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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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ABSORPTION: the passage of digested nutrients from the gut
lumen into the blood or lymph, which distributes them through
the body.
ELIMINATION: the expulsion of indigestible residues from
the body.
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 esophagus 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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Hormones that Control Digestion:
Hormone
Released by What Part/
in response to what?
upper part of stomach/in
response to protein in the
stomach
Acts on What
Part?
Gastric juice
secreting cells at
top of stomach
SECRETIN
Small intestine/Acid
chyme from stomach
Pancreas and
parietal cells of
stomach
CCK
(Cholecysto
kinin)
Small intestine/Acid
chyme in stomach
Pancreas and
Liver (gall
bladder)
GASTRIN
What does it do?
Causes secretion of
gastric juices
Also increases the
churning action of the
stomach
Is inhibited by the
presence of HCl,
Secretin, and GIP –
negative feedback loop
Causes pancreas to
release NaHCO3 and
pancreatic enzymes
Also inhibits gastric acid
secretion by the
stomach
Causes liver to secrete
bile and pancreas to
secrete pancreatic juice.
Also acts as a hunger
supressant
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
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 9.2
9.2 Accessory Organs of Digestion
The accessory digestive organs are the pancreas, liver, and
gall bladder.
Figure 9.8 Liver, gall
bladder, and pancreas. a.
The liver makes bile, which
is stored in the gall bladder
and sent (black arrow) to
the small intestine by way
of the common bile duct.
The pancreas produces
digestive enzymes that are
sent (black arrows) to the
small intestine by way of
the pancreatic duct.
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UNIT B Chapter 9: Digestive System
The Pancreas
The pancreas is an organ that has both
endocrine and exocrine functions.
• Exocrine function of the pancreas
o Pancreatic cells produce pancreatic
juice (to neutralize stomach acid)
and digestive enzymes
• Endocrine function of the pancreas
o Pancreas secretes insulin and
glucagon, hormones that regulate
blood glucose (sugar) levels
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Section 9.2
UNIT B Chapter 9: Digestive System
Structure and Function of the Pancreas
The pancreas contains pancreatic islets (islets of
Langerhans), which are clusters of at least three types of
endocrine cells:
• Alpha cells: produce glucagon
• Beta cells: produce insulin
• Delta cells: produce somatostatin
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Section 9.2
UNIT B Chapter 9: Digestive System
Section 9.2
Structure and Function of the Pancreas
Insulin
• Hormone secreted when blood glucose level is high
• Stimulates the uptake of glucose by cells (liver, muscle,
adipose tissue) to lower blood glucose
Glucagon
• Hormone secreted when blood glucose level is low
• Stimulates the liver to break glycogen down into glucose to
increase blood glucose
• Stimulates adipose tissue to break fat down to glycerol and
fatty acids (to make glucose)
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UNIT B Chapter 9: Digestive System
The Liver
The liver is the largest gland in the body.
The liver has many functions, including:
•
•
•
•
Detoxifying blood
Making plasma proteins
Maintaining blood glucose levels
Producing bile, which contains bile salts
that emulsify fat in the small intestine
• Producing urea, a nitrogenous waste
product from the breakdown of amino
acids
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Section 9.2
UNIT B Chapter 9: Digestive System
Section 9.2
Structure and Function of the Liver
The liver contains about 100 000 lobules that serve as its
structural and functional units.
Three structures are located between the lobules:
• Bile duct: takes bile away from the liver
• Hepatic artery branch: brings oxygen-rich blood to the liver
• Hepatic portal vein: transports nutrients from the intestines
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UNIT B Chapter 9: Digestive System
Section 9.2
Figure 9.8 b. A hepatic lobule. The liver contains over 100 000 lobules, each lobule
composed of many cells that perform the various functions of the liver. They remove and
add materials to the blood and deposit bile in a duct.
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UNIT B Chapter 9: Digestive System
Figure 9.10 Hepatic
portal system. The
hepatic portal vein takes
the products of digestion
from the digestive system
to the liver, where they are
processed before entering
a hepatic vein.
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Section 9.2
UNIT B Chapter 9: Digestive System
The Gall Bladder
The gall bladder is a muscular sac
attached to the surface of the liver.
• Excess bile from the liver is stored
in the gall bladder
• Bile leaves the gall bladder and
proceeds to the duodenum via the
common bile duct
o Bile emulsifies fat to prepare it
for further breakdown by
digestive enzymes
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Section 9.2
UNIT B Chapter 9: Digestive System
Section 9.3
9.3 Digestive Enzymes
Digestive enzymes help break down the major components of
food: carbohydrates, proteins, nucleic acids, and fats.
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UNIT B Chapter 9: Digestive System
Section 9.3
Carbohydrate Digestion by Enzymes
The digestion of starch (a carbohydrate) begins in the mouth.
• Salivary amylase (produced by the salivary glands) digests
starch into maltose (a disaccharide)
• Pancreatic amylase (produced by the pancreas) and maltase
(produced by the small intestine) then convert maltose in the
small intestine to glucose (a monosaccharide). Glucose can be
absorbed by the small intestine.
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UNIT B Chapter 9: Digestive System
Figure 9.11 Digestion and
absorption of nutrients.
a. The breakdown of
carbohydrates, such as starch,
involves amylase enzymes.
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Section 9.3
UNIT B Chapter 9: Digestive System
Section 9.3
Carbohydrate Digestion by Enzymes
Other disaccharides, such as lactose, have their own enzyme
that digests them in the small intestine.
• Lactase is an enzyme that digests lactose, a sugar found in
milk.
o Individuals who have a lactase deficiency often have
symptoms of lactose intolerance (diarrhea, gas,
cramps) caused by the fermentation of non-digested
lactose by intestinal bacteria
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UNIT B Chapter 9: Digestive System
Section 9.3
Protein Digestion by Enzymes
The digestion of proteins begins in the stomach.
• Pepsin is an enzyme produced by gastric glands that acts on
proteins to produce peptides.
• Trypsin (produced by the pancreas) and peptidases (produced
in the small intestine) break down peptides into amino acids
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UNIT B Chapter 9: Digestive System
Figure 9.11 Digestion and
absorption of nutrients.
b. Protein digestion involves
the action of protease
enzymes.
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Section 9.3
UNIT B Chapter 9: Digestive System
Section 9.3
Fat Digestion by Enzymes
• Lipase (produced by the pancreas) acts in the small intestine
and digests fat molecules in the fat droplets after they have
been emulsified by bile salts
• Glycerol and fatty acids enter the cells of the villi, where they
are rejoined and repackaged as lipoprotein droplets
(chylomicrons) before entering the lacteals
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UNIT B Chapter 9: Digestive System
Figure 9.11 Digestion and
absorption of nutrients.
c. For fat digestion, bile salts
emulsify the fats so that lipase
enzymes can digest the
particles.
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Section 9.3
UNIT B Chapter 9: Digestive System
Section 9.3
Regulation of Digestive Enzymes
Enzymes function best at an optimum temperature and pH
that helps maintain the proper shape to fit their substrate.
• Since the digestive system is maintained at a constant
37ºC, enzymatic activity is largely controlled by pH
o The pH of the stomach is between 1 and 2 but can
increase to around 7.4 to 7.8 when sodium bicarbonate
in pancreatic juice is released from the pancreas
o This increase in pH occurs after chyme enters the
duodenum, and allows different digestive enzymes to
be active depending on the pH
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