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OTHER RESPIRATORY STRUCTURES
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THE NASAL CAVITY In most fish, there is no connection between
the nasal cavity and the oral cavity.
This is true of lampreys, most cartilaginous fish, and virtually
all actinopterygians. There
are a few exceptions. For example,
in hagfish, a nasopharyngeal pouch (present in lampreys) communicates
with the pharynx. In some cartilaginous
fish, there are oronasal grooves which connect
the nasal cavity with the mouth. In
the teleost genus Astroscopus, there is a channel which connects the nasal and
oral cavities (Weichert, 1970). Sarcopterygians evolved a connection between the nasal cavity
and the oral cavity, called an internal naris
(Romer, p. 325; Weichert,
1970). In none of these fish, not
even the lungfishes, are the nostrils used to
breathe (Weichert, 1970). Tetrapods utilize
their nasal cavity for the intake of air. In reptiles, the length of the nasal cavity is
typically longer than in amphibians. Crocodiles,
like mammals, developed a secondary palate in which the nasal and oral
cavities fuse are separated by shelves of bone
(Weichert, 1970). A hard and
soft palate separate the air of the nasal cavity from food in the
oral cavity (Romer, p. 327). |
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LUNGFISH |
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In
cynodonts, the ancestors of mammals, the maxillary and palatine
bones in the roof of the mouth fuse to form a hard palate. This secondary palate separates the oral and
nasal cavities which allowed mammals (with their higher metabolisms) to
breathe even while chewing. |
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Amphibians evolved cartilage around their
nostrils and the ability to change the size of their nostril openings (Weichert, 1970). Some
amphibians possess a cartilaginous projection into nasal cavity, the first
known concha. (Weichert,
1970, p. 204). All amniotes
(except perhaps turtles) possess nasal conchae,
homologous to the inferior nasal concha in mammals.
(Gauthier, 1988; Weichert, 1970, p. 205-7). Some people have what might be erectile tissue
over their concha which may interfere with breathing
and may even respond to erotic stimuli (Weichert,
1970, p. 207). |
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CAT |
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LARYNX Amphibians evolved a
intrapulmonary duct and amniotes evolved cartilaginous support of this
duct (Perry, 2004). Amphibians
may possess a short trachea and larynx as well as the cricoid
and arytenoids cartilages of the larynx.
Mucus membrane folds in the larynx form vocal cords and the pitch
of the sounds produced is increased with additional tension on these folds. These are not considered to be homologs of mammalian vocal cords (Romer,
p. 370; Dutta, p. 246). In reptiles the hyoid apparatus became more
closely associated with the larynx. Only
mammals and crocodiles possess a thyroid cartilage of the larynx. In some lizards, mucus membrane folds may represent
the early stages the epiglottis of mammals. (Weichert, 1970, p. 230-1; Gauthier, 1988). The thyroid
cartilages fuse in marsupials and placentals
but not in monotremes (Weichert,
1970, p. 231). Some mammals possess
additional corniculate and cuneiform cartilages. Vocal cords evolved in mammals (Romer, p. 370). Elephants
lack false vocal cords while hippos lack true vocal cords. Howler monkeys have a larynx which is modified
to form a resonating chamber (Weichert, 1970). In human infants, the larynx and hyoid bone
are located in a position similar to that observed in other mammals although
both structures subsequently descend.
The resulting positions of the larynx, hyoid and tongue and the
shape of the pharynx enable human speech.
This change seems to have evolved in two steps.
The first step, which occurred in the last common ancestor of chimps
and humans, was the descent of the larynx.
The descent of the hyoid occurred in human ancestors, perhaps in
conjunction with the changes in the jaw or cranial flexion which also
occurred in the lineage. While
this allowed for speech, it had the disadvantage of increasing the risk
of choking on food (Nishimura, 2003; Kardong,
p. 496). |
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TURTLE |
ALLIGATOR![]() |
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OPOSSUM |
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CAT |
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SHEEP |
GOAT |
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MONKEY |
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HUMAN MODEL |
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TRACHEA In amphibians, the trachea is extremely short.
This is especially true in frogs but the trachea may reach a few
cm in length in some salamanders. In amniotes, the trachea is longer. Partial rings of cartilage exist around the
trachea in some amphibians (such as caecilians). Snakes may have fused tracheal cartilages. (Weichert, 1970, p. 233). Some lemurs have complete rings in the upper
portion of the trachea (Hartman). Many
birds have ossified rings in their trachea and penguins have a double
trachea. Bronchi developed in amniotes. In some snakes, only the right bronchus and
lung are present. (Weichert,
1970, p. 233). In pigs,
whales, and other ruminants a third primary bronchus (the apical or eparterial
bronchus) forms. Mammalian bronchi
are supported by cartilage. (Weichert,
1970, p. 235). |
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TURTLE |
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ALLIGATOR |
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CHICKEN |
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SHEEP |
PIG |
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CAT |
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MONKEY |
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HUMAN MODEL |
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PULMONARY
VENTILATION AND HEMOGLOBIN More active teleost
fish generally have a larger surface area in their gills. Tuna, which are very active and can even perform
some thermoregulation, have a respiratory surface area which approaches
the lung surface area in mammals. They
also have blood hemoglobin levels (20g/100 ml blood) which are higher
than most fish (Hoar, 1970, p. 270-1).
Fish which either swim rapidly or live in rapidly moving water
simply allow water to pass over their gills to obtain oxygen.
Other fish must actively ventilate their gills.
Thus, the energy spent in respiration can vary greatly in fish.
(Hoar, 1970) Hypoxia leads to increase in hematocrit in fish as it does in mammals. O2 dissociation
curves similar in fish and mammals (Hoar, 1970). Even in an invertebrate (such as in a worm),
the hemoglobin saturation curve has a steep slope and flat upper portion
as in higher animals (Hoar, 1983, p. 533).
Hagfish hemoglobin shows no Bohr effect
and has a lower oxygen affinity than lamprey hemoglobin (Hoar, 1970). RESPIRATORY MUSCLES The contraction
of a number of muscles can allow for greater gas exchange at respiratory
surfaces. Fish depend on the muscles
which act on the buccal cavity to drive respiration
at the gills. The same muscles
of the buccal cavity are used to drive respiration
in both lungfishes and amphibians. (Johansen, Kjell from Hoar, 1970). Lungfish also use skeletal muscle and aspiratory
forces help venous blood return to the heart, as in tetrapods (Hoar, 1970). Amphibians
do not use the muscles between the ribs (intercostals muscles) to facilitate
respiration. Because the ribs of
amphibians do not contact the sternum as in amniotes, ventilating the
lungs cannot be accomplished by changing the volume of the thoracic cavity. The inefficiency of the resultant amphibian
respiration is perhaps best illustrated in the loss of the lungs in the
very successful family of salamanders Plethodontidae
which perform all gas exchange through their skin.
In amniotes, the ribs attach to the sternum and
allow the ventilation of the lungs (Romer, p.
366-70). Intercostal
and abdominal muscles are used in breathing to create negative pressure
to inflate the lungs (Kardong, p. 421-2). With a more efficient pulmonary respiration,
cutaneous respiration was reduced and the skin of amniotes
became less permeable (to limit water loss). Although crocodiles do not possess the mammalian
diaphragm, they do possess a transverse septum which separates the pleural
cavities from the peritoneum. Crocodiles
also possess a muscle, the diaphragmaticus,
which can pull on this septum to expand the pleural cavities. (Webster, 1974, p. 381). Note
the absence of a diaphragm in the following animals |
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FROG |
ALLIGATOR |
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CHICKEN |
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While
muscles of the body wall can be used in respiration in amphibians, the
use of negative pressure for inhalation is unique to amniotes (Perry,
2004). While only mammals possess a completely muscularized diaphragm which separates the thoracic and abdominopelvic body cavities, septa and the presence of some
muscle also exists in at least some crocodiles, birds, turtles, lizards,
and snakes (Gauthier, 1988). In mammals, the evolution of the diaphragm
allowed the thorax to increase its volume further and thus allow for the
inhalation of more air. Mammals
also lost their lumbar ribs so that the organs of the abdominal cavity
could be pushed outward when the diaphragm pushed downward. These modifications for greater gas exchange
were important as mammals evolved a higher metabolism (Romer,
p. 368). |
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PIG |
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Many invertebrates, including worms, can
use autorhythmic neurons to control respiration
and use respiratory reflexes, as do vertebrates (Hoar, 1983, p. 521). In all gnathostomes,
the medulla is the region of the brain which is responsible for rhythmic breathing (Hoar,
1970, p. 321). |
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