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Types of Digestive Systems
The Digestive System
Single-celled organisms as well as sponges
digest their food intracellularly
-Other multicellular animals digest their
food extracellularly, within a digestive cavity
-Cnidarians and flatworms, have a
gastrovascular cavity
-Has only one opening, and no
specialized regions
Chapter 48
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Types of Digestive Systems
Food
Mouth
Specialization occurs when the digestive tract
has a separate mouth and anus
-Nematodes have the most primitive
digestive tract
-A tubular gut lined by an epithelial
membrane
-More complex animals have a digestive
tract specialized in different regions
Wastes
Tentacle
Body stalk
Gastrovascular
cavity
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Types of Digestive Systems
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Types of Digestive Systems
Ingested food may be stored or first subjected
to Physical digestion
-Chemical digestion occurs next, involving
hydrolysis reactions that liberate the subunit
molecules from food
-Products pass through gut’s epithelial
lining into the blood (absorption)
-Wastes are excreted from the anus
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Vertebrate Digestive Systems
The digestive system consists of a tubular
gastrointestinal tract and accessory organs
-Mouth and pharynx = Entry
-Esophagus = Delivers food to stomach
-Stomach = Preliminary digestion
-Small intestine = Absorption
-Large intestine = Concentration of wastes
-Cloaca or rectum = Waste storage
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Vertebrate Digestive Systems
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Vertebrate Digestive Systems
Accessory organs
-Liver
-Produces bile
-Gallbladder
-Stores and concentrates bile
-Pancreas
-Produces pancreatic juice and
bicarbonate buffer
The gastrointestinal tract has four layers
-Mucosa = Epithelium that lines the interior,
or lumen, of the tract
-Submucosa = Connective tissue
-Muscularis = Double layer of smooth
muscles
-Serosa = Epithelium that covers the
external surface of the tract
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Gland outside gastrointestinal tract
Blood vessel
Mouth and Teeth
Many vertebrates have teeth used for
chewing or mastication (PD)
Birds, which lack teeth, break up food in their
two-chambered stomach
-Gizzard = Muscular chamber that uses
ingested pebbles to pulverize food (PD)
Lumen
Mucosa
Submucosa
Gland in submucosa
Submucosal plexus
Muscularis
Circular layer
Longitudinal layer
Serosa
Epithelial
tissue layer
Nerve
Myenteric
plexus
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Mouth and Teeth
For food
storage
Vertebrate teeth are adapted to their
nutritional source
-Carnivorous mammals have pointed teeth
that lack flat grinding surfaces
-Herbivores have large flat teeth suited for
grinding cellulose cell walls of plant tissues
-Humans have carnivore-like teeth in the
front and herbivore-like teeth in the back
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Mouth and Teeth
Mouth and Teeth
Inside the mouth, the tongue mixes food with
saliva
-Moistens and lubricates the food
-Also contains salivary amylase, which
initiates the breakdown of starch (CD)
Salivation is controlled by the nervous system
-Tasting, smelling, and even thinking or
talking about food stimulate salivation
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Herbivore
Horse
Incisors
Canines
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Carnivore
Omnivore
Lion
Human
Premolars
Molars
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Mouth and Teeth
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Mouth and Teeth
Swallowing is initiated by voluntary action,
then is continued under involuntary control
-When food is ready to be swallowed, the
tongue moves it to the back of the mouth
-Soft palate seals off nasal cavity
-Elevation of the larynx (voice box)
pushes the glottis against the epiglottis
-Keeps food out of respiratory tract
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Peristalic
movement
The Esophagus
The esophagus is a muscular tube
connecting the esophagus to the stomach
-It actively moves a processed lump of food,
called a bolus, through muscular action
-Swallowing center in brain stimulates
successive waves of contraction
-Peristalsis moves the food down to
the stomach
Esophagus
Relaxation
Contraction
Food Bolus
Relaxation
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The Stomach
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The Stomach
The stomach is the saclike portion of tract
-Has convoluted surface, allowing expansion
-Contains an extra layer of smooth muscles
for mixing food with gastric juice
-Has two kinds of secretory cells
-Parietal cells – Secrete HCl and
intrinsic factor (for vita. B12 absorption)
-Chief cells – Secrete pepsinogen, the
inactive form of pepsin
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The Stomach
The Small Intestine
The low pH in the stomach helps denature
food proteins
-No significant digestion of carbohydrates or
fats occurs
The mixture of partially digested food and
gastric juice is called chyme
-Leaves the stomach through the pyloric
sphincter to enter the small intestine
The small intestine is about 4.5 m long
-Consists of duodenum, jejunum and ileum
Epithelial wall is covered with villi, which in
turn are covered by microvilli
-Greatly increase surface area
Microvilli also participate in digestion
-Many adults lack the enzyme lactase
-Have lactose intolerance
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The Small Intestine
Accessory Organs
Pancreas
-Pancreatic fluid is secreted into the
duodenum through the pancreatic duct
-Host of enzymes: trypsin, chymotrypsin,
pancreatic amylase, and lipase
-Digest proteins into smaller polypeptides,
polysaccharides into shorter sugars, and
fats into free fatty acids & monoglycerides
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Pancreatic islet
(of Langerhans)
Accessory Organs
Liver
-The liver is the body’s largest internal organ
-It secretes bile into the duodenum during
digestion of a meal
-Consists of bile pigments (waste products)
and bile salts (for emulsification of fats)
Gallbladder
-Stores and concentrates bile
β cell
From liver
α cell
Common Pancreas
bile duct
Gallbladder
Pancreatic
duct
Duodenum
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Absorption
Amino acids and monosaccharides are
transported through epithelial cells to blood
-Blood carries these products to the liver via
the hepatic portal vein
Fatty acids and monoglycerides diffuse into
epithelial cells
-They are reassembled into chylomicrons
-Enter the lymphatic system and later
join the circulatory system
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Absorption
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Absorption
The Large Intestine
About 9 liters of fluid pass through the small
intestine per day
-Only about 50 g of solid and 100 mL of
liquid leave the body as feces
-The normal fluid absorption efficiency
of the human digestive tract is 99%!
The large intestine, or colon, is much shorter
than small intestine, but has larger diameter
Small intestine empties directly into the large
intestine at a junction where two vestigial
structures, cecum and appendix, remain
No digestion occurs
Only 4% absorption
-Water, remaining electrolytes & vitamin K
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The Large Intestine
Main function is waste material concentration
-Compacted feces are stored in the rectum,
until it can be eliminated through the anus
Ascending portion
of large intestine
Most mammals have a rectum
-Most vertebrates have a common cavity,
the cloaca, where the urinary, reproductive,
and gastrointestinal tracts join
Ileocecal valve
Last portion of
small intestine
Cecum
Appendix
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Variations in Digestive Systems
The digestive tracts of some animals contain
bacteria and protists that convert cellulose
into substances the host can absorb
Ruminants have a four-chambered stomach
-Rumen, reticulum, omasum, abomasum
-Rumen has cellulose-degrading microbes
-Contents can be regurgitated and
rechewed
-Rumination
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Variations in Digestive Systems
Animals, such as horses, deer and rabbits,
digest cellulose in the cecum
-Regurgitation of contents is not possible
-However, such animals practice
coprophagy
-Eat their feces to absorb nutrients
on the second passage of food
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Variations in Digestive Systems
Some animals digest waxes with the help of
microorganisms
All mammals rely on intestinal bacteria to
synthesize vitamin K, which is required for
blood clotting
-Birds, which lack these bacteria, must
consume the required quantities of vitamin
K in their diet
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Accessory Organ Function
Liver
-Chemically modifies the substances
absorbed from the digestive tract before
they reach the rest of the body
-Removes toxins, pesticides, & carcinogens,
converting them to less toxic forms
-Regulates levels of steroid hormones
-Produces most proteins found in plasma
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Accessory Organ Function
Regulation of blood glucose
-After a meal, increased secretion of insulin
promotes deposition of glycogen and fat
-During fasting or exercising, an increased
secretion of glucagon promotes the
breakdown of glycogen (glycogenolysis)
-If this continues, liver begins to convert
other molecules into glucose
-Gluconeogenesis
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Metabolism
Eating carbohydraterich meal
Fasting or
exercise
Increasing blood glucose
Decreasing blood glucose
Pancreatic Islets
Pancreatic Islets
Insulin secretion
Insulin secretion
Glucagon secretion
Glucagon secretion
Build up of
glycogen (in liver)
and fat (in adipose
tissue)
Breakdown of
glycogen (in liver)
and fat (in adipose
tissue)
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Food Energy
Food Energy
Ingestion of food serves two primary functions
1. Source of energy
2. Source of raw material
Basal metabolic rate is the minimal amount
of energy consumed under defined resting
conditions
-Continued ingestion of excess food energy
results primarily in accumulation of fat
When total kilocalories ingested exceeds the
metabolic rate for a sustained period, a
condition called obesity occurs
-Obesity is currently classified using the
metric body mass index (BMI)
-A ratio of height and weight that
indirectly measures body fat
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Regulation of Food Intake
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Regulation of Food Intake
Obesity in mice is caused by a mutation in a
single gene named ob (for obese)
-Encodes a peptide hormone named leptin
-The main satiety factor
Mice homozygous for the recessive mutant
allele are obese
-Become normal when injected with leptin
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Regulation of Food Intake
Regulation of Food Intake
Leptin is produced by adipose tissue in
response to feeding
Leptin has also been found in humans,
where it functions much as it does in mice
-Leptin produced by obese people appears
to be normal
-Most cases of human obesity may
result from reduced sensitivity to the
actions of leptin in the brain
Other hormones involved in the control of
feeding and energy include:
-Insulin, GIP, and CCK, which signal satiety
-Ghrelin which stimulates food intake
The efferent control involves the
hypothalamus and two neuropeptides
-Neuropeptide Y (NPY), which induces
feeding activity, and melanocyte-stimulating
hormone (α-MSH) which suppresses it 48
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Essential Nutrients
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Essential nutrients are substances that an
animal cannot manufacture for itself but
which are necessary for health and so must
be obtained in the diet
-These include certain:
-Vitamins
-Amino acids
-Long-chain unsaturated fatty acids
-Minerals
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