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Human Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 8 Part 1
Digestive
System and
Nutrition
Lecture Outline
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Points to Ponder
1. What are the digestive system structures and their
functions?
2. Where does carbohydrate, protein, and fat digestion and
absorption occur?
3. What are proteins, lipids, carbohydrates, minerals and
vitamins needed for?
4. What is an essential vs. a nonessential nutrient?
5. What are the 3 accessory organs of digestion?
6. What is obesity and why is it a problem?
7. What is LDL and HDL?
8. What are the components of a healthy diet?
9. Name and explain 4 eating disorders.
8.1 Overview of digestion
What are the main steps in the digestive
process?

Ingestion: intake of food by the mouth.

Digestion: mechanically or chemically breaking down
food into their subunits

Movement: food must be moved along the GI tract in
order to fulfill all functions.

Absorption: movement of nutrients across the GI tract
wall to be delivered to cell via the blood.

Elimination: removal of indigestible molecules.
8.1 Overview of digestion
What are the 4 major layers that make up
the wall of the GI tract?
 Mucosa: innermost layer that produces mucus that
protects the lining and also produces digestive enzymes.
 Sub mucosa: 2nd layer of loose connective tissue that
contains blood vessels, lymphatic vessels, and nerves.
 Muscularis: 3rd layer made of 2 layers of smooth muscle
that move food along the GI tract.
 Serosa: outer lining that is part of the peritoneum.
8.1 Overview of digestion
Visualizing the layers of the GI
tract
Mucosa
nerve lymph
supply vessel
inner mucous membrane
layer modified according
to the digestive organ
Submucosa
broad band of loose
connective tissue that
contains nerves, blood,
and lymphatic vessels
Lumen
central space containing
food being digested
Serosa
thin, outermost tissue
that is the visceral
peritoneum
Muscularis
two layers of
smooth muscle
artery
vein
8.1 Overview of digestion
An overview of the digestive system
Accessory organs
Salivary gland
secrete saliva: contains digestive enzyme
for carbohydrates
Digestive tract organs
Mouth
teeth chew food; tongue tastes and
pushes food for chewing and swallowing
Pharynx
passageway where food is swallowed
Esophagus
passageway where peristalsis pushes
food to stomach
Liver
major metabolic organ:
processes and stores nutrients;
produces bile for emulsification of fats
Gallbladder
Stomach
secretes acid and digestive enzyme
for protein; churns, mixing food with
secretions, and sends chyme to
small intestine
stores bile from liver; sends it to
the small intestine
Small intestine
Pancreas
produces pancreatic juice: contains
digestive
enzymes, and sends it to the small intestine;
produces insulin and secretes it into the
blood after eating
mixes chyme with digestive enzymes
for final breakdown; absorbs nutrient
molecules into body; secretes
digestive hormones into blood
Large intestine
absorbs water and salt to form feces
Rectum
stores and regulates elimination
of feces
Anus
8.1 Overview of digestion
What is the pathway that food follows?
1.
2.
3.
4.
5.
6.
7.
8.
Mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Rectum
Anus
8.2 First part of the digestive tract
The mouth
 3 pairs of salivary glands secrete salivary amylase that
begins carbohydrate digestion.
 Tonsils at the back of the mouth and other lymphatic
tissues are important in fighting disease.
 Contains teeth that begin the mechanical breakdown of
food.
 The tongue is covered in taste buds and also assists in the
mechanical breakdown and movement of food.
 The tongue forms a bolus(mass of chewed food) and
moves it toward the pharynx.
8.2 First part of the digestive tract
Anatomy of the mouth
hard palate
soft palate
uvula
tonsil
molars (3)
premolars (2)
canine (1)
incisors (2)
8.2 First part of the digestive tract
Teeth
 32 in adults used for mechanical breakdown of food
(20 deciduous teeth in babies).
 Each tooth is made of a crown and a root.
 A hard covering called enamel and dentin covers the
crown with an inner pulp area with nerves and blood
vessels.
 Dental caries (cavities) occur when bacteria metabolize
sugars and produce acids, so limiting sugar intake and
brushing teeth reduces these.
8.2 First part of the digestive tract
Anatomy of a tooth
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enamel
crown
dentin
pulp
gum
jawbone
root canal
root
periodontal
membrane
cementum
8.2 First part of the digestive tract
The pharynx and esophagus
• Pharynx: is a cavity between the mouth and
esophagus that serves as a passageway
for food (and air).
• Esophagus: is a long, muscular tube that
carries food to the stomach.
8.2 First part of the digestive tract
How do we swallow food?
• Voluntary phase:
– The beginning of food being swallowed from the mouth
into the pharynx.
• Involuntary phase:
– Once the food is in the pharynx swallowing becomes a
reflex action perform automatically.
– The epiglottis covers the voice box to make sure food
is routed into the esophagus.
– Food moves down the esophagus through peristalsis
(rhythmic contraction).
8.2 First part of the digestive tract
How do we swallow food?
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hard palate
food bolus
nasopharynx
Soft palate closes
off nasopharynx.
soft
palate
bolus
peristaltic
wave
epiglottis
glottis
Esophagus
is closed.
trachea
(airway)
open
a. Swallowing
Larynx rises and
epiglottis covers
glottis.
esophagus
opens
Lower gastroesophageal
esophagus spincter relaxes and
food enters stomach.
b. Peristalsis
stomach
8.2 First part of the digestive tract
Health focus: Heartburn
 Occurs when acids from the stomach pass into the
esophagus (acid reflux).
 Burning sensation in the esophagus.
 Chronic heartburn is called gastro esophageal reflux
disease (GERD).
 Tips for decreasing heartburn:
 Avoid high fat meals.
 Don’t overeat.
 Eat several small meals rather than the standard 3 larger meals
each day.
 Exercise lightly.
The stomach
• The stomach is an enlarged part of alimentary
canal and receives food from esophagus.
• The stomach like J shape with a thick wall.
• Mucous membrane lining stomach contains
folds called rugae.
• Folds disappear as stomach fills with food.
o Its structure is divided
into 4 main region:
o Funds: bulge
o Body : largest region
o Antrum:
o Pylorus : pyloric
sphincters
o It has two sphincters:
 Cardiac sphincter.
 Pyloric sphincter.
Function of stomach
store food, start digestion of proteins
controls movement of chyme into the small intestine.
 Mixing of the food in the stomach produces Chyme.
 Mixing movement mainly occurs in the Antrum.
 Controlled emptying of gastric content into
duodenum.
HCl Secretion gives the stomach a pH of 2 which
activates pepsin and helps kill bacteria found in food.
8.3 The stomach and small intestine
Anatomy of the stomach
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esophagus
lower gastroesophageal
sphincter
pyloric
sphincter
Muscularis layer
has three layers
of muscle.
Mucosa layer
has rugae.
c. Gastric pits in mucosa
gastric pit
SEM 3,260x
lower gastroesophageal
sphincter
a. Stomach
gastric pit
gastric gland
cells that secrete
gastric juice
b. Gastric glands
pyloric
sphincter
d. How the stomach empties
c: © Dr. Fred Hossler/Visuals Unlimited
8.3 The stomach and small intestine
The small intestine
 Averages 6m (18 ft) in length
 Enzymes secreted by the pancreas into the small intestine
digest carbohydrates, proteins, and fats.
 Bile is secreted by the gallbladder into the small intestine to
emulsify fats.
 Digested food is absorbed through large surface area
created by numerous villi (finger-like projections) and
microvilli.
 Amino acids and sugars enter the capillaries while fatty
acids and glycerol enter the lacteals (small lymph vessels).
8.3 The stomach and small intestine
Anatomy of the small intestine
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Small intestine
Section of intestinal wall
villus
lumen
lacteal
blood
capillaries
villus
microvilli
goblet
cell
lymph
nodule
venule
lymphatic
vessel
Villi
arteriole
(villi): © Manfred Kage/Peter Arnold/Photolibrary; (microvilli):Reprinted from Medical Cell Biology, Charles Flickinger, copyright 1979, with permission from Elsevier.
8.3 The stomach and small intestine
How are nutrients digested and transported
out of the small intestine?
carbohydrate
protein
pancreatic amylase
+
bile salts fat globules
trypsin
emulsification
droplets
peptides
maltase
cell of
intestinal
villus
peptidase
lipase
glucose
monoglycerides
and free
fatty acids
amino acids
pH = basic
pH = basic
pH = basic
blood capillary
a. Carbohydrate digestion
blood capillary
b. Protein digestion
lymphatic
capillary
c. Fat digestion
chylomicron
8.3 The stomach and small intestine
What are the major digestive enzymes?
8.4 Three accessory organs and regulation of secretions
The three accessory organs
• Pancreas
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bile canals
• Liver
bile
branch of
hepatic
artery
central
vein
common hepatic duct
• Gallbladder
pancreatic duct
pancreas
pancreatic
juice
gallbladder
b.
common bile duct
duodenum
a.
bile duct
branch of
hepatic
portal vein
8.4 The accessory organs and regulation of secretions
The pancreas
• Fished-shaped spongy organ behind the stomach
• Functions of the pancreas:
1. Secretes enzymes into the small intestine
• Trypsin: digests proteins.
• Lipase: digests fats.
• Pancreatic amylase: digests carbohydrates.
2. Secretes bicarbonate into the small intestine to
neutralize stomach acids
3. Secretes insulin into the blood to keep blood sugar
levels under control
8.4 The accessory organs and regulation of secretions
The liver and gallbladder
• Large metabolic organ that lies under the diaphragm and is
made of 100,000 lobules.
Function:
1. Filters blood from the GI tract thus acting to remove poisons
and detoxify the blood.
2. Removes iron, vitamins A, D, E, K and B12 from the blood and
stores them.
3. Stores glucose as glycogen and breaks it down to help retain
blood glucose levels.
4. Makes plasma proteins and helps regulate cholesterol levels by
making bile salts.
5. Makes bile that is then stored in the gallbladder to be secreted
into the small intestine to emulsify fats.
6. Breaks down hemoglobin.
8.4 The accessory organs and regulation of secretions
Liver disorders
• Hepatitis:
– Inflammation of the liver
– Caused by Hepatitis A, B, and C
– This can lead to liver damage, cancer and/or death
• Cirrhosis:
– The liver becomes fatty and eventually this tissue is
replaced by fibrous scar tissue.
– Seen in alcoholics and obese people.
– This can lead to liver failure in which the liver
cannot regenerate as fast as it is being damaged.
8.4 The accessory organs and regulation of secretions
How do hormones control digestive
gland secretions?
gallbladder
liver
stomach
pancreas
duodenum
blood vessel
secretin
CCK
gastrin
8.5 The large intestine and defecation
The large intestine
• Includes the cecum, colon, rectum
and anal canal
• Larger in diameter but shorter than
the small intestine
• The cecum has a projection known
as the appendix that may play a
role in fighting infections
• Functions to:
transverse colon
ascending
colon
small
intestine
descending
colon
1. Absorb water to prevent
dehydration.
2. Absorbs vitamins (B complex and
cecum
orifice of
appendix
vermiform
appendix
rectum
K) produced by intestinal flora.
3. Forms and rids the body of feces
through the anus.
internal anal sphincter
anal canal
sigmoid colon
external anal sphincter
anus
8.5 The large intestine and defecation
Disorders of the colon and rectum
1. Diarrhea: increased peristalsis and water is not reabsorbed due
to either an infection or nervous stimulation.
1. Constipation: condition when feces are dry and hard that may be
controlled with water and fiber.
1. Hemorrhoids: enlarged and inflamed blood vessels of the anus
due to chronic constipation, pregnancy, aging, and anal
intercourse.
8.6 Nutrition and weight control
To understand weight and nutrition we first
have to understand nutrients
• Nutrients are a component of food that is needed
to perform a physiological body function.
• Nutrients include:
Carbohydrates
Proteins
Lipids
Minerals
Vitamins