The lungs of
archosaurs (crocodilians, birds, and dinosaurs) are constructed rather differently from those of mammals.
Mammals (and amphibians, and non-archosaur reptiles) have what's called
tidal respiration, meaning that there is a 2-way flow of air in our respiratory passages. Air first flows in during
inspiration, and then out through the same passages during
expiration.
Mammals use a unique muscle called the
diaphragm to expand the chest cavity during inspiration. The
intercostal muscles between the ribs also help to expand the chest cavity. Some of the neck and shoulder muscles can also help to raise the clavicles (collarbones) to further expand the chest cavity during inspiration.
As the chest cavity expands, the pressure inside the lungs drops, and so air is sucked in until the pressure of the air inside the lungs is equal to the outside air pressure. When the diaphragm and intercostal muscles relax, the chest cavity contracts and the lungs are compressed. This raises the pressure inside the lungs, forcing air out until the pressure inside the lungs is equal to the pressure in the outside air.
This is called
negative-pressure breathing, since you bring air into your lungs by decreasing the internal pressure. One consequence of this is that there's no way to force air into your lungs. Another consequence is that you cannot empty your respiratory passages of air. That is, you cannot exchange all of the "stale" air in your lungs with "fresh" air, even if you take several deep breaths in quick succession.
Our respiratory passages end in small, blind-ended sacs known as
alveoli. This is where gas exchange occurs (oxygen is absorbed from the air in the alveoli and into the blood; carbon dioxide is absorbed from the blood and into the air to be exhaled). But as you can imagine, this is a relatively inefficient process, since so much "stale" air remains in the alveoli and the rest of the respiratory passages.
By the way, just for comparison, most amphibians and some reptiles are
positive-pressure breathers. Instead of expanding the lungs to suck air in, they draw air into throat pouches and then compress their throats to
force air into their lungs. (That's why a frog's throat is always moving up and down; it's a mechanism for pumping air into its lungs.)
An archosaur's respiratory system works somewhat differently. They don't have tidal flow in their respiratory systems, they have
one-way flow, meaning that air moves through the system in a continuous loop. One important consequence of that is that, during a 2-breath cycle, a bird can
completely replace the air in its respiratory passages. This means that a bird has a much more efficient respiratory system than does a mammal, and this is part of the reason why birds can breathe and function normally in air that has so little oxygen that a mammal would asphyxiate.
A bird's respiratory system is rather more complicated than a crocodilian's, so we'll look at that.
A bird has relatively small lungs (as does a crocodilian), but 7 (or 9, depending on the species) relatively large
air sacs. Many of a birds' bones are hollow, and some of the air sacs extend into the bones. (I saw an interesting demonstration once; somebody drilled a hole into both humerus bones of an albatross, down into the air sacs, and then plugged the bird's nostrils. It was able to continue breathing just fine.)
A crocodilian doesn't have anywhere near as elaborate a respiratory mechanism as does a bird, but it works on the same basic principle. Also, there is good fossil evidence that at least some dinosaurs had air sacs much like those of a modern bird and, presumably therefore, the same sort of respiratory mechanism. This isn't really too surprising, given the fact that birds are apparently the direct descendants of theropod dinosaurs. Some people have speculated that dinosaurs' much more efficient respiratory systems help to explain why dinosaurs so completely dominated mammals for over 100 million years. (Contrary to popular impressions, mammals have been around for as long as dinosaurs. In fact, the ancestors of what would become true mammals appeared
first and were the dominant land animals for awhile, before being replaced as the dominant land animals by the early dinosaurs.)
The respiratory system of a bird.
Anyway, a bird breathes in a 4-step cycle that involves 2 complete breaths.
During Step 1 (the "first inspiration"), the bird draws air into its
posterior (
caudal)
air sacs through the nostrils. (A bird does not have a diaphragm; only mammals have a true diaphagm. Contraction of muscles in the bird's chest move the breastbone outward to expand the chest cavity.)
During Step 2 (the "first expiration"), the bird compresses the posterior air sacs to force air into the
lungs. As the air flows through the lungs, it flows not into blind-ended alveoli but through tiny tubes (
air capillaries) that are open at both ends. This is where gas exchange occurs.
During Step 3 (the "second inspiration"), the bird expands its
anterior (
cranial)
air sacs and the air flows into them from the lungs.
Finally, during Step 4 (the "second expiration"), the anterior air sacs are compressed and air flows out of the respiratory passages, via the nostrils.
You can see that the air moves in a continuous loop, not in the back-and-forth way that it moves through the respiratory passages of a mammal. Because of this, the bird is able to completely replace the air in its respiratory passages with every 2 breaths -- something no mammal can come close to doing. The air cannot flow backwards because of various valves which ensure one-way flow.
The 4-step respiratory cycle of a bird.
A crocodilian doesn't have the intricate series of air sacs that a bird has, but a crocodilian's lung is divided internally into several different chambers. Apparently, air flowing from one lung chamber to another moves in much the same way that air flows between the air sacs and lungs of a bird. What's more, in a crocodilian's lungs, gas exchange occurs not in blind-ended alveoli, but in open-ended tubes similar to those in the lungs of a bird. (Somewhat confusingly, a crocodilian's lungs
do contain sacs that are sometimes called "alveoli." They aren't true alveoli, however; only mammals possess true alveoli, which are blind-ended sacs that serve as the primary sites of gas exchange.)
A crocodilian lacks a true diaphragm, but it has a muscle called the
diaphragmaticus that performs much the same function. The diaphragmaticus is attached to the liver and some of the other internal organs, and when a crocodilian inspires, the diaphragmaticus pulls the liver and other organs back toward the hips, causing the chest cavity to expand. At the same time, intercostal muscles pull on the ribs to help expand the chest cavity (just as in mammals).
During expiration, the diaphragmaticus relaxes and the visceral organs move forward, compressing the lungs and forcing air out.
Cheers,
Michael
Re: A Question For The Lone Ranger
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