The Digestive System
MDC_BSC2086
Luis H. Santamaria
The Digestive System
Organs of the Digestive System
• Two main groups
–Alimentary canal – continuous coiled
hollow tube, extends from the oral
entrance up to the anal exit.
–Accessory digestive organs, enclosed
or connected to the alimentary canal.
Digestive Mucosa
• The Digestive Epithelium
– Mucosal epithelium is simple or stratified
– Depending on location, function, and stresses:
– oral cavity, pharynx, and esophagus are subjected to
mechanical stress. They are lined by stratified squamous
epithelium
– stomach, small intestine, and most of large intestine are
sites for digestion and absorption. They are lined by
simple columnar epithelium with mucous (goblet) cells.
Alimentary Canal
• It consists of the long, continuous tube that
extends from the oral cavity to the anus.
• Along this tube, the breakdown of food or
digestion occurs, absorption of nutrients and
elimination of waste products also takes place.
• The final product of this process, a material
called feces is discarded through the anal
aperture.
Organs of the Alimentary Canal
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine (duodenum, jejunum, ileum)
• Large intestine (cecum, ascending colon,
transverse colon, descending colon, sigmoid
colon, rectum)
• Anus
Accessory Digestive Organs
• Salivary glands (Parotids, Sublingual and
Submandibular)
• Tongue
• Teeth
• Liver and Gallbladder
• Pancreas
Mouth (Oral Cavity) Anatomy
• Lips (labia) – protect the anterior
opening
• Cheeks – form the lateral walls
• Hard palate – forms the anterior roof
• Soft palate – forms the posterior roof
• Uvula – fleshy projection of the soft
palate
Figure 14.2a
Oral Cavity
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Actions of the Mouth
• Mastication (chewing) of food
• Mixing masticated food with saliva
• Initiation of swallowing by the tongue
• Allowing for the sense of taste
Salivary Glands
• Parotid Glands: Located anterior to each ear, between the
skin and the masseter muscle. They drain their secretion into
the oral cavity via the parotid ducts (Stensen’s ducts) whose
opening is adjacent to the upper second molar.
• Submandibular glands: located medial to the mandible,
extending from the mandibular arch to the ramus. They drain
their salivary secretion via the Wharton’s ducts whose
openings are at both sides of the lingual frenulum
• Sublingual glands: Located deep to the base of the tongue.
Their multiple ducts (Rivinus’ ducts) open along the base of
the tongue
• The salivary glands secrete a watery fluid,(saliva), that
contains mucin, a sugar digestive enzyme named Ptyaline
and a mild antiseptic substance named Lysozyme.
Salivary Glands
Salivary Secretions
• Salivary Glands
Produce 1.0–1.5 liters of saliva each day
• 70% by submandibular glands
• 25% by parotids
• 5% by sublingual glands
Primary and Secondary Dentitions
Deciduous teeth (20)
Adult teeth (32)
Adult Dentition
Pharynx Anatomy
• Nasopharynx – not part of the digestive
system
• Oropharynx – posterior to oral cavity
• Laryngopharynx – below the oropharynx
and connected to the esophagus
Figure 14.2a
The Pharynx
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Pharynx Function
• Serves as a passageway for air and food
• Food is propelled to the esophagus by two
muscle layers
– Longitudinal inner layer
– Circular outer layer
• Food movement is by alternating contractions
of the muscle layers (peristalsis)
Esophagus
• Runs from pharynx to stomach through the diaphragm;
it is around 10 inches (25 cm) long
• Conducts food by peristalsis (slow rhythmic squeezing)
• Passageway for food only
• It connects to the stomach at the cardial region, a
lower esophageal sphincter prevents the reflux of
gastric acid content into the esophagus
The Esophagus
Stomach Anatomy
The Stomach
Gastric Mucosa, its secretions
• Simple columnar epithelium lines the interior of the
stomach
• Mucous Cells: secrete a thick mucus to protect the
gastric mucosa
• Parietal cells: Secrete Hydrochloric acid a potent
antiseptic and the initiator of the breakdown of
proteins. Also secrete Intrinsic factor for the absortion
of Complex B vitamines
• Chief cells: Secrete a non active enzyme, Pepsinogen,
once activated to Pepsin in the gastric acid medium, is
one of the several digestors of proteins
Gastric Mucosa
Stomach: Functions
• Acts as a storage tank for food
• Site of food breakdown, mainly by mechanical
digestion
• Chemical breakdown of protein begins
• Delivers chyme (processed food) to the small
intestine
Small Intestine
• The body’s major digestive organ
• Site of nutrients absorption into the blood
• Muscular tube extending form the pyloric
sphincter to the ileo-cecal valve
• Suspended from the posterior abdominal wall
by the mesentery
Subdivisions of the Small Intestine
• Duodenum
– Attached to the stomach
– Curves around the head of the pancreas
• Jejunum
– Attaches anteriorly to the duodenum
• Ileum
– Extends from jejunum to large intestine
Small Intestine
Small Intestine: Duodenum
• The C shaped duodenum is just around 10 inches (25
cm.) long.
• It forms a “ picture frame” around the head of the
pancreas
• It receives chyme from the stomach and is the site
where most of the digestive process takes place
• The pancreatic duct and the common bile duct open
into the duodenum bringing their digestive
secretions into its lumen
• Trypsinogen, a proenzyme secreted by the pancreas
activates in the duodenum to Trypsin, the final
digestor of proteins
Small Intestine: Jejunum & Ileum
• Jejunum: About 8 feet (2.5 m) long. The bulk of
absorption of nutrients occurs in this segment of
the small intestine. The presence of villi and
microvilli in the jejunal mucosa increases greatly
its surface of absorption
• Ileum: The longest segment of the small intestine,
around 11.5 feet (3.5 m) long. Reabsorption of
water and electrolytes occurs in the ileum. It ends
opening into the proximal end of the colon, the
Cecum. A sphincter (ileo-cecal valve) controls the
flow of material from the ileum into the cecum
Intestinal Mucosa
Villi of Small intestine
• Fingerlike projections
• Lined by simple
columnar epithelium
carpeted by microvilli
• Each villus contains:
• Absorptive cells
• Blood capillaries
• Lacteals (specialized
lymphatic capillaries)
Pancreas
• A “fishlike” organ located in transverse orientation in the abdomen
• It is around 15 cm long and weights around 80 gm
• Formed of a head (fitting in the duodenal frame), body and a lateral tail,
adjacent to the spleen
• Its excretory duct, (Wirsung’s duct) runs along the full length of the organ
and opens into the Duodenum, after merging with the Common Bile duct
(Ductus Choledocus)
• The pancreatic sphincter regulates the drainage of pancreatic fluid into
the papilla of Vater. At the opening of this papilla, the Hepato-pancreatic
sphincter (Oddi’s sphincter) controls the entrance of bile and pancreatic
juice into the duodenum
• An Accessory pancreatic duct (Santorini’s duct) often opens into the
duodenum, usually at a proximal location from the main pancreatic duct.
• The exocrine cells of the pancreas (acinar and epithelial cells) secrete
sodium bicarbonate and the main lipid digestive enzyme, Lipase
The Pancreas
Liver
• Located in the right upper abdominal quadrant, it is the
most voluminous viscera in the body
• Consists of four lobes: Right lobe, left lobe, quadrate
lobe and caudate lobe
• Right and left lobes are separated by the Falciform
ligament.
• Each lobe is organized in around 100.000 lobules, each
one is a secretory unit formed of thousands of
hepatocytes, the productors of Bile, a catabolite of
hemoglobin, whose action is the breakdown of the large
lipid particles in the chyme, a process called fat
emulsification.
Liver: Anterior & Inferior Views
Gallbladder
• A small, muscular sac, located inferior to the
right hepatic lobe, its function is to store and
concentrate the bile between meals, and
pouring it into the duodenum for the
breakdown of lipids.
• Contractions of the gallbladder wall occur
under the stimulus of a hormone released by
the duodenal mucosa (Cholecystokinin). This
hormone also stimulates pancreatic secretion
and relaxes the hepato-pancreatic sphincter
Bile Tree
• The small hepatic intralobular ducts join to form the
two Hepatic ducts (right and left), these emerge from
the right and left hepatic lobes and merge to form the
Common hepatic duct
• The common hepatic duct joins the Cystic duct coming
from the gallbladder to form the Common Bile duct
(Ductus Choledocus)
• The common bile duct enters into the tissue of the
pancreatic head to join the Main Pancreatic duct
(Wirsung’s duct)
• The two form the hepato-pancreatic ampulla (ampulla
of Vater) that opens into the duodenum as the
Duodenal papilla (Papilla of Vater)
Duodenum & Bile Tree
Bile Tree
A) Right & Left hepatic ducts
B) Common hepatic duct
C) Cystic duct
D) Common bile duct (Ductus
Choledocus)
E) Main Pancreatic Duct
(Wirsung’s duct)
F) Duodenal Papilla (Papilla
of Vater)
G) Duodenum
H) Accesssory pancreatic
duct (Duct of Santorini)
I) Gallbladder
A
I
B
D
C
G
F
H
E
Duodenal Papilla
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Structures of the Large Intestine
• Around 4.9 feet (1.5 m) long, and 3 inches (7.5 cm) in diameter
• Colon
-Cecum
-Ascending
-Transverse
-Descending
-“S-shaped” sigmoid
• Rectum
• Anus – external body opening, guarded by two sphincters; the
internal anal sphincter formed of smooth muscle and thus
involuntary, and the external anal sphincter, a ring of skeletal
muscle and hence under voluntary control.
The Large Intestine
Functions of the Large Intestine
• Reabsorption of water, small amounts of bile
salts and some vitamins (like Vitamin K,
synthesized by colonic bacteria)
• Eliminates indigestible food from the body as
feces
• Does not participate in digestion of food
• Goblet cells produce mucus to act as a
lubricant
Control of Digestive Functions
The activities of the digestive system are regulated by...
• Neural mechanisms: Peristaltic movements of the digestive tract are triggered
by sensory receptors in the wall of the GI tube. Motor neurons, mainly
parasympathetic, interact with sensory neurons and interneurons to produce
an excitatory effect in the muscle layers of the digestive tube. GI secretions
are also released under parasympathetic stimulus.
• Hormonal mechanisms: The sensitivity of the smooth muscle cells to neural
commands can be enhanced or inhibited by digestive hormones, such as
gastrin, secretin, cholecystokinin and others
• Local mechanisms: Chemicals, such as histamine and prostaglandins, released
into intestinal fluid may affect adjacent cells within a small segment of the
intestinal tract. These local messengers can coordinate a response to changing
conditions in a limited portion of the GI tract. For example, the release of
histamine in the stomach stimulates the secretion of acid by cells in the gastric
mucosa