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LUNGS
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BREATHING FISH At first, it may seem as if the ability to
breathe atmospheric oxygen in the ancestors of the amphibians was an almost
insurmountable challenge. Upon
closer consideration, however, the evolution of air-breathing seems well
within the capabilities of aquatic organisms.
The ability of aquatic animals to breath oxygen from the air is
thought to have evolved at least 67 times (Maina,
1998). A number of crustaceans can breathe air. There are lung-like structures in some snails.
Some oligochaete
and polychaete worms perform gas exchange through
their anus (Maina, 1998). The sea cucumber Holothuria tubulosa can
breathe in stagnant water by rising through the surface and performing
gas exchange through its cloaca (Hoar, 1983,
p. 505). Some fish perform gas exchange with water moved
through their highly vascularized rectum. Some aquatic turtles possess 2 sacs opening
into the cloaca which can function in gas exchange
(Weichert, 1970, p. 244). A
considerable number of fish can breathe oxygen from air. The majority do so
as a supplement to the oxygen supplied by their gills but some are actually
obligate air breathers that would drown in water without access to air. Different groups of fish have evolved different
mechanisms for air breathing. Increased vascularization of caudal and
pelvic fins function in respiration in the walking goby (Periopthalmus) and the South American
lungfish, respectively (Weichert, 1970, p. 244). Some fish perform gas exchange with the air
through their buccopharyngeal apparatus (mouth
and throat) such as Electrophorus
electricus of the family Electrophoridae. Some do so through using an opercular structures, such as Hypopomus brevicostris of the family Sternarchidae, Symbranchus marmotatus and Monopterus javanesis of the family Symbranchidae,
Pseudapocryptes lanceolatus of
the family Gobiidae, Heteropneustes fossilis of
the family Saccobranchidae, Clarias lazera, Clarias magur, and Clarias mossambicus of the family Clariidae,
and Macropodus cupanus, Colisa fasciata, Betta, Osphronemus gorami, and Anabas
testudineus from the family Anabantidae
(Maina, 1998).
While the fine filaments of most fish gills collapse in air, a few
fish, such as Symbranchus
and Hypopomus,
possess modified gills which do not collapse in air in addition to vascularized mouth and pharyngeal cavities which permit gas
exchange. Gill breathing in air
occurs in eels and Periopthalmus. In electric eels, the folding of the oral cavity
increases the respiratory surface to about 15% of the total body surface.
This area apparently performs sufficient gas exchange since the
gills in adults are almost nonfunctional (Johansen, Kjell
from Hoar, 1970). Some fish perform gas exhange
in their intestine, such as Misgurnus fossilis and Lepidocephalichthys guntea from the family Cobitidae
and Doras
of the family Doridae. Some members of the family Ophicephalidae possess pharyngeal lungs such as Opicephalus (Channa) puntatus, O. marulius, O. striatus, O. gachua, and Amphipnous cuchia. Anguilla anguilla, A. bengalensis,
and A. japonicus
of the family Anguillidae can perform gas
exchange through their skin. Eels
which are removed from water can absorb about 60% of their oxygen uptake
in water. While most of this oxygen
exchange in air occurs through the skin, a significant portion occurs
across the gills, indicating that their gills have also been adapted for
air-breathing (Hoar, 1970; Maina, 1998). Some members
of the families Loricaridae and Callichthyidae possess modifications of their gastrointestinal
tract which allow gas exchange using swallowed air. Some of these fish are obligate air breathers
and may travel for short distances over land (Maina,
1998). An air-breathing species
of the family Loricaridae is pictured in the
following photos. |
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Bony fish possess a sac which opens into
the esophagus. While it is called
a swim bladder and the buoyancy it generates is an adaptation for swimming,
this structure can also be used for respiration and seems to be homologous
to tetrapod lungs. Some
actinopterygian fish perform gas exchange using
their swim bladder, such as Arapaima
gigas of the family Arapaimidae,
Gymnarchus
of the family Gymnarchidae, Erythrinus unitaeniatus of the family Characinidae, Umbra
from the family Umbridae, and Notopterus notopterus and Notopterus chitala of the family Notopteridae. Maina, 1998). This sac is used for the respiration of oxygen
from air in dipnoan sarcopterygians and in the
most primitive actinopterygians, suggesting
that this was its primitive function in the ancestral bony fish. In other words, swim bladders seem to have evolved
from lungs rather than lungs from swim bladders. (Perry, 2001). Basal actinopterygians even possess blood vessels which correspond to the pulmonary circuit of lungfish and tetrapods, although there is no separation of this blood from that of the systemic circuit in the heart (Johansen, Kjell from Hoar, 1970). The blood hematocrit (the percentage of the blood composed of red blood cells) of some air-breathing fishes even reaches that observed in humans (such as that of Symbranchus, 47%, and of Electrophorus, 41%) (Johansen, Kjell from Hoar, 1970). (Perry, 2001). The brainstem activity in swallowing is similar in the gar (which possesses a swim bladder) and tetrapods (which have lungs) (Perry, 2001). The first fish to breathe oxygen from air may have taken advantage of muscular mechanisms of gulping food which are similar to those used in breathing (Dutta, p. 138). |
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GAR |
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BOWFIN |
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Note
that there are capillaries in the swim bladder of fish which bring the
blood close enough to the epithelial lining of the bladder to perform
gas exchange with the air contained within. |
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LUNGS If a lung is defined as a vascularized sac which performs gas exchange with the air,
then lungs are known in some spiders, decapods, chilopods,
isopods, and snails (and in all scorpions).
In invertebrates, gas exchange occurs through diffusion only, in
contrast to vertebrate lungs which require muscle contractions to transport
air through respiratory surfaces. (Hoar, 1983, p. 508) The ancestors of bony fish evolved lungs
as an endodermal outpocket
of the esophagus (Romer, p. 363). Although advanced actinopterygians
(teleosts) converted this outpocket
to a swim bladder to control buoyancy, the earliest actinopterygians
possess lungs (such as Polypterus)
whose texture is similar to amniote lungs. The gar pike and Amia will suffocate without atmospheric
air; as will sarcopterygian lungfish (Romer, p. 360, 364). The
placoderm Bothriolepis possessed a pair of
pharyngeal pouches which extended posteriorly
and probably functioned as simple lungs.
Some sharks possess a vestigial structure which may be related
to this (Weichert, 1970; Kardong). |
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Among vertebrates, lungs are not unique
to tetrpods, they actually evolved in fish before the adaptation to life
on land. Some fish alive today,
appropriately called lungfish, possess true lungs, the homologs
of tetrapod lungs.
The Australian lungfish, the most primitive of the three lungfish,
possesses a single lung while the African and South American lungfish
possess paired lungs. Once African
lungfish (Protopterus) reach
more than 100g in size, more than 85% of their oxygen is obtained through
gas exchange in the lung (Orgeig, 1995). While the Australian lungfish is a facultative
air breather, African and South American lungfish are primary air breathers
during several months of the year which they spend on land (Hoar, 1970).
The African lungfish secretes a mucus cocoon where it waits in
the mud for the next rainy season. During this time, it depends on oxygen from
the air which is breathed through its lungs.
Experimentally, these lungfish have been able to survive 5 years
in this state.(Webster, 1974, p. 372) The lungs of lungfish actually function better
than those of many amphibians. (Weichert,
1970, p. 221). In some lungfishes, the lungs possess adaptations
such as internal divisions and alveoli-like chambers which resemble the
lungs of amphibians. Electron microscopic
examination has demonstrated that lungfish and amniote
lungs share the same basic structure. (Johansen, Kjell from Hoar, 1970; Romer, p.
364-6). |
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Amphibian lungs are similar to the lungs of
lungfish and empty into the esophagus, as do those of lungfish. Many amphibian lungs have alveoli, at least
in the basal areas. In reptiles,
there is an increased internal complexity in the lungs as septa further
divide them (Romer, p. 363; 366 Amphibian
and reptilian lungs are located in the pleuroperitoneal
cavity (Weichert, 1970). Mammalian ancestors evolved separate pleural
cavities (also present in some reptiles) (Romer,
p. 320). Reptiles possess both
type I and type II cells in their lungs (Maina,
1998). Birds possess the most efficient respiratory
system among tetrapods. Air sacs in bones also
help lower specific gravity— a preadaptation
for flight which was achieved by some feathered dinosaurs. The majority
of the air in the avian respiratory system is outside the lungs (Webster,
1974, p. 384). Most snakes a vestigial
left lung. (Torrey, p. 334). |
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FROG |
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TURTLE |
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ALLIGATOR |
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Marsupial lungs are more reptilian at birth
in that they possess a separate capillary in each air space and a cuboidal epithelial lining of the alveoli. Later in life marsupial lungs acquire classical
mammalian characteristics. (Stonehouse, 1977) |
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OPOSSUM |
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SHEEP |
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PIG |
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CAT |
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The number of lobes of the lungs varies in
primates. The rhesus monkey possesses
an accessory lobe (lobus azygos)
on the posterior side of the right lung.
Tarsiers have additional lobes and orangutan lungs usually lack
lobes. (Hartman) |
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MONKEY |
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HUMAN MODEL |
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SURFACTANT Human lungs depend on surfactant proteins to decrease water tension in air sacs. The epithelia of an air sac is pictured below. |
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