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The Living World
Fourth Edition
GEORGE B. JOHNSON
26
The Path of Food
Through the
Animal Body
PowerPoint® Lectures prepared by Johnny El-Rady
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26.1 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
Fig. 26.1 The pyramid
of nutrition
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Sources
Energy
content
Use
Cereals, grains,
4.1 calories
Carbohydrates breads, fruits
per gram
and vegetables
For energy
and raw
materials
Dairy products, 4.1 calories
poultry, fish,
per gram
meat and grains
For energy
and raw
materials
Proteins
Fats
Oils, butter,
9.3 calories For energy
Margarine, fried per gram
storage and
foods and chips
raw materials
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The body mass index is used to determine who’s overweight
body weight in kg
BMI =
(height in m)2
(body weight in lbs) X 703
=
(height in in)2
Fig. 26.3
Obesity
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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
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Essential Substances for Growth
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
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26.2 Types of Digestive Systems
Heterotrophs are divided into three main groups
based on food sources
Herbivores – Eat plants exclusively
Carnivores – Eat meat exclusively
Omnivores – Eat both plants and meat
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Single-celled organisms and sponges digest their
food intracellularly
Other animals digest their food extracellularly,
within a digestive cavity
In cnidarians and
flatworms, this
gastrovascular
cavity has only one
opening
Therefore, no
specialization
Fig. 26.4
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Specialization occurs when the digestive tract has
a separate mouth and anus
This allows transport of food in one direction
Ingested food may be stored in a specialized
region of the digestive tract
Or it may be subjected to physical
fragmentation, followed by chemical digestion
Products are then absorbed into the blood
Molecules that are not absorbed are excreted
as wastes from the anus
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The most primitive
Fig. 26.5
One-way
digestive
tracts
Specialization in
different regions
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26.3 Vertebrate Digestive Systems
The vertebrate digestive system 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 (mammals) – Waste excretion
Cloaca (other vertebrates) – Waste excretion
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Fig. 26.6
The human
digestive
system
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26.3 Vertebrate Digestive Systems
In general, herbivores have relatively longer
intestines than carnivores
Plant cellulose resists digestion
Herbivores solicit the help of bacteria
1. Ruminants (such as cows) have stomach
with multiple chambers
2. Other herbivores (horses and rabbits) have
a cecum at the beginning of the large intestine
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The gastrointestinal tract has a characteristic
layered structure
Fig. 26.7
Mucosa
Epithlelium
Submucosa
Connective tissue
Muscularis
Smooth muscles
Serosa
Connective tissue
Regulate
gastrointestinal
activities
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26.4 The Mouth and Teeth
Specializations of the digestive system reflects
differences in the way vertebrates live
Birds, for example,
lack teeth
They break up
food with the
help of stones
in their twochambered
stomachs
For food
storage
Fig. 26.9
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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
Fig. 26.8 Diagram of generalized
vertebrate dentition
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Fig. 26.10 Human teeth
Incisors
Premolars
and molars
Cuspids
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The tongue mixes food with a solution called saliva
Saliva moistens and lubricates food
It 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
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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
This keeps food out of the respiratory
tract, and into the esophagus
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Fig. 26.11 The human pharynx, palate and larynx
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26.5 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
Fig. 26.12
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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
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Fig. 26.13 The stomach and gastric glands
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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
It 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
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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
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26.6 The Small and Large Intestines
The small intestine is the body’s true digestive vat
It breaks down large molecules into smaller ones
These 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
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26.6 The Small and Large Intestines
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
These increase the absorptive surface
Indeed, the absorptive efficiency
approaches 99%
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Fig. 26.14 The small intestine
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26.6 The Small and Large Intestines
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
It collects and compacts solid wastes
Feces pass through the rectum as a result
of peristalsis
Leave the body through the anus
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26.7 Variations in Vertebrate
Digestive Systems
Most animals lack the enzymes that digest cellulose
Their digestive tracts contain prokaryotes and
protozoa that have such enzymes
Herbivores tend to have long digestive tracts with
specialized pouches for breakdown of plant matter
Insectivorous and carnivorous mammals have short
digestive tracts with few specialized pouches
Protein diets are more easily digested
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Ruminants have stomachs with four chambers
Rumen
Reticulum
Omasum
Abomasum
The contents
of the rumen
can be
regurgitated
and rechewed
(rumination)
Fig. 26.15
Contains prokaryotes
and protozoa
Only chamber equivalent
to the human stomach
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In some animals digestion of cellulose by microbes
occurs in the cecum
Regurgitation of contents is not possible
However, these animals engage in coprophagy
They eat their feces to absorb the nutrients on
the second passage of food
All mammals rely on intestinal bacteria to synthesize
vitamin K
Necessary for blood clotting
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Fig. 26.16 The digestive system of different mammals
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Fig. 26.16 The digestive system of different mammals
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26.8 Accessory Digestive Organs
Pancreas
Functions as an exocrine gland in digestion
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
Functions as an endocrine gland
Cell clusters called Islets of Langerhans secrete
Insulin and glucagon
Regulate sugar levels in blood
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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
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Fig. 26.17 The pancreatic and bile ducts empty into the duodenum
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26.9 Homeostasis
Homeostasis is the dynamic constancy of the
internal environment
Conditions fluctuate continuously within
narrow limits
Homeostasis is essential for life
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Regulating Body Temperature
Humans, like other mammals, are endothermic
They maintain a body temperature of 37oC (98oF)
The hypothalamus coordinates the regulation of the
body temperature
When body temperature rises
Heat is dissipated via sweating and dilation of
skin blood vessels
When body temperature drops
Heat is conserved via shivering and constriction
of skin blood vessels
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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
Fig. 26.19
Stimulate
glycogen
breakdown
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Eliminating Nitrogenous Wastes
Catabolism produces nitrogenous wastes that must
be eliminated from the body
The first by-product is ammonia, which is toxic
It is released through the gills of bony fish
Mammals convert ammonia into urea (less toxic)
It is water-soluble and thus, excreted in urine
Reptiles and birds convert ammonia to uric acid
It is largely insoluble in water, and is excreted
as a semisolid
White paste in bird droppings
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Fig. 26.20 Nitrogenous wastes
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26.10 Osmoregulatory Organs
Osmoregulation is the regulation of the body’s
osmotic (water and salt) composition
Sponges use contractile vacuoles
Invertebrates use a system of tubules from which
water and metabolites are reabsorbed
Wastes are excreted through excretory pores
In flatworms, the tubules are called protonephridia
In earthworms, the tubules are called nephridia
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Insects use Malpighian tubules for excretion
K+ is secreted into these tubules, and water is
drawn osmotically
Much of this is reabsorbed in the hindgut
Waste products are secreted from the rectum
Vertebrates use kidneys for excretion
Unlike in insects, tubular fluid is created by
filtration of the blood under pressure
Selective reabsorption of water and molecules
The final waste product, urine, is expelled
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26.11 Evolution of the
Vertebrate Kidney
The kidney is a complex organ made of thousands
of repeating disposal units called nephrons
Blood pressure forces the fluid in blood through
a capillary bed called the glomerulus
It retains large molecules and allows water
and small molecules to pass through
Useful sugars and ions are then recovered
What remains forms urine
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Fig. 26.25
Basic
organization of
the vertebrate
nephron
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Freshwater Fish
Face two serious problems
1. Water tends to enter body from environment
The fish don’t drink water
They also excrete a large volume of dilute urine
2. Solutes tend to leave body to the environment
The fish reabsorb ions across the nephron
tubules
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Marine Bony Fish
Their body fluids are hypotonic to seawater
Thus, water leaves by osmosis across gills
They compensate by drinking large amounts of
seawater
Most monovalent ions are actively transported out of
blood across gills
Divalent ions that enter blood are secreted into
nephron tubules and excreted in urine
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Fig. 26.26
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Cartilaginous Fish
Most common are elasmobranchs – sharks and rays
Reabsorb urea from the nephron tubules
Maintain a blood urea concentration that is 100
times higher than that of mammals
The higher urea makes their blood about
isotonic with seawater
Water loss is prevented
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Fig. 26.27 Osmoregulation in elasmobranchs
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Amphibians
Amphibians produce very dilute urine
Compensate for loss of Na+ by active transport of
Na+ across their skin from the surrounding water
Fig. 26.28a
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Reptiles
Marine reptiles drink seawater and excrete an
isotonic urine
Excess salt is secreted by salt glands
Fig. 26.28b
Terrestrial reptiles reabsorb salt and water in
nephron tubules
Reabsorb additional water in the cloaca
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Mammals and Birds
The only vertebrates able to produce urine with
higher osmotic concentrations than body fluids
They can thus excrete waste products in a small
amount of water, and thus retain more water
The kidneys of the
kangaroo rat are so
efficient, it does not
have to drink water!
Fig. 26.29
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Hypertonic urine is produced by the loop of Henle
Most mammals have nephrons with long tubes
Birds have relatively few nephrons with long tubes
Produce less concentrated urine than mammals
Marine birds minimize
water loss by drinking
seawater and excreting
salt through glands
Additional water is
absorbed in the cloaca
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Fig. 26.30
26.12 The Mammalian Kidney
Each kidney receives blood from a renal artery and
from this it produces urine
Urine drains from each kidney through a ureter to
a urinary bladder
Renal tissue is divided into
Renal cortex – on the outside
Renal medulla – on the inside
The kidney is composed of ~ 1 million nephrons
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Fig. 26.31 The mammalian urinary system
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Each nephron is composed of three regions
1. Filter
Bowman’s capsule located on top
Contains a fine network of capillaries called a
glomerulus
2. Tube
Loop of Henle is bent back on itself in the center
Reabsorption device
3. Duct
Tube empties into a collecting duct
Water conservation device
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Fig. 26.31 The mammalian urinary system
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The formation of urine occurs in five steps
1. Pressure filtration
Blood pressure pushes water and small molecules across
the glomerulus walls and into the Bowman’s capsule
This glomerulus filtrate contains water, urea, nutrients
and ions
2. Reabsorption of water
The walls of the descending arm of the loop of Henle are
impermeable to salts and urea but not to water
Water exits the loop leaving behind a concentrated
fitrate
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3. Selective reabsorption
The walls of the ascending arm of the loop of Henle
become permeable to salts & nutrients but not to water
Nutrients pass out to the surrounding tissue
NaCl is actively exported out of the loop
4. Tubular excretion
Active transport excretes into the urine other nitrogenous
wastes such as uric acid and ammonia
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5. Further reabsorption of water
Lower portions of collecting duct are permeable to urea
Urea leaves and water follows
Fig. 26.33
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