The Digestive System
• Digestion
o Breakdown of ingested food
o Absorption of nutrients into the blood
• Metabolism
o Production of cellular energy (ATP)
Organs of the Digestive
System
• Two main groups
▫ Alimentary canal
 a/k/a gastrointestinal (GI) tract
 continuous coiled hollow muscular
tube open at both ends
▫ Accessory digestive organs
Organs of the
Alimentary Canal
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Mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Anus
Mouth Anatomy
• Lips (labia) – protect
the anterior opening
• Cheeks – form the
lateral walls
• Hard palate – forms
the anterior (front) roof
• Soft palate – forms
the posterior (back) roof
• Uvula – fleshy
projection of the
soft palate
• Vestibule – space
between lips externally
and teeth and gums
internally
Figure 14.2a
Mouth Anatomy
• Oral cavity – area
contained by the teeth
• Tongue – attached at
hyoid and styloid
processes of the skull,
and by the lingual
frenulum
• Tonsils
▫ Palatine tonsils
 Back of the throat
▫ Lingual tonsil
 Base of the tongue
Figure 14.2a
Salivary Glands
• Saliva-producing glands
o Parotid glands
o Submandibular glands
o Sublingual glands
• Saliva (spit)
o Contains salivary amylase to begin starch
digestion
Processes of the Mouth
• Mastication (chewing) of food
• Tongue mixes chewed up food with saliva
o Saliva contains digestive enzymes, so digestion
starts in the mouth.
• Tongue helps us swallow the food
• Tongue has papillae (bumps) that contain taste
buds
o allow for our sense of taste
Functions of the Pharynx
• 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 by peristalsis
• Wavelike contractions of the muscle layers
Esophagus
• Runs from pharynx to stomach through the
diaphragm
• Propels food by peristalsis
• Passageway for food only (respiratory
system branches off after the pharynx)
Layers of Alimentary Canal Organs
1. Mucosa
▫ Innermost layer
▫ Mucous membrane made of columnar epithelium
 Goblet cells secrete mucous
2. Submucosa
▫ a thin layer of loose connective tissue just beneath the
columnar epithelium
3. Muscularis – smooth muscle layer
▫ inner circular layer
▫ outer longitudinal layer
4. Serosa
▫ outermost layer
▫ smooth membrane consisting of a thin layer of cells
that secrete serous fluid which reduces friction from
muscle movement
Layers of Stomach
Stomach
• Located on the left side of the abdominal cavity
• Sort of hidden by liver and diaphragm
• Food enters at the esophageal sphincter
• sometimes called cardioesophageal sphincter
Stomach Anatomy
• Regions of the stomach
o Cardia region – near the heart
o Fundus
o Body
o Pylorus – funnel-shaped terminal end
• Food empties into the small intestine at the pyloric
sphincter
• Rugae – internal folds of the mucosa
Stomach Anatomy
• External regions
o Lesser curvature
o Greater curvature
• Layers of peritoneum
attached to the stomach
o Lesser omentum –
attaches the liver to the
lesser curvature
o Greater omentum –
attaches the greater
curvature to the
posterior body wall
Functions of the Stomach
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Acts as a storage tank for food
Site of food breakdown
Chemical breakdown of protein begins
Delivers chyme (processed food that looks like thick
cream) to the small intestine
Structure of the Stomach Mucosa
• Gastric pits formed by folded mucosa turn into the
gastric glands
• Gastric glands are made up of specialized cells that
perform specific functions
Specialized Cells of the Stomach
• Simple columnar epithelium
o Mucous neck cells – produce a sticky
alkaline mucus
o Gastric glands – secrete gastric juice
o Chief cells – produce pepsinogens (proteindigesting enzymes)
o Parietal cells – produce hydrochloric acid
o Endocrine cells – produce gastrin
(hormone)
Ulcers
• An ulcer is a sore, which means it's an
open, painful wound. Peptic ulcers are
ulcers that form in the stomach or the
upper part of the small intestine.
What causes ulcers?
• Most peptic ulcers are caused by a particular
bacteria (Helicobacter pylori) that lives in the
stomach and upper intestine
• Ulcers are also caused by the overuse of nonsteroidal anti-inflammatory drugs (NSAIDs), such as
aspirin or ibuprofen (Motrin & Advil)
• Other factors that may put you at risk for
developing an ulcer:
o Smoking
o Drinking alcohol
o Uncontrolled Stress
Symptoms of ulcers
loss of appetite
sudden, sharp stomach pains
nausea
frequent burping or hiccuping
weight loss
vomiting (if blood is in the vomit or the vomit looks
like coffee grounds, which only happens with
severe ulcers, call a doctor right away)
• bloody or blackish bowel movements (this could
indicate a serious problem, so call a doctor right
away if you see this)
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Small Intestine
 MAJOR SITE OF NUTRIENT ABSORPTION
 Extends from the pyloric sphincter to the
ileocecal valve
 8-18 feet long
 3 sections:
 duodenum
 jejunum
 ileum
Special Structures of the
Small Intestine
• villi
o Fingerlike structures
formed by the mucosa
o Give the small intestine
more surface area
• microvilli
o Small projections of the
plasma membrane
• plicae circulares
o Deep circular folds of
the mucosa and
submucosa
o Do not disappear
when filled with food
• Peyer’s patches
o collections of
lymphatic tissue in
ileum
o Immune surveillance
Large Intestine
• Larger in diameter, but shorter than the small
intestine
• Frames the internal abdomen
• Cecum – saclike first part of the large intestine
o Appendix
• Hangs from the cecum
• Colon
o ascending
o transverse
o descending
o sigmoid
• Rectum
• Anus – external body opening
Functions of the
Large Intestine
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Absorption of water
Does not participate in digestion of food
Goblet cells produce mucus to act as a lubricant
Eliminates indigestible food from the body as feces
Accessory Organs of the
Digestive System
Pancreas
 Produces digestive enzymes
 pancreatic amylase → starch digestion
 trypsin → protein digestion
 lipases → fat digestion
 nucleases → nucleic acid digestion
 Enzymes are secreted into the duodenum
 Produces hormones
 insulin
 glucagon
Insulin
• Insulin is secreted by the pancreas in response to
high blood sugar (glucose).
• After you eat, blood glucose levels rise and in
response to this, insulin is secreted into the blood
• In response to insulin, cells (muscle, red blood cells,
and fat cells) take glucose in from the blood to use
to generate energy (ATP)
• This lowers the blood glucose levels back to the
normal range
• As blood glucose falls, insulin secretion by the
pancreas decreases.
Diabetes
 A person with diabetes does not produce enough
insulin or produces no insulin at all.
 Without insulin, body cells can’t take the glucose out
of the blood
 Blood glucose levels become high which leads to
other health problems.
 So, even though the blood has plenty of glucose,
the cells are not getting it for their essential energy
and growth requirements.
 Diabetes Type 1 - You produce no insulin at all.
 Diabetes Type 2 - You don't produce enough
insulin, or your insulin is not working properly.
 Gestational Diabetes - You develop diabetes just
during your pregnancy.
Glucagon
• Glucagon is secreted when blood glucose is low
like between meals and during exercise.
• Glucagon's function is to cause the liver to release
stored glucose from its cells into the blood.
• When blood glucose is high, no glucagon is
secreted
Liver
• Largest gland in the body
• Manufactures bile
o Bile breaks fat into smaller globs for lipases (fat-digesting
enzymes)to work on
• The liver consists of four lobes suspended from the
diaphragm and abdominal wall by the falciform
ligament
• It’s connected to the gall bladder via the cystic
duct
Gall Bladder
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Small sac under the liver
Connected to the liver by the cystic duct
Stores bile
When you eat fatty foods, bile is released from the
gallbladder into the duodenum through the
common bile duct
What are gallstones?
o Made from cholesterol and other things found in
the bile
o They can be smaller than a grain of sand or as
large as a golf ball.
o People who are overweight or who are trying to
lose weight quickly are more likely to get
gallstones.
o You can live without your gallbladder
o The liver just dumps bile directly into the
duodenum through the common bile duct
o Body can no longer store bile between meals,
but this has little effect on digestion