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When I see diagrams of cellular structures such as the layers of the skin, these diagrams show the cells as being closely packed together. For example here is a typical such diagram:

skin structure

So, it shows the capillaries of the vascular system going only to the bottom of the many layers of tightly packed cells. So, I assume that all the cells need oxygen, so they all need to be in contact with the blood cells that bear oxygen, so how does this happen? Obviously each individual cell can't have its own personal capillary. If blood were to be released just loosely around the cells that would not work either because the blood would have no way to get back. All the blood has to be in vessels of some kind so it can circulate. But these vessels are not shown as reaching the upper skin cells in any way, so how are those cells getting oxygen?

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The skin is highly vascularized up to the epidermis, with dense vessels right at the top of the dermis (your picture omits these, but you will find them in better diagrams like this one). The epidermis itself gets substantial oxygenation from the outside air (see Stücker et al, for example), though the very outermost layers are not metabolically active ("dead") and do not require oxygen.

As a gas, oxygen moves quite well through tissue, and cells can get sufficient oxygen from sources up to a few hundred micrometers away. No need to be in direct contact with blood cells (and typically no tissues of the body are in direct contact, besides vascular endothelial cells that line all blood vessels). Many small animals have no need for a circulatory system because their entire body is close enough to the outside for gas diffusion.


Stücker, M., Struk, A., Altmeyer, P., Herde, M., Baumgärtl, H., & Lübbers, D. W. (2002). The cutaneous uptake of atmospheric oxygen contributes significantly to the oxygen supply of human dermis and epidermis. The Journal of physiology, 538(3), 985-994.

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  • $\begingroup$ What is the largest "small animal" which doesn't have a need for a circulatory system? (Approximately, to an order of magnitude, I'm just trying to get a feel for how much depth you're talking about.) $\endgroup$ – davidbak Dec 20 '20 at 17:55
  • $\begingroup$ @davidbak Flatworms and roundworms are probably a good place to start. Some of these can be quite large (some flatworms can be meters long) but in that case they are also quite thin. Jellyfish, too, can be quite large but their bodies are also quite thin despite the space they take up; they also don't have a lot of metabolic need. $\endgroup$ – Bryan Krause Dec 20 '20 at 18:22
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It's also worth noting that the ~15-60 most superficial layers of keratinocytes (epidermal epithelial cells) are not metabolically active (i.e., "dead") and therefore do not need to receive nutrients/gasses or eliminate wastes. Therefore, exchange with blood is not needed beyond 10-15 layers of "living" epithelial cells in the deeper epidermal strata.

  • The deeper, active strata are approximately 20-100 μm thick 1, which is within the "few hundred" μm range for interstitial gas exchange that @BryanKrause mentions in his answer.

See here and here for more info.

enter image description here

Modified From: wikimedia


Citations:

1. Olatunji, O. and D.B. Das. 2011. "5.48 - Drug Delivery Using Microneedles." Pages 625-642 in Moo-Young, M., editor. Comprehensive Biotechnology (Second Edition), Academic Press, ISBN 9780080885049.

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  • $\begingroup$ Ok, so you are saying that the oxygen just diffuses out of the hemocytes through multiple layers of cells? What about nutrients, can nutrients diffuse the same way? I guess my question applies to muscle as well as skin. Like when I eat say a chicken breast, I don't see any blood vessels in the chicken breast. $\endgroup$ – Imprisoned Rhesus Dec 19 '20 at 20:18
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    $\begingroup$ @ImprisonedRhesus My post is really more of an extension of BryanKrause's answer and is only meant to point out the anatomy of the skin (i.e., that superficial layers aren't alive). I wasn't gong to extend my post with further physiology (e.g., interstitial exchange and transport) due to current personal time constraints. I hope someone else can provide you an adequate response in a separate answer $\endgroup$ – theforestecologist Dec 19 '20 at 20:31
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    $\begingroup$ @ImprisonedRhesus Also, I would keep your question focused on skin here. If you're interested in muscles or tissues more generally, it might be worth taking what you've learned here and asking another question that builds from this one. $\endgroup$ – theforestecologist Dec 19 '20 at 20:31

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