Download Summary of Chapter 2 – Digestion and Absorption

Summary of Chapter 2 – Digestion and Absorption
Food enters the mouth and travels down the esophagus and through the lower esophageal sphincter to the stomach, then through the
pyloric sphincter to the small intestine, on through the ileocecal valve to the large intestine, past the appendix to the rectum, ending at the
anus. The wavelike contractions of peristalsis and the periodic squeezing of segmentation keep things moving at a reasonable pace.
To digest food, the salivary glands, stomach, pancreas, liver (via the gallbladder), and small intestine deliver fluids and digestive
enzymes.
The many folds and villi of the small intestine dramatically increase its surface area, facilitating nutrient absorption. Nutrients pass
through the cells of the villi and enter either the blood (if they are water soluble or small fat fragments) or the lymph (if they are fat
soluble).
Nutrients leaving the digestive system via the blood are routed directly to the liver before being transported to the body’s cells. Those
leaving via the lymphatic system eventually enter the vascular system but bypass the liver at first. Within the circulatory system, lipids
travel bundled with proteins as lipoproteins. Different types of lipoproteins include chylomicrons, very-low-density lipoproteins
(VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Elevated blood concentrations of LDL are associated
with a high risk of heart disease. Elevated HDL are associated with a low risk of heart disease.
Fiber
Carbohydrate
Fat
Protein
Vitamins
Minerals and
Water
The salivary glands
secrete a watery fluid
into the mouth to
moisten the food. The
salivary enzyme
amylase begins
digestion.
Glands in the
base of the
tongue secrete
a fat-digesting
enzyme known
as lingual
lipase. Some
hard fats begin
to melt as they
reach body
temperature.
Chewing and
crushing moisten
protein-rich foods
and mix them with
saliva to be
swallowed.
No action.
The salivary glands
add water to
disperse and carry
food.
Stomach acid and
enzymes start to
digest salivary
enzymes, halting
starch digestion. To a
small extent, stomach
acid hydrolyzes
maltose and sucrose.
The acid-stable
lingual lipase
splits one bond
of triglycerides
to produce
diglycerides and
fatty acids. A
gastric lipase
accesses and
hydrolyzes a
very small
amount of fat.
Stomach acid
uncoils protein
strands and
activates stomach
enzymes.
Intrinsic factor (see
Chapter 8)
attaches to vitamin
B12.
Stomach acid
(HCl) acts on iron
to reduce it,
making it more
absorbable. The
stomach secretes
enough watery
fluid to turn a
moist, chewed
mass of solid food
into liquid chyme.
The pancreas
produces enzymes
and releases them
through the
pancreatic duct into
the small intestine.
Then enzymes on the
surfaces of the small
intestinal cells break
disaccharides into
monosaccharides,
and the cells absorb
them.
Bile flows in
from the liver
and gallbladder
(via the
common bile
duct).
Pancreatic
lipase flows in
from the
pancreas (via
the pancreatic
duct); broken
down to
monoglycerides,
Broken down to
amino acids and
absorbed.
Bile emulsifies fatsoluble vitamins
and aids in their
absorption with
other fats. Watersoluble vitamins
are absorbed.
The small intestine,
pancreas, and liver
add enough fluid
so that
approximately 2
gallons are
secreted into the
intestine in a day.
Many minerals are
absorbed. Vitamin
D aids in the
absorption of
calcium.
Mouth
The mechanical
action of the mouth
and teeth crushes
and tears fiber in
food and mixes it
with saliva to
moisten it for
swallowing.
Stomach
Fiber is unchanged.
Small intestine
Fiber is unchanged.
glycerol, fatty
acids and
absorbed.
Large intestine
(colon)
Most fiber passes
intact through the
digestive tract to
the large intestine.
Here, bacterial
enzymes digest
some fiber. Fiber
holds water;
regulates bowel
activity; and binds
cholesterol and
some minerals,
carrying them out of
the body as it is
excreted with feces.
Some fat and
cholesterol,
trapped in fiber,
exit in feces.
Bacteria produce
vitamin K, which is
absorbed.
More minerals and
most of the water
are absorbed.