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CHAPTER 21
Nutrition and
Digestion
Nutrition->life process by
which an organism obtains
and utilizes food
OBTAINING AND PROCESSING
FOOD


Animals ingest their food
in a variety of ways
Animal diets are highly
varied



Herbivores are plant-eaters
Carnivores are meat-eaters
Omnivores eat both plants
and other animals
Overview: Food processing occurs
in four stages
1.
2.
3.
4.
Ingestion: taking in food
Digestion: mechanical and chemical
breakdown of food so that it can be absorbed
by the cells
Absorption: cells lining the digestive tract take
up (absorb) small nutrient molecules
Elimination: undigested material passes out of
the digestive tract
HUMAN DIGESTIVE SYSTEM
consists of
1. alimentary canal (GI
gastrointestinal tract)
~ continuous one way
food tube (mouth to anus)
2.accessory glands
~pancreas, liver, &
gallbladder

HUMAN DIGESTIVE SYSTEM

When food is swallowed, it is moved
through the alimentary canal by
peristalsis
 Peristalsis
is rhythmic muscle contraction in
the walls of the digestive tract
 Ringlike sphincter muscles regulate the
passage of food
Digestion begins in the oral cavity




The teeth break up food
(mechanical digestion)
Saliva moistens it
Salivary enzymes begin
the hydrolysis of starch
(amylase) (chemical
digestion)
The tongue pushes the
chewed food into the
pharynx (throat)
The food and breathing passages
both open into the pharynx

The swallowing reflex moves food from the
pharynx into the esophagus
 At
the same time, food is kept out of the
trachea; epiglottis is a flap that prevents
choking
 Food is now in the form of a bolus
The esophagus squeezes food
along to the stomach

Peristalsis in the esophagus moves food boluses
into the stomach
The stomach mechanically churns food into liquid
and further chemically digests some of the food by
secreting gastric juice

The stomach mixes food with
gastric juice:
1. water-solvent
2. mucus-lubrication
3. pepsin- enzyme that begins
chemical digestion of protein
4. Hydrochloric acid ( HCl) makes food acidic, (pH=2)
activates pepsin
Food now in liquid form –chyme
NOT all digestion has occurred
Connection: Bacterial infections
can cause ulcers

evidence suggests that a spiral-shaped
prokaryote causes many ulcers
 Helicobacter
pylori growth erodes protective
mucus and damages the stomach lining
The small intestine is the major
organ of chemical digestion and
nutrient absorption

Alkaline pancreatic juice
neutralizes stomach acids


Its enzymes digest
polysaccharides, proteins,
nucleic acids, and fats
Bile emulsifies fat
droplets for attack by
pancreatic enzymes

It is made in the liver and
stored in the gall bladder
Enzymes from the walls of the small intestine
complete the digestion of many nutrients
Absorption

The lining of the small intestine is folded and covered with tiny,
fingerlike villi


Villi increase the absorptive surface
Nutrients pass through the epithelium of the villi and into the blood

The blood flows to the liver
 The liver can store nutrients and convert them to other substances the
body can use
 Center of villi contains lymph vessel called lacteals which absorb fatty
acids
The large intestine reclaims water

Undigested material
passes to the large
intestine, or colon





Water is absorbed
Feces are produced
Absorption of vitamins
produced by bacteria that
live in LI
Storage and elimination of
feces
rectum-last part of GI,
feces eliminated through
anus
Large
intestine
(colon)
End
of small
intestine
Small
intestine
Rectum
Anus
Nutrient
flow
Cecum
Some Digestive Homeostasis
Disorders





1. Constipation– person has uncomfortable or infrequent
bowel movements results from sluggish peristalsis that
allows excess water to be removed from feces (fecal
matter hardens)- may result from insufficient fiber in diet
2. Diarrhea– opposite of constipation– associated with
intestinal disturbances caused by infections or stress–
prolonged diarrhea may result in severe dehydration
3. Gall stones– small hard particles made of cholesterol
which form & collect in gall bladder- may block the bile
duct and cause pain
4. acid reflux -backflow of stomach contents upward into
esophagus
5. appendicitis- inflammation of appendix
NUTRITION

Overview: A healthful diet satisfies three
needs
 fuel
for its activities
 raw materials for making the body’s own
molecules
 essential nutrients that the body cannot make
Chemical energy powers the body

Once nutrients are inside cells, they can
be oxidized by cellular metabolism to
generate energy
 This
energy is in the form of ATP
Chemical energy powers the body

The energy a resting
animal requires each
day to stay alive is its
basal metabolic rate
(BMR)
Chemical energy powers the body

More energy is
required for an
active life
 Excess
energy
is stored as
glycogen or fat
Connection: Body fat and fad diets



The human body tends to store excess fat molecules
instead of using them for fuel
A balanced diet includes adequate amounts of all
nutrients
Fad diets are often ineffective and can be harmful
Connection: Vegetarians must be sure
to obtain all eight essential amino acids

The eight essential amino acids that adults
require must be obtained from food
 They
are easily
obtained from
animal protein
 They can also be
obtained from the
proper combination
of plant foods
A healthful diet




includes 13 vitamins
Most of these vitamins function as coenzymes
Essential minerals are required for many body
functions
A sound diet supplies
 enough
raw materials to make all the macromolecules
we need
 the proper amounts of prefabricated essential
nutrients
 enough kilocalories to satisfy our energy needs
Diet can influence cardiovascular
disease and cancer