So I'm asking this in reference to the injection of insulin, which is commonly done subcutaneously (in the hypodermis, a fatty part of skin). Now I know proteins usually get into the blood when digested through the stomach/intestines - but I was wondering how they manage to get into the blood when injected into muscles/fat? I know there are capillaries pretty much everywhere, but from what I've researched proteins are too large to get through capillary walls.

That begs the question then, how do proteins manage to get into the blood stream when injected? I can't really envisage large capillaries, because surely they'd have huge holes in them or whatever to allow large proteins like insulin in. Any help on this one? Cheers

  • $\begingroup$ maybe from lymphatic capillaries(they have larger pores) ... I am not sure.... $\endgroup$
    – biogirl
    Commented Nov 20, 2013 at 9:02

1 Answer 1


Found the answer - the lymph capillaries. I've not researched them too in depth but from what I've read thus far, their job is to essentially pick up fluids not picked up by the blood capillaries. They have walls that allow proteins to enter but make it very hard for fluid to flow out.

Wiki states that an average of 20 litres of blood gets processed each day by the blood, with roughly 17 litres through the blood capillaries/system (1), with the remaining 3 litres through the lymphatic system.

These are also prevalent around skin. 70% of lymph capillaries are found near the skin (2).

1) http://en.wikipedia.org/wiki/Lymphatic_system 2) http://www.lymphnotes.com/article.php/id/151/

  • $\begingroup$ Could you expand on this slightly to make it more useful for people viewing it in the future? $\endgroup$
    – Rory M
    Commented Nov 20, 2013 at 19:25
  • $\begingroup$ Apologies, didn't see the comment above suggesting the lymphatic system, and have done a bit more research so edited. $\endgroup$
    – Mark Khan
    Commented Nov 20, 2013 at 20:41

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