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 and sinuses ( cavities).
- The urethra, the channel in which urine and sperm flow, which runs along the underside of the corpora cavernosa.
- Erectile tissue, which is contained within the corpora cavernosa above 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 penile blood vessels to relax so that blood can flow freely into the penis. Once blood is in the penis, high pressure traps it within both corpora cavernosa to sustain an erection. This causes the penis to expand, creating an erection.
Erection is reversed when the inflow of blood is stopped and outflow channels open, allowing the penis to become soft.
How does ejaculation occur?
Ejaculation, the release of semen at climax, 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 causing ejaculation.
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 prostate gland and urethra, and seminal vesicles release secretions that 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 five spurts.