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.
The capillaries then join and widen to become venules, which in turn widen and converge to become veins, which then return blood back to the heart through the great veins. Capillaries do not function on their own, but instead in a capillary bed, an interweaving network of capillaries supplying organs and tissues. The more metabolically active a cell or environment is, the more capillaries are required to supply nutrients and carry away waste products.