Digestive System
What is digestion?
The body carries out digestion of food to convert large
insoluble food molecules into smaller soluble ones.
carbohydrate
molecule
protein
molecule
fat
molecule
Small food molecules can pass through the walls of the
small intestine and then dissolve into the blood stream.
Large food molecules cannot do this.
Anatomy of the Digestive
System

Mouth

Esophagus
 Stomach

Small Intestine
 Large Intestine
Functions of the Digestive
System
 Ingestion:
intake of nutrients
 Digestion: breakdown of large
particles into smaller ones
 Absorption: uptake of nutrient
molecules
 Defecation: elimination of undigested
residues
Stages of Digestion
 Mechanical:
physical breakdown of food
 Chemical: digestive enzymes break large
food molecules into smaller ones
Some nutrients are absorbed without digestion:
– Vitamins, minerals, cholesterol, water
Mechanical digestion
Digestion begins in the mouth where food is broken down by
the teeth. This is called mechanical breakdown. The small
parts of food are mixed with saliva and swallowed.
The stomach
After food is swallowed it enters the stomach, which is basically a
muscular bag filled with hydrochloric acid.
food enters
from the
esophagus
food leaves
the stomach
Two things happen to food in the stomach:
 the chemical breakdown of food begins;
 microbes are destroyed.
muscle
tissue
Accessory Organs

Teeth
 Tongue
 Salivary Glands

Liver
 Gall Bladder
 Pancreas

Mouth/Oral Cavity
Functions:
– A. Ingestion: cheeks lips
and tongue mobilize food
– B. Digestion:
 Mechanical
–Chewing
 Chemical
–3 salivary glands
–Digests some starches
and fat
Pharynx
Motility:
muscles
force food
down during
swallowing
Straight
Esophagus
muscular
tube about 1 foot
long
Motility:
muscular
contraction moves
food towards
stomach

www.mywebmd.ca.com
Stomach
Muscular sac on the

left side of the
abdominal cavity

Functions:
– Food storage
– Mechanical
digestion
– Chemical digestion
Rugae in the Stomach
 Rugae-
folds in the
stomach lining that
allow it to expand
with more food
 How
do we vomit?

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion
Digestion in the
Stomach
 Mechanical
Digestion:
– Muscular contraction
 Chemical
Digestion:
– Secretes digestive enzymes and
HCl
Small Intestine

http://www.afns.ualberta.ca/bbo/1/ANATOMY/SI1.asp

2 meters long, 1 inch
in diameter, 5-6 meters
in cadaver

Functions:
– Chemical Digestion
– Nutrient
Absorption
– Motility
Functions of the Small
Intestine
 Motility: smooth
muscle contract

Digestion: chemical
enzymes received from
the pancreas, liver and
gall bladder

Nutrient re-absorption:
microvilli increase
surface area
Microvilli
http://www.emc.maricopa.edu/faculty/farabee/biobk/humdigest_2.gif
Large Intestine
 Functions
– Motility
– Re-absorption of Water and
Electrolytes
 Symbiotic
bacteria
– Synthesize Vitamins B & K
Anatomy of the Large
Intestine
Enzymes Involved in
Chemical Digestion
 Saliva:
– Amylase: breaks down starch
– Lipase: breaks down fats when it
enters the stomach
 Low pH of the stomach activates
the enzyme
How do digestive enzymes help the
process of digestion?
Digestive enzymes are the chemicals that break large
insoluble food molecules into smaller soluble molecules.
Digestive enzyme
animation
Multiple-choice quiz