The penis has been described by Desmond Morris (The Naked Ape, The Human Animal) as "a marvel of fluid mechanics". To be sure the penis has adequate plumbing to provide and subside erection, the skin, including the skin of the prepuce, is embedded with a vast array of veins. Blood to the penis is provided by an artery protected within the inner structure of the penis itself. A network of veins allows blood to exit. This exit of blood is restricted during arousal so that the erection can be maintained.
As you see in the photo to the left, this network of veins branches throughout the foreskin. Even when soft (above) the veins are very noticeable and seem to carry a high volume of blood most of the time.Circumcision seriously damages this veinous network. One has to wonder what affect this has on erection and performance not to mention the overall nutrition of the cells in the various tissues affected by this massive destruction of the circulatory system. There are no known detailed studies of this to date, however Dr. Paul Fleiss, MD mentions that this causes a circulatory condition called backflow, in his article, "Where is my Foreskin?".
Notice that even during erection, with pressure on the base of the penis, the frenar band keeps the skin forward. This exemplifies the previous note that it keeps the foreskin over the penis during nocturnal erection to protect the glans and keep it moist.