Mill Finish is the natural appearance of the aluminum as it comes from the rolling mill (Alcoa, Reynolds, Kaiser, or Alcan) or the extrusion mill. It is as is with no external mechanical or chemical finishing. Extruded metal is considered mill finish. All aluminum has an oxide of some varying thickness. Anodizing is a very heavy controlled oxide.
As they are so small of approximately 5-20 micro-metres in diameter, they do not need valves, as there is a distinct flow from the arterioles supplying the capillary with blood, and the venules draining the capillary within the double circulation.
Veins have much thinner walls, because the blood they carry is under low pressure. VEINS have valves that aid the return of blood to the heart and prevent blood from going backwards. ARTERIES do not have valves, except for the semi-lunar valves of the pulmonary artery and the aorta. ARTERIES have relatively narrow lumens, VEINS have relatively wide lumens.