Whenever we hear the phrase "blood vessels", we always think of arteries and how they distribute blood to cells and tissues. However, we hardly think about the billions of microscopic capillaries, the real vessels that supply cells and tissues with blood and nutrients. It is no doubt that arteries branch into arterioles and capillaries, but it is mainly through capillary walls that gases, nutrients, and metabolic wastes diffuse. The tiniest blood vessels, capillaries have a diameter ranging from .008 to .01 mm, roughly 1/1000 the diameter of arteries and 1/18th the diameter of a strand of hair. Capillaries lack a both a tunica adventitia and tunica media, and consist of a tunica intima so small in diameter that they can fit only one red blood cell at a time, forcing the RBCs to move in a single file. Although capillaries are found in most tissues, some tissues lack these vessels. For instance, the cornea of the eye does not contain many capillaries. It acquires oxygen and nu