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.
What are capillaries? Capillaries Tiny blood vessels connecting arteries to veins. These  blood vessels carry oxygen and nutrients to individual cells.  Capillaries are the smallest blood vesselsâ¦ in the body, and have  walls only one or two cells thick.