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Misteri napas kadal : aliran udara satu arah mungkin berusia 270 juta tahun yang lalu

Air mengalir terutama di satu arah loop melalui paru-paru dari biawak - metode pernapasan yang sama oleh burung , buaya dan mungkin dinosaurus , menurut sebuah studi baru yang dapat mendorong evolusi dari sifat ini kembali ke 270 juta tahun yang lalu ....more 

The mystery of lizard breath: One-way
air flow may be 270 million years old

Date:

December 11, 2013

Source:

University of Utah

Summary:

Air flows mostly in a one-way loop through the lungs of monitor lizards --
a breathing method shared by birds, alligators and presumably dinosaurs,
according to a new study that may push the evolution of this trait back to 270
million years ago.

.............................

Air flows mostly in a one-way loop through the lungs of monitor lizards --
a breathing method shared by birds, alligators and presumably dinosaurs,
according to a new University of Utah study.

The findings -- published online Dec. 11 in the journal Nature --
raise the possibility this breathing pattern originated 270 million years ago,
about 20 million years earlier than previously believed and 100 million years
before the first birds. Why remains a mystery.

"It appears to be much more common and ancient than anyone
thought," says C.G. Farmer, the study's senior author and an associate
professor of biology at the University of Utah. "It has been thought to be
important for enabling birds to support strenuous activity, such as flight. We
now know it's not unique to birds. It shows our previous notions about the
function of these one-way patterns of airflow are inadequate. They are found in
animals besides those with fast metabolisms."

But Farmer cautions that because lizard lungs have a different structure
than bird and alligator lungs, it is also possible that one-way airflow evolved
independently about 30 million years ago in the ancestors of monitor lizards
and about 250 million years ago in the archosaurs, the group that gave rise to
alligators, dinosaurs and birds. More lizard species, such as geckos and
iguanas, must be studied to learn the answer, she says.

Farmer conducted the study with two University of Utah biologists -- first
author and postdoctoral fellow Emma Schachner and doctoral student Robert Cieri
-- and with James Butler, a Harvard University physiologist.

The research was funded by the American Association of Anatomists, the
American Philosophical Society, the National Science Foundation and private
donor Sharon Meyer.

Tidal Versus One-Way Airflow in the Lungs

Humans and most other animals have a "tidal" breathing pattern:
Air flows into the lungs' branching, progressively smaller airways or bronchi
until dead-ending at small chambers called alveoli, where oxygen enters the
blood and carbon dioxide leaves the blood and enters the lungs. Then the air
flows back out the same way.

Birds, on the other hand, have some tidal airflow into and out of air sacs,
but their breathing is dominated by one-way airflow in the lung itself. The air
flows through the lung in one direction, making a loop before exiting the lung.

In 2010, Farmer published a study showing that a mostly one-way or
"unidirectional" airflow controlled by aerodynamic valves exists in
alligators. That means the breathing pattern likely evolved before 250 million
years ago, when crocodilians -- the ancestors of alligators and crocodiles --
split from the archosaur family tree that led to the evolution of flying
pterosaurs, dinosaurs and eventually birds.

The new study found a mostly one-way, looping air flow in African savannah
monitor lizards, Varanus exanthematicus -- one of roughly 73
species of monitor lizards -- although there was some tidal airflow in regions
of the lungs. That means one-way airflow may have arisen not among the early
archosaurs about 250 million years ago, but as early as 270 million years ago
among cold-blooded diapsids, which were the common, cold-blooded ancestors of
the archosaurs and Lepidosauromorpha, a group of reptiles that today includes
lizards, snakes and lizard-like creatures known as tuataras.

One-way airflow may help birds to fly without passing out at high
altitudes, where oxygen levels are low. Before the new study, Farmer and others
had speculated that the one-way airflow may have helped dinosaurs' ancestors
dominate the Earth when atmospheric oxygen levels were low after the
Permian-Triassic mass extinction -- the worst in Earth's history -- 251 million
years ago.

"But if it evolved in a common ancestor 20 million years earlier, this
unidirectional flow would have evolved under very high oxygen levels," she
says. "And so were are left with a deeper mystery on the evolutionary
origin of one-way airflow."

How the Study was Performed

As in her earlier research on alligators, Farmer and colleagues
demonstrated predominantly one-way airflow in the lungs of monitor lizards in
several ways. They performed CT scans and made 3-D images of lizard lungs to
visualize the anatomy of the lungs. They surgically implanted flow meters in
the bronchi of five monitor lizards to measure airflow direction.

Using lungs removed from 10 deceased lizards, the researchers measured air
flow as they pumped air into and out of the lungs. They also pumped water laden
with sunflower pollen particles or plastic microspheres through lizard lungs,
and the movement of the pollen and spheres also showed the unidirectional
airflow.

Savannah monitor lizards were used in the research because they are
relatively large and thus easier to study, weighing about a pound and measuring
roughly 15 inches from head to tail tip. Monitor lizards also have some of the
highest rates of oxygen consumption, partly because they breathe using not only
their trunk muscles and ribs, but also using "gular pumping," which
is when the lizards flare out their throat and pump air into their lungs.

Monitor lizards' lungs have more than a dozen chambers or bronchi in each
lung. The primary airway runs the length of the lung, with lateral bronchi
branching off of it.

The study showed that air enters the lizard's trachea or windpipe, then
flows into the two primary airways, which enter the lung. But then, instead of
flowing tidally back out the same way, the air instead loops back in a
tail-to-head direction moving from one lateral airway to the next through small
perforations between them.

The walls containing perforations that allow air to flow from one chamber to
the next "are like lace curtains," Farmer says.

There appear to be no mechanical valves or sphincters, so the one-way
airflow appears "to arise simply from jetting," or aerodynamic valves
created when air flows around bends within the lung airways. That is supported
by the fact that one-way airflow was observed even in lungs removed from dead
lizards.










Story Source:

The above story is based on materials provided by University
of Utah. Note: Materials may be edited for content and length.










Journal Reference:

1.   
Emma R. Schachner, Robert L. Cieri, James P. Butler & C. G.
Farmer. Unidirectional pulmonary airflow patterns in the savannah
monitor lizard. Nature, 2013 DOI:10.1038/nature12871
http://www.sciencedaily.com/releases/2013/12/131211133943.htm

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