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THE DIGESTIVE SYSTEM
By: Jocelyn Pearce
THE MOUTH
Inside the mouth, food is mixed
with saliva. Humans have three pairs
of salivary glands that do this. The
parotid, submandibular, and
sublingual glands.
The nervous system controls the
secretions
Although our mouth is constantly
secreting saliva, it increases when
food enters the mouth
When food is ready to be
swallowed, the tongue pushes it to
the back of the mouth
THE MOUTH CONT.
Swallowing occurs when the
soft palate moves up and pushes
against the pharynx
Pressure on the pharynx
stimulates a reflex, where the
larynx is raised, pushing the
glottis against the epiglottis.
All of this directs the food
into the esophagus, making sure
it does not enter the nasal
passage or respiratory tract
THE ESOPHAGUS
After being swallowed, food
enters the esophagus, a
muscular tube connecting the
pharynx to the stomach
Food is moved down the
esophagus by rhythmic waves
of muscle contractions called
peristalsis
Sphincters in the stomach let
food in but prevents it from
exiting
THE STOMACH
It is like a sack that can fold up
when empty and expand when filled
Contains an extra layer of smooth
muscle that mixes food with gastric
juices (an acidic substance secreted
by the walls of the stomach)
Two kinds of secretory glands,
parietal and chief cells
Parietal cells secrete hydrochloric
acid
Chief cells secrete pepsinogen
THE STOMACH CONT.
The pepsinogen a weak protein-digesting enzyme that is
activated by the HCL’s low pH
When activated they cleave each other producing a more
active protein- digesting enzyme called pepsin
This prevents the chief cells from eating themselves
Pepsin makes food proteins into shorter polypeptides that
are digested in the small intestines
The parietal cells also secrete intrinsic factor, a
polypeptide needed for the intestines to absorb vitamin B12
THE STOMACH CONT.
The mixture of food and gastric juice is called
chyme
The acidic solution on the stomach kill most
of the bacteria that we ingest in food
The bacteria not killed grows and multiplies in
the intestines
Chyme leaves through the pyloric sphincter to
travel to the small intestine
THE SMALL INTESTINE
Neural and hormonal signals are
used as communications between the
stomach and the small intestines
Limited room in the intestines and
the digestion takes a long time
Approx. 4.5 meters long in living
person
The first 25 cm is the duodenum
The rest is divided into the
jejunum and the ileum
THE SMALL INTESTINE CONT.
The duodenum receives
chyme from the stomach
It also takes in digestive
enzymes and bicarbonate from
the pancreas, and bile from the
liver and gallbladder
These juices help to digest
food into smaller molecules
This happens primarily in
the duodenum and jejunum
THE SMALL INTESTINE CONT.
The inner wall of the small
intestine is covered with villi
The villi are covered in
microvilli
The villi and microvilli
increase the surface area of the
intestine
They also contain digestive
enzymes which help with
digestion of disaccharides like
lactose and sucrose
ABSORPTION IN THE SMALL
INTESTINES
When we digest proteins and carbohydrates, amino acids and
monosaccharides are left
They are transported across the microvilli and then across the membrane,
into the blood capillaries within the villi
These products are then taken into the liver by way of the hepatic portal vein
Fats are absorbed in a different way
Fats are broken down into fatty acids which are absorbed into the epithelial
cells and then are reassembled into fats. The fats then combine with proteins
called chylomicrons and then absorbed into the lymphatic capillaries
THE PANCREAS
A large gland near the
connection of the stomach and
small intestine
Pancreatic fluid is secreted into
the duodenum through the
pancreatic duct
This fluid contains many
enzymes that digest proteins,
starches, etc.
Enter duodenum as inactive
zymogens and activate when they
come in contact with the microvilli
THE PANCREAS CONT.
Pancreatic fluid contains bicarbonate
neutralizing the HCL from the stomach.
Produced from acini
The pancreas also secretes hormones
into the the blood that control the levels of
glucose and other nutrients
THE LIVER
Secretes bile made up of bile
pigments and bile salts that are
delivered to the duodenum during
digestion
Bile pigments do not help in
digestion! They are just waste
products and in the end are
eliminated with feces
Bile salts help digest fat by
making fat into smaller droplets
After bile is produced in the liver
it is stored in the gallbladder
THE GALLBLADDER
When fatty foods
arrive, in the duodenum,
they trigger trigger
neural reflexes
The reflexes cause the
gallbladder to contract,
injecting bile into the
duodenum
THE LARGE INTESTINE
Called large because of its
larger diameter, not length
(which is shorter than the small
intestine)
Makes up the last meter of
the digestive tract
No digestion occurs here
Primary function is to
concentrate waste materials,
although it does absorb some
water, sodium, and vitamin K
THE LARGE INTESTINE CONT.
Undigested material is compacted and stored
here
Many bacteria live and reproduce here and are
incorporated into feces
Compacted feces pass from the large intestines
into the rectum, driven by peristaltic contractions
From the rectum, the feces then exit through
the anus
WHY IS IT NECESSARY?
This system is necessary because it is our way
of breaking down foods and getting the nutrients
out of them. If we didn’t have the digestive
system then we would be in trouble. There
would be no way to really eat, therefore we would
starve and also die from the lack of nutrients.
We would have to inject ourselves with nutrients
or something like that.
INTERDEPENDENCY
Many parts of this system are controlled by
neural and hormonal signals, for example the
movement of fluids from the stomach to the
small intestine
Also, the secretions by the salivary glands are
controlled by the nervous system
Works with the endocrine and nervous systems
EVOLUTIONARY DEVELOPMENT
Single-celled organisms digest their food intracellularly
while other organisms digest their food extracellularly in
digestive cavities
The most primitive digestive tract is in nematodes, a
tubular gut lined with epithelial membrane.
Earthworms have specialized systems for the
ingestion, storage, fragmentation, digestion, and
absorption of foods
Vertebrates have similar specialization, more complex
DISEASES/DISORDERS
Gastric Ulcersoverproduction of gastric acid.
The acid eats a hole through the
lining of the stomach
Gallstones- formed from a
hardened precipitate of
cholesterol. These block the
bile duct.
Herpes esophagitis is a viral
infection of the esophagus,
caused by the herpes simplex
virus