Download Functions of the Digestive System

Chapter 16
The Digestive System
Organs of the Digestive System
• Two main divisions:
1) Alimentary canal (digestive tract) –
continuous coiled hollow tube
2) Accessory digestive organs – salivary
glands, gallbladder, liver, pancreas
Organs of the Alimentary Canal
1) Mouth
2) Pharynx
3) Esophagus
4) Stomach
5) Small intestine
6) Large intestine
7) Rectum/Anus
 Food passes through these organs
Accessory Digestive Organs
1) Salivary glands
2) Teeth
3) Pancreas
4) Liver
5) Gall bladder
 Food does not pass through these
organs, they contribute secretions
which aid digestion (except teeth)
Organs of the Digestive System
Functions of the Digestive System
1) Ingestion – taking in food
2) Mechanical Processing – physical breakdown
of food into smaller particles
Examples:
a. Chewing by teeth and
movement of food by tongue
b. Churning of food in the
stomach
c. Segmentation in the small
intestine
Functions of the Digestive System
3) Digestion – breakdown of food particles
into chemical components
 Using different enzymes:
a. Carbohydrates are broken to simple sugars
b. Proteins are broken to amino acids
c. Fats are broken to fatty acids and
alcohols (glycerol)
4) Secretion – release of fluids including
water, acids, enzymes, & buffers into the
digestive tract
Functions of the Digestive System
5) Absorption – moving nutrients and fluid
into the blood and lymph from the
digestive tract
6) Elimination – getting rid of undigested
and unabsorbed waste molecules
through defecation
Functions of the Digestive System
7) Metabolism – performs catabolic
reactions to provide reactants for
anabolic reactions and to make ATP
Processes of the Digestive System
 Propulsion – moving foods
from one region of the
digestive system to another
 Peristalsis – alternating
waves of muscle contraction
Peristalsis
 Segmentation – moving
materials back and forth to
aid in mixing
Figure 14.12
Segmentation
A little peristalsis video…
Figure 14.12
Segmentation
Processes of the Digestive System
Layers of Alimentary Canal Organs
Digestive tract has four layers of tissue:
1) Mucosa
 Innermost layer
 Consists of:
a) Moist epithelium that is folded to
increase surface area
b) Small smooth muscle layer to move the
folds
Layers of Alimentary Canal Organs
2) Submucosa
 Just beneath the mucosa
 Consists of soft connective tissue with
blood vessels, nerve endings, and
lymphatics
Layers of Alimentary Canal Organs
3) Muscularis externa – two layers of
smooth muscle
 Inner circular layer
 Outer longitudinal layer
 Network of nerves between to coordinate
their actions
Layers of Alimentary Canal Organs
4) Serosa, a.k.a. visceral peritoneum
 Outermost layer that is in contact with the
body cavities (parietal peritoneum)
 Layer of serous fluid-producing cells to
reduce friction between organs
 Mesenteries – epithelial and connective
tissue that:
a) Suspend & stabilize portions of the
digestive system
b) Allow passageway for blood vessels &
nerves
Mesenteries
Layers of Alimentary Canal Organs
Mouth (Oral Cavity) Anatomy
 Formed by the lips,
cheeks, hard & soft
palates, uvula, and
tongue
Mouth (Oral Cavity) Anatomy
 Tongue
 Functions:
1) Manipulates food to
prepare it for
swallowing
2) Covered with
sensory receptors
(touch, temperature,
taste)
3) Initiates swallowing
Mouth (Oral Cavity) Anatomy
 Tonsils – composed
of lymph tissue
 Function: help resist
infections
Figure 14.2a
Functions of the Mouth
1) Mechanical breakdown
 Food is physically broken down by chewing
(mastication)
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Teeth
 The role is to masticate (chew) food
 Humans have two sets of teeth –
deciduous and permanent
 A full set is 32 teeth, but some people
do not have wisdom teeth
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Classification of Teeth
 Incisors – for clipping and cutting (ex.
carrot)
 Canines – for tearing and slashing (ex.
first bite of meat)
 Premolars/Molars – for crushing,
mashing, and grinding (ex. harder foods
like nuts)
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Classification of Teeth
Figure 14.9
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Functions of the Mouth
2) Chemical digestion
 Food is mixed with saliva
 Break down of starch into simple sugars by
salivary amylase
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Salivary Glands
 Produce saliva
 Saliva:
 Mixture of mucus and serous fluids moisten &
lubricate food
 Contains salivary amylase to begin starch
digestion
 Dissolves chemicals that stimulate the taste
buds
 Reduces oral bacteria
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Pharynx Anatomy
 The “throat”
 Three major divisions
Functions of the Pharynx
 Serves as a common passageway for
air and food
 Propels food into the esophagus by
swallowing/peristalsis
Esophagus
 Runs from pharynx to stomach through
the diaphragm
 Conducts food by peristalsis
 Passageway for food only (respiratory
system branches off after the pharynx)
What is a hiatal hernia?
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Activities of the Pharynx and Esophagus
 These organs have no digestive function
(mechanical or chemical)
 Serve as passageways to the stomach
Deglutition (Swallowing)
**Before swallowing can occur, food must
have the right texture and consistency.**
1) Oral phase
 Voluntary
 Occurs in the mouth
 Food is formed into a bolus
 The bolus is forced into the pharynx by the
tongue
Deglutition (Swallowing)
2) Pharyngeal phase
 Involuntary “swallowing reflex” initiated when
the bolus hits sensory receptors in the
pharynx
 All passages except to the stomach are
blocked:
a) Tongue blocks off the mouth
b) Soft palate (uvula) blocks the nasopharynx
c) Epiglottis blocks the trachea
What causes choking?
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Deglutition (Swallowing)
3) Esophageal phase
 Involuntary
 Peristalsis moves the bolus toward the
stomach
 The sphincter (muscular ring) between
the esophagus and the stomach opens
when the bolus presses against it
Deglutition (Swallowing)
Figure 14.13
Deglutition (Swallowing)
Figure 14.13
Swallow Study
Click Here
Stomach Anatomy
 Located on the left side of the abdominal
cavity
 Food enters at the cardioesophageal
sphincter
 Four regions of the stomach
 Food empties into the small intestine at the
pyloric sphincter
Stomach Anatomy
Figure 14.4a
Stomach Anatomy
 Rugae – internal folds of the mucosa and
submucosa that allow it to expand
 An empty stomach holds ~50 mL, but a
full stomach can expand to hold up to 4 L!
 Has three layers of muscle (longitudinal,
circular, and OBLIQUE) that allow the
stomach to churn and mix the food
Rugae
Specialized Cells of the Stomach
 Goblet cells – produce alkaline mucus
 Gastric pits lead to gastric glands which
produce gastric juice made by:
 Mucus neck cells – produce sticky acidic mucus
 Chief cells – produce protein-digesting enzymes
(pepsinogens)
 Parietal cells – produce hydrochloric acid
 Endocrine cells – produce hormone gastrin
which regulates stomach activity
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Structure of the Stomach Mucosa
Endocrine cell
Figure 14.4b, c
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Stomach Functions
 Storage tank for food
 Mechanical breakdown of tougher foods
 Chemical breakdown of protein begins
 Delivers chyme (food + gastric juice) to
the small intestine
 Absorbs ONLY pain medications and
alcohol
Food Breakdown in the Stomach
 Sight, smell, taste, or thought of food
stimulates the production of gastric juice
 Production of gastric juice is regulated by
neural and hormonal factors
 Gastrin is released when food arrives in
the stomach and causes stomach
glands to secrete pepsinogens and HCl
 Food is mixed with secretions to
produce chyme
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Acid Environment in the Stomach
The body needs HCl to:
 Activate pepsinogen to pepsin for
protein digestion
 Provide a hostile environment for
microorganisms
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Stomach Protection: Mucosal Barrier
Stomach protects itself from this acid
environment by creating a barrier that:
1) Contains bicarbonate-rich mucus
2) Has tight junctions between mucosal cells
3) Has deep gastric gland cells are
impermeable to HCl
4) Sheds and renews damaged cells every
3-6 days
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Chemical Digestion in the Stomach
 Enzymes:
 Pepsin – digests proteins
 Rennin – digests milk protein (children
only…why?)
Propulsion in the Stomach
• Food must first be well mixed before it
moves to the small intestine…why?
• The pylorus meters out chyme into the
small intestine (3 mL at a time)
• The stomach empties in four to six
hours (carbs are quicker, fats take
longer)
Figure 14.14
Propulsion in the Stomach
Figure 14.14
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A little film about digestion…
Click Here
Vomiting
Causes:
• Extreme stretching of stomach or intestine
• Presence of irritants – bacterial toxins, excessive
alcohol, spicy foods, certain drugs
Excessive vomiting can result in:
• Dehydration
• Electrolyte imbalance
• Acid-base balance (lose HCl, blood becomes
alkaline)
Figure 14.14
Small Intestine
Anatomy:
 Muscular tube extending from the
pyloric sphincter to the ileocecal valve
(where it connects to the large intestine)
 20 feet in actual length if relaxed, but
only 6 feet as a result of muscle tone
Functions:
 Body’s major digestive organ
 Site of nutrient absorption into the blood
and lymph
Divisions of the Small Intestine
1) Duodenum
 First and shortest portion
 Receives chyme and secretions from liver
(bile) and the pancreas (pancreatic juice)
2) Jejunum
 Most chemical digestion and nutrient
absorption occurs here
 Question: Drastic weight loss surgery
involves a removal of this section…why?
3) Ileum
 Longest part, connects to large intestine
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Entering the Small Intestine
• Intestinal juice has a slightly alkaline
pH…why?
• Production stimulated by stretching
of small intestine or irritation by
chyme
• Composed of water and mucus
• Does not contain enzymes
• Enzymes are secreted by other organs
(we will learn about that later)
Chemical Digestion in the Small Intestine
Enzymes complete the digestion of all
organic molecules:
Brush border enzymes (attached to
membrane of microvilli)
Pancreatic enzymes
**Bile from liver enters via gallbladder
**Bile is not an enzyme (not a protein), but it
helps to break up the fats.
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Chemical Digestion in the Small Intestine
Figure 14.6
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Absorption in the Small Intestine
The small intestine absorbs:
1) Water along the entire length
2) The end products of digestion
 Most substances are absorbed by active
transport (protein carriers) through cell
membranes
 Some move by facilitated diffusion or
endocytosis
 Lipids are absorbed by diffusion
 Substances are transported to the liver by
the blood or lymph (fats)
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Modifications for Absorption: Circular Folds
 a.k.a. plicae circulares
 Deep folds (1 cm) of the mucosa and
submucosa
 Do not disappear when filled with food
 Cause the chyme to “spiral” through, mixing it
and slowing it down
 The submucosa has Peyer’s patches –
collections of lymphatic tissue that keep
bacteria from large intestine out of the blood
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Modifications for Absorption: Circular Folds
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Peyer’s Patches
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Modifications for Absorption: Villi
 Fingerlike structures formed by the
mucosa (1 mm)
 Give the small intestine more
surface area
 Contain blood vessels and lymph
vessels, and smooth muscle which
causes them to pulsate
 Contain lacteals – modified lymph
capillaries (transport lipids)
Modifications for Absorption: Villi
Modifications for
Absorption: Villi
Modifications for Absorption: Lacteals
Modifications for Absorption: Microvilli
 Small projections of the plasma
membrane (look like a hair brush)
 Surface contains enzymes that complete
digestion of proteins and carbohydrates
 Found on absorptive cells
Modifications for Absorption: Microvilli
Propulsion in the Small Intestine
 Peristalsis is the major means of moving
food only after absorption is completed
 Segmental movements – most common
movement
 Mix chyme with digestive juices (“massage”)
 Helps to slightly propel chyme
 Movement takes 2 hours from duodenum
to end of the ileum
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A little film of a toothpick being
removed from the small
intestine…
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Pancreas
 Located behind the stomach
 Secretes pancreatic juice (into the duodenum)
that contains:
 Enzymes that break down ALL four types of
organic molecules
 Alkaline fluid to neutralize acidic chyme and
activate enzymes
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Regulation of Pancreatic Secretion
Secretion is mostly regulated by intestinal
hormones that act on the pancreas:
 Secretin – released in the presence of HCl in
the intestine, prompting the release of
bicarbonate-rich pancreatic juice
 Cholecystokinin – released in response to the
presence of proteins and fats, prompting the
release of enzyme-rich pancreatic juice
Figure 14.15
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Regulation of Pancreatic Secretion
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Liver
 Largest gland in the body, filled with
blood vessels…why?
 Located on the right side of the body
under the diaphragm
 Only organ that can completely
regenerate itself
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Liver
Functions:
 Makes bile
 Manufactures blood proteins
 Stores nutrients including
carbohydrates, lipids, and vitamins
 Removes toxic substances from the
blood (or converts them to a non-toxic
form)
Bile
 Yellowish-green alkaline liquid
containing:
 Bile salts
 Bile pigment (mostly bilirubin from the
breakdown of hemoglobin)
 Cholesterol
 Emulsifies fats – breaks fats down into
smaller droplets so enzymes (lipases)
are more effective
Gall Bladder
 Thin-walled, green, muscular sac found
in the hollow space behind the liver
 Stores and concentrates (10x) bile that
comes from the liver
 Bile is secreted into the duodenum in
the presence of fatty food
 Gallstones (crystallized cholesterol) can
cause blockages
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Large Intestine
Characteristics:
 Larger in diameter, but shorter than the small
intestine
 Frames the internal abdomen
 Muscular walls are formed into pocketlike sacs
called haustra
Functions:
 Absorbs water
 Eliminates indigestible material as feces
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Large Intestine
Figure 14.8
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Slide 14.28
Digestion and Absorption in the Large Intestine
 No digestive enzymes are produced
 Resident bacteria digest remaining nutrients
1) Produce some vitamin K (needed to make
proteins for blood clotting) and vitamin B
2) Release gases as a result of breaking down
indigestible carbohydrates (ex. cellulose)
 Water, some vitamins, and electrolytes are
absorbed**
**This is not its main job contrary to popular
belief – propulsion of feces is!
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Propulsion in the Large Intestine
 Sluggish peristalsis or haustral contractions
occur about every 30 minutes (stimulated by
distension of haustra from food residue)
 “Mass movements” are slow and powerful
contractions that occur 3-4 times per day
(usu. during or right after eating)
 Bulk (fiber) increases strength of
contractions which softens the feces
 Presence of feces in the rectum causes a
defecation reflex
A little film of the large intestine…
Control of Digestive Activity
 Mostly controlled by reflexes via the
parasympathetic division of the
autonomic nervous system
 Chemical and mechanical receptors are
located in organ walls that trigger
reflexes
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Control of Digestive Activity
 Stimuli include:
 Stretch of the digestive organ
 pH of the contents
 Presence of breakdown products
 Reflexes include:
 Activation or inhibition of secretions
 Activation or inhibition of smooth muscle
activity
Effects of Aging on the Digestive System
 Rate of epithelial cell replacement
declines
 Smooth muscle tone decreases
 Effects of cumulative damage appear
(ex. loss of teeth, cirrhosis of liver)
 Cancer rates increase
 Decline in smell and taste sensitivity
What potential problems could these changes
lead to?
Integration with Other Systems
 Nervous system
 Endocrine system
 Cardiovascular system
 Lymphatic system
 Muscular system
How do these systems relate to the
digestive system?