Can someone with real medical competence tell me why this weird idea wouldn't work?

[GaYellowDawg]

9. I don't think it would work.

There are a couple of main reasons I don't think that would work.

1. Too thick a layer to allow diffusion of oxygen into the bloodstream, I think. We're reliant on hemoglobin because oxygen doesn't dissolve into water very easily, especially at our body temperature. In order to load oxygen onto red blood cells, the oxygen molecules have to pass through alveolar cells, which are very thin (a type of tissue called simple squamous epithelium), then a thin protein basement membrane below it, then the walls of the alveolar capillaries, then across the red blood cell membrane. Only then is O2 loaded onto hemoglobin.

In the large intestine, the oxygen would have to pass through a mucus layer much thicker than that in the lungs. Then the cells themselves are much thicker - simple columnar cells. They are, as the name implies, column-shaped instead of flat. Then after the columnar epithelium, the O2 would have to pass through a protein basement membrane, then through a thin layer of smooth muscle called the muscularis mucosae, then through at least a little connective tissue to make it into the capillaries in the submucosa. It's unlikely you'd be able to put the oxygen at a high enough partial pressure to drive it into those capillaries.

2. The hemoglobin would probably have less affinity for oxygen. In the lungs, the local environment is more alkaline than in peripheral tissues. This pH causes hemoglobin to adopt a form that has a higher affinity for oxygen, and it's easier to load the O2 onto hemoglobin. Out at peripheral tissues, where cellular respiration is producing a lot more CO2, the local environment is more acidic. That pH causes hemoglobin to adopt a form that has a lower affinity for oxygen, and in that case, it's easier to unload oxygen and more difficult to load oxygen than it is in the alveolar capillaries.

The diffusion distance and pH, I think, would prevent this from working well enough to make a difference. It's not bad thinking, though, and I would love to field a question like that from my students.