Transcript Digestive System - El Camino College

Digestive System
Human Anatomy
Chapter 22
Organs of the digestive system are either part of the gastrointestinal tract
(alimentary canal) or accessory digestive organs. The GI tract includes the mouth,
esophagus, stomach and intestines, it is considered to be part of the outside of the
body because the mouth and anus both open to the outer environment. The
accessory organs include the teeth, tongue, salivary glands, pancreas, and liver.
These organs assist in the digestion of food.
A. Digestive Processes- there are six food
processing activities:
1. Ingestion- brining food into the mouth (eating)
2. Propulsion- moving food through the GI tract,
peristalsis (contractions of smooth muscle in GI
tract) helps to move the food along.
3. Mechanical digestion- physical change of the
food particles from large to small, this helps to
increase the surface area to make chemical
digestion more effective. Actions include
chewing, churning of food in stomach, and
mixing food with digestive juices.
4. Chemical digestion- chemical change of the
food particles, bonds are broken to change a large
molecule into a smaller one so absorption can
happen more effectively. This involves the use of
enzymes, hydrochloric acid, and other digestive
juices.
5. Absorption- transport of digested food
molecules from the GI into the blood and
lymphatic vessels
6. Defecation- elimination of feces (indigestible
substance/digestive waste)
Peristalsis
Segmentation
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B. Abdominal Regions and
Quadrants- Review the
regions and quadrants as
presented in Chapter 1.
Digestive organs are located
in the abdominal cavity. Be
familiar with the approximate
location of the intestinal
organs within the nine regions
regions and four quadrants as
shown.
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II. Anatomy of the digestive SystemThis section focuses on the gross anatomical and microscopic anatomical
characteristics of the digestive organs.
A. The peritoneal cavity and peritoneum- The peritoneum is an
extensive serous membrane in the abdominal cavity, the visceral
peritoneum covers the surfaces of digestive organs. A cavity is found in
between the visceral and parietal peritoneum. The parietal peritoneum lines
the wall of the abdominal cavity. The peritoneal cavity is a slit like space
filled with serous fluid. A double layer of serous membranes that hold
digestive organs in place is called the mesentery. Organs surrounded by
mesentery and suspended in the cavity are called intraperitoneal or
peritoneal organs. Some are not surrounded by mesentery; they are called
retroperitoneal
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B. Histology of the alimentary Canal Wall- the following are the layers
found in all organs of the gastrointestinal tract. The thickness of each layers
varies with location or organ.
1. The Mucosa- inner most layer, closest to the lumen, it is called a mucous
membrane and has three layers- 1) lining epithelium that secretes mucus
and absorbs nutrients, it contains intrinsic glands; 2) a lamina propia that
has loose connective tissue rich in capillaries and lymphoid tissue; 3)a
muscularis mucosa made up of smooth muscle.
2. The submucosa- contains connective tissue that helps to reshape the
organs once food passes by, major blood and lymphatic vessels, and some
gland tissue
3. The muscularis Externa- two layers of smooth muscle: the circular and
longitudinal layers that help to contract the tube so food passes by
(peristalsis)
4. The serosa-outermost layer, actually the visceral peritoneum, organs that
don't have a serosa have a layer of fibrous connective tissue called the
adventitia, such as the esophagus. Some organs have both a serosa and
adventitia.
5. Nerve plexuses- the GI tract and accessory organs are innervated by the
autonomic nervous system (sympathetic and parasympathetic). There are
two plexuses: the myenteric nerve plexuses that innervates the smooth
muscle in tunica muscularis externa and controls contraction. The
submucosa nerve plexus innervates the submucosa and controls glandular
secretion.
• C. The mouth and associated Organs- the mouth is primarily used
for mechanical digestion.
• 1. Mouth- also known as the oral cavity, its opening is the oral
orifice, and it is the site of ingestion. The lips are also known as
vestibule and the oral cavity proper is the space inside the mouth
(internal to teeth).
• a. Mouth tissue- the mouth is lined by stratified squamous epithelium
and this is also called a mucous membrane. The mouth also has
layers called internal mucosa, submucosa, and external layer of
muscle and bone instead or a serosa. The tongue, lips, gums, and
palate are lined with cells that are slightly keratinized for extra
protection.
• b. Cheeks- made up primarily by the buccinator muscle, they help to
keep food within oral cavity.
• c. Lips- muscular flaps that have a translucent area called the red
region. This is the area that can become dried or chapped because
there are no lubricating glands. The labial frenulum connects the lips
to the gums.
• 2. Tongue- composed of skeletal muscle, it repositions food during
chewing and shapes it into a bolus. The tongue contains taste buds
and also helps to shape sounds when one is speaking. The tongue
is attached to the floor of the mouth by the lingual frenulum. The
tongue also contains lingual tonsils.
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3. Salivary glands- compound tubular glands that produce a liquid
containing water, mucus, proteins, a buffer, and enzymes. The purpose
of saliva is to moisten, begin the digestion of starch, neutralize acids,
stimulate growth of beneficial bacteria in the mouth and dissolve food
chemicals so we can taste them (stimulate the chemoreceptors of the
mouth).
a. Salivary glands include the parotid, submandibular, and
sublingual.
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4. Teeth- The first set of
teeth humans develop is the
deciduous (20 milk teeth)
and then the permanent
teeth (32 teeth). Teeth are
pecialized structures found
within sockets of the gum
called alveoli and are held in
place by the periodontal
ligament. They form an
immoveable joint. Teeth are
composed of three layers
called enamel (hardest
substance in the body),
dentin (bone like tissue that
is harder than bone), and
the pulp cavity (hollow
centered filled with blood
vessels and nerves). The
teeth can also be divided
into three sections, the
crown, the neck, and the
root. Every tooth has its own
blood and nervous supply
(pulp).
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D. Pharynx- once food leaves the mouth it passes into the pharynx (the cavity
behind the mouth) and moves from the oropharynx into the laryngopharynx. There
it is routed to enter the esophagus by contractions of the pharyngeal constrictor
muscles. Swallowing is avoluntary actions that is controlled by skeletal muscle.
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E. Esophagus- long muscular tube that moves food through peristalsis. The
lumen is typically collapsed.
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1. Gross anatomy- It extends from the laryngopharyx, through the thorax, exits via
the esophageal hiatus, and meets the stomach at the cardiac orifice. The cardiac
sphincter controls the entering and exiting of substances into/ out of the stomach.
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2. Microscopic anatomy- It has non-keratinized stratified epithelium, the mucosa
and submucosa form folds when esophagus is empty (pg 623). Tthe submucosa
contains glands that lubricate the food as it passes by. The upper part of the
esophagus contains skeletal muscle and the rest contains smooth muscle. The
esophagus contains an adventitia layer not a serosa layer.
C.S. of Esophagus
Junction at stomach (L.S.)
• F. Stomach- A pouch with sphincter muscles at both ends:
superiorly is the cardiac sphincter and inferiorly is the pyloric
sphincter. They contract to contain the food within the stomach while
it is liquefied, mixed with stomach acid (hydrochloric acid) and
turned into chyme. This is the site where proteins are first digested
by pepsin enzymes. Substances absorbed through the stomach are:
water, electrolytes and some drugs. Food is process in the stomach
for about 4 hours.
• 1. Gross anatomy- It is inferior to the diaphragm and anterior to
spleen and pancreas, The stomach is generally J shaped and has
the following features: cardiac sphincter, cardiac region, fundus,
body, pyloric antrum, pyloric canal, and pyloric sphincter. The curves
of the stomach are described as the greater and lesser curvature.
Internally large folds called rugae are visible. These folds allow the
stomach to expand so it can hold up to 4L of food at one time. It has
three layers of smooth muscle: longitudinal layer, circular layer, and
oblique layer.
2. Microscopic anatomy- the lining
is composed of simple columnar
epithelial cells and many goblet
cells that secrete bicarbonate
buffered mucus. The mucus
protects the stomach lining.
Gastric pits are also visible, these
open into gastric glands. There
are some enteroendocrine cells
that release hormones related to
digestion and undifferentiated
stem cells that divide to replace
the damaged lining
a. Mucous neck cells- secret a
mucus different of that
secreted by goblet cells.
b. Parietal (oxyntic) cellsproduce the hydrochloric
acic
c. Chief (zymogenic) cellssecrete enzyme called
pepsinogen that becomes
pepsin when activated by the
hydrochloric acid.
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G. Small intestine- this is the
longest section, an area where
chemical digestion is
completed and most nutrients
are absorbed. Food remains
here for about 3-6 hours. The
small intestine can be divided
into three section: duodenum,
ilelium, jejunum. It uses
peristalsis to move chyme
along and for segmentation
movements.
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1. Gross anatomy- the tube
runs from the pyloric sphincter
of the stomach to the
beginning of the large intestine.
The first division is the
duodenum (5%) short area
where the digestive chemicals
from the pancreas and gall
bladder are received. The
second division is the jejunum
(40%) a longer region where
food is digested and nutrients
are absorbed. The last section
is the ileum (60%) it connects
to the large intestine.
2. Microscopic anatomy- the small intestine is designed for absorption
of nutrients, thus its modifications increase its surface area by 200
square feet.
a. Circular folds- transverse ridges (1 cm tall) that force the chyme to
spiral as it slows it down to increase absorption.
b. Villi- these are finger-like projections (1mm tall) made up of simple
columnar epithelial cells specialized for absorption (enterocytes).
They are in close contact with capillaries and lacteal (lymph
vessels that absorb fat)
c. Microvilli- microscopic folds on the apical side of the simple
columnar epithelial cell in the mucosa. The folds increase the
surface area of the cell's membrane allowing for greater
interaction. This is also the area where chemical digestion is
completed.
Intestinal crypts contain cells that produce digestive juice in the small
intestine and stem cell replace the epithelium every 3-6 days.
Duodenal glands also secrete a solution that neutralizes stomach
acid.
H. Large intestine- Area where food remains for 12-24 hours. Some absorption and
digestion occurs (by bacteria) it is primarily designed to reabsorb water and electrolytes.
1. Gross anatomy- It is wider than the small intestine but shorter. It is divided into the
cecum (opening of ileum to large intestine, has iliocecal valve), the vermiform appendix
(opens to the cecum, contains lymphoid tissue), the colon (has at least four sections),
the rectum (has rectal valves that prevent feces from being passed along with gas) and
the anal canal (external to abdominal pelvic cavity, has external anal canal). The
sections of the colon and bends are: ascending colon, right colic (hepatic) flexure,
transverse colon, left colic (splenic) flexure, descending colon, and sigmoid colon and
rectum. Some of the feature are: the teniae coli (longitudincal strips that run along the
colon), the haustra (pockets or sacs) and the epiploic appendages (hanging fat filled
pouches)
2. Microscopic anatomy- has
no villi or microvilli, has
intestinal crypts with goblet
cells that provide mucus to
lubricate the feces. Stem cells
are present to fully replace the
epithelium every week. Has
more lymphoid tissue than
other GI tract organs. At the
anal canal the epithelium
changes to stratified squamous
and eventually merges with true
skin surrounding the anus.
Rectum and anal canal
Defication Reflex
• I. Liver- considered the largest gland, it performs over 500 functions
that influence several systems beside the digestive system. In
regards to the digestive system it produces bile. A substance that
emulsifies fat and makes it accessible to fat digesting enzymes. The
liver is involved in balancing blood glucose levels, it responds to
hormones, and clears toxins or drugs from the blood and makes
blood proteins.
• 1. Gross anatomy- It is protected by the rib cage. It has a
diaphragmatic surface and a visceral surface, the bare area is fused
to the diaphragm. It has four lobes: right and left lobes that are
divided by the falciform ligament, the caudate lobe, and the quadrate
lobe. The posterior and inferior view also shows the round ligament
(ligamentum teres) and the ligamentun venosum. The port heptis is
also on this side. Major blood vessels, nerves, and ducts enter
through the porta hepatis. Bile is carried from the liver by the right
and left hepatic ducts that merge into the common hepatic duct that
also connects to the cystic duct of the gall bladder. Bile reaches the
small intestine through the common hepatic duct.
Anterior View of liver (diaphramatic surface)
Posterior View of Liver (Visceral side)
• 2. Microscopic anatomy- liver tissue shows
multiple hexagonal sections called liver
lobules. Each lobule contains a central
vein, hepatocytes arranged into “plates”,
and is surrounded by connective tissue. At
the corners there are portal triads
containing a hepatic vein, artery, and bile
duct. Liver cells are called hepatocytes
and aside from the many functions they
perform they have the ability to regenerate
the liver.
•2. Microscopic
anatomy- liver tissue
shows multiple
hexagonal sections
called liver lobules.
Each lobule contains
a central vein,
hepatocytes arranged
into “plates”, and is
surrounded by
connective tissue. At
the corners there are
portal triads
containing a hepatic
vein, artery, and bile
duct. Liver cells are
called hepatocytes
and aside from the
many functions they
perform they have the
ability to regenerate
the liver.
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J. Gall bladder- muscular sac that stores bile, it is attached to the
cystic duct and when the muscle contract bile exits through this duct
and into the common hepatic duct.
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K. Pancreas- a gland that is both exocrine (has a duct) and endocrine
(releases hormones into the blood stream). It produces 22 different
enzymes for digestion by acinar cells and buffers to neutralized
stomach acid by epithelial cells. It joins gall bladder secretions and
both enter the duodenum via the hepatopancreatic ampulla at the
major duodenal papilla.
• L. Mesenteries- double layer sheets of serosa membranes that hold
organs in place by connecting them to the dorsal and ventral
abdominal walls. Sometimes they are referred to as ligaments.
• Ventral Mesenteries1) falciform ligament- connects liver to anterior wall
2) lesser omentum- runs from the liver to the lesser
curvature
• Dorsal Mesenteries• 1) Greater omentum- from the greater curvature to the
posterior wall, however it also connect to the
intestines. It also holds a lot of fat
2) Mesentery proper- support and hold the jejunum and ileum.
• 3) Transverse mesocolon- attaches the transverse colon, it fuses
with the greater omentum.
• 4) Sigmoid mesocolon- connects sigmoid colon to posterior pelvic
wall.