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.
With the exception of the lungs, where the opposite is true, capillaries bring oxygenated blood, blood carrying oxygen, to organs and carry away deoxygenated blood, blood with the oxygen removed. Their walls are very thin to allow substances to easily and quickly diffuse, or pass through them.