Download The Digestive System

The Digestive System
Digestive System
Built around an alimentary canal (one-way
tube)
 Includes

 Mount,
pharynx, esophagus
 Stomach
 Small intestine
 Large intestine
 Salivary glands, pancreas, liver
The Digestive
System
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Mouth

Begins mechanical digestion with chewing
 Teeth
crush food to make a fine paste for
swallowing

Begins chemical digestion with saliva
 Digestive
enzymes begin breakdown of
carbohydrates into smaller molecules
Your Mouth
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Teeth
Anchored in jaw bones
 Protected by mineralized enamel
 Used to cut, tear, and crush food into
smaller fragments

Tooth
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Saliva
Secreted by salivary glands in mouth to
moisten food and make it easier to chew
 Controlled by nervous system
 Begins chemical digestion

 Enzyme
amylase breaks down starches and
releases sugars
 Lysozyme fights infection by digesting
bacteria that may enter mouth with food
Esophagus
Food (called a bolus) from mouth is swallowed
(forced from mouth into esophagus by combined
action of tongue and throat muscles)
 Bolus moved through esophagus by contractions
(peristalsis) of smooth muscles to the stomach
 Cardiac sphincter at the bottom of esophagus
prevents food in stomach from moving back into
esophagus

Peristalsis
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Stomach
Muscular sac that continues mechanical
and chemical digestion
 Three layers of smooth muscle thoroughly
churn and mix food to help with digestion

Stomach
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Chemical digestion in stomach
Glands in the stomach lining release
lubricants (to protect the stomach wall),
hydrochloric acid (HCl), and pepsin
(enzyme)
 Pepsin is activated by HCl to begin protein
digestion to break protein into smaller
pieces or amino acids
 Other enzymes are denatured by HCl and
stop carbohydrate digestion

Mechanical digestion in the stomach
Stomach muscles contract to churn and
mix stomach contents
 This makes chyme
 After 1-2 hours the pyloric sphincter opens
so chyme can pass into the small intestine

Small intestine
Chyme enters the first part of the small
intestine called the duodenum
 Digestive enzymes and fluids from the
pancreas and liver also enter at the
duodenum
 Most chemical digestion and absorption of
nutrients occurs in the small intestine

Pancreas
& Liver
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Effects of digestive enzymes
Salivary amylase
(mouth)
 Pepsin (stomach)

Amylase (sm. Int.
from pancreas)
 Trypsin (sm. Int. from
pancreas)
 Lipase (sm. Int. from
pancreas)






Breaks down starch
and disaccharides
Breaks down proteins
into large peptides
Continues breakdown
of starch
Continues breakdown
of protein
Breaks down fat
Effects of digestive enzymes

Maltase, sucrase,
lactase (sm. Int.)

Peptidase (sm. Int.)
Breaks down
remaining
disaccharides into
single sugars
 Breaks down
dipeptides into amino
acids

Accessory structures of digestion

Pancreas (behind the stomach) has 3 roles
 Releases
hormones to regulate blood sugar
levels
 Releases enzymes that break down
carbohydrates, proteins, lipids, nucleic acids
 Produces sodium bicarbonate that neutralizes
stomach acids so the enzymes can function
(without neutralization, enzymes denature
and don’t function properly)
Accessory structures cont.
Liver-(above and to the left of the
stomach) produces bile loaded with lipids
and salts
 Bile dissolves fats in fatty foods
 Bile is stored in the gallbladder

Absorption in the small intestine
Last 2 parts of the small intestine are the
jejunum and ileum, together are 6 meters long
 When chyme enters these it is a mixture of
medium and small nutrient molecules (most
chemical digestion is competed in the
duodenum)
 Lining of the small intestine is covered with villi
(fingerlike projections) each of which is covered
with microvilli, both are needed to increase the
surface area for absorption

Villi
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Absorption in the small int. cont.
Cells of small intestine absorb nutrients
 Products of carbohydrate and protein digestion
are absorbed into capillaries in villi
 Undigested fat and fatty acids are absorbed by
lymph vessels
 When food leaves small intestine and enters the
large intestine it is nutrient free and contains
only water, cellulose, and undigestible
substances

The large intestine
Also called the colon
 Primary function is to remove water from
undigestible material
 Water is absorbed quickly through the wall
of the colon
 Colonies of bacteria produce useable
compounds such as vitamin K
 Concentrated waste that remains passes
through the rectum to be excreted
