Because this shape works for the body. Somewhere in the evolution time line the creature which had the first cylindrical blood vessel survived and that information has been stored in the DNA and has past it on to its offspring.
The reason I believe it works is that it is the easiest shape to dilate or constrict.
Tagalog of cylindrical: mabilog
Bending a rectangular sheet into a cylindrical shape.
Yes, they're usually cylindrical in shape.
The cylindrical barrel floated down the stream.
10 figure we make from cylindrical shape
There are three major categories of veins: superficial veins, deep veins, and perforating veins. All varicose veins are superficial veins; they lie between the skin and a layer of fibrous connective tissue called fascia.
Non-cylindrical objects are objects that do not have a cylindrical shape. This includes objects with irregular shapes, such as spheres, cubes, pyramids, and cones among others.
There are three types of veins, superficial veins that are just beneath the surface of the skin, deep veins that are large blood vessels found deep inside muscles, and perforator veins that connect the superficial veins to the deep veins.
Pulmonary veins
Maps with cylindrical projections are designed as if a cylinder has been wrapped around a globe
They are cylindrical
Volume of a cylindrical tank in cubic units: pi*radius2*height