T
9.4 – 9.6
THE DIGESTIVE SYSTEM
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Digestive System
– breaks down food into useful substances that
can be absorbed by the circulatory system and
transported to individual body cells
It takes about 24-33 hours
to fully digest a large meal
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Digestive Enzymes
Enzymes help in the breakdown of food, in a
process called chemical digestion.
Food contains carbohydrates, proteins and
lipids, so a wide range of enzymes is
needed. See Table 10.2 of a list of
digestive enzymes
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• What is a substrate?
– A molecule on which an enzyme acts
• Ex: the enzyme MALTASE acts on the sugar
MALTOSE
• Carbohydrases break down carbohydrates
• Proteases break down protein
• Lipases break down lipids
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Complete digestive
apparatus
Upper
Respiratory
Tract
Esophagus
Liver
Gall
bladder
Duodenum
Colon
Abdominal aorta
Stomach
Spleen
Pancreas
Small intestine
Ileum
Appendix
Rectum
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Parts of the Human Digestive Tract
1. Mouth
• teeth begins mechanical breakdown of food
(chewing breaks up food into smaller pieces)
• tongue pushes the food towards the back of the
mouth
• chewed food is called a bolus
• Saliva – begins digestive process
• Stimulates taste buds
• Produced by salivary glands
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• food is mixed with saliva to moisten and
lubricate it
– secreted by the 3 salivary glands
• parotid,
• sublingual,
• submandibular
• saliva contains enzymes such as amylase,
which begins the chemical breakdown of
carbohydrates (starch)
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Salivary glands
Parotid gland
Sublingual gland
Submaxillary gland
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Which type of digestion is the following?
1. Chewing a saltine? 2. Saliva breaking the saltine down into molecules of
glucose? 3. Your tongue breaking pieces of a hamburger apart?
4. Pepsin (an enzyme) in your stomach breaking the
hamburger into amino acids?
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2. ESOPHAGUS
• when we swallow, bolus passes through the
pharynx and the epiglottis closes to prevent the
bolus from entering the trachea
• esophagus is long tube lined with muscles that
work to push the bolus down into the stomach
through a process known as peristalsis
(wavelike contractions & relaxation of muscles)
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Swallowing (& not choking)
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Peristalsis
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Stomach
Esophagus
Fundus
Body
Pylorus
Pyloric Den
Duodenum
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3. Stomach
•
muscular, J-shaped, sac-like
•
muscle fibers squeeze to continue the
mechanical break down of food
•
valves at the beginning & end of the stomach,
the esophageal sphincter & the pyloric
sphincter, control the flow of food entering &
leaving the stomach
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• lined with millions of gastric glands that secrete gastric
juices essential for the breakdown of food
• Hormone: Gastrin released into the bloodstream
• enzyme pepsin breaks down protein
• acidic environment (hydrochloric acid) also helps break
down food
– mucous lining surrounds interior of stomach to protect it from
acidity
• result: thick liquid known as chyme (partially digested
food)
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4. Small Intestine
• Chyme enters the small
intestine (small in diameter)
– actually about 7 meters long
in an adult human
• Peristalsis helps move the
chyme along
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•
Divided into
3 regions:
– Duodenum
– Jejunum
– Ileum
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2 Main functions:
1. further chemical breakdown of chyme
•
•
•
Lining secretes intestinal juices which contain many
enzymes
Pancreatic duct from pancreas releases enzymes here
Bile duct from gall bladder releases bile here
2. absorption of nutrients
•
Huge surface area (lining has folds called villi and
microvilli)
•
At the end, it pushes remaining undigested material
into the large intestine
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Duodenum
• Duodenum - most enzymes are added and
digestion in the small intestine begins.
• Jejunum - digestion continues and some
nutrients are absorbed.
• Ileum - Majority of nutrients are absorbed
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• Secretin – a hormone
secreted by the
duodenum that
stimulates pancreatic
and bile secretions
• Also encourages
pancreas to secrete
lipid and protein
enzymes
• stimulates the liver to
make more bile
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• Primary function is to stimulate the
pancreas to release bicarbonate ions to
neutralize the acidic chyme.
• Therefore, secretin protects the small
intestine from stomach acids.
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• Pepsin is only active in acidy env’t
• Stops in Small Intestines
• Protein digestion is carried on by
trypsinogen, the inactive form of trypsin
• Pancreas  duodenum  enterokinasis
comes in to convert it into active trypsin
• Further breakdown of protein is done now
(carries the work of pepsin as if it were in
the stomach)
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Absorption in the Small
Intestines
Passive Transport:
• Passive transport is the movement of materials
across a cell membrane without the use of energy
from the cell.
• Simple diffusion, osmosis, and facilitated diffusion.
• Diffusion will follow the concentration gradient: high
concentration  low concentration
• Another type of diffusion that plays a role in
digestion is facilitated diffusion: The diffusion of
molecules across a membrane through transport
proteins.
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Active Transport
• Materials are moved across a cell
membrane, from an area of lower
concentration to an area of higher
concentration, using energy provided by
the cell
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Absorption of nutrients in
Capillary networks
• It doesn’t matter which type of transport
system you use, all nutrients make their
way through the mucosa of the small
intestine and into the capillary networks in
the villi.
• Then carry nutrients through the
bloodstream to the rest of the body
• Different substances need different forms
of transport
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Small intestine (2)
Circular creases
with villi
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Action of enzymes
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5.
Large Intestine
•
large in diameter, however, shorter than the small
intestine
•
peristalsis takes place
•
no digestive enzymes production
•
consists of 4 parts
–
Caecum
–
Colon
–
Rectum
–
Anal Canal
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Colon
Transverse
colon
Ascending
colon
Descendant
colon
Caecum
Appendix
Sigmoid colon
Rectum
Anus
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a)
caecum
– sac-like part near small intestine, contains appendix
b) colon
- undigested food passes through here
- water and dissolved minerals are absorbed from the
undigested food
- bacteria help to break down the nutrients further and
produce vitamins
- result is a damp mass of indigestible material, feces
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Rectum
c) rectum - feces
stored
d) anal canal – feces
is led to external
environment
through anus, has
sphincter muscle to
control timing of
elimination
egestion the removal of waste food
materials from the body
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Other Associated Organs
LIVER:
•
•
•
•
•
Largest internal organ
produces bile salts, which help break up fat (micelles) in
the small intestines
– Bile is not considered an enzyme. Bile rather allows
enzymes to interact more effectively with enzymes
secreted by the pancreas.
Stores glycogen & vitamins
detoxifies foreign chemicals/poisons ingested by
food/drink
**everyday life example: soap detergent allows for blobs
of fat to float up the dirty dish if placed in hot water – bile
does the same thing; literally breaks down big fat
droplets into smaller ones
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PANCREAS
• source of enzymes needed to break down
carbohydrates, fats & peptides
• releases a basic solution into the small intestine
to neutralize the acidic chyme coming from the
stomach
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• The enzyme amylase, found in saliva, is also
secreted by the pancreas.
• Amylase continues the digestion of starch that
was started in the saliva
• When fat-rich chyme enters the duodenum, a
hormone called cholecystokinin (CCK) is
released into the bloodstream. This hormone
signals the pancreas to secrete a variety of
substances, including ones that control the pH of
the intestine.
• CCK also signals the stomach to slow down
• the speed of digestion so that the small intestine
can effectively digest the fats.
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GALL BLADDER
• stores bile produced in the liver and releases it
into the small intestine when needed
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• Lipids entering the duodenum stimulate the
gall bladder to contract, which causes bile
to be squeezed out from the gall bladder
into the duodenum through the bile duct.
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Digestive Homeostasis
Disorders
• ULCERS – erosion of the surface of
the alimentary canal generally
associated with some kind of irritant
• Helicobacter pylori (commonly
known as H. pylori), is able to
survive by secreting acidneutralizing enzymes and by
burrowing through the mucosa
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•
•
These bacteria
prevent mucusproducing cells
from producing
enough mucus to
protect the
stomach lining.
destruction of the
gastric or
intestinal mucosal
lining of the
stomach by
hydrochloric acid,
an acid normally
present in the
digestive juices of
the stomach.
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Digestive Homeostasis
Disorders
• DIARRHEA – a
gastrointestinal
disturbance characterized
by decreased water
absorption and increased
peristaltic activity of the
large intestine.
• This results in increased,
multiple, watery feces.
• This condition may result
in severe dehydration,
especially in infants
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CONSTIPATION
• a condition in which the large intestine is
emptied with difficulty.
• too much water is reabsorbed
• solid waste hardens
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• GALLSTONES – an accumulation of
hardened cholesterol and/or calcium
deposits in the gallbladder
• Can either be “passed” or surgically
removed
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Gastroesophageal reflux
disease (GERD)
Esophageal sphincter opens
spontaneously, or does not close
properly and stomach contents
rise up into the esophagus.
Closed sphincter
Persistent reflux that occurs more
than twice a week is considered
GERD
Heart Burn
Solution:
Eat smaller, more frequent
meals.
Limit your intake of acidstimulating foods and beverages.
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Open sphincter
Cirrhosis of the liver
Replacement
of liver tissue
by fibrous scar
tissue,
leading to
progressive
loss of liver
function.
Can be
caused by
alcoholism,
hepatitis and
fatty liver
disease.
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Stages of the Digestion Process
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Video: Digestive
System
Homework: Pg.
Pg. 411 #1,3,4,5
Pg. 420 #1,2,6,8
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• I have made a decision: Your final test
from me, which will sum up my 3 month
teaching career with you all, will ONLY be
on digestion. I may add in one or two
circulatory or respiratory questions but the
majority (about 97%) of the test will be
from digestion. I don’t want you to study
too much over the holidays 
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