Transcript Animals Regulation and Body Plans

Regulation and Body Plans
Regulation: Mechanisms of homeostasis
moderate changes in the internal environment
The internal environment of vertebrates is called the
interstitial fluid.
• This fluid exchanges nutrients and wastes with blood
contained in microscopic vessels called capillaries.
• While a pond-dwelling hydra is powerless to affect the
temperature of the fluid that bathes its cells, the human
body can maintain its “internal pond” at a more-or-less
constant temperature of about 370C.
• Similarly, our bodies control the pH of our blood and
interstitial fluid to within a tenth of a pH unit of 7.4.
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• There are times during the course of the
development of an animal when major changes in
the internal environment are programmed to occur.
• For example, the balance of hormones in human blood
is altered radically during puberty and pregnancy.
• Actually the internal environment of an animal always
fluctuates slightly.
• Homeostasis is a dynamic state, outside forces tend to
change the internal environment and internal control
mechanisms oppose such changes.
• The stability of the internal environment is remarkable.
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2. Homeostasis depends on feedback
circuits
• Any homeostatic control system has three functional
components: a receptor, a control center, and an
effector.
• The receptor detects a change in some variable in the
animal’s internal environment, such as a change in
temperature.
• The control center processes the information it receives
from the receptor and directs an appropriate response by
the effector.
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Regulating and Conforming
• A regulator uses internal control mechanisms to
moderate internal change in the face of external,
environmental fluctuation
• A conformer allows its internal condition to
vary with certain external changes
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Fig. 40-7
40
Body temperature (°C)
River otter (temperature regulator)
30
20
Largemouth bass
(temperature conformer)
10
0
10
20
30
40
Ambient (environmental) temperature (ºC)
Homeostasis
• Organisms use homeostasis to maintain a
“steady state” or internal balance regardless of
external environment
• In humans, body temperature, blood pH, and
glucose concentration are each maintained at a
constant level
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Mechanisms of Homeostasis
• Mechanisms of homeostasis moderate changes
in the internal environment
• For a given variable, fluctuations above or below
a set point serve as a stimulus; these are
detected by a sensor and trigger a response
• The response returns the variable to the set point
Animation: Negative Feedback
Animation: Positive Feedback
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Fig. 40-8
Response:
Heater
turned
off
Room
temperature
decreases
Stimulus:
Control center
(thermostat)
reads too hot
Set
point:
20ºC
Stimulus:
Control center
(thermostat)
reads too cold
Room
temperature
increases
Response:
Heater
turned
on
Feedback Loops in Homeostasis
• The dynamic equilibrium of homeostasis is
maintained by negative feedback, which helps
to return a variable to either a normal range or a
set point
• Most homeostatic control systems function by
negative feedback, where buildup of the end
product shuts the system off
• Positive feedback loops occur in animals, but
do not usually contribute to homeostasis
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• Negative-feedback is analogous to a system that
controls the temperature in a room.
• In this case, the control center, called a thermostat, also
contains the receptor, a thermometer.
• When room temperature
falls, the thermostat
switches on the heater,
the effector.
Fig. 40.9a
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• Our own body
temperature is kept
close to a set point
of 37oC by the
cooperation of
several negativefeedback circuits
that regulate energy
exchange with the
environment.
Fig. 40.9b
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• In contrast to negative feedback, positive
feedback involves a change in some variable that
trigger mechanisms that amplify rather than
reverse the change.
• During childbirth, the pressure of the baby’s head
against sensors near the opening of the uterus stimulates
uterine contractions.
• These cause greater pressure against the uterine
opening, heightening the contractions, which cause still
greater pressure.
• Positive feedback brings childbirth to completion, a
very different sort of process from maintaining a steady
state.
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Alterations in Homeostasis
• Set points and normal ranges can change with
age or show cyclic variation
• Homeostasis can adjust to changes in external
environment, a process called acclimatization
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Homeostatic processes for thermoregulation
involve form, function, and behavior
• Thermoregulation is the process by which
animals maintain an internal temperature within
a tolerable range
• Endothermic animals generate heat by
metabolism; birds and mammals are endotherms
• Ectothermic animals gain heat from external
sources; ectotherms include most invertebrates,
fishes, amphibians, and non-avian reptiles
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• In general, ectotherms tolerate greater variation
in internal temperature, while endotherms are
active at a greater range of external temperatures
• Endothermy is more energetically expensive
than ectothermy
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Variation in Body Temperature
• The body temperature of a poikilotherm varies
with its environment, while that of a
homeotherm is relatively constant
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Balancing Heat Loss and Gain
• Organisms exchange heat by four physical
processes: conduction, convection, radiation, and
evaporation
Radiation
Convection
Evaporation
Conduction
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• Heat regulation in mammals often involves the
integumentary system: skin, hair, and nails
Hair
Epidermis
Sweat pore
Dermis
Muscle
Nerve
Sweat
gland
Hypodermis
Adipose tissue
Blood vessels
Oil gland
Hair follicle
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• Five general adaptations help animals
thermoregulate:
• Insulation
• Circulatory adaptations
• Cooling by evaporative heat loss
• Behavioral responses
• Adjusting metabolic heat production
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Insulation
• Insulation is a major thermoregulatory
adaptation in mammals and birds
• Skin, feathers, fur, and blubber reduce heat flow
between an animal and its environment
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Circulatory Adaptations
• Regulation of blood flow near the body surface
significantly affects thermoregulation
• Many endotherms and some ectotherms can alter
the amount of blood flowing between the body
core and the skin
• In vasodilation, blood flow in the skin increases,
facilitating heat loss
• In vasoconstriction, blood flow in the skin
decreases, lowering heat loss
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• The arrangement of blood vessels in many
marine mammals and birds allows for
countercurrent exchange
• Countercurrent heat exchangers transfer heat
between fluids flowing in opposite directions
• Countercurrent heat exchangers are an important
mechanism for reducing heat loss
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Fig. 40-12
Canada goose
Bottlenose
dolphin
Blood flow
Artery Vein
Vein
Artery
35ºC
33º
30º
27º
20º
18º
10º
9º
• Some bony fishes and sharks also use
countercurrent heat exchanges
• Many endothermic insects have countercurrent
heat exchangers that help maintain a high
temperature in the thorax
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Cooling by Evaporative Heat Loss
• Many types of animals lose heat through
evaporation of water in sweat
• Panting increases the cooling effect in birds and
many mammals
• Sweating or bathing moistens the skin, helping
to cool an animal down
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Behavioral Responses
• Both endotherms and ectotherms use behavioral
responses to control body temperature
• Some terrestrial invertebrates have postures that
minimize or maximize absorption of solar heat
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Adjusting Metabolic Heat Production
• Some animals can regulate body temperature by
adjusting their rate of metabolic heat production
• Heat production is increased by muscle activity
such as moving or shivering
• Some ectotherms can also shiver to increase
body temperature
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• While some aspects of the internal environment are
maintained at a set point, regulated change is
essential to normal body functions.
• In some cases, the changes are cyclical, such as the
changes in hormone levels responsible for the menstrual
cycle in women.
• In other cases, a regulated change is a reaction to a
challenge to the body.
• For example, the human body reacts to certain
infections by raising the set point for temperature to a
slightly higher level, and the resulting fevers helps
fight infection.
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• Over the short term, homeostatic mechanisms can
keep a process, such a body temperature, close to a
set point, whatever it is at that particular time.
• But over the longer term, homeostasis allows
regulated change in the body’s internal
environment.
• Internal regulation is expensive and animals use a
considerable portion of their energy from the food
they eat to maintain favorable internal conditions.
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Acclimatization in Thermoregulation
• Birds and mammals can vary their insulation to
acclimatize to seasonal temperature changes
• When temperatures are subzero, some
ectotherms produce “antifreeze” compounds to
prevent ice formation in their cells
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Physiological Thermostats and Fever
• Thermoregulation is controlled by a region of
the brain called the hypothalamus
• The hypothalamus triggers heat loss or heat
generating mechanisms
• Fever is the result of a change to the set point for
a biological thermostat
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Fig. 40-16
Sweat glands secrete
sweat, which evaporates,
cooling the body.
Body temperature
decreases;
thermostat
shuts off cooling
mechanisms.
Thermostat in hypothalamus
activates cooling mechanisms.
Blood vessels
in skin dilate:
capillaries fill;
heat radiates
from skin.
Increased body
temperature
Homeostasis:
Internal temperature
of 36–38°C
Body temperature
increases; thermostat
shuts off warming
mechanisms.
Decreased body
temperature
Blood vessels in skin
constrict, reducing
heat loss.
Skeletal muscles contract;
shivering generates heat.
Thermostat in
hypothalamus
activates warming
mechanisms.
Body Plan
• An animal’s size and shape, often called body plans
or designs, are fundamental aspects of form and
function that significantly affect the way an animal
interacts with its environment.
• The terms plan and design do not mean that animal body
forms are products of a conscious invention.
• The body plan or design of an animal results from a
pattern of development programmed by the genome, itself
the product of millions of years of evolution due to natural
selection.
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1. Physical laws constrain animal form
• Physical requirements constrain what natural
selection can “invent,” including the size of single
cells.
• This prevents an amoeba the size of a pro wrestler
engulfing your legs when wading into a murky lake.
• An amoeba the size of a human could never move
materials across its membrane fast enough to satisfy such
a large blob of cytoplasm.
• In this example, a physical law - the math of surface-tovolume relations - limits the evolution of an organism’s
form.
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• Similarly, the laws of hydrodynamics constrain the
shapes that are possible for aquatic organisms that
swim very fast.
• Tunas, sharks, penguins, dolphins, seal, and whales are all
fast swimmers and all have the same basic shape, called a
fusiform shape.
• This shape
minimizes drag
in water, which is
about a thousand
times denser
than air.
Fig. 40.6
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• The similar forms of speedy fishes, birds, and
marine mammals are a consequence of convergent
evolution in the face of the universal laws of
hydrodynamics.
• Convergence occurs because natural selection shapes
similar adaptations when diverse organisms face the
same environmental challenge, such as the resistance of
water to fast travel.
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2. Body size and shape affect interactions
with the environment
• An animal’s size and shape have a direct effect on
how the animal exchanges energy and materials with
its surroundings.
• As a requirement for maintaining the fluid integrity of the
plasma membrane of its cells, an animal’s body must be
arranged so that all of its living cells are bathed in an
aqueous medium.
• Exchange with the environment occurs as dissolved
substances diffuse and are transported across the plasma
membranes between the cells and their aqueous
surroundings.
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• For example, a single-celled protist living in water
has a sufficient surface area of plasma membrane
to service its entire volume because it is so small.
• A large cell has less surface area
relative to its volume than a
smaller cell of the same shape.
• These considerations place a
physical constraint on cell size.
Fig. 40.7a
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• Multicellular animals are composed of microscopic
cells, each with its own plasma membrane that acts
as a loading and unloading platform for a modest
volume of cytoplasm
• This only works if all the cells of the animal have
access to a suitable aqueous environment.
• For example, a hydra, built on
the sac plan, has a body wall
only two cell layers thick.
• Because its gastrovascular
cavity opens to the exterior,
both outer and inner layers
of cells are bathed in water.
Fig. 40.7b
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• Another way to maximize exposure to the
surrounding medium is to have a flat body.
• For instance, a tapeworm may be several meters long,
but because it is very thin, most of its cells are bathed in
the intestinal fluid of the worm’s vertebrate host, from
which it obtains nutrients.
• While two-layered sacs and flat shapes are designs
that put a large surface area in contact with the
environment, these solutions do not lead to much
complexity in internal organization.
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• Most animals are more complex and made up of
compact masses of cells, producing outer surfaces
that are relatively small compared to their volume.
• Most organisms have
extensively folded or
branched internal surfaces
specialized for exchange
with the environment.
• The circulatory system
shuttles material among
all the exchange surfaces
within the animal.
Fig. 40.8
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Fig. 40-4
External environment
CO2
Food
O2
Mouth
Respiratory
system
0.5 cm
50 µm
Animal
body
Lung tissue
Nutrients
Heart
Cells
Circulatory
system
10 µm
Interstitial
fluid
Digestive
system
Excretory
system
Lining of small intestine
Kidney tubules
Anus
Unabsorbed
matter (feces)
Metabolic waste products
(nitrogenous waste)
• Although exchange with the environment is a
problem for animals whose cells are mostly
internal, complex forms have distinct benefits.
• Because the animal’s external surface need not be
bathed in water, it is possible for the animal to live on
land.
• Also, because the immediate environment for the cells
is the internal body fluid (interstitial fluid), the animal’s
organ systems can control the composition of the
solution bathing its cells.
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