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Mechanical and Chemical Breakdown of
Ingested Food
Most food is solid and in the form of large complex
Talk
to your neighbor.
Tell them
you think itand
is important
that we inert
molecules
which
arewhy
insoluble
chemically
break
foodreadily
into smaller
pieces.
(not
usable)
 As food was synthesized by other organisms, it
contains materials not suitable for human tissue these need to be separated and removed
 Large molecules need to be broken down into
smaller molecules that can be readily absorbed
across membranes and into cells
 Small molecules can be reassembled into new
products (e.g. amino acids can be reassembled to
make new proteins)

Digestion - breakdown of food into
particles/molecules small enough to pass
into the blood stream.
1.
2 types of digestion
◦


2.
3.
mechanical (Mastication)
chemical (Enzymes)
Absorption of nutrients into the blood
stream
Elimination of indigestible nutrients
 There
are two major groups of organs
that comprise the human digestive
system:
Alimentary Canal: Contains organs through
which the food actually passes (esophagus,
stomach, small intestine, large intestine, etc.)
Accessory Organs: Organs that assist in
digestion but no food passes through them
(liver, pancreas, gall bladder, salivary glands,
etc.)
Alimentary Canal
Accessory Organs



Enzymes are biological catalysts which speed up
the rate of a chemical reaction (e.g. digestion) by
lowering the activation energy
Enzymes allow digestive processes to occur at body
temperature and at sufficient speed to meet the
organism's survival requirements
Enzymes are specific for a given substrate and so
can allow digestion of certain molecules to occur
independently of others

Mechanical breakdown of nutrients begins in
the mouth by chewing (Mastication).
◦ The purpose of chewing is to increase the surface area
of food.



Chemical breakdown of starch also begins in
mouth. Starch is converted into glucose by
salivary amylase (secreted by the salivary
glands)
This mixture of food and saliva is made into a
mass of food called a bolus and then pushed
into the pharynx by the tongue which triggers
involuntary swallowing.
The esophagus is a muscular tube whose
muscular contractions (peristalsis) propel food
to the stomach.
◦ Peristalsis – the wave action of muscle that moves
food through the digestive system.

Oral Phase: mastication
◦ Food is prepared into a food bolus (pellet of food)

Pharyngeal Phase: involuntary part (reflex)
◦ Food is pushed further into the pharynx and
esophagus. Involuntary closure of larynx prevents food
from going “down the wrong pipe”

Esophageal Phase
◦ Food is passed through esophagus to stomach
◦ upper sphincter prevents food from being
regurgitated; lower sphincter prevents acid and
stomach contents from traveling backwards






The stomach acts as a temporary storage tank and
is where protein digestion begins
The stomach contains gastric glands which secrete
digestive juices for chemical digestion
Acids create a low pH environment (pH~1-2) that
denatures proteins, while proteases like pepsin
hydrolyse large proteins
The stomach also releases a hormone (gastrin) that
regulates stomach secretions
The mechanical action of the stomach (churning)
also promotes digestion by mixing the food
The stomach turns food into a creamy paste called
chyme
Specialized Mucosa of the
Stomach
 Simple columnar epithelium
 Mucous neck cells – produce a sticky
alkaline mucus
 Gastric glands – secrete gastric juice
 Chief cells – produce protein-digesting
enzymes (pepsinogens)
 Parietal cells – produce hydrochloric acid
 Endocrine cells – produce gastrin
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Structure of the Stomach Mucosa
 Gastric pits formed by folded mucosa
 Glands and specialized cells are in the
gastric gland region
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Structure of the Stomach Mucosa
Figure 14.4b, c
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




The small intestine is where usable food
substances (e.g. nutrients) are absorbed into the
bloodstream
The pancreas and gall bladder (via the bile duct)
both secrete substances into the small intestine to
aid in digestion
The small intestine is lined with smooth muscle to
allow for the mixing and moving of digested food
products (via segmentation and peristalsis)
It also contains small pits (crypts of lieberkuhn)
that secrete intestinal juices
The small intestine contain infoldings called villi, to
increase surface area and optimise the rate of
absorption
Subdivisions of the Small Intestine
“Dogs Just Itch!
 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
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

The small intestine is lined with villi and
microvilli
The purpose of villi and microvilli are to
increase surface area in the small intestines.
Increase the speed at which nutrients are
absorbed
◦ The small intestines a 6 meter long tube has a surface
area of 300 square meters or the surface area of a
500-600m long tube.

Each villus has a capillary network supplied by
a small artery. Absorbed nutrients pass
through the microvilli into the capillary (blood
stream), usually by passive transport
(diffusion).







The large intestine is made up by the cecum, appendix,
colon, and rectum.
Digested food is pushed from the small intestines into
the large intestine.
No digestion takes place in the large intestines
Water, and salts are absorbed, the remaining contents
form feces (mostly cellulose, bacteria, bilirubin).
Bacteria in the large intestine, such as E. coli, produce
vitamins (including vitamin K) that are absorbed.
The large intestine absorbs water and dissolved
minerals from the indigestible food residues, and by
doing so converts what remains from a fluid state into a
semi-solid feces
The feces is stored in the rectum and eliminated out the
anus
There are 3 parts to
the Colon
1. Ascending Colon
2. Transverse Colon
3. Descending Colon
Accessory Digestive Organs
 Salivary glands
 Teeth
 Pancreas
 Liver
 Gall bladder
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Slide
Salivary Glands
 Saliva-producing glands
 Parotid glands – located anterior to ears
 Submandibular glands
 Sublingual glands
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Saliva
 Mixture of mucus and serous fluids
 Helps to form a food bolus
 Contains salivary amylase to begin
starch digestion
 Dissolves chemicals so they can be
tasted
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Teeth
 The role is to masticate (chew) food
 Humans have two sets of teeth
 Deciduous (baby or milk) teeth
 20 teeth are fully formed by age two
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Teeth
 Permanent teeth
 Replace
deciduous teeth
beginning
between the ages
of 6 to 12
 A full set is 32
teeth, but some
people do not
have wisdom
teeth
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Pancreas
 Produces a wide spectrum of digestive
enzymes that break down all categories of food
 Enzymes are secreted into the duodenum
 Alkaline fluid introduced with enzymes
neutralizes acidic chyme
 Endocrine products of pancreas
 Insulin
 Glucagons
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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

When the acidic chyme is pushed out of the
stomach into the small intestines. It
stimulates the pancreas to send pancreatic
juice, which neutralizes the chyme, begins
digestions of carbohydrates, lipids and
continues digestion of protein.
Pancreatic juice also contains Lipase which
digested emulsified Lipids
Liver
 Largest gland in the body
 Located on the right side of the body
under the diaphragm
 Consists of four lobes suspended from
the diaphragm and abdominal wall by
the falciform ligament
 Connected to the gall bladder via the
common hepatic duct
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Role of the Liver in Metabolism
 Several roles in digestion
 Detoxifies drugs and alcohol
 Degrades hormones
 Produce cholesterol, blood proteins
(albumin and clotting proteins)
 Plays a central role in metabolism
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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Gall Bladder
 Sac found in hollow fossa of liver
 Stores bile from the liver by way of the
cystic duct
 Bile is introduced into the duodenum in
the presence of fatty food
 Gallstones can cause blockages
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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


The Liver makes Bile and stores it in the gall
bladder.
The Gall bladder sends bile to the small
intestine when the acidic chyme from the
stomach. Bile contains bile salts, which
emulsify fats, making them susceptible to
enzymatic breakdown.
The liver also stores excess glucose in the
form of glycogen.
Processes of the Digestive System
1. Digestion
• There are three major parts to digestion:
• Digestion Part 1
•Swallowing: process of using smooth and skeletal
muscles in the mouth, tongue, and pharynx to
push food out of the mouth, through the pharynx,
and into the esophagus.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide

Digestion Part 2
◦ Peristalsis: a muscular wave that travels the
length of the GI tract, moving partially digested
food a short distance down the tract. It takes
many waves of peristalsis for food to travel from
the esophagus, through the stomach and
intestines, and reach the end of the GI tract.

Digestion Part 3
◦ Segmentation: occurs only in the small intestine
as short segments of intestine contract like hands
squeezing a toothpaste tube. Segmentation helps
to increase the absorption of nutrients by mixing
food and increasing its contact with the walls of
the intestine.
Processes of the Digestive System
 Mechanical digestion
 Mixing of food in the mouth by the tongue
 Churning of food in the stomach
 Segmentation in the small intestine
 Bile produced by liver breaks fats into
smaller globules
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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Processes of the Digestive System
 Chemical Digestion
 Begins in mouth (saliva), continues in stomach
(enzymes and acids), most occurs in small intestine
(pancreatic juices secreted into S.I. by pancreas)
 Enzymes break down food molecules into their building
blocks
 Each major food group uses different enzymes
 Carbohydrates are broken to simple sugars
 Proteins are broken to amino acids
 Fats are broken to fatty acids and alcohols
 Water is degraded into hydrogen and oxygen
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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Processes of the Digestive System
2. Absorption
 End products of digestion are absorbed in the
blood or lymph
 Begins in stomach, ends in large intestine
 Stomach: water and alcohol
 Small intestine: most absorption takes place
here
 Large intestine: water and vitamins B and K
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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3. Defecation/Excretion
 Elimination of indigestible substances
as feces
 Controlled voluntarily but must be
accomplished on a regular basis

The following compounds are not
absorbed, but rather defecated
◦ Bile pigments (bilirubin)
◦ Epithelial cells of the intestinal lining
◦ Lignin (found in root vegetables, wheat, and
berry seeds)
◦ Cellulose (found in bran, legumes, nuts, peas,
roots, cabbage and apple skins)
◦ Human flora / bacteria
Processes of the Digestive System
Figure 14.11
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Control of Digestive Activity
 Relationship between Digestive System
and Nervous system:
 Mostly controlled by reflexes via the
parasympathetic division
 Chemical and mechanical receptors
are located in organ walls that trigger
reflexes
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