Mammalian digestive systems evolved to be (AFAIK) a single path where food enters at one end, the digestive processes occur, and waste exits at the other end (let’s leave urine out of this for now). The movement of the bolus through the system is facilitated by peristalsis.
The respiratory system, in contrast, is an in-and-out arrangement where air comes in, the necessary gas exchange occurs, then waste is expelled the same way the fresh air came in.
Would there be any advantage to a respiratory system that worked like the digestive system, where you had a continuous flow of air in one end and out the other? One challenge I can think of is that peristalsis might not work as well for a gas as it does for solids, slurries, and fluids. OTOH it might allow a more efficient extraction of oxygen and dumping of CO2 since these two processes would not have be done in the same place.
Has this never occurred because there would be no advantage? Or because there just has not been an appropriate evolutionary path to lead to it? Or is it one of those things about evolution where we can only shrug?
This is rarely a safe thing to say in biology! Bird respiration is unidirectional continuous flow through the lungs, enabled by air sac reservoirs - although they still inhale and exhale. There’s a good description and diagram here - it’s a simple but clever way to arrange continuous unidirectional flow where it matters without needing separate intake/exhaust tubes.
Unidirectional breathing would require at least one additional valve. There wouldn’t be much point if inhalation and exhalation couldn’t operate concurrently so a more complex structure is needed. Something like birds and dinos do, although it could be done with a sequence of lungs with valving in between but that kind of complexity is tough to reach starting with mammalian bidirectional breathing to start with. Dino ancestors may have started that process in the during the transformation from gills to lungs.
It’s surely worth mentioning that mammals need a much greater volume of air to keep their blood oxygenated than they need food.
There are wide varieties in nature, both digestive and respiratory. Flatworms, for example, only have one opening serving both as mouth and anus. Fish, as mentioned above, extract oxygen from water by a continuous flow through gills.
I don’t think so. Whales breathe in and out through the blowhole. The mouth remains under water and is not used for breathing at all. In fact, the whale’s trachea is not even connected to the whales throat. Whales couldn’t breathe though the mouth even if they tried. But it would be a hell of a belch!
Lungs and gills evolved separately in fish, both the common ancestor of the lobe- and ray-finned fishes had lungs as well as gills. Some fish still have both. Land animals lost the gills, retained the lungs. Cite.
Fish do it by alternating between open mouth / closed gills and closed mouth / open gills. The mouth needs to point forward for feeding, and when a fish is swimming forward, the position of the mouth facilitates “inhaling” water but would inhibit “exhaling” water through the mouth. That effect not a significant factor for mammals that are not continuously moving forward and use air rather than water.
If you are using musculature to expand and contract the lungs to pump air in and out, absent an external flow effect like the fish, is there much advantage to separate intake/exhaust pipes? You need the valve arrangement, so separate intake/exhaust pipes do not give you continuous flow. The advantage would be that if the “dead volume” in the pipes themselves is significant, you are not wasting effort pumping it back and forth.
Weighed against that advantage you have added complexity. If the valve arrangement is compromised, you are dead. And I guess another hole is one more place for pathogens to get in.
I think nature is telling us that when you need to facilitate more rapid metabolism, the bird arrangement is preferred. They are still pumping the dead volume in the trachea back and forth, but they have achieved continuous flow across the lungs.