Food As a Source of
Nutrients
• Humans obtain nutrients from food.
• These nutrients are the source of raw
materials that your body uses to build
tissue and fuel cellular work.
• There are six types of nutrients in
food: carbohydrates, fats, proteins,
vitamins, minerals, and water.
Four Stages of Food
Processing
• The process of how your body
obtains raw materials from food
is called nutrition.
• This process involves the four
stages.
• First is eating.
• Eating or drinking is called
ingestion.
After food enters the body through
ingestion, the next stage of food processing
begins.
Digestion is the process of breaking food
down into molecules small enough for the
body to absorb.
For example, polysaccharides such as
the starch in the pizza crust are broken down
into monosaccharides (simple sugars).
Digestion occurs in two steps.
Mechanical digestion (such as chewing)
chops and grinds food into smaller pieces,
increasing its surface area.
Chemical digestion breaks the chemical bonds
within the large molecules that make up food,
producing smaller building-block molecules.
An example of chemical digestion is the
action of acids and enzymes in your stomach.
The last two stages of food processing occur
after food is digested. In the third stage,
absorption, certain cells take up (absorb)
the small molecules.
The circulatory system then transports
the nutrients throughout the body.
Finally, undigested material passes out of the
body in the stage called elimination.
Digestion Occurs in a Tube
• Digestion occurs in a tube
called the alimentary canal.
–Food moves in one direction
through the alimentary canal,
which is organized into
specialized regions that carry
out digestion and absorption
in a step-by-step process.
Epithelial tissue lines the alimentary canal.
One function of the epithelial cells is to
secrete mucus that lubricates the canal and
helps prevent the body from digesting itself.
This is important because the digestive juices
in your stomach, for example, have a pH of
2—acidic enough to dissolve iron nails.
Despite the mucous layer, the epithelial cells
in the stomach are constantly eroded.
Enough new cells are generated through
mitosis to completely replace your stomach
lining every three days.
Organs of the Digestive
System
• Six main organs make up the
alimentary canal: the mouth,
pharynx, esophagus, stomach,
small intestine, and large intestine.
• Accessory glands and organs
include the salivary glands,
pancreas, liver, and gallbladder,
which secrete digestive juices into
the alimentary canal.
Mouth
• Functions both in ingestion and in
the beginning of digestion.
–Your teeth and tongue are
responsible for mechanical
digestion.
•The various shapes of different
types of teeth cut, smash, and
grind food into smaller pieces.
This makes the food easier to
swallow and exposes more
surface area to digestive
enzymes.
Chemical digestion also begins in your
mouth.
Salivary glands in your mouth region
secrete more than one liter of liquid.
This liquid is called saliva,
contains digestive enzymes, mucus,
and other chemicals.
The salivary enzyme called amylase
begins the chemical digestion of the
polysaccharide starch in foods like pizza
crust, pasta, and bread.
Other chemicals in saliva kill bacteria and
neutralize certain acids in foods,
protecting your teeth from decay.
Pharynx
• The tongue pushes each chewed clump of
food, called a bolus, down the throat.
• The upper portion of the throat, is the
pharynx,
– the function of the alimentary canal
and the passageway by which air
enters the lungs. When you swallow, a
cartilage flap called the epiglottis
temporarily seals off the airway and
prevents food from entering.
Esophagus
• The bolus enters a long, muscleencased tube called the
esophagus.
–This connects the pharynx to
the stomach. Although the
esophagus is oriented vertically
in your body, gravity is not the
reason that food moves toward
your stomach.
•Food is pushed through the esophagus by
a series of muscle contractions called
peristalsis.
•The muscles at the very top of the
esophagus are striated (voluntary),
which means that swallowing begins
voluntarily.
•The muscle layers around the rest of the
esophagus are smooth (involuntary).
•Once a bolus of food reaches the
pharynx these smooth muscles trigger the
swallowing reflex.
•The smooth muscles contract in a wavelike motion that forces the bolus of food
toward the stomach.
•Food continues to move along the
alimentary canal by peristalsis.
Stomach
• The stomach is an elastic,
muscular sac capable of stretching
to hold up to 2 liters of food.
• A liquid called gastric juice,
secreted by glands in the stomach
lining, bathes the bolus after it
enters the stomach.
• Gastric juice is a mixture of
mucus, hydrochloric acid, and
enzymes.
•Hydrochloric acid breaks apart the cells
in food. It also kills many of the bacteria
swallowed with food.
•One of the gastric enzymes, pepsin,
hydrolyzes large protein molecules into
smaller polypeptides.
Meanwhile, mechanical digestion turns
the bolus into an acidic liquid called
chyme.
Stomach muscles contract, creating a
churning motion that stirs the chyme and
eventually forces it into the small
intestine.
Most of the time, the stomach is pinched
closed at both ends
The passageway between the esophagus and
the stomach opens when peristalsis delivers
a bolus.
But in some people, the passageway may
open at inappropriate times, allowing acidic
chyme to flow backward into the esophagus.
This creates a burning sensation called
heartburn.
At the opposite end of the stomach, a
muscular valve called the pyloric
sphincter
This regulates the flow of chyme
into the small intestine.
It typically takes 2 to 6 hours after a
meal for the stomach to empty.
Small Intestine, Liver, and
Pancreas
• From the stomach, liquid chyme passes into the
small intestine.
– The small intestine is a long (6 m), narrow
(2.5 cm) tube where digestion is completed
and absorption of most nutrients takes place.
– Peristalsis moves chyme along the small
intestine.
– Digestion mostly occurs in the first portion of
the small intestine, while absorption occurs
along the rest of its length.
The first section of the small intestine is
called the duodenum.
As chyme enters the duodenum, it mixes
with several digestive juices.
One of these digestive juices is a liquid
called bile.
Bile
is produced outside of the small
intestine by the body's largest
internal organ, the liver.
Bile is stored in a sac-like
structure called the gallbladder
until it is secreted into the
duodenum.
Fats tend to clump together into globs,
making it difficult for enzymes to reach
the molecules.
Bile separates small fat droplets,
preventing them from clumping into
globs.
This enables digestive enzymes to
break the fats down more efficiently
The pancreas produces and secretes
pancreatic juice into the duodenum.
Pancreatic juice neutralizes the acid chyme
and also contains enzymes that hydrolyze
carbohydrates, proteins, and lipids.
Large Intestine
• By the time food reaches the end of
the small intestine, the nutrients
have all been broken down and
absorbed.
• Undigested material passes through
another sphincter from the small
intestine into the large intestine.
Also called the colon, the large intestine
is a wide (5 cm), short (1.5 m) tube from
which water is absorbed into the body.
The large intestine also contains certain
bacteria that produce vitamin K and
several B vitamins.
a major function of the large intestine is
to reabsorb water.
Saliva, gastric juice, and other digestive
juices all contain large amounts of
water.
Much of that water is reabsorbed along
with nutrients in the small intestine.
The large intestine finishes the job by
absorbing most of the remaining water.
Together the small intestine and large
intestine reclaim 90 percent of the water
that enters the alimentary canal.
Undigested food material and other waste
products are called feces.
Reabsorption of water causes the feces to
become more solid as it moves through the
large intestine.
Once again, peristalsis is the mechanism
that moves this material through the large
intestine. It generally takes 12 to 24 hours
for waste material to travel through the
colon.