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I have read that small molecules like oxygen and carbon dioxide diffuse through the capillaries, but larger structures can't fit through.

This answer indicates that some proteins go through via transcytosis.

The wikipedia article on capillaries shows that there are 3 types of capillaries, and it looks like LDL could fit through fenestrated or sinusoid, but sinusoid seem to only be found in certain organs, and if it did go through the holes in fenestrated capillaries, how would the LDL get to cells that are nearby only continuous capillaries? Would fenestrated capillaries be regularly dispersed so one is always nearby, or would the LDL just diffuse through the interstitial fluid originating wherever the fenestrations are?

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  • $\begingroup$ I love these nobel prize questions! This is the same sort of question George Palade had, some 60 years ago :) $\endgroup$ – De Novo supports GoFundMonica Jul 20 '18 at 22:35
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Many biologists had the same question for a number of years. In the 1950s, Nobel Laureate George Palade identified small vesicles in endothelial cells and postulated a mechanism whereby these vesicles would take in macromolecules from the luminal side of the endothelial membrane and transport them across the cell to release them to the tissues. His hypothesis has been tested, confirmed, and is now well documented.

Endothelial cells have LDL receptors that bind LDL in the capillary lumen, cause the cell to form a vesicle, and then transport it across to the membrane facing the tissues. There, the vesicle fuses with the tissue facing membrane, and LDL is released for uptake by peripheral tissue.

enter image description here

This process is called transcytosis. Endothelial cells use receptor mediated transcytosis to deliver many different large molecules to peripheral tissues in a regulated manner. You can read about this in some detail here (also the source of the figure), and more generally here.

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