Here’s that hen respiratory system segmentation you ordered

Two and a half years ago, I posted a glorious hemisected hen, taken (with permission) from a poster by Roberts et al. 2016, and supplied by Ray Wilhite, best known in this parish for his work on sauropod appendicular material.

At the end of that post, I blithely promised “More from this poster in a subsequent post!”, and then — predictably — forgot all about it. My apologies. Here is the fulfilment of that promise, in glorious colour:

There’s lots to love here, not least the sheer extent of the respiratory system — it almost seems there is no space in the hen’s torso for any actual soft tissue. But the big thing for me is how tiny a part of the respiratory system the lung contributes. It’s almost an afterthought: it’s a fool’s game judging 3d volumes from a single perspective, but here it seems that the lung makes up at most 20% of the system.

And yet it’s the only part of the system that has parenchymal tissue — the only place where gas exchange takes place. The air-sacs are not doing anything: they just sit there, moving air through the lung as they expand and contract but otherwise inert. Isn’t that strange? Doesn’t it seem wasteful? Why not respire though the entire air-sac system?

And of course this raises questions about how the system worked in sauropods. Long-time followers of this blog, or indeed of Matt’s research output, will know that there is very good evidence that sauropods had an air-sac system similar to that of birds, but since the air-sacs themselves do not fossilise we can’t know the details of the soft-tissue anatomy — only what we can infer from fossilised vertebrae. So I can’t help speculating about whether the greater metabolic demands of sauropods compelled them to evolve more extensive gas-exchange in their respiratory systems.

[“Greater metabolic demands”? Yes, because metabolic throughput scales roughly with body mass to the 3/4 power (Kleiber 1932) but air gets into an animal though a gas-exchange surface whose area, if isometric, goes with the square of linear dimension, i.e. body mass to the 2/3 power. So metabolic demand relative to gas-exchange area goes with body mass to the power 3/4 / 2/3 = 3*3/4*2 = 9/8. All numbers very subject to debate.]

Long, long ago (2004), in an email, I asked Matt this same question. His response, in part:

Blue whales, of up to 209 tons, get by just fine with the horribly inefficient mammalian design, so why couldn’t 100 ton sauropods get by with the avian one?

Which is a good point. But as I responded at the time:

Maybe the real mystery here is what the heck are whales doing that we’re not? And the answer would seem to be “swimming in water, which is an order of magnitude less energetically demanding than walking on land”. Hmm.

And there, my record of our discussion fizzles out. If we discussed further, history does not record what was said. And I feel this is still worthy of some exploration. In short, whales are big blubbery cheats, and nothing they say or do can be taken at face value.

Bonus content! Here is the whole poster!

References

• Kleiber, M. 1932. Body size and metabolism. Hilgardia 6:315–353.
• Roberts, John, Ray Wilhite, Gregory Almond, Wallace D Berry, Tami Kelly, Terry Slaten, Laurie McCall and Drury R. Reavill. 2016. Gross and histologic diagnosis of retrograde yolk inhalation in poultry. The American Association of Avian Pathologists, San Antonio, Texas. doi:10.13140/RG.2.2.28204.26246