Transcript Chapter 15

Chapter 15
The Gastrointestinal
System
Gastrointestinal System (GI)
• Takes in (ingests) raw materials
• Breaks them down (digests) both physically
and chemically to usable elements
• Absorbs those elements
• Eliminates what isn’t usable
GI System
• The processes of the GI incorporate several
main and accessory organs and substances
Something to think about
• The food that enters your mouth, travels
through you digestive system, and is
eventually eliminated is never once inside
your body
• It remains in a tube like “highway” with
certain materials exiting the ramp at different
locations in the body.
Tube Like Highway
• The tube begins at the mouth and ends at the
anus
• In between the mouth and anus are:
»
»
»
»
The Pharynx
Esophagus
Stomach
Small and large intestine
• In addition, there are accessory organs
Accessory Organs
• Accessory Organs include:
»
»
»
»
»
Teeth
Salivary glands
Liver
Pancreas
Fallbladder
• ALL ARE NECESSARY FOR PROCESSING
MATERIALS INTO USABLE SUBSTANCES.
GI overview from 4/27/10
• The digestive tract, often called the alimentary
tract or canal, is a muscular tube that contains
the organs of digestion.
• The tube begins with the mouth and ends at the
anus.
• In between these two points are the pharynx,
esophagus, stomach, small intestines, and large
intestines.
• Accessory organs of digestion include the teeth,
salivary glands, liver, pancreas, and gallbladder.
Functions of the Gastrointestinal Tract
• Ingestion: food enters the mouth
• Mastication (chewing): mechanically grinding
food with the teeth and tongue – beginning
the process of physically breaking it down
• Digestion: the chemical act of breaking down
food into small molecules
Mouth and Oral Cavity
•
•
•
•
•
Your mouth leads to the buccal (oral) cavity.
Your lips act as the door to this cavity.
The hard and soft palate create the roof.
The tongue acts as the floor.
The cheeks form the walls.
Mouth and Oral Cavity (cont’d)
• The tongue’s base (area of attachment) and
the uvula are the barrier to the next part of
the system, the pharynx.
• The uvula aids in swallowing, directing food
toward the pharynx and blocking food from
entering your nose.
• The mouth receives, tastes, mechanically
breaks down, and begins the process of
chemical breakdown of food, adding saliva.
Tongue
• Your tongue is a muscle that provides taste
stimuli to your brain, determines temperature,
manipulates food, and aids in swallowing.
• As the tongue moves food around in the oral
cavity, saliva is added to moisten and soften it,
while teeth crush the food.
Tongue
• The tongue pushes the food into a ball-like
mass, called a bolus, so it may be swallowed passed to the pharynx.
• The lingual frenulum, a membrane under the
tongue, keeps you from swallowing your
tongue and aids in speaking.
Clinical Application:
Sublingual Medication
• The area under the tongue has many blood
vessels. This sublingual blood vessel network
readily absorbs substances and is a rapid
means of administering medication.
• One medication given by this route is
nitroglycerine, used to treat angina. Angina
develops as a result of poor oxygen supply to
the myocardium because of diminished blood
flow.
Clinical Application:
Sublingual Medication (cont’d)
• Nitroglycerine dilates arteries, improving
blood supply and oxygenation – hopefully
relieving angina symptoms.
Salivary Glands
• There are 2 pairs of salivary glands controlled
by the autonomic nervous system.
• A large parotid salivary gland is found slightly
inferior and anterior to each ear. These are the
ones that swell when you get mumps.
• The ducts from these glands empty into the
upper portion of the oral cavity.
Salivary Glands
• The sublingual salivary glands are found under
the tongue.
• The submandibular salivary glands are located
along both sides of the inner surface of the
mandible, or lower jaw.
Saliva
• The salivary glands produce 1–1.5 liters of
saliva daily.
• Small amounts of saliva keep the mouth
moist, but the idea or presence of food
increase production significantly.
• Saliva is 99.4% water, and contains antibodies,
buffers, ions, waste products, and enzymes.
Saliva (cont’d)
• Enzymes act as organic catalysts to speed up
chemical reactions.
• One enzyme, salivary amylase, speeds the
chemical activity of breaking down
carbohydrates.
• After eating, saliva cleans the oral surfaces,
reducing the amount of bacteria that grows in
your mouth.
Teeth
• The first set of teeth you grow as a baby are
the deciduous teeth, falling out in time.
• The 1st tooth appears around 6 months of
age. The lower central incisors appear first,
with all 20 teeth in place by age 2½.
• Between 6 and 12 years these teeth fall out
and are replaced by 32 permanent teeth.
• Wisdom teeth appear by the time we turn 21.
Types of Teeth
• Incisors are located at the front of the mouth, are
blade shaped, and are used to cut food.
• Canine teeth are for holding, tearing, or slashing
food. They are also known as eyeteeth or cuspids,
and are located next to incisors.
• Bicuspids, or premolars, are transitional teeth.
• Molars have flattened tops. Both bicuspid and
molars are responsible for crushing and grinding
food.
Tooth Structure
• Teeth have a crown, neck, and root.
• The crown is the part you normally see and is
covered by the hardest biologically
manufactured substance, enamel.
• The neck is the transitional section that leads
to the root.
• Most teeth are made up of dentin, a
mineralized bone-like substance.
Tooth Structure (cont’d)
• The next layer is connective tissue, pulp,
located in the pulp cavity.
• The pulp cavity contains blood vessels and
nerves providing nutrients and sensation. The
nerves and blood vessels get to the pulp cavity
via the root canal.
The Root
• The root is nestled in a bony socket and is held in
place by fibers of the periodontal ligament.
• In addition, cementum covers the dentin of the
root, aiding in securing the periodontal ligament.
• Cementum is a soft version of bone.
• Healthy gums, or gingiva, help hold the teeth in
place.
• Epithelial cells form a tight seal around the tooth
to prevent bacteria from coming into contact with
the tooth’s cementum.
Pharynx
• There are 3 parts to the pharynx:
– The nasopharynx is primarily part of the respiratory
system, blocked by the soft palate.
– The oropharynx and the laryngopharynx act as a
passageway for food, water, and air. The epiglottis covers
the trachea to prevent food from entering the lungs,
forcing food into the opening for the esophagus.
Esophagus
• The esophagus is approximately 10 inches
long and is connected to the stomach.
• It extends from the pharynx, through the
thoracic cavity, through the diaphragm,
connecting to the stomach in the peritoneal
cavity.
• The esophagus is normally a collapsed tube
until a bolus of food is swallowed.
Esophagus (cont’d)
• Rhythmic contractions, called peristalsis,
pushes food down the esophagus.
• The esophageal walls are lined with stratified
squamous epithelium that secrete mucus to
make the walls slippery. These cells make the
lining resistant to abrasion, temperature
extremes, and irritation.
Esophageal Sphincters
• A muscular ring at the top of the esophagus,
called the pharyngoesophageal sphincter,
relaxes to open the esophagus so food can
enter.
• At the entrance to the stomach is the lower
esophageal sphincter, or cardiac sphincter,
opening the door to the stomach and closing
to prevent acidic gastric juices from splashing
into the esophagus – causing heartburn.
• The whole process of swallowing food takes
about 9 seconds. Fluid takes less time.
GERD
• See Video
Walls of the Alimentary Canal
• Four basic types of tissue line the entire
alimentary canal from the esophagus onward.
• The innermost layer, the mucosa, lines the lumen
of the canal.
• This layer is composed mostly of surface
epithelium with some connective tissue and has a
thin smooth muscle layer surrounding it.
• The mucosa also possesses cells that secrete
digestive enzymes to break down foodstuffs and
goblet cells that secrete mucus for lubrication.
Walls of the Alimentary Canal (cont’d)
• The submucosa is the next layer, and is composed of
soft connective tissue.
• This layer contains:
– Blood vessels
– Lymph vessels
– Lymph nodes (called Peyer patches; are similar to your
tonsils)
– Nerve endings
Walls of the Alimentary Canal (cont’d)
• The next layer is called the muscularis externa,
and is composed of two layers of smooth
muscle.
• The innermost layer of muscle encircles the
canal, while the outer layer of muscle is
longitudinal in nature, so it lies in the direction
of the canal.
• The stomach is surrounded by a third layer of
oblique smooth muscle.
Walls of the Alimentary Canal (cont’d)
• The outermost layer, the serosa, is composed
of a single, thin layer of flat, serous fluid
producing cells supported by connective
tissue.
• The serosa is called the visceral peritoneum in
most of the canal.
Walls of the Alimentary Canal (cont’d)
• The fluid secreted is important to keep the
outer surface of the intestine moist and allow
friction free movement of the intestine against
the abdominopelvic cavity.
• The esophagus differs in that it only possesses
a loose layer of connective tissue called the
adventitia.
Stomach
• The stomach is located in the left side of the
abdominal cavity, under the diaphragm, and is
covered completely by the liver.
• It is approximately 10 inches long with a
diameter that depends on how much you just
ate.
• It can hold up to 4 liters when filled.
• Rugae, or folds, help the stomach expand and
contract.
Stomach Functions
• The stomach has 4 functions:
– Temporary holding area for received food
– Secretes gastric acids and enzymes that mix with
food, performing chemical digestion
– Regulates the rate the new, partially digested food
(a thick, heavy, cream-like liquid called chyme)
enters the intestine
– Absorbs small amounts of water and substances
on a very limited basis (the stomach does absorb
alcohol)
Movement of Food
in the Stomach
• It takes about 4 hours for the stomach to
empty following a meal.
• Liquids pass through fairly quickly.
• Carbohydras move through quickly.
• Proteins take more time to pass through.
• Fats take the longest, usually between 4–6
hours.
Regions of the Stomach
• The stomach is divided into 4 regions.
• Near the heart is the cardiac region,
surrounding the lower esophageal sphincter.
• The fundus, laterally and slightly superior to
the cardiac region, temporarily holds the food
as it enters the stomach.
Regions of the Stomach (cont’d)
• The body is the mid-portion of the stomach.
• The funnel shaped, terminal end of the
stomach is called the pylorus. Most of the
work of the stomach is performed here. This is
where food passes through the pyloric
sphincter into the small intestine.
• The 2 curves are the lesser curvature
(concave) and the greater curvature (larger
convex curve).
Muscle Layers
of the Stomach
• The muscular action of the stomach works like
a cement mixer, achieved by 3 layers of
muscles.
• The three layers are the longitudinal layer,
circular layer, and oblique layer.
• The stomach churns food as it mixes with
gastric juices excreted by gastric glands in the
gastric pits of the columnar epithelial lining of
the stomach.
Muscle Layers
of the Stomach (cont’d)
• The stomach works the food toward the
pyloric sphincter through peristaltic activity of
the muscles.
• Both chemical and physical digestion occurs
here.
Gastric Juice
• Gastric juice is comprised of hydrochloric acid
(HCl), pepsinogen, and mucus. About 1500
mls is produced daily by gastric glands.
• Pepsinogen is secreted by the chief cells while
HCl is secreted by parietal cells combining to
produce pepsin, the chief digestive enzyme.
• Pepsin breaks down protein. HCl breaks down
the connective tissue.
Gastric Juice (cont’d)
• HCl has a pH of 1.5–2, and is effective at killing
pathogens.
• Mucous cells generate a thick layer of mucus
shielding the stomach from the effects of the
stomach acids.
• The stomach also secretes intrinsic factor,
allowing vitamin B12 to be absorbed.