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Lecture

bioa02 chapter 48


Department
Biological Sciences
Course Code
BIOA02H3
Professor
Mary Olaveson

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Chapter 48
48.1 โ€“ What physical factors govern respiratory gas exchange?
-the respiratory gases that animals must exchange are oxygen and carbon dioxide
-cells need to obtain oxygen from the environment to produce an adequate supply
of ATP by cellular respiration
-there are no active transport mechanism to move respiratory gases across
biological membranes
osince diffusion is a physical process, knowing the physical factors that
influence rates of diffusion helps us understand the diverse adaptations of
gas exchange systems
-since diffusion results from the random motion of molecules, the net movement of
a molecule is always down its concentration gradient
-barometer pressure at sea level is 760 mmHg
-the partial pressure of oxygen at sea level is 20.9% of 760 mmHg ๎€ 159 mmHg
-the actual amount of a gas in a liquid depends on the partial pressure of that gas in
the gas phase in contact with the liquid as well as on the solubility of that gas in
that liquid
-Fickโ€™s law of diffusion ๎€ all environmental variables that limit respiratory gas
exchange and all adaptations that maximize respiratory gas exchange are reflected
in one or more components of this equation
-Oxygen can be obtained more easily from air than from water for several reasons
oThe O2 content of air is must higher than the O2 content of an equal
volume of water
oOxygen diffuses about 8000 times more rapidly in air than in water ๎€๎€‚
thatโ€™s why the O2 content of a stagnant pond can be zero only a few
millimeters below the surface
oWhen an animal breathes, it does work to move water or air over its
specialized gas exchange surfaces. More energy is required to move water
than to move air since water is 800 times more dense
-Diffusion of O2 in water is so slow that even animal cells with low rates of
metabolism can be no more than a couple of millimeters away from a good source
of environmental O2
oTherefore there are severe size and shape limits on the many species of
invertebrates
oA critical factor enabling larger, more complex animal bodies has been the
evolution of specialized respiratory systems with large surface areas for
enhancing respiratory gas exchange
-Most water breathers are ectotherms ๎€ their body temperatures are closely tied to
the temperature of the water around them
-As temperature gets warmer, their body temperature and metabolic rate rise,
making them need more O2
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-Therefore, as water temperature goes up, the water breather must extract more and
more O2 from an environment that is increasingly O2 deficient, and a lower
percentage of that O2 is available to support activities other than breathing
-An increase in altitude reduces the O2 supply for air breathers
-As you go up in altitude, the total amount of gas per unit volume decreases, as
reflected in the barometric pressure
oTherefore, the drastically reduced PO2 in the air at high altitudes contains
O2 uptake
-The partial pressure of CO2 in the atmosphere is close to zero both at sea level
and on top of Mount Everest
oIn general, getting ird of CO@ is not a problem for water-breathing
animals because CO2 is much more soluble in water than is O2
48.2 โ€“ What adaptations maximize respiratory gas exchange?
-Many anatomical adaptations maximize the specialized body surface area over
which respiratory gases can diffuse
-External gills ๎€ are highly branched and folded extension s of the body surface
that provide a large surface area for gas exchange with water
oAre found n larval amphibians and in the larvae of many insect species
oBecause they consist of thin, delicate tissues, the minimize the length of
the path traverse by diffusing molecules of O2 and CO2
oBut they are very vulnerable
-Internal gills ๎€ are found in many mollusks and arthropods, and in all fishes
-Lungs ๎€ are internal cavities for respiratory gas exchange with air
oHave a large surface area because they are highly divided, and they are
elastic so that they can be inflated with air and deflated
-Tracheae ๎€ air-f8illed tubes that branch through all tissues of the insectโ€™s body
oSince the tubes are so numerous, it creates an enormous surface area
-Transporting gases to and form the exchange surfaces optimizes partial pressure
gradients
oMinimization of path length ๎€ thin tissues in gills and lungs that reduce
the diffusion path lengths
oVentilation ๎€ actively moving the respiratory medium over the gas
exchange surfaces (breathing) exposes those surfaces regularly to fresh
respiratory medium containing maximum O2 and minimum CO2
concentrations
oPerfusion ๎€ circulating blood over the internal side of the exchange
surfaces transports CO2 to those surfaces and O2 away from those
surfaces, thus maximizing the concentration gradients driving diffusion
-Gas exchange system ๎€ is made up of its gas exchange surfaces and the
mechanisms it uses to ventilate and perfuse those surfaces
-Insects have airways throughout their bodies
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oThe tracheal system extends to all tissues in the insect body
oIts respiratory system communicates with the outside environment through
gated openings called ๎€ spiracles in the sides of the abdomen
oThe spiracles can open to allow gas exchange, and then close to decrease
water loss
oSpiracles open into tubes called tracheae that branch into even finer tubes
๎€ tracheoles, until they end in tiny air capillaries
-Fish gills use countercurrent flow to maximize gas exchange
oThe internal gills of fish are supported by gill arches that lie between the
mouth cavity and the protection opercular flaps on the sides of the fish just
behind the eyes
oWater flows uni-directionally into the fishโ€™s mouth, over the gills, and out
from under the opercular flaps
oThis one-way flow of water moving over the gills maximizes the PO2 on
the external gill surfaces
oOn the internal side of the gill membranes, the circulation of blood
minimizes the PO2 by sweeping the O2 away as rapidly as it diffuses
across
oGills have enormous surface area for gas exchange
oEach gills consists of hundreds of leaf-shaped ๎€ gill filaments
๎€ƒThe upper and lower flay surface are covered with rows of evenly
spaced folds ๎€ lamellae ๎€ the actual gas exchange surfaces
๎€ƒThe structure minimize the path length for diffusion of gases
between blood and water
๎€ƒSurfaces of lamellae consist highly flattened epithelial cells
oAfferent blood vessels bring blood to the gills, while efferent blood
vessels take blood away from the gills
oBlood flows through the lamellae in the direction opposite of the flow of
water over the lamellae
oCountercurrent flow ๎€ optimized the PO2 gradient between water and
blood, making gas exchange more efficient than it would be in a system
using concurrent (parallel) flow
oThese adaptations allow fish to extract an adequate supply of O2 from
meager environmental sources by maximizing the surface are for
diffusion, minimizing the path length for diffusion, and maximizing the
PO2 gradient by means of constant, uni-directional, countercurrent flow of
blood and water over the opposite sides of their gas exchange surfaces
-Birds use uni-directional ventilation to maximize gas exchange
oTheir lungs expand and contract less during a breathing cycle than do
mammalian lungs
oBird lungs contract during inhalation and expand during exhalation
oTheir lungs allow air to flow uni-directionally through the lungs, rather
than bidirectionally through all of the same airways, like a mammal
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