Blood supply to the penis
When a man becomes aroused, his brain sends nerve impulses that increase blood flow to his penis. The blood fills the spongy chambers (corpora cavernosa) in the penis, causing it to expand and become rigid. The veins that would normally drain blood from the penis constrict, trapping the blood inside and maintaining the erection.
After ejaculation or when the man is no longer sexually aroused, the arteries narrow, the veins expand, and the blood drains out of the penis. The penis returns to its normal size and is no longer rigid.
My Penis Stuff » Blog Archive » Erection Problems:
[...] cause of erection problems may be physical (such as injury to nerves or loss of blood supply to the penis) or psychological (such as anxiety or depression). Medications you are taking for other conditions, [...]
17 December 2007, 1:11 pm