Download Digestion in the Small Intestine

What is the function
of your Digestive
System?
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 23.1
Digestive System
Brainstorm what each of the following
organs/structures does regarding digestion:



The alimentary canal or gastrointestinal (GI) tract
digests and absorbs food
Alimentary canal consists of – mouth, pharynx,
esophagus, stomach, small intestine, and large
intestine
Accessory digestive organs – teeth, tongue,
salivary glands, liver, gallbladder and pancreas
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Digestive Process

The GI tract is a “disassembly” line


Nutrients become more available to the body in each step
(example: proteins broken into amino acids)
There are six essential activities:
1. Ingestion
2. propulsion by peristalsis
3. mechanical digestion
4. Chemical digestion
5. absorption
6. defecation
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 23.2
Peritoneum and Peritoneal Cavity

Peritoneum – serous (moist) membrane of the
abdominal cavity
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Peritoneal Cavity

Mesentery – double layer
of peritoneum (connective
tissue) that provides:

Vascular and nerve
supplies to the viscera
(organs)

Hold digestive organs in
place and store fat
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Histology of the Alimentary Canal
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Figure 23.6
Oral Cavity and Pharynx: Anterior View
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Figure 23.7b
Salivary Glands
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Figure 23.9a
Salivary Glands

Produce and secrete saliva that:
1. Cleanses the mouth
2. Moistens and dissolves food chemicals
3. Aids in bolus formation
4. Contains digestive enzymes that break down
starch (sugar)
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Digestive Processes in the Mouth

Food is ingested

Mechanical digestion begins (chewing)

Propulsion is initiated by swallowing

The pharynx and esophagus serve as conduits to
pass food from the mouth to the stomach
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Bolus of food
Tongue
Uvula
Pharynx
Epiglottis
Bolus
Epiglottis
Glottis
Esophagus
Trachea
(a) Upper esophageal
sphincter contracted
Bolus
(b) Upper esophageal
sphincter relaxed
Relaxed
muscles
Bolus of
food
Longitudinal
muscles
contract,
shortening
passageway
ahead of bolus
Gastroesophageal
sphincter closed
(d)
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(c) Upper esophageal
sphincter contracted
Circular muscles
contract,
constricting
passageway
and pushing
bolus down
Relaxed
muscles
Gastroesophageal
sphincter open
Stomach
(e)
Figure 23.13
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Figure 23.14a
Microscopic Anatomy of the Stomach

Epithelial (tissue) lining is composed of:


Goblet cells that produce a coat of alkaline mucus
Gastric pits contain gastric glands that secrete
gastric juice (acidic) and mucus
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Microscopic Anatomy of the Stomach
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Figure 23.15a
Did You Know That?
Your stomach acid is so
strong that you grow a new
stomach lining every three
days?

You can eat standing on
your head, because your
esophagus pushes food into
your stomach, even
uphill?


Your body makes about one
liter of saliva every day?
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Digestion in the Stomach

The stomach:

Holds ingested food

Degrades this food both physically and chemically

Delivers chyme to the small intestine

Enzymatically digests proteins with pepsin
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Small Intestine: Gross Anatomy

Has three subdivisions:
duodenum, jejunum, and
ileum

Structural modifications
increase surface area:

Villi

Microvilli
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Small Intestine: Microscopic Anatomy
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Figure 23.21
Functions of Small Intestine:
1) Chemical
digestion & 2) Absorption of nutrients into blood

Intestinal juice secreted by intestinal glands in
response to distension (food entering the area)

Slightly alkaline and isotonic with blood plasma

The majority of chemical digestion occurs in the
duodenum

Absorption occurs in the jejunum & ileum
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Duodenum linked to Pancreas, Gallbladder & Liver
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Figure 23.20
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Liver (largest organ in body) & Gallbladder

Bile is produced in the liver and the Gallbladder stores
bile
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Gallbladder and Associated Ducts
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Figure 23.20
Composition of Bile

A yellow-green, alkaline solution containing bile
salts, bile pigments, cholesterol, neutral fats,
phospholipids, and electrolytes

Bile salts are cholesterol derivatives that:

Emulsify fat

Facilitate fat and cholesterol absorption

Help solubilize cholesterol
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Pancreas

Location

Lies deep to the greater curvature of the stomach

The head is encircled by the duodenum and the tail abuts
the spleen
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Pancreas

Exocrine function


Secretes pancreatic juice which breaks down all
categories of foodstuff
The pancreas also has an endocrine function –
release of insulin and glucagon (hormones)
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Pancreatic Activation
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Figure 23.27
Digestion in the Small Intestine

As chyme enters the duodenum:

Carbohydrates and proteins are only partially
digested

No fat digestion has taken place
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Digestion in the Small Intestine

Digestion continues in the small intestine

Chyme is released slowly into the duodenum

Secretions from the Gallbladder and Pancreas
aid in the breakdown of chyme (Duodenum)

Virtually all nutrient absorption takes place in the
small intestine
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Motility in the Small Intestine

After nutrients have been absorbed:

Peristalsis begins with each wave starting distal to
the previous

Meal remnants, bacteria, mucosal cells, and
debris are moved into the large intestine
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Large Intestine

Has three unique features:
1. three bands of smooth muscle
2. pocketlike sacs
3. fat-filled pouches
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Large Intestine

Is subdivided into the cecum, appendix, colon,
rectum, and anal canal

The saclike cecum:

Lies below the ileocecal valve

Contains a wormlike vermiform appendix
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Large Intestine
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Figure 23.29a
Valves and Sphincters of the Rectum and
Anus
 Three valves of the rectum stop feces from being
passed with gas


The anus has two sphincters:

Internal anal sphincter composed of smooth muscle

External anal sphincter composed of skeletal
muscle
These sphincters are closed except during
defecation
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Bacterial Flora


The bacterial flora of the large intestine consist of:

Bacteria surviving the small intestine that enter the
cecum and

Those entering via the anus
These bacteria:

Colonize the colon

Ferment indigestible carbohydrates

Release irritating acids and gases (flatus)

Synthesize B complex vitamins and vitamin K
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Functions of the Large Intestine

no further digestion takes place
1. Vitamins, water, and electrolytes are
reabsorbed
2. Its major function is propulsion of fecal material
toward the anus

Though essential for comfort, the colon is not
essential for life
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Defecation


Distension of rectal walls caused by feces:

Stimulates contraction of the rectal walls

Relaxes the internal anal sphincter
Voluntary signals stimulate relaxation of the
external anal sphincter and defecation occurs
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Developmental Aspects

Digestive system has few problems until the onset
of old age

During old age the GI tract activity declines,
absorption is less efficient, and peristalsis is
slowed
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Cancer

Stomach and colon cancers rarely have early signs
or symptoms

Metastasized colon cancers frequently cause
secondary liver cancer
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Cancer

Colon cancer is the 2nd largest cause of cancer
deaths in males (lung cancer is 1st)

Forms from benign mucosal tumors called polyps
whose formation increases with age

Regular colon examination should be done for all
those over 50
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

http://www.mic.stacken.kth.se/Diseases/C06.html
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings