Why 'blood vessels' and not 'blood tubes'?
Vessels usually contain something, most often liquid, for the purpose of carrying it from one place to another. Blood vessels serve exactly this purpose for blood. I'm not very good at biology but still, this definition seems useful:
- In anatomy, any tube or canal, in which the blood and other humors are contained, secreted or circulated, as the arteries, veins, lymphatics, spermatics, &c.
Merriam Webster Online seems to still support the canal/tube definition in this sense. As such, it seems to indicate that the word vessel indicates broader category, referring to more than just than just the tubular ones. Since you've already mentioned tube, I'll focus on canal first:
- In anatomy, a duct or passage in the body of an animal, through which any of the juices flow, or other substances pass; as the neck of the bladder, and the alimentary canal.
In this sense, perhaps the heart chambers could be considered "blood vessels," and they're not exactly tube-like. Although we also have this definition of Tube:
A pipe; a siphon; a canal or conduit; a hollow cylinder, either of wood, metal or glass, used for the conveyance of fluids, and for various other purposes.
- A vessel of animal bodies or plants, which conveys a fluid or other substance.
This almost brings us back to where we started. However, it doesn't say 'any vessel' so perhaps the broader context of the definition should be read to only applies to the ones that are tube shaped, given the overall context of the entry.
In short, it seems like the reason for it was to convey a broader category and that meaning may have been lost somewhere along the line.
Except where otherwise stated, all of the referenced definitions are from Noah Webster's 1828 American Dictionary of the English Language.
The blood in a human body is circulated by blood vessels, but it is also contained within the blood vessels. There is no other reservoir of blood.
I suspect it is the latter that led to the use of vessels to refer to veins and arteries. From Oxford English Dictionary 1971:
1398 Veynes ben the vessels of blode.
1495 There is no more difference betweene these two vessels of blood, but that the Artere is a vessel of blood spiritual or vytal.
1635 A nimble thrust his active En'emy made, .. And opened wide those secret vessels where Life's Light goes out, when first they let in aire.
I don't see any references in the OED that imply circulation much before the 19th century.
1793 Vessels everywhere penetrate the bones, supplying them with juices and marrow.
A vessel originally is a ship (vascello in Italian) and ships were once the only way to carry goods in large quantities. In Marine Insurance in the City of London the following phrase is commonly used on Marine Policies (policies for any type of transport): "any one vessel". It follows the Sum Insured, for instance: US$ 10,000,000 any one vessel, any one location. That means that the Assured is covered for shipments up to 10 million dollars. Blood vessels "carry" all the blood that all parts of our body need. We have not invented yet another suitable "means of transport".