send impulses into
intrinsic nervous system; and activates motor neurons.
 Motilin:
 Stimulates contraction of the duodenum and stomach
antrum.
 Guanylin:
 Activates guanylate cyclase, stimulating the production of
cGMP.
 cGMP stimulates the intestinal cells to secrete Cl- and H20.
 Inhibits the absorption of Na+.
 Uroguanylin:
 May stimulate kidneys to secrete salt in urine.

Intestinal Reflexes
 Intrinsic and extrinsic regulation controlled by
intrinsic and paracrine regulators.
 Gastroileal reflex:
 Increased gastric activity causes increased motility
of ileum and movement of chyme through
ileocecal sphincter.
 Ileogastric reflex:
 Distension of ileum, decreases gastric motility.
 Intestino-intestinal reflex:
 Overdistension in 1 segment, causes relaxation
throughout the rest of intestine.

 Secretion of pancreatic juice and bile is stimulated by:
 Secretin:
 Occurs in response to duodenal pH < 4.5.
 Stimulates production of HC03
- by pancreas.
 Stimulates the liver to secrete HC03
- into the bile.
 CCK:
 Occurs in response to fat and protein content of
chyme in duodenum.
 Stimulates the production of pancreatic enzymes.
 Enhances secretin.
 Stimulates contraction of the sphincter of Oddi.
Secretion of Pancreatic Juice

Digestion and Absorption of
Carbohydrates
 Salivary amylase:
 Begins starch
digestion.
 Pancreatic amylase:
 Digests starch to
oligosaccharides.
 Oligosaccharides
hydrolyzed by brush
border enzymes.
 Glucose is transported
by secondary active
transport with Na+
into the capillaries.
Insert fig. 18.32

 Digestion begins in the stomach when pepsin
digests proteins to form polypeptides.
 In the duodenum and jejunum:
 Endopeptidases cleave peptide bonds in the
interior of the polypeptide:
 Trypsin.
 Chymotrypsin.
 Elastase.
 Exopeptidases cleave peptide bonds from the ends
of the polypeptide:
 Carboxypeptidase.
 Aminopeptidase.
Protein

Protein (continued)
 Free amino acids
absorbed by
cotransport with
Na+.
 Dipeptides and
tripeptides
transported by
secondary active
transport using a
H+ gradient to
transport them into
the cytoplasm.
 Hydrolyzed into
free amino acids
and then secreted
into the blood.
Insert fig. 18.33

 Arrival of lipids in the duodenum serves as a
stimulus for secretion of bile.
 Emulsification:
 Bile salt micelles are secreted into duodenum to
break up fat droplets.
 Pancreatic lipase and colipase hydrolyze
triglycerides to free fatty acids and
monglycerides.
 Colipase coats the emulsification droplets and
anchors the lipase enzyme to them.
 Form micelles and move to brush border.
Lipids

 Free fatty acids, monoglycerides, and
lysolecithin leave micelles and enter into
epithelial cells.
 Resynthesize triglycerides and
phospholipids within cell.
 Combine with a protein to form chylomicrons.
 Secreted into central lacteals.
Lipids (continued)

 In blood, lipoprotein lipase hydrolyzes
triglycerides to free fatty acids and glycerol
for use in cells.
 Remnants containing cholesterol are taken to
the liver.
 Form VLDLs which take triglycerides to cells.
 Once triglycerides are removed, VLDLs are
converted to LDLs.
 LDLs transport cholesterol to organs and blood vessels.
 HDLs transport excess cholesterol back to
liver.
Transport of Lipids

Absorption of Fat
Insert fig. 18.36

Digestion

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