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Lecture 18
The Digestive System
Food for Energy and Growth
 Food provides animals with energy and essential nutrients for growth
A healthy diet contains more
carbohydrates than fats
It also contains a lot of
proteins
The pyramid of nutrition
Who Is Overweight?
 The body mass index is used to determine who’s overweight
BMI =
body weight in kg
(height in m)2
=
(body weight in lbs) X 703
(height in in)2
Obesity
Essential Substances for Growth
 Many vertebrates are unable to manufacture one or more of the 20 amino
acids needed to make proteins
 Humans are unable to synthesize 8 amino acids
 These essential amino acids must be obtained through food
 In addition, all vertebrates cannot synthesize certain polyunsaturated fats
 Trace elements
 Minerals required in small amounts
 Iodine, cobalt, zinc, molybdenum, manganese
 Vitamins
 Essential organic substances required in small amounts
 Humans require at least 13 different vitamins
 Vitamin C (ascorbic acid) if not in diet, the disease scurvy will develop
Evolution of the Digestive System
Digestive System Overview
 Consists of tubular gastrointestinal
tract and accessory digestive
organs
 Mouth and pharynx
 Esophagus – Delivers food to
the stomach
 Stomach – Some preliminary
digestion
 Small intestine – Digestion and
absorption
 Large intestine – Water and
mineral absorption
 Rectum – Waste excretion
General Structure of Digestive Organs
 The gastrointestinal tract has a characteristic layered structure
Mucosa
Epithelium
Submucosa
Connective tissue
Muscularis
Smooth muscles
Serosa
Connective tissue
Breaking Down Food in the Mouth
 Many vertebrates have teeth that are used for chewing (mastication)
Carnivores have
pointed teeth
adapted for cutting
and shearing
Herbivores have
large flat teeth well
suited for grinding
plant cellulose
Omnivores have
carnivorous teeth in
front and herbivorous
teeth in the back
The Structure of a Tooth
Tooth and Gum Disease: Periodontitis
 Dental caries – gradual demineralization of enamel and dentin by
bacterial action




Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth
Acid produced by the bacteria in the plaque dissolves calcium salts
Without these salts, organic matter is digested by proteolytic enzymes
Daily flossing and brushing help prevent caries by removing forming plaque
 Gingivitis – as plaque accumulates, it calcifies and forms calculus, or
tartar
 Accumulation of calculus:
 Disrupts the seal between the gingivae and the teeth
 Puts the gums at risk for infection
 Periodontitis – serious gum disease resulting from an immune
response
 Immune system attacks intruders as well as body tissues, carving pockets
around the teeth and dissolving bone
Tongue

Superior surface bears three types of papillae
 Filiform – give the tongue roughness and provide friction
 Fungiform – scattered widely over the tongue and give it a reddish hue
 Circumvallate – V-shaped row in back of tongue

Sulcus terminalis – groove that separates the tongue into two areas:
 Anterior 2/3 residing in the oral cavity
 Posterior third residing in the oropharynx
In The Mouth
 The tongue mixes food with a
solution called saliva
 Saliva moistens and
lubricates food and
contains amylase which
initiates breakdown of
starch into maltose
 The secretions of the salivary
glands are controlled by the
nervous system
 A continuous secretion of
about 0.5 milliliters per
minute keeps the mouth
moist
 The presence of food in
the mouth triggers an
increased rate of secretion
Swallowing
 Prior to swallowing, the tongue moves food to the back of the mouth
 The soft palate elevates, pushing against back wall of pharynx
 This stimulates neurons to send impulses to the swallowing center in
the brain
 Muscles contract and raise the larynx
 The glottis is pushed against the epiglottis which keeps food out of the
respiratory tract, and into the esophagus
The Esophagus and Stomach
 The esophagus is a muscular tube
that connects the pharynx to the
stomach
The swallowing center stimulates
successive waves of contraction
Peristalsis moves food along the
esophagus to the stomach
The stomach and gastric glands

Movement of food from
esophagus into stomach is
controlled by a ring of
smooth muscle, the
sphincter
 Humans lack a true
sphincter and thus, can
regurgitate

The stomach is a saclike
portion of the tract with a
convoluted surface
enabling expansion
 It contains an extra layer
of smooth muscles for
mixing food with gastric
juices
 Two kinds of secretory
cells
 Parietal cells –
Secrete hydrochloric
acid
 Chief cells – Secrete
pepsinogen

The human stomach produces about 2 liters of
HCl and other gastric secretions every day
 This gastric juice has a pH of ~ 2
 It kills most bacteria ingested with food and
also denatures food proteins
 The mixture of partially digested food and
gastric juice is termed chyme

Chyme leaves the stomach to the small
intestine through the pyloric sphincter
Ulcers
 The hormone gastrin regulates the synthesis of HCl
 Overproduction of gastric acid can occasionally eat a hole
through the stomach wall
 These gastric ulcers are rare
 Susceptibility increases when mucosal barriers are
weakened by Helicobacter pylori infection
 Over 90% of gastrointestinal ulcers are duodenal ulcers
 Caused by too much chyme in the small intestine
The Small Intestine

The small intestine is the body’s
true digestive vat
 It breaks down large
molecules into smaller ones
which are then absorbed into
the bloodstream

The small intestine is ~ 6 m long
 The first 25 cm (~ 4%)
constitute the duodenum

The duodenum is the actual site
of digestion
 The pancreas secretes
digestive enzymes into it
 The liver secretes bile salts
into it, to make fats easier to
digest

The ileum is the rest of the small intestine (~
96%)
 It is devoted to absorption
 The lining is covered with finger-like
projections called villi
 Each cell covering a villus has cytoplasmic
projections called microvilli which increase
the absorptive surface
Anatomy of the Small Intestine
The Large Intestine
 The large intestine, or colon is only 1 meter long
 But has a larger diameter than the small intestine
 No digestion and little absorption take place in the large intestine
 Its primary function is to act as a refuse dump by collecting and compacting
solid wastes
 Feces pass through the rectum as a result of peristalsis and leave the body
through the anus
Accessory Digestive Organs

Pancreas
 Functions as both an exocrine & endocrine
gland
 Exocrine: Cell clusters called acini secrete
 Trypsin and chymotrypsin which digest
proteins
 Amylase which digests starch
 Lipase which digests fats
 Bicarbonate which neutralizes HCl in
chyme
 Endocrine: Cell clusters called Islets of
Langerhans secrete
 Insulin and glucagon which regulate sugar
levels in blood

Liver
 Largest internal organ of the body
 Its main exocrine secretion is bile
 Aids in the digestion of fats in the
duodenum
 Chemically modifies substances absorbed in
the gastrointestinal tract
 Converts poisons into less toxic forms
 Produces most of the proteins found in plasma

Gall bladder
 Stores and concentrates bile
 Delivers it to the duodenum
via the bile duct
Microscopic Anatomy of the Liver

Hepatocytes’
functions include:
 Production of bile
 Processing
bloodborne
nutrients
 Storage of fatsoluble vitamins
 Detoxification

Secreted bile flows
between hepatocytes
toward the bile ducts
in the portal triads


Liver sinusoids – enlarged, leaky
capillaries located between hepatic
plates
Kupffer cells – hepatic macrophages
found in liver sinusoids
Homeostasis
 Homeostasis is the dynamic constancy of the internal
environment
 Conditions fluctuate continuously within narrow limits
 Homeostasis is essential for life
Play
Hormones & Gastric Secretion
Regulating Blood Glucose
 Blood glucose levels are monitored by the Islets of Langerhans in the
pancreas
When levels are high, insulin is
released
When levels are low, glucagon
is released