Ms. Rafferty Biology
Digestive System
Digestive Processes
-In order for our bodies to use the food we eat to grow, energize, and repair our cells, 4
digestive processes have to occur.
-These include:
I .
I n g e s t i o
I I .
D i g e s t i o
I I I .
A b s o r p t o
I V .
E x c r e t i o
n
n
n
n
-These processes occur at different locations along the digestive tract, which is a
continuous tube from mouth to anus.
Digestive System
-Organs which perform digestion tasks:
 Digestive Tract (Gastrointestinal Tract, G. I. Tract) includes:
m o u t h ,
e s o p h a g u s , s t o m a c h , s m a l l
i n t e s t i n e , l a r g e i n t e s t i n e o r c o l o n , a n d
a n u s
.
 Accessory Structures include:, t o n g u e , s a l i v a r y g l a n d s ,
l i v e r, g a l l b l a d d e r a n d p a n c
r e a s .
Digestion
Definition: The process in our bodies that breaks down food for use by our cells. This is
done in two ways:
 Mechanical: o u r t e e t h c h e w i n g t h r o u g h t h e f o o d
 Chemical: e n z y m e s b r e a k d o w n f o o d i n t o a
b
Mouth
Four main parts of the mouth
V .
T
e e t h
V I .
T o n g
u e
V I I .
S a l i v a r y G l a n d s
V I I I .
H a r d a n d S o f
t
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1) Teeth (Chewing)
I X .
M e c h a n i c a l
2)




D i g e s t i o n
Tongue
Floor of oral cavity
Composed of voluntarily controlled muscle tissue
Role: Guides food (bolus) during chewing and swallowing
Major taste buds are embedded here.
3) Salivary Glands
 Release s a l i v a in response to the smell, taste, and thought of food
 First example of chemical digestion:
o Contains A m y l a s e : an enzyme that begins breakdown of starch into
simple sugars, like glucose or table sugar.
4) Hard & Soft Palates
 Separate mouth from n a s a l c a v i t y , so food doesn’t go up your nose!
 Located at the roof of your mouth.
Uvula (dangles)
 Just beneath the soft palate
 Prevents f o o d ( b o l u s ) from entering nasal passages by rising up.
Epiglottis
 Soft tissue that is pushed down as bolus moves into e s o p h a g u s to block
the entrance to the trachea
Esophagus
 Muscular tube extends from pharynx (throat) to stomach
 Swallowing initiates p e r i s t a l s i s , wave of contraction which initiates just
above the “bolus” (substance) and moves it towards the stomach.
 Secretes m u c u s (along with the rest of the digestive tract)
o Protects esophagus from acid and enzymes in gastric juice if reflux were to
occur.
Stomach
 Separated from esophagus by the gastro- esophageal sphincter
 Expandable m u s c u l a r J-shaped sac that can hold up to 2-4 liters of food
and liquid
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 Secretes mucous to protect lining cells
 Contents are held in the stomach by the p y l o r i c s p h i n c t e r , ring
of circular muscle, which separates the stomach from upper small intestine
3 Stomach Functions
1) Stores food and releases it gradually into the small intestine
2) Assist in m e c h a n i c a l breakdown of food by p e r i s t a l s i s and
contractions of muscular walls
3) C h e m i c a l breakdown of food by enzymes and other substances.
-Secretions
o G a s t r i n : hormone that stimulates secretion of Hydrochloric acid
(HCl)
o P e p s i n o g e n : inactive form of enzyme is activated by HCl and
transformed into pepsin which helps breakdown proteins into amino
acids (smaller units).
 Food is gradually converted into a thick liquid called c h y m e which consists of
partially digested food and digestive secretions.
o Chyme is propelled towards small intestine
o The Pyloric sphincter lets a teaspoon of chyme into the small intestine during
each contraction
Small Intestine(S.I.)
 Narrow tube (1-2” diam)
 Longest part of digestive tract
 Split into three parts:
X .
d u o d e n u m ,
X I .
j e j u n
u m ,
X I I .
i l e
u m
Function:
 D i g e s t i o n of food into small molecules
 A b s o r p t i o n of nutrients into blood stream and lymph vessels.
Accessory Organs
1. Liver
 In digestion: produces b i l e which is stored in the gall bladder, then released in
the s m a l l i n t e n s t i n e made up of bile salts, water, and other salts
and assist in the breakdown of l i p i d s .
2. Gall Bladder
 Stores b i l e until needed for digestion
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3. Pancreas
 Located between stomach and S. I.
 Produces pancreatic juice which n e u t r a l i z e s the acidic chyme
 Digests carbohydrates, proteins, and lipids
 Contains water, sodium bicarbonate, and many enzymes
 Enzymes include:
Amylase >
C a
r b o h y d r a t e s
Lipase >
L i p i
d s
Proteases > P r o t e i n s a n d p e p t i d e s
Small Intestine (structure)
 Specialized cells within the wall complete the digestion process. At the same time,
nutrients are absorbed into the blood stream.
 The numerous folds and projections, called v i l l i , give the internal surface area a
much larger surface for absorption.
 The area of absorption is also increased by the millions of microvilli.
 While the nutrients are being absorbed by these cells, chyme is being mixed back and
forth by segmentation movements within the small intestine.
 When absorption is complete, undigested material moves into the l a r g e
i n t e s t i n e .
Large Intestine (L.I.)
 Is about 5’ long and 3”diameter
 Two parts:
1) C o l o n (most of its length)
2) R e c t u m : final 6”
 Receives leftovers of digestion: water mixture, undigested fat and proteins,
indigestible fibers
 Contains large populations of b a c t e r i a that thrive on the unabsorbed
nutrients and are also responsible for s y n t h e s i z i n g important
vitamins for the body (B12, riboflavin, K).
 Remaining w a t e r , s a l t s , a n d v i t a m i n s are reabsorbed
by cells in the L. I.
 Leftover material is condensed into semisolid feces which are transported to the
rectum by p e r i s t a l t i c movements.
 Expansion of the rectum stimulates the urge to defecate which is initiated
voluntarily after age 2.