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 in the penis, high pressure traps the blood within both corpora cavernosa. This causes the penis to expand and sustain an erection.
An erection is reversed when the inflow of blood is stopped and opening 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 where other chemical messages are sent back to the penis — via the spinal cord — causing ejaculation.
Ejaculation has two phases — During the first phase, the vas deferens, which are the tubes which store and transport sperm from the testes, contract to squeeze sperm toward the back of the urethra. The seminal vesicles also release secretions into the back of the urethra. During the second phase, the posterior urethra senses the secretions within it (at the height of sexual excitement) and sends signals to the spinal cord to, in turn, send powerful signals to the muscles at the base of penis to vigorously contract every 0.8 seconds and force the semen out of the penis.