Download The Pancreas

ANIMAL ANATOMY UNIT 3
LESSON PLAN 4
DIGESTION SYSTEM Part A
THE STRUCTURES AND FUNCTIONS OF THE HUMAN
DIGESTIVE SYSTEM
The main function of the human digestive system is to breakdown large food
molecules into smaller molecules that can be absorbed by the body.
Terms:
Alimentary Canal or
Gastrointestinal(GI) Tract
 Is a one way "tube system with specialization
 Performs "extracellular" digestion (digestion that
takes place outside the cell)
Major Sites of Digestion
 Mouth, stomach small intestine, large intestine
Accessory
Digestive Organs
 Liver, pancreas, gall bladder, salivary glands
 Assist by adding secretions to the digestive tract
through ducts (small tubes that connect these organs
to the digestive system
Digestion
 Involves the conversion of food from its original form
in the external environment into nutrient molecules
small enough to enter body cells and participate in
cell functions
Ingestion
 Intake of food via the mouth
Propulsion
 Movement of food along digestive tract
Physical Digestion
 The breaking of large food pieces into smaller ones
Chemical Digestion
 Chemically changing large complex food molecules
(lipids, carbohydrates, proteins) into smaller simpler
ones (fatty acids, glycerol, sugars, amino acids) by
enzymes or acid (HCl)
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Absorption
 The transport of nutrient molecules from the GI tract
thru the walls, into the blood stream for distribution to
cells
Egestion
Elimination of undigested food/waste in the form of
feces (defecation)
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HUMAN DIGESTIVE TRACT
STRUCTURE
Mouth
Teeth
Salivary Glands
FUNCTION
 entrance to the digestive tract
 ingestion of food (digestive process starts)
 aids in the physical breakdown of food (mechanical digestion)
 teeth structure:
 incisors -- cuts food into smaller pieces
 canines & bicuspids -- pierce and tears food
 molars -- crush and grinds food
 secrete saliva (water & mucus) and
 amylase (enzyme) breaks down starch (carbohydrates) into
maltose
Tongue
 a muscular organ that moves the food around for chewing and
mixing with saliva until it forms a bolus which is then pushed
back into the pharynx
Pharynx
 is a passage at the back of the mouth that receives both food and
air
Epiglottis
 a flap of tissue that prevents food from entering the trachea
Esophagus
 is a muscular tube that connects the mouth to the stomach
 its main function is to push the bolus into the stomach by
peristalsis
Structure
 two muscle layers
i) circular muscle fibres
ii) longitudinal muscle fibres
 the contractions and relaxation of these muscles causes the food
to be pushed down into the stomach
Cardiac
Sphincter
 is a ring-like muscle that controls the entry of food into the
stomach
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Stomach
 is a muscular organ that has three types of muscle layers
i) circular
ii) longitudinal
iii) oblique
 the rhythmic contraction and relaxation of these muscles layers
churns the partially digested food and mixes it with gastric
juices
 gastric juice: is a mixture of
i) hydrochloric acid (HCl):
 activates digestive enzymes
 breaks down fibrous tissue
 kills germs
ii) pepsin(enzyme):
 breaks down proteins into long chains of amino
acids called polypeptides (ie. peptones and protease)
iii) mucus layer:
 protects the stomach from pepsin and HCl (without
this layer the stomach would be digested resulting in
a gastric ulcers
 pylorus:
i) lower region of the stomach
ii) contains partially digested food now called chyme
 chyme: is a mixture of food, water, and gastric juice
Pyloric Sphincter  controls the passage of chyme (food) from the stomach to the
intestine
Small Intestine
 aprox. 6.2 m in length
 2.5 cm in diameter
 two muscle layers (circular & longitudinal)
 movement of food is by peristalsis
 is divided into three main sections:
a) duodenum (25 cm long):
i)
ii)
high digestive activity
connected to the liver and pancreas by the bile
duct and the pancreatic duct which releases their
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digestive juices in this location
iii) the acidic chyme is neutralized by bicarbonates
released b the pancreas
b) jejunum:
i)
has an increase number of folds compared to the
duodenum
ii) secretes enzymes that breakdown any remaining
proteins (peptidase) into amino acids and
carbohydrates (maltase, sucrase, lactase) into
glucose
iii) has a number of finger-like structures called villi
which absorbs the nutrients and transfers them to
the circulatory system
c) ileum (3.5 m):
i)
ii)
iii)
iv)
similar to the jejunum
smaller and fewer villi
produces few enzymes
main function is absorption and to push any
undigested material into the large intestine
ACCESSORY ORGANS
Liver
 largest gland of the body (1.5 kg)
 produces bile
 bile:

i) is an emulsifying agent needed for the
physical digestion of fat
large fat globules + bile  small fat globules
(small surface
(large surface
area/volume ratio)
area/volume ratio)
ii) it exposes a large number of fat molecules for
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digestion
iii) is secreted continuously and is stored in the gall
bladder
Pancreas
 produces several digestive enzymes
i)
bicarbonates: neutralizes stomach acids which allow
the intestinal enzymes to continue the process of
chemical digestion of food
ii)
trypsin & chymotrypsin: the breakdown of proteins
into dipeptides an amino acids
iii) carboxypeptidase: cleaves peptides
iv) pancreatic amylase: cleaves starch, glycogen
v)
lipase: cleaves lipids into glycogen and fatty acids
vi) nucleases: cleaves nucleic acids
 these juices are carried to the duodenum by the pancreatic duct
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Large Intestine
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aprox. 1.5 m long
5 -7 cm in diameter
reabsorbs water, ions, vitamins
production of vitamin K and some B vitamins are produced by
bacteria (E. coli)
 the liquid waste becomes firmer and is called feces
 feces: consists of living and dead bacteria, cellulose and water
 as the feces is formed it pushed through the large intestine by
peristalsis into the rectum
Rectum
 expels waste
 weak sphincter muscle at its entrance prevents fecal matter from
entering until it is ready for elimination
Anus
 contains two sphincter muscles ( an internal and an external) that
controls the final elimination of feces
 when the rectum is full the internal sphincter relaxes and sends
a message to the brain
 the external sphincter remains contracted until it relaxes
voluntarily for elimination
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The Movement of Food
The movement of food is accomplished by a series of wavelike muscular
contractions and relaxations known as peristalsis.
Peristalsis involves the circular and longitudinal muscles that surround the
various parts of the digestive tract.
To move food,

the circular muscles over a food mass relax while the longitudinal muscles
immediately in front of it contract.

the circular muscles immediately behind the food mass then contract while
the longitudinal muscles over the food mass relax.

As succeeding muscular regions relax and contract, the food is pushed
along
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ANIMAL ANATOMY UNIT 3
LESSON PLAN 4
DIGESTION SYSTEM Part B
Chemical Digestion and Enzymes
The major nutrients taken in by the body through eating are carbohydrates, proteins,
fats and nucleic acids. These large macromolecules need to be broken down by the
digestive system into their simpler building blocks.
In order to breakdown these nutrient they under go a chemical reaction called
HYDROLYSIS. Hydrolysis is the use of water to break the bonds between the
monomers (building blocks) of the macromolecules.
Hydrolysis is a slow reaction and needs to be sped up. The body solves this problem by
making a biological catalyst called enzymes. Enzymes are special proteins produced by
the cell. Enzymes break the bonds between monomers at very specific locations
(linkages).
Enzymes often require special conditions in order to act
i)
ii)
iii)
iv)
some work best in an acidic environment
(i.e. pepsin in the stomach)
other work in a neutral or alkaline (basic) environment
(i.e.. the digestive enzymes in the small intestine)
enzymes are affected by temperature changes
(i.e. higher temperatures will denature the enzyme so that it is no longer
functional)
in order for the enzyme to function properly it may require the presence of
metallic ions
(i.e. cobalt and magnesium), vitamins, or
coenzymes in order to function properly.
There are three groupings of enzymes that are formed by secretory cells and then secreted
into the digestive tract.
Macromolecule
carbohydrates
proteins
fats
Enzyme
Carbohydrases
Proteases
Lipases
Monomers
monosaccharides (glucose, galactose,
fructose)
amino acids
glycerol + 3 fatty acids
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Digestive Enzymes
Most of the digestive enzyme names start with the name of the substrate they react on
and have the ending "ase" added.
Example: maltose is broken down by an enzyme called maltase.
Some enzymes still use their older names such as pepsin.
A Summary of Chemical Digestion
In humans and other mammals, chemical digestion begins in
mouth


Salivary glands release the enzyme called salivary amylase,
Amylase breaks down starch into smaller disaccharide sugar molecules

gastric juice secreted by the stomach contains hydrochloric acid and the enzyme,
pepsin.
Pepsin
begins the breakdown of protein in the stomach
Pepsin functions well within a pH range of 1 to 2.
stomach

duodenu
m
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
is 25 cm long
secretes enzymes from its lining,
the pancreas and liver both empty their enzymes into the duodenum to complete the
process of digestion.

Pancreatic juice is alkaline and thus neutralizes the acidity of partially digested food
coming from the stomach, stopping any further action of pepsin


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produces no enzymes
water is absorbed from indigestible material through its walls
anaerobic bacteria living there,
digest some of this material, and some of it is absorbed for use by the body
synthesize some B vitamins and Vitamin K which is then used by the body.
The total digestion of a large meal takes about 24-33 h
pancreas
Large
intestine

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The Regulation of Digestive Secretions
The secretion of digestive enzymes is regulated by both nerves and hormones.
A hormone is a chemical regulator that is secreted in one part of the body and
transported by the bloodstream to another part, where it causes a response.
Examples
a) Secretion of Gastric Juices



Nerve impulse stimulates the lower part of the stomach to produce the
hormone gastrin
Gasterin enters the circulatory system and is carried back to the upper
part of the stomach
Gasterin then stimulates the digestive glands lining the walls of the upper
part of the stomach to secrete mucin pepsin, hydrochloric acid, lipases.
b) Neutralization of Chyme


Chyme entering the duodenum stimulates the glands of the duodenum to
secrete the hormone secretin into the bloodstream
Secretin stimulates the duct cells of the pancreas to release an alkaline
fluid containing sodium carbonate and sodium bicarbonate into the
duodenum which changes the pH from 2 to 8
c) The hormone secretin and cholecystokinin (CCK) are secreted by the duodenum
and stimulates the pancreas to release its digestive enzymes into the duodenum.
These enzymes also stimulate the gall bladder to secrete bile salt.
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Enzymes of the Human Digestive System
Place where
enzyme acts
Substrate
Enzyme
mouth
starch,
glycogen
salivary amylase
stomach
protein
fats
fats
peptides
peptides
peptides
starch
small
intestine
ribonucleic acid
deoxyribonucleic acid
peptides
tripeptides
maltose
sucrose
lactose
pepsin
Products
Origin of
Enzyme
maltose
salivary
glands
peptides
lipase
glycerol + 3 fatty
acids
pancreatic lipase
glycerol + 3 fatty
acids
trypsin
chymotrypsin
carboxypeptidase
pancreatic amylase
ribonuclease
deoxyribonuclease
aminopeptidase
tripeptidase
maltase
sucrase
lactase
stomach
glands
simpler peptides
simpler peptides
simpler peptides
pancreas
maltose
ribonucleotides
deoxyribonucleotides
simpler peptides
dipeptide + amino
acid
glands in the
walls of the
small intestine
glucose + glucose
glucose + fructose
intestinal
glands
glucose + galactose
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The Roles of Related Organs
Three other organs associated with the digestive tract are the liver, gall bladder, and
pancreas.
These organs play vital roles in the digestive process.
The Liver
The liver is the largest organ in the body and has been identified as having over 500
different functions that are essential to the body's general good health.
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produces bile salts from cholesterol which is released into the small intestine to
emulsify fat globules (break up ) to allow lipase to chemical break the fat up into
glycerol and fatty acids
breaks down old red blood corpuscles, and hemoglobin
the decomposed hemoglobin molecules are used to make bile salts.
the liver collects from the bloodstream chemicals that are in excess of the amount
needed by the body at any given time.
The liver converts and stores all monosaccharides (except glucose), into
glycogen
The liver will convert the stored glycogen back into glucose when the glucose
level in the bloodstream become low
The liver also stores fat-soluble vitamins A, D, E, and K
the liver works to detoxify various poisons ingested with food and drink in
addition to those produced in the intestines.
The liver is highly evolved organ that performs a wide range of important, specialized
functions for the body.
The Gall Bladder
The gall bladder serves as the




storage warehouse for bile produced in the liver.
bile contains a number of chemicals that include cholesterol
bile salts are used in fat digestion
bile salts are reabsorbed by the small intestine and is carried back to the liver to
be recycled back into bile
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The Pancreas
The pancreas is the source of


several enzymes that act on carbohydrates, fats, and peptides (smaller segments
of proteins)
produces and releases a basic solution that changes the pH of chyme (from a
strongly acid mixture to a weakly basic one) after it enters the duodenum.
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