Lloyd’s Comment: This article has quite a bit to say about the smooth muscles. Apparently, not only do the smooth muscles allow blood for erection by relaxing, their tension level REGULATES the blood supply all the time! So an abnormally high constant tension would cause abnormal retraction and low blood flow! Especially during puberty, this my cause abnormally weak penis growth. GOOD PICTURES AT LINK.
The Sexual Basics Menu
PHYSIOLOGY OF ERECTIONS:
HOW THEY OCCUR
Which is why various aids can help or not.
(For a different description and line drawing, click here.)
The penis is composed of three different regions: a pair of parallel spongy columns called the corpa cavernosum and the central corpa spongiosum, which contains the urethra (the tube that carries urine from the body).
All three regions are made up of erectile tissue. Erectile tissue is rich in tiny pool-shaped blood vessels called cavernous sinuses, which are surrounded by smooth muscles and supported by elastic fibrous tissue.
The natural erection is prompted by the corpa cavernosa nerves (also known as cavernous nerves) which come to the penis from the prostate gland. Removal of the gland without sparing the nerves breaks the neural connection. The tissue cannot relax, allowing blood to flow in.
In the flaccid, or unerect, normal penis, the small arteries leading to the cavernous sinuses contract, reducing the inflow of blood. The smooth muscles regulating the many tiny blood vessels within the penis also contract.
When a man becomes aroused, his central nervous system stimulates the release of a number of chemicals that relax the smooth muscles in the penis, allowing blood to flow into the tiny pool-like sinuses and flood the penis.
The spongy chambers almost double in diameter due to the increase in blood flow. The veins surrounding the corpa cavernosum and corpus spongiosum are squeezed almost completely shut by the pressure of the erectile tissue; they are unable to drain blood out of the penis, causing it to become rigid.