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Chapter 3
Digestion, Absorption and
Transport
The process of
digestion
transforms all
kinds of foods into
nutrients.
By the end of this chapter
you should be able to
follow the ingestion of food
from the mouth all the way
to the the end of the
gastrointestinal (GI) tract.
You will need to describe
the mechanical,
biochemical, and
regulatory processes along
the way.
As food is passed into the mouth,
it is MASTICATED by the teeth
and tongue (process referred to
as mastication).
Simultaneously, the salivary
glands excrete SALIVA. Saliva
contains starch-digesting
enzymes which begin to break
down the starch to sugars.
As you swallow, the food is
guided by the PHARYNX into
the ESOPHAGUS.
A flap of tissue called the
EPIGLOTTIS prevents the food
(usually) from entering the
TRACHEA which leds to the
lungs.
As the food/saliva mixture enters
the ESOPHAGUS, it is now
called a BOLUS.
The ESOPHAGUS has
SPHINCTER muscles at each end
(Upper and Lower).
The UPPER ESOPHAGEAL
SPHINCTER opens to allow the
food to enter the ESOPHAGUS.
As the BOLUS moves downwards,
the ESOPHAGUS passes through
an opening in the DIAPHRAGM
towards the STOMACH.
The LOWER ESOPHAGEAL
SPHINCTER opens to allow food
to enter into the STOMACH.
The LOWER ESOPHAGEAL
SPHINCTER closes to prevent
the BOLUS from re-entering
the ESOPHAGUS.
As the BOLUS enters the
STOMACH, GASTRIC JUICES
are secreted by cells of the
STOMACH.
These JUICES contain water,
enzymes and HCl (acid).
This acid lowers the pH of the
STOMACH to about 1.5 – 2.
The BOLUS / JUICE mixture is now referred to as CHYME.
The acid kills bacteria in the BOLUS and activates the
enzymes that digest PROTEIN.
The entire GI tract
is lined with multidirectional muscles
that move the food
along (referred to
as
SEGMENTATION)
The STOMACH
MUSCLES flex and
relax to grind the
CHYME until it no
longer appears as
food.
The STOMACH cells
would also be digested
under these conditions.
However, there is a
resistant lining of cells in
the STOMACH and
special GOBLET CELLS
produce MUCOUS that
coats the lining protecting
the STOMACH from
digestion
At the bottom of the STOMACH, the PYLORIC SPHINCTER
opens to allow the CHYME into the SMALL INTESTINE.
The SMALL INTESTINE is
about 3 meters long. It is
composed of 3 segments –
Duodenum
Jejunum
Ileum
The SMALL INTESTINE does
not have the protective lining or
GOBLET CELLS to protect it.
It is very susceptible to damage.
Therefore, as the CHYME enters the DUODENUM, several
things happen:
1. PANCREAS produces PANCEATIC JUICE (NaHCO3
(bicarbonate) and ENZYMES) through the same DUCT.
2. BILE produced in the LIVER, stored in the
GALLBLADDER is added through the BILE DUCT.
The pH Scale:
Comparison of
Digestive Juices
The enzymes produced by the PANCREAS act to
further breakdown ALL of the major energy nutrients:
CARBOHYDRATES
PROTEINS
FAT
as the food moves along the small intestine.
Now ADSORPTION also begins to take place. Small
molecules are transported into cells lining the small
intestine.
Simple Diffusion
Facilitated Diffusion
Active Transport
There are three ways that these small nutrient molecules can
move into cells.
Simple Diffusion, Facilitated Diffusion, Active Transport
Villi
Muscles
Capillaries in the
Vascular System
(red - blue)
Lymphatic System
(yellow)
Electron
Micrograph of
Cells
Microvilli