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Erectile Dysfunction
How an Erection Occurs
What
Is the Penis?
The penis
is the male sexual organ. It contains:
Two cylinder
shaped chambers called the corpora cavernosa, which run the
length of the penis and contain a maze of blood vessels.
The urethra, the channel in which urine and sperm flow, which
runs along the underside of the corpora cavernosa.
Erectile tissue, which surrounds the urethra, two main arteries
and several veins and nerves
The shaft, the longest part of the penis.
The head (glans), located at the end of the shaft.
The meatus, the opening at the tip of the head where urine
and semen are discharged.
How
Does an Erection Occur?
An erection
begins in the brain. Physical and/or mental stimulation cause
nerves in the brain to send chemical messages to nerves in
the penis telling the penis to relax so that blood can flow
freely into the penis. Once in the penis, high pressure traps
the blood. This causes the penis to expand, thereby creating
an erection.
A membrane
surrounding the corpora cavernosa helps to trap the blood
in the corpora cavernosa, thereby sustaining the erection.
Erection is reversed when muscles in the penis contract, or
get smaller, stopping the inflow of blood.
How
Does Ejaculation Occur?
Ejaculation,
the release of semen, is triggered when the man reaches a
critical level of excitement. Sexual stimulation causes nerves
in the penis to send chemical messages or impulses to the
spinal cord and into the brain. There, more chemical messages
are sent back to the penis telling it to ejaculate.
Ejaculation
has two phases -- in the first, the vas deferens, the
tubes that store and transport sperm from the testes, contract
to squeeze the sperm toward the base of the penis and the
prostate gland and seminal vesicles release secretions to
make semen. In the second phase, muscles at the base of penis
contract every 0.8 seconds and force the semen out of the
penis in up to 5 spurts.
Reviewed
by the doctors at the Glickman Urological Institute at The
Cleveland Clinic.
Edited
by Charlotte Grayson, MD, WebMD, May 2004
Portions of this page © The Cleveland Clinic 2000-2004
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