Download Chapter 9: Digestive System

UNIT B: Human Body Systems
Chapter 8: Human Organization
Chapter 9: Digestive System: Section 9.1
Chapter 10: Circulatory System and
Lymphatic System
Chapter 11: Respiratory System
Chapter 12: Nervous System
Chapter 13: Urinary System
Chapter 14: Reproductive System
UNIT B Chapter 9: Digestive System
Chapter 9: Digestive System
In this chapter, you will learn
about the structure and function
of the digestive system.
What is our current understanding
of the structure and function of the
digestive system, including the
stomach?
Alexis St. Martin was an unexpected live test subject
for Dr. William Beaumont’s research on how the
digestive system works.
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Which substances produced by the
stomach and other organs help with
the chemical breakdown of foods?
UNIT B Chapter 9: Digestive System
Section 9.1
9.1 The Digestive Tract
The digestive system is involved in the ingestion and
digestion of food and elimination of indigestible material.
• Digestion takes place within the digestive tract, which
begins with the mouth and ends with the anus.
• Digestion involves mechanical and chemical digestion.
o Mechanical digestion: chewing of food, and churning
and mixing of food in the stomach
o Chemical digestion: enzymes break macromolecules
down into small organic molecules
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UNIT B Chapter 9: Digestive System
Figure 9.1 The human
digestive tract. The upper
part of the tract includes the
mouth, pharynx, esophagus,
stomach, and small intestine.
The large intestine consists
of the cecum, the colon
(ascending, transverse,
descending, and sigmoid
colons), the rectum, and the
anus. Note also the location
of the accessory organs of
digestion: the pancreas, the
liver, and the gall bladder.
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Section 9.1
UNIT B Chapter 9: Digestive System
Section 9.1
The Mouth
The mouth takes food into the body.
• Teeth: involved in chewing food
• Tongue: composed of skeletal muscle and involved in forming
the bolus (a mass of food that is ready for swallowing)
• Roof of the mouth: composed of a hard palate and a soft palate;
prevents ingested food from entering the nasal cavity
• Tonsils: contain lymphoid tissue that protect against infections
• Salivary glands: produce saliva to keep the mouth moist; saliva
contains an enzyme that digests starch
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UNIT B Chapter 9: Digestive System
Section 9.1
The Pharynx
The pharynx is a passageway that
receives air from the nasal cavities
and food from the mouth.
Swallowing (a reflex action) occurs in the pharynx.
• The soft palate moves back to close off the nasopharynx
• The trachea moves up under the epiglottis to cover the glottis
(the opening to the larynx (voice box))
• During swallowing, food enters the epiglottis because the air
passages are blocked
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UNIT B Chapter 9: Digestive System
Figure 9.2 Swallowing.
When food is swallowed,
the soft palate closes off
the nasopharynx and the
epiglottis covers the
glottis, forcing the bolus to
pass down the esophagus.
Therefore, a person does
not breathe while
swallowing.
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Section 9.1
UNIT B Chapter 9: Digestive System
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Section 9.1
UNIT B Chapter 9: Digestive System
The Esophagus
The esophagus is a long
muscular tube that moves food
from the mouth to the stomach by
peristalsis.
• Peristalsis (rhythmic muscular
contractions) pushes food
along the esophagus and
through the digestive tract to
the stomach.
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Section 9.1
UNIT B Chapter 9: Digestive System
Section 9.1
The Stomach
The stomach is an organ that receives food from the
esophagus, mechanically and chemically digests food, and
moves food into the small intestine.
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UNIT B Chapter 9: Digestive System
Section 9.1
Structure and Function of the Stomach
The human stomach has thick walls with folds (called rugae)
that allow it to expand and fill with food.
• Lining of the stomach: has gastric glands that secrete gastric
juice containing mucus and digestive enzymes
• Wall of the stomach:
o Three muscle layers (longitudinal, circular, oblique)
o Responsible for moving and churning food (mechanical
digestion), and mixing food with gastric juice to break it
down (chemical digestion)
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UNIT B Chapter 9: Digestive System
Section 9.1
• Alcohol and other liquids are absorbed in the stomach,
but most solid food is not.
• When food leaves the stomach, it is a thick, soupy
liquid called chyme.
o Chyme enters the small intestine by way of the
pyloric sphincter.
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UNIT B Chapter 9: Digestive System
Figure 9.3 Anatomy of
the stomach. a. The
stomach has a thick wall
with folds that allow it to
expand and fill with food.
b. The mucosa contains
gastric glands, which
secrete gastric juice
containing mucus and
digestive enzymes.
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Section 9.1
UNIT B Chapter 9: Digestive System
The Small Intestine
The small intestine receives chyme from the stomach and
completes the digestion of food. Macromolecules are broken
down into nutrients, which are absorbed in the small intestine
and pass into the blood.
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Section 9.1
UNIT B Chapter 9: Digestive System
Section 9.1
Structure and Function of the Small Intestine
The small intestine is composed of three parts.
• Duodenum: upper part of the small intestine
o Contains a bile duct that delivers bile from the liver and
pancreatic juice from the pancreas
− Enzymes in pancreatic juice complete food digestion
• Jejunum: middle part of small intestine
• Ileum: lower part of small intestine
o Contains lymphoid tissues involved in immune response
to intestinal pathogens
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UNIT B Chapter 9: Digestive System
Section 9.1
Structure and Function of the Small Intestine
The wall of the small intestine contains villi, which increase the
surface area to improve the absorption of nutrients.
• Villi: fingerlike projections that contain microvilli
o Microvilli increase the surface area of the villus for the
absorption of nutrients
o Each villus contains blood capillaries and a small
lymphatic capillary called a lacteal
o Nutrients are absorbed into the blood capillaries and the
lacteals, which carry them to body cells
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UNIT B Chapter 9: Digestive System
Section 9.1
Figure 9.4 Anatomy of the small intestine. The wall of the small intestine has folds that bear
fingerlike projections called villi. Microvilli, which project from the villi, absorb the products of
digestion into the blood capillaries and the lacteals of the villi.
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UNIT B Chapter 9: Digestive System
Section 9.1
Regulation of Digestive Secretions
Digestive secretions are controlled by the nervous system and
hormones.
After eating a meal:
• The stomach produces the hormone gastrin
o Gastrin: stimulates the gastric glands to secrete more
gastric juice
• The duodenal wall produces the hormones secretin and CCK
o Secretin and CCK: stimulate the pancreas to secrete
pancreatic juice and the gall bladder to secrete bile
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UNIT B Chapter 9: Digestive System
Figure 9.5 Hormonal control
of digestive gland secretions.
Gastrin (blue), produced by the
lower part of the stomach,
enters the bloodstream and
thereafter stimulates the
stomach to produce more
digestive juices. Secretin
(green) and CCK (purple),
produced by the duodenal wall,
stimulate the pancreas to
secrete its digestive juices and
the gall bladder to release bile.
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Section 9.1
UNIT B Chapter 9: Digestive System
Section 9.1
The Large Intestine
The large intestine absorbs water, salts, and some vitamins. It
also stores indigestible material until it is eliminated as feces.
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UNIT B Chapter 9: Digestive System
Section 9.1
Structure and Function of the Large Intestine
The large intestine includes the
cecum, colon, rectum, and anal
canal.
• Cecum: a small pouch that
forms the first part of the
large intestine
o Contains the appendix or
vermiform appendix,
which may play a role in
fighting infection
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Figure 9.6 Junction of the small intestine
and the large intestine. The cecum is the
blind end of the large intestine. The appendix
is attached to the cecum.
UNIT B Chapter 9: Digestive System
Section 9.1
Structure and Function of the Large Intestine
• Colon: includes the ascending, transverse, descending, and
sigmoid colon
• Rectum: the last part of the large intestine; opens at the anus
• Anus: rectum opening; site of defecation (expulsion of feces)
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UNIT B Chapter 9: Digestive System
Defecation Reflex
• When feces are forced
into the rectum by
peristalsis, a defecation
reflex occurs
Figure 9.7 Defecation
reflex. The accumulation of
feces in the rectum causes it
to stretch, which initiates a
reflex action resulting in
rectal contraction and
expulsion of the fecal
material.
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Section 9.1
UNIT B Chapter 9: Digestive System
Section 9.1
Defecation Reflex
• Eliminating feces from the body is a way the digestive system
maintains homeostasis
• Feces are about three-quarters water and one-quarter solids
o Solids: bacteria, fibre, and other indigestible materials
o Bacteria break down some indigestible material, and
produce some vitamins that our bodies can absorb
o Water that is unsafe for drinking has a high number of
coliform (intestinal) bacteria, indicating that a significant
amount of feces has entered the water
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UNIT B Chapter 9: Digestive System
Section 9.1
Check Your Progress
1. List the parts of the human digestive tract that food
passes through, and note whether chemical and/or
mechanical digestion occurs in each.
2. Compare the major functions of the small intestine and
the large intestine.
3. Discuss strategies that might help obese people lose
weight by inhibiting the secretion and/or activity of
certain hormones.
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UNIT B Chapter 9: Digestive System
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Section 9.1
UNIT B Chapter 9: Digestive System
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Section 9.1