Penis

Penis: Male humans possess a multipurpose external organ called the penis (PEE-nis). It contains the urethra (yuu-REE-thruh), which is the tube used for expelling urine from the body (in a female human, the urethra is in the abdomen). That same male urethra is the passage for sperm-containing semen during copulation.

Size and location Although the penis normally is soft and flexible, it needs to be hard and stiff to enter a woman's vagina during copulation. To achieve this transformation, called erection, the penis is made from three different masses of spongy tissue filled with many blood vessels. During erection, which can take place spontaneously (usually during sleep) or when there is sexual stimulation, the arteries enlarge to admit more blood while the veins shut down to keep the blood in the penis. Fluid pressure produces the erection.
Before erection a typical adult penis is about four inches long; during erection it is about six inches long.

Role During copulation muscular contractions at the end of the process expel about a teaspoonful or two of semen, which can contain as many as 500 million sperm.

Conditions that affect the penis Inability to achieve erection is termed impotence or erectile dysfunction. Certain sexually transmitted diseases, especially syphilis and genital herpes, often have a sore on the exterior of the penis as an early symptom. Warts may be sexually transmitted or may simply develop on the penis. Because cancer and syphilitic sores can resemble warts, consult a physician about any growth on the penis.  Do not use over-the-counter wart treatments on the penis as its thin and sensitive skin is easily damaged. Cancer of the penis is rare, but it does occur and can spread into the lymphatic system.
Other sexually transmitted infections, such as gonorrhea   or trichomonas, are primarily located inside the urethra. Often the cause of infection of the urethra is unknown, and may be labeled nonspecific urethritis by a physician. Infection of the urethra can result in blockage, known as urethral stricture, which is corrected by scraping its interior.
Sometimes an erection begins and will not go away, a condition called priapism (PRY-uh-piz-uhm). This painful condition is a serious disorder that requires emergency treatment. If not corrected within three or four hours, priapism can cause permanent impotence.