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I know that the vascular system of the human body is organized in the two, "ever branching", structures of the veins and the arteries. These two structures stem from their connections to the heart and stretch and branch throughout the body.

The two structures are interconnected through capillary beds, responsible for the interactions between the blood and the extracellular liquid, surrounding the tissue cells of the body.

The capillary beds are connected to the arteries and the veins through small vessels, venules, branching out from veins and arterioles, branching out from the arteries.

The capillary beds consist of a web of capillaries, vessels small and thin enough to allow exchange of nutrients and waste between the blood in the vessels and the extracellular liquid surrounding the vessels as well as the cells of the body.

Now, I realize that the capillary beds must be ubiquitous throughout the body, sustaining every part of the body but the circulatory system itself.

I am eager to find out more about the venules and arterioles. Looking at the structure of veins and arteries, how prevalent are the smaller blood vessels? Are the arteries and veins "fuzzy" with arteriole and venules or are the small vessels more sparse? Do they emanate from large vessels as well as smaller ones? How do their presence interfere with vascular interventions or even drawing blood for blood tests?

I'd be happy for information in these matters. Cheers Mats

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I think this question is a bit too broad to write a comprehensive answer to, and there will certainly be some variation between tissues.

However, here is one reference that might give you what you are looking for.

I used to make acrylic arterial casts, mostly of coronary arteries but also some dabbling in the pulmonary, renal, and hepatic systems. My experience with the coronary system is that most of the arterioles come off in a tree-like branching pattern from quite small arteries, but there are always exceptions. I think the "fuzziness" you refer to definitely depends on the scale of observation: the further away you are, the more "fuzzy" arterioles look: of course they will never be as "fuzzy" as capillaries, but certainly once you enter the muscles themselves there is a lot of branching, but that branching isn't necessarily uniform. That is, there may be larger branches running in one dimension, and smaller branches in another dimension.

In larger vessels you also have the vasa vasorum: arteries that feed the large arteries and veins themselves. Some of those vessels are certainly the same size as arterioles, and some branch directly off the main vessel, but I am not sure whether anatomists always consider those to be arterioles or if they are considered separate.

From the perspective of interventions such as surgery, etc, smaller arterioles aren't really any different from capillaries, and larger arterioles might need to be clamped off or cauterized, but there isn't really any way to avoid them if you are cutting into a vascularized tissue. One wouldn't avoid these vessels they way they might avoid larger vessels.

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  • $\begingroup$ I would appreciate an explanation for the downvote, I am happy to make edits if suggested! $\endgroup$ – Bryan Krause Dec 17 '16 at 1:30
  • $\begingroup$ Thanks Bryan. Yes, I apologize for a poorly stated question. In spite of this, you helped me shed a little more light on the subject. I hope I am not responsible for any down vote. If I am please explain and I will rectify. I will accept this answer. It helped me. I will restate and ask a new, clearer question later on. Thanks again for your answer. $\endgroup$ – Mats Dec 17 '16 at 9:04

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