Transcript Chapter 21

Chapter 21: Gas exchange in
animals
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
21-1
Air and water
•
All animals exchange O2 and CO2 in the process of
respiration
• Animals undertake exchange in air or water
• Composition of air is stable under normal
conditions
• Proportions of dissolved gases in water vary with
depth, salinity and temperature
(cont.)
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Air and water (cont.)
•
Concentration of gas in medium depends on
– partial pressure of gas

proportion of total pressure of gas mixture (e.g. air) provided
by nominated gas (e.g. O2 or CO2)
– solubility of gas

different gases do not dissolve at the same rate in all media
 water has greater capacitance for CO2 than for O2
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Exchanging gases
•
•
O2 must pass from external environment to
mitochondria
CO2 must pass from mitochondria to external
environment
– process of diffusion (passive, requires no energy)
•
Dissolved gases diffuse across membranes
provided that there is a partial pressure gradient
– example: mitochondria utilise O2, so the partial pressure
of O2 inside a mitochondrion (PO2) is zero
(cont.)
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Exchanging gases (cont.)
•
Rate of diffusion across a surface depends on
– difference in partial pressures of gas on either side of
membrane
– properties of membrane

permeability
 surface area
 thickness
•
Gases pass most rapidly across large, highlypermeable thin membranes
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Surface area and volume
•
Surface area is important in gas exchange
• Diffusion is only effective in small organisms or
over small distances
• As organisms increase in volume, the surface area
does not increase at the same rate
– diffusion becomes ineffective
– alternative mechanisms for transporting gases
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Ventilation
•
•
Passive transfer of gas across a surface depends
on difference in partial pressure of gas on either
side of that surface
If medium (air, water) is stagnant, then partial
pressure may drop (O2) or increase (CO2)
– reduces difference in partial pressure
– decreases diffusion rate
(cont.)
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Ventilation (cont.)
•
•
•
•
Animals may take advantage of the natural flow
(convection) of a medium
Animals may set up currents to circulate air or
water (ventilation)
Internal convection (perfusion) transports gases
through body fluids
Animals have internal circulatory system to
transport gases
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21-8
Fig. 21.3: Pathways of O2 and CO2
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Respiratory systems
•
Respiratory systems for gas exchange
– may involve ventilation of respiratory surfaces
•
Circulatory system for gas transport
– distributes O2 to cells and removes CO2 from cells
•
Dissolved gases pass across membranes by
diffusion
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Water breathers
•
•
Low O2 content of water means that animals
require constant movement of water across
respiratory surfaces
Boundary layer of water around an aquatic
organism forms a layer that is quickly depleted of
O2
– animals must disturb or dispel the boundary layer
•
Example: sponges circulate water using flagellated
cells (choanocytes)
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Cutaneous exchange
•
Gas exchange across the body surface
• Ventilation is achieved by moving the surface
through the water
• Area for gas exchange may be reduced if body
surface has protective covering
– development of specialised respiratory structures
– cutaneous respiration becomes secondary method of gas
exchange
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Gas-exchange structures
•
Specialised structures for gas exchange vary from
tiny outgrowths on body wall to elaborate gills
• Despite differences in form, all structures have a
large surface area and are thin to maximise
diffusion of gas
• Gills are usually ventilated by muscular action
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Fish gills
•
Fish gills are large and elaborate structures
• Gills supported by gill arches
– filaments projecting from gill arches are folded into
lamellae
– increases surface area
•
•
Gills are ventilated by forcing water from the
buccal cavity into the operculum cavity
Many fast-swimming fish use ram ventilation
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Countercurrent mechanisms
•
•
•
Countercurrent flow increases efficiency of gas
exchange
Water passes over gill in opposite direction to
blood (or haemolymph) flow within gill
Maximises difference in partial pressure of O2 in
water and blood
– blood entering gill has low PO2
– water leaving gill also has low PO2, but there is a gradient,
so O2 passes across
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21-15
Fig. 21.8a: Countercurrent flow
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21-16
Fig. 21.8b: Co-current flow
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21-17
Transition to land
•
Gills are ineffective in air
– they are likely to collapse under their own weight
– surface tension causes surfaces to adhere
– reduce area for gas exchange
•
Intertidal and semi-terrestrial invertebrates have
strengthened gills to prevent collapse
– reduced surface area is offset by higher O2 concentration
of air
– gills enclosed to reduce water loss through evaporation
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Air-breathing fish
•
Many fish gulp air from the surface
• Use a buccal-force pump to push air down into
gas-bladder
– outgrowth from alimentary tract
•
Some fish use internal surface of gut for gas
exchange
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Lungs
•
Specialised for respiration in air
– internal, paired sac-like structures
– in most vertebrates, air moved over surface in tidal
ventilation
•
Amphibians
– ventilate lungs using buccal-force pump
– also use cutaneous respiration
•
Reptiles
– ventilate lungs using aspirating pump
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Mammal lungs
•
Gas exchange takes place in alveoli
– alveolar walls are thin and highly vascularised
– coated with phospholipid surfactant, which prevents
collapse of alveoli
•
•
Air in dead space between the trachea and
alveolar ducts is not involved in gas exchange
Lungs are ventilated when muscular diaphragm
contracts to increase space around lung
– creates negative pressure allowing air to enter lungs
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Bird lungs
•
Air flow in birds is unidirectional not tidal
• Parabronchi (gas exchange structures) are
associated with air sacs
– air sacs have no respiratory function but act as bellows
•
•
Air enters through parabronchi and passes into
vascularised air capillaries
Air is shunted through the air sacs and
parabronchi during inspiration and expiration
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Fig. 21.20a: Flow of air through bird lung
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21-23
Fig. 21.20b: Flow of air through bird lung
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21-24
Tracheae
•
•
•
Insects possess tracheae that carry gases to and
from tissues
Air enters through spiracles on the side of an
insect’s thorax and abdomen
Tracheae branch into tracheoles
– tracheole have chitinous walls to prevent collapse
– deliver O2 to with a few μm of cells
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Transporting oxygen
•
Respiratory pigments increase amount of O2
carried in a fluid
– examples: haemoglobin, haemocyanin
•
In many vertebrates, haemoglobin is found in red
blood cells (erythrocytes)
– presence of respiratory pigment in cells prevents osmotic
problems
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Oxygen-carrying capacity
•
Respiratory pigments reversibly bind to O2
• Amount of O2 bound to pigment depends on partial
pressure
• Oxygen equilibrium curve
– also known as oxygen dissociation curve
– relationship between PO2 and total O2 content
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Fig. 21.25: Oxygen equilibrium curve
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Oxygen affinity
•
Different pigments have different affinities for O2
– pigments with high affinity for O2 become saturated at low
partial pressures
•
Example: fetal haemoglobin has a higher affinity
for O2 than maternal haemoglobin
– fetal haemoglobin has to bind O2 at a lower partial
pressure because maternal tissues have already
depleted some of the available O2
(cont.)
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Oxygen affinity (cont.)
•
Bohr effect
– O2 affinity of haemoglobin decreases with pH
– binds O2 in regions of high pH (lungs or gills)
– releases O2 in regions of low pH (areas high in CO2)
•
Root effect
– O2 affinity of haemoglobin decreases if CO2 is also bound
to the pigment molecule
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Transport of carbon dioxide
•
•
CO2 is carried in solution in plasma or combined
with haemoglobin in erythrocytes
CO2 hydrated to form carbonic acid (H2CO3)
– dissociates to bicarbonate (HCO3–) and hydrogen (H+)
ions
– reaction rate is increased by carbonic anhydrase
•
HCO3– diffuses from erythrocytes and is replaced
by Cl–
– at lungs, Cl– is replaced by HCO3–
– chloride shift
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Control of ventilation
•
Convection requirement of water-breathing
animals is substantially higher than that of airbreathing animals
– reflects low availability of O2 in water
– ventilation rate depends on O2 concentration
•
Air-breathing animals face problem of higher level
of internal CO2
– smaller air volume required in respiration means that CO2
may not be removed at sufficient rate
– ventilation rate depends on CO2 concentration
(cont.)
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21-32
Control of ventilation (cont.)
•
•
Chemoreceptors detect changes in blood
chemistry
Chemoreceptive tissue in medulla
– Responds to changes in CO2, pH
•
Carotid bodies in carotid arteries contain glomus
cells
– Respond to changes in O2 (also CO2 and pH)
•
Result in increased ventilation
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21-33