I teach anatomy and sometimes I introduce a topic by looking at that same structure in other organisms, especially from an evolutionary stand point. I have two questions - are bird lungs "better" than ours because they have additional air sacs or reservoirs that allow gas exchange while they are exhaling as well as inhaling,
Gas exchange occurs when all air breathing animals inhale or exhale. The difference is that with mammals the air just sits in the lungs as C02 builds up in it and its 02 levels drop. This means that the concentration of 02 and C02 in the air get closer to those in the blood. Since gas exchange occurs via diffusion and since the rate of diffusion is proportional to the relative differences in gas concentrations you can see that the rate of gas exchange declines the longer it takes before a new breath is taken.
Birds don't have this problem, 'fresh' air is constantly flowing through the gas exchange tissues and stale air is collected in the anterior air sacs away from the gas exchange tissues.
or are the tiny alveoli found in human lungs actually more efficient because there is more surface area for gas exchange.
The gas exchange tissues of birds are more efficient than mammalian alveoli.
To get efficient gas exchange you need a very large surface area and a very thin barrier between air and blood.
All gas exchange tissues need a fluid lining. Surface tension effects make very small, wet, cavities harder to inflate. It take more mechanical effort to breathe. This matters because the best way to increase surface area is to have very large numbers of very small subdivisions and surface tension effects put a lower limit on alveolar size. So gas exchange surface area is constrained in mammals.
Mammalian alveoli have to be able to handle repeated stretching and contraction. This limits how thin the air/blood interface can get because it needs to be mechanically robust.
The gas exchange tissues of birds suffer from neither of these restrictions.
They are, essentially, 3D nets of capillaries that have air on all sides.
Each capillary is extremely fine bored so you get a very large total surface area.
Because the lungs are fixed volume (~1.4% volume change during the respiratory cycle) the capillaries are not mechanically stressed and their cell walls are extremely thin.
So birds have a larger surface area per unit volume of lung and thinner barriers to gas diffusion.http://jeb.biologists.org/content/203/20/3045.full.pdf
See the contents on page 3050. The images show sections of bird and bat tissues so check the text to tell which image is which.
Its funny how you get in the habit of assuming humans are top of the line evolution wise. I have to remind myself that an eagle's eyes are sharper than mine.
In reptiles, dinosaurs vs alligators, they talk about uni-directional or bi-directional air flow - does that mean air flows out a different way than it came in?
No, the majority of the avian respiratory pathways are bi directional. It is only the airflow through SOME of the gas exchange tissues of the lungs that is one way.
What structure allows this? Or are they refering to those same kind of air reservoirs found in birds? They implied that reptiles like alligators, and birds, survived better in a lower oxygen environment, but it looks like dinosaurs had lungs more like mammals, so I dont get it. Unless all mammals were just smaller and could get by for a while.
That's a lot of questions :-)
The key to unidirectional air flow in bird and alligator lungs is the intrapulmonary primary bronchus and the alternate pathways for air flow through the chambers of the multicameral lung. Google multicameral lung.
You probably know that our trachea bifurcates into two primary bronchi that enter the lungs. In multicameral lungs the primary bronchus runs through the lung and can exit the far end of the lung into an air sac or terminates near the far end. The bronchus has openings along its length that supply air to chambers that lead away from the bronchus.
Figure 3 http://physiologyonline.physiology.org/content/19/2/55.full
shows some examples of multicameral lungs. You can make the image bigger (twice).
Note that multicameral lungs are generally not homogeneous. There are regions of large compliant chambers and regions of small, less compliant chambers where the majority of gas exchange occurs.
Now, if you have connections between chambers then you have the possibility that air can take different paths when inhaling and exhaling. For example air can travel down the bronchus when inhaling but travel from chamber to chamber whilst exhaling.
Now imagine a multicameral lung with large highly complaint chambers at front and back, joined by smaller, fixed-size, gas exchange chambers in the middle. You might start to recognize the arrangement found in birds.
When the animal inhales both sets of compliant chambers expand. This causes the air pressure in those chambers to drop and so higher pressure air from the atmosphere blows into them. Imagine that the air travels down the bronchus to fill the posterior air chambers and that it also enters the gas exchange chambers near the posterior end and passes through them to reach the anterior air chambers.
When the animal exhales imagine the air in the anterior chambers emptying directly into the anterior end of the bronchus whilst air from the posterior air chambers passes through the gas exchange chambers and enters the anterior end of the bronchus.
In both inhalation and exhalation air has traveled through the gas exchange chambers in a posterior to anterior direction. It was unidirectional.
If you are interested in dinosaur lungs then check out these linkshttp://svpow.com/papers-by-sv-powsketeers/wedel-2009-on-air-sacs/http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003303
But probably your best port of call (on the web) for bird lung related info is here http://people.eku.edu/ritchisong/birdrespiration.html
There are some flash animations that show air flow through the avian respiratory system and LOTS of diagrams and pictures of bird lungs and respiratory anatomy.