Insects
Insects don't have a circulatory system as they have a highly branching tracheal system that can take air all the way to the cells. The tracheal system begins at one of many spiracles, which are little "air holes" on the outer surface (for lack of a better word) of the insect. These spiracles connect to tracheae, which branch out into tracheoles, and so on. Reliance on this system limits insect size. That being said, around 300 million years ago there was a giant dragonfly called Meganeura with a 70cm wingspan. It's been suggested that it was able to survive because atmospheric oxygen then was higher then than it is now (~30% as compared to ~21%).
Fish
As you probably know, fish have gills on the outside of their bodies, which allow them to breathe underwater. Gills are made up of highly vascularised thin parts known as lamellae, which create a very large surface area for gas exchange. Blood travels in the opposite direction to water, creating a "counter-current system" that is very efficient at absorbing oxygen. Despite the large surface area and high efficiency, lots of water needs to be moved past the gills in order for adequate oxygen to be obtained. Lamellae collapse when exposed to air, which is why fish can't breathe when out of water.
Amphibians
Amphibians have lungs, but they look quite different to ours. They are simple inverted bags and don't have as much folding as mammal lungs. Amphibians use "positive pressure" ventilation, which I think means that they mechanically pump something (the buccal cavity, which is like the mouth cavity of the frog) in order to move air around.
More "primitive" amphibians use 4-stroke ventilation which, as you may guess, has four steps. Steps 1 and 2 are for inspiration, whereas steps 3 and 4 are for expiration.
- Nostrils open and buccal cavity expands, allowing air to enter the buccal cavity.
- Nostrils close, glottis opens and buccal cavity contracts, pushing air into the lungs.
- Lungs contract, pushing air back into the buccal cavity.
- Glottis closes, nostrils open and buccal cavity contracts, pushing air out.
More "modern" amphibians use 2-stroke ventilation:
- Buccal cavity expands. Air is drawn in from outside and from the lungs.
- Buccal cavity contracts. Air is forced out outside and into the lungs.
One of the downsides to 2-stroke ventilation is that there is some mixing of old and new air.
Reptiles
Reptiles have a "negative pressure" ventilation, just like us. However, most do not have a diaphragm which can contract in order to produce this "negative pressure." Instead, many have muscularised ribs that can do this job. Because of this, some reptiles can't walk and breathe at the same time. Crocodiles are a bit different: they do have a diaphragm-like structure called the diaphragmaticus, which connects the pelvic girdle to the liver (which is connected to the lungs). (Crocodile hearts are also a bit different to those of other reptiles, as I'll talk about in a later post.)
More primitive reptiles have "bag-like" lungs, just like amphibians. More modern ones, however, have folding in order to increase the surface area for gas exchange. Hence we're starting to get closer to the mammalian lung... yay I guess?
Oh and one more thing. Crocodiles and some lizards have something called "unidirectional airflow." Air coming down the trachea can go from the trachea into the dorsobronchi (an air sac), through some parabronchi (where gas exchange happens) and then into the ventrobronchi (another air sac) before leaving. In short, air goes around in one direction (rather than going down into the lungs and then back up the way it came). Birds also have something similar, as I'll talk about later.
Mammals
I won't talk too much about mammals, as I've already spoken quite a lot about the lungs of one mammal in particular (*cough*humans*cough*). They have "negative pressure" ventilation and alveolar lungs. One of the "cons" of the mammalian lung is that it has dead space, which is where air passes through but is not exchanged. There are two types of dead space: alveolar dead space and anatomic dead space. Alveolar dead space occurs when there is no blood flowing through the surrounding capillaries of an alveolus, whereas anatomic dead space occurs in the conducting zones of the lung where air passes through but is not exchanged.
Birds
Bird lungs are kind of weird in that they are calcified. They also have the parabronchial structure and unidirectional airflow that I talked about when talking about reptiles. They do not have a diaphragm but they do have some air sacs that help in pushing the air around. Their blood-gas barrier is very thin, making gas exchange efficient, while the calcification of the lung prevents tearing. Birds have a "cross-current" system where the blood runs perpendicular to the direction of airflow. This is not as efficient as the counter-current system in fish, but it's a lot more efficient than gas exchange without a fancy current system. In fact, they can more efficiently extract oxygen as the oxygen content of the air decreases. (They do this by taking big, slow breaths.)
There are two cycles in bird breathing. In cycle 1, air goes from outside to the posterior air sac during inspiration, and into the lung during exhalation. In cycle 2, air goes from the lung to the anterior air sac during inspiration, and leaves the body during expiration.
Air breathing
A random note on the issues with breathing air (as opposed to water, like fish). Even though air is usually not saturated with water vapour, gas exchange surfaces are moist and water is required in order for diffusion capacity to reach a suitable level. Additionally, inspired air is usually cooler than the body, so heating is also required. The other challenge is that the amount of water required to dissolve oxygen increases as temperature increases, so lots of water needs to be added. This water can be recuperated by decreasing the temperature of the air during expiration, allowing the water to condense out. Thankfully, we have a "nasal turbinate" system in our noses, which acts as a kind of "air-con."
"Arse breathing"
Basically what it says on the box. Some animals, like the Fitzroy River turtle, can breathe through their arses. Yay!
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