Download The Digestive System

8/26/2016
Chapter 24: The Digestive
System
Digestive System

Digestive Tract:







mouth
pharynx
esophagus
stomach
small intestine
large intestine
Accessory Organs:



pancreas
liver
gall bladder
1
8/26/2016
Organs of the digestive system
Digestive System Functions
 ingestion
 mechanical
and chemical
digestion
 Absorption
 concentration
 excretion
2
8/26/2016



Digestive System Histology
Mucosa is the inner epithelium of the
digestive tract.
Glands in the lamina propria secrete through
ducts opening into the epithelium
stratified squamous epithelium





Buccal cavity
Pharynx
Esophagus
anus
Remainder lined by simple columnar
epithelium
Digestive System Histology
 Submucosa - loose connective
tissue surrounding the muscularis
mucosae (smooth muscle).
 Large blood vessels and lymphatic
vessels .
 Submucosal plexus

Network of nerves that regulates
contractions/secretions
3
8/26/2016
Layers of the gastrointestinal tract




Digestive System Histology
Muscularis Externa: smooth muscle cells
 inner circular layer
 outer longitudinal layer.
Serosa is a serous membrane covering the
muscularis externa inside the peritoneal cavity.
 There is no serosa covering the muscularis external
of the oral cavity, pharynx, esophagus and rectum.
Autonomic reflexes of the myenteric plexus which is
between the inner and outer smooth muscle layers.
Parasympathetic stimulation increases muscluar tone
and activity and sympathetic stimulation promotes
muscular inhibition and relaxation.
4
8/26/2016
Organization of the enteric nervous
system
Digestive System


Pacesetter cells in the smooth muscle of the
digestive tract trigger waves of contraction.
Peristalsis



circular muscles contract behind digestive
contents.
longitudinal muscles contract shortening
adjacent segments
Segmentation


muscles churn and fragment digestive
materials to mix them with intestinal
secretions.
Does not move material in a particular
direction.
5
8/26/2016
Buccal Cavity
 Analysis of potential food.
 Mechanical digestion (mastication).
 Lubrication by mucus and salivary
secretions (parotid, sublingual,
submandibular glands).
 Chemical Digestion.
 Bolus: partially digested food for
deglutition.
Buccal Cavity
labia
 vestibule
 gingivae
 hard palate
 soft palate
 lingual frenulum
 uvula

6
8/26/2016
Structures of the mouth (oral
cavity)


Salivary Glands
Produce 1-1.5 L of saliva daily

Bolus secretion



99.4% water with ions, buffers, and enzymes

lubricate the mouth
dissolve chemicals in food & stimulate the taste
buds
make deglutition possible
Background secretion


flushes the oral surfaces
salivary IgA and lysozymes control oral bacteria.
7
8/26/2016
The three
major
salivary
glandsparotid,
sublingual,
and
submandibular


Salivary Glands
Parotid glands produce a secretion rich in
salivary amylase.
Submandibular and sublingual glands
produce saliva that contains less enzymes but
more buffers and mucus.



Salivary production can reach 7 ml per minute
during eating
When eating the pH rises from about 6.7 to
about 7.5.
Salivary secretions are normally controlled by
the ANS
8
8/26/2016


Teeth
Neck of the tooth marks the boundary
between the root and the crown.
Crown is covered by enamel containing a
crystalline form of calcium phosphate.


Calcium phosphate is the hardest biologically
manufactured substance.
Bud of the tooth consists of dentin - a
mineralized matrix containing no living
cells.
A typical tooth
9
8/26/2016












Teeth
Pulp cavity receives blood vessels and nerves
through the root canal at the root of the tooth.
The root sits in the alveolus.
Collagen fibers of the periodontal ligament
extend from the dentin of the root to the
surrounding bone.
Cementum covers the dentin of the root
providing protection and anchoring the
periodontal ligament.
Epithelial cells in the gum form tight
attachments to the tooth preventing bacterial
access to the cementum.
Teeth
Incisors
Cuspids (canine)
Bicuspids (premolar)
Molars
Deciduous teeth (20) are replaced by
secondary dentition.
Periodontal ligaments and roots of the
deciduous teeth erode and are pushed out by
the eruption of the secondary teeth (32).
Third molars are the last teeth to appear.
10
8/26/2016
Teeth
Pharynx & Esophagus

The pharynx is the passageway of
solid food, liquids, and air.
 Pharyngeal muscle contractions
during swallowing propel the
bolus into the esophagus.
11
8/26/2016

Pharynx & Esophagus
The esophagus is about 25 cm
 Upper 1/3 contains skeletal muscle.
 Lower third contains smooth muscle.
 Center third is a mixture.
 Passes through the mediastinum in the
thoracic cavity and enters the peritoneal
cavity through the esophageal hiatus in the
diaphragm.
 Lined with a stratified squamous epithelium
protects from abrasion, temperature
extremes and chemicals.
 Lubricated with mucous glands
Histology of the esophagus
12
8/26/2016



Deglutition
Oral Phase begins with compression of the
bolus against the hard palate and the
movement into the pharynx. Under conscious
control.
Pharyngeal phase begins when bolus comes
in contact with sensory receptors around the
pharynx and posterior pharyngeal wall and
initiates involuntary swallowing reflex.
Followed by the closure of the glottis by the
epiglottis.
Esophageal Phase begins with the passage
through the upper esophageal sphincter then
peristalsis through the cardiac sphincter.
Deglutition (swallowing)
13
8/26/2016




?????
What is the importance of the mesenteries?
Whick would be more efficient in propelling
intestinal contents from one place to another
peristalsis or segmentation?
What effect would a drug that blocks
parasympathetic stimulation of the digestive
tract have on peristalsis?
What is occuring when the soft palate and
larynx elevate and the glottis closes.
Stomach
ULQ of abdominopelvic cavity.
 Functions

Bulk storage of ingested food.
 Mechanical digestion.
 Chemical digestion (acid and enzymes).
 Production of intrinsic factor.


Structure
cardia
 fundus
 body
 Pyloris.

14
8/26/2016
External and internal anatomy of the
stomach
Stomach
Third layer of muscle, the inner oblique,
adds strength.
 Visceral peritoneum of the stomach is
continuous with mesenteries

greater omentum
 lesser omentum

Rugae are ridges and folds in a relaxed
stomach.
 Bolus is turned to chyme.
 Can contain 1-1.5 L

15
8/26/2016
Peritoneal Folds
Gastric Wall of stomach
Is lined by a mucous epithelium.
 Alkaline mucus protects the stomach
lining from HCl and enzymes.
 Gastric pits open into the gastric surface
and contain mucous cells.
 Gastric glands produce about 1500 ml of
gastric juice per day from secretory cells

G cells produce hormone stimulated gastric
acid secretion, gastrin
 Parietal cells make intrinsic factor and HCl.
 Chief cells secrete pepsinogen which is
converted to pepsin by HCl.

16
8/26/2016
Histology of the stomach
Gastric Activity
Cephalic phase parasympathetic fibers
under the control of the vagus nerve
innervate mucous cells , parietal cells,
chief cells and endocrine cells of the
stomach.
 Gastric Phase which begins with the
arrival of the bolus in the stomach.
 Intestinal Phase begins when chyme
begins to enter the small intestine.

17
8/26/2016


Gastric Activity
Intestinal Phase begins when chyme begins to enter
the small intestine.
 Controls the rate of gastric emptying to ensure
secretory, digestive, and absorptive function of the
small intestine.
Local endocrine control reduces gastric activity.




Secretin
cholecystokinin (CCK)
gastric inhibitory peptide (GIP)
Inhibitory reflexes depress gastric activity when the
proximal small intestine becomes too full, or acidic.
 slows stomach wall contractions
 inhibits PNS, stimulates SNS
Small Intestines
Small intestines includes the duodenum,
the jejunum, and the ileum.
 The ileocecal valve is the sphincter
between the small and large intestines.
 Plicae are the transverse folds of the
intestinal mucosa.
 Villi are small projections into the lumen
of the small intestines.


Microvilli cover the villi to increase surface
area.
18
8/26/2016
Anatomy of the small intestine
Small Intestine
Villi contain a network of capillaries to
transport respiratory gases and carry
absorbed nutrients to the hepatic portal
circulation to the liver.
 At the bases of the villi are entrances to
intestinal glands that secrete alkaline
intestinal juice and intestinal
hormones.

19
8/26/2016
Histology of the duodenum and
ileum
Small Intestines
 Intestinal juice includes mucus and
hormones to buffer acids and
dissolve digestive enzymes.
 Intestinal hormones include
cholecystokinin (CCK) and gastric
inhibitory peptide (GIP) to control
the rate of the intestinal phase.
 90% of absorption occurs in the small
intestines.
20
8/26/2016
Small Intestine
 Duodenum - 25 cm. Receives chyme
from the stomach and exocrine
secretions of the pancreas and liver
 Jejunum - 2.5 m supported by a
sheet of mesentery attached to the
dorsal body wall. Most chemical
digestion and nutrient absorption.
 Ileum - 3.5 m ends at the ileocecal
valve.
Histology of the small intestine
21
8/26/2016
Histology of the small intestine
Small Intestine
 Weak peristaltic contractions move
material as it is being absorbed.
 Gastroenteric reflex is initiated by
distension of the stomach and
accelerates glandular secretion and
peristaltic activity in all segments.
 Gastroileal reflex is a response to
circulating levels of gastrin which
relaxes the ileocecal valve.
 Transit time through small intestines
is about 5 hours.
22
8/26/2016
Small Intestine
1.8 liters of intestinal juice is
produced each day.
 Hormones and CNS regulate
secretions.
 Secretions are increased through
parasympathetic stimulation of the
vagus nerve.


Intestinal Hormones
Secretin


released when the pH falls in the duodenum
Increases the secretion of bile and buffers by the
liver and pancreas

Cholecystokinin (CCK)

Gastric inhibitory peptide (GIP)




Secreted when chyme arrives with lipids and
partially digested proteins
Targets the pancreas and gall bladder
Released when fats and carbohydrates enter the
small intestine
Inhibits gastric activity and stimulates insulin
secretion
23
8/26/2016
?????
Which muscle regulates the flow of
chyme from the stomach to the small
intestine?
 When a person suffers from chronic
ulcers in the stomach treatment
sometimes involves cutting the branches
of the vahus nerve to the stomach. Why?
 How would a meal high in fat affect the
level of cholecystokinin in the blood?





Pancreas
The pancreas is about 15cm long & about 80g.
Pancreatic islets secrete insulin and glucagon
account for 1% of the pancreas.
Is connected to the duodenum by the
pancreatic duct and secretes the exocrine
hormones.
Pancreatic acini are sacs where small ducts
originate.


Enzymes and buffers are secreted by the acinar
cells.
Pancreatic lipases, carbohydrases, nucleases
and proteases do most of the digestion in the
small intestine.
24
8/26/2016
Pancreas
Secretes about 1000 ml of pancreatic juice
per day.
 Primarily controlled by hormones from
the duodenum.
 Acid chyme stimulates the duodenum to
release secretin that triggers pancreatic
production of fluid with a pH of 7.5-8.8.
 CCK controls the production and
secretion of pancreatic enzymes.

Relation of the pancreas to the liver,
gallbladder, and duodenum
25
8/26/2016
Relation of the pancreas to the liver,
gallbladder, and duodenum
Liver
The liver is in the right hypochondriac
and epigastric abdominopelvic regions.
 Four lobes: L, R, caudate, quadrate
 Falciform ligament divides the L and R
lobes.
 The hepatocytes secrete bile.


Bile is released into bile canaliculi
common hepatic duct  common bile
duct duodenum or the cystic duct into the
gallbladder.
26
8/26/2016
Liver

Metabolic Regulation:

Hematological Regulation:


All blood leaving the absorptive areas of the
digestive tract flow through the liver before
reaching the general circulation.
Largest blood reservoir in the body





Kupffer cells
synthesize plasma proteins
synthesize clotting factors
Synthesizes bile made of ions, bilirubin,
cholesterol & lipids (bile salts).
Bile dilutes and buffers acids in chyme
Bile salts are synthesized from cholesterol in
the liver and serve to emulsify fat

Histology of the Liver
27
8/26/2016
Histology of the Liver







Gallbladder
Muscular pear shaped organ
Stores and concentrates bile
Liver cells produce about 1 L of bile /day.
The gallbladder can store 40-70 ml.
Bile secretion occurs continuously but is
released into the duodenum only under
stimulation of CCK.
CCK relaxes the hepatopancreatic sphincter
of the common bile duct.
Gallbladder contractions eject bile.
28
8/26/2016
?????
 Narrowing of the ileocecal valve
would hamper the movement of
materials to what organ?
 The digestion of which nutrient
would be most impaired by damage
to the exocrine pancreas?
 How would a decrease in the
amount of bile salts in bile affect the
digestion and absorption of fat?




Large Intestine
The large intestine is 1.5 m and 7.5 cm in
diameter.
Sections
 Cecum
 Colon
 Rectum
Reabsorption, compaction and storage.
Bacteria in the large intestine break down
undigested food liberate some nutrients and
produce gas.
29
8/26/2016
Large Intestine


About 1500 ml of watery material arrives
in the colon per day.
 About 1300 ml of water is reabsorbed.
 About 200 ml of feces is ejected.
 Average feces is 75% water, 5%
bacteria, and 20% indigestible
materials, inorganic matter, and
epithelial cells.
Large Intestines
Cecum collects and stores material from the
ileum and begins the process of compaction.
 The veriform appendix is attached to the
cecum, about 9 cm long.


Walls of the appendix are dominated by
lymphoid nodules.
Colon has thinner wall than the small
intestine.
 Haustra are pouches in the colon produced
by tension in the teniae, longitudinal bands
of smooth muscle.
30
8/26/2016

Large Intestines
Rectum
 Anal canal contains anal columns.
 The opening of the anal canal becomes
keratinized.
 Internal anal sphincter is the circular muscle
layer of the muscularis externa.
 External anal sphincter consists of skeletal
muscle and guards the exit of the anorectal
canal.
Anatomy of the large intestine
31
8/26/2016
Large Intestine
The gastroileal and gastroenteric reflexes
move material into the cecum while you
eat.
 Movement from the cecum to the
transverse colon is slow.
 Movement from the transverse colon
through the rest of the large intestine
results from powerful peristaltic
contractions called mass movements.

Large Intestine
Mass movements are stimulated by
distension of the stomach and
duodenum.
 Contractions are a result of the intestinal
nerve plexuses.
 Fecal material is forced into the rectum.

32
8/26/2016
Histology of the large intestine
Histology of the large intestine
33
8/26/2016
The gastric phase of digestion
Daily volumes of fluid ingested,
secreted, absorbed, and excreted
from the GI tract
34
8/26/2016




Defecation
Distension of the rectal wall triggers the
defecation reflex resulting in a series of local
peristaltic contractions in the colon & rectum.
The movement of feces through the anal canal
requires relaxation of the internal anal
sphincter which shuts the external sphincter
automatically.
Release of feces requires conscious effort of
open the external sphincter voluntarily.
If the command does not arrive peristaltic
contractions cease until additional rectal
expansion triggers the defecation reflex a
second time.
Absorption of digested nutrients in
the small intestine
35
8/26/2016
Clinical Application
• inflammation of
Hepatitis
the liver
• most commonly caused by viral infection
• can be caused by reactions to drug, alcoholism or
autoimmunity
Hepatitis A – not washing hands or
Signs and Symptoms
eating raw shellfish
• headache
Hepatitis B – chronic; serum
• low fever
Hepatitis C – serum
• fatigue
• vomiting
Hepatitis D – very severe; only
• rash
produces symptoms if infected with
• foamy urine
B; serum
• pale feces
Hepatitis E, F, G – more rare
• jaundice
• pain
36