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.
Capillaries are the smallest of all blood vessels and form the connection between veins and arteries. As arteries branch and divide into arterioles and continue to reduce in size as they reach the muscle they become capillaries. Here the capillaries form a capillary bed, which is a vast expanse of very small vessels forming a network throughout the muscle. However, unlike veins and arteries, their main function is not transporting blood.