Like birds, green iguanas have unidirectional airflow in their lungs: When they breathe in or out, air flows in a one-way loop. In humans and other mammals, lungs have tree-like airways: The main trunk in each lung splits into branches and "twigs.
The respiratory system of birds is strikingly distinct from all other animals with a few rare exceptions. In their lungs, air loops efficiently in one direction through a series of tubes lined with blood vessels for gas exchange, and aerodynamic forces act like valves to sustain the one-way flow as they inhale and exhale.
This one-way flow and its corresponding suite of anatomical features were thought to be vital to their ability to fly, an energetically demanding activity. Recent work revealed that alligators and monitor lizards also have a bird-like pattern of airflow.
Now, Farmer and colleagues show that green iguanas -- who are not known for their high-capacity aerobic fitness, and who lack all those avian adaptions above -- also have unidirectional airflow in their simple lungs. By adding yet another group of reptiles to the bird-like breathing list, a new study in Proceedings of the National Academy of Sciences suggests that we need to rethink the evolution of the vertebrate respiratory system.
Oxygen and carbon dioxide pass to and from blood in tiny air sacs, called alveoli, at the tips of the smallest airway branches. In bird lungs, air loops in one direction through a series of tubes lined with blood vessels for gas exchange. Aerodynamic forces act like valves to sustain the one-way flow through cycles of inhalation and exhalation.
Iguanas don't fly. Alligators also have a bird-like pattern of airflow. Farmer and Kent Sanders, a radiologist at the U, revealed that in a study. It was the first evidence that one-directional lung ventilation might be an innovation pre-dating the origin of birds. Earlier this year, Farmer along with Emma Schachner and Robert Cieri at the U, and James Butler of Harvard University reported that monitor lizards have one-directional airflow through their lungs, too.
Those discoveries left open the possibility that crocs and monitor lizards evolved their bird-like lungs independently, that is, their evolution converged on a design similar to birds. The finding of bird-like lungs in yet another group of reptiles builds a stronger case for an origin in the remote past in a common ancestor.
To make the discovery, Farmer and co-authors Cieri, Schachner and Brent Craven of Pennsylvania State University had to find a way to visualize air moving through iguana lungs. In one set of experiments, they used a surgical scope to look inside the lungs of live iguanas as the lizards inhaled harmless smoke from a theatrical fog machine.
They also used probes that measure air speed and volume in dissected lungs. Working from 3-D X-ray imaging of the contours of iguana lungs, Craven made a computer model simulating airflow. The model's predictions closely matched the patterns observed in real lungs. The revelations make clear that scientists have much to learn about the physiology of lungs in species other than mammals. Textbooks generally assert that air moving in and out of lungs flows down a pressure gradient from a point of higher pressure to one of lower pressure, but Farmer says her group's findings show that in iguana lungs "that's not what's going on at all.
The mechanics aren't fully known yet, but Farmer says a better understanding could inspire new ways to design devices that circulate or filter blood or other fluids without using mechanical valves. More from Biology and Medical. Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page. For general inquiries, please use our contact form. For general feedback, use the public comments section below please adhere to guidelines.
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More information Privacy policy. The finding bolsters the case that uni-directional bird-like flow evolved long before the first birds, arising nearly million years ago in a common ancestor of lizards, snakes, crocodiles and dinosaurs. Air flows in and out in a tidal fashion. Oxygen and carbon dioxide pass to and from blood respectively in tiny air sacs, called alveoli, at the tips of the smallest airway branches. In bird lungs, air loops in one direction through a series of tubes lined with blood vessels for gas exchange.
Aerodynamic forces act like valves to sustain the one-way flow through cycles of inhalation and exhalation. That is all wrong.
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