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Tubular cavities for the transport of blood.

All blood vessels except the [capillary] have four layers.

1. The tunica interna (intima) is the innermost wall and consists of a continuous single row of endothelium cells lying in the longitudinal direction of the vascular flow. It serves as a barrier to the tissue. The endothelium is separated from the wall by a basement membrane. It is a very smooth layer that ensures unimpeded blood flow.
2. The second layer (Membrana elastica interna) is a thin layer of connective tissue (subendothelium). Here, Arteries have more elastic fibers.
3. The tunica media (media) consists mainly of smooth muscles, in the veins thinner than in the large arteries, between which elastic material and collagenous fibrils lie. The collagen fibrils reduce the elasticity of the venous tube
4. The tunica externa (adventitia) has important functions in the supply and innervation of the blood and lymph vessel system. In addition, it forms a displacement bearing and a demarcation from neighbouring structures. Depending on the type of vessel, collagene and elastic fibres occur to varying degrees. It does not necessarily set itself off cleanly from its surroundings because its collagen fibres radiate into the surrounding tissue.

The walls of the larger arteries must also be nourished, therefore small arteries are built into the outer wall, which take over the nourishment and come from the closer environment. Veins and lymphatic vessels have valves from intimate pockets that protrude into the lumen as protuberances. They form valves and direct the blood stream or lymph flow to the center.

A possible backflow (e.g. through the muscle pump) unfolds the flaps and prevents this.

https://en.wikipedia.org/wiki/Blood_vessel _{(Wikipedia CC-by-sa-3.0)}

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