Function

During sexual activity, the penis becomes erect, enabling penetration during sexual intercourse. An erection results from a complex interaction of neurologic, vascular, hormonal, and psychologic actions. Pleasurable stimuli cause the brain to send nerve signals through the spinal cord to the penis. The arteries supplying blood to the corpora cavernosa and corpus spongiosum respond by dilating. The widened arteries dramatically increase blood flow to these erectile areas, which become engorged with blood and expand. Muscles tighten around the veins that normally drain blood from the penis, slowing the outflow of blood and elevating blood pressure in the penis. This elevated blood pressure causes the penis to increase in length and diameter.

At the climax of sexual excitement (orgasm), ejaculation usually occurs, caused when friction on the glans penis and other stimuli send signals to the brain and spinal cord. Nerves stimulate muscle contractions along the seminal vesicles, prostate, and the ducts of the epididymis and vas deferens. These contractions force semen into the urethra. Contraction of the muscles around the urethra further propels the semen through and out of the penis. The neck of the bladder also constricts to keep semen from flowing backward into the bladder.

Once ejaculation takes place--or the stimulation stops--the arteries constrict and the veins relax. This reduces blood inflow and increases blood outflow, causing the penis to become limp (detumescence). After detumescence, erection cannot be obtained for a period of time (refractory period), commonly about 20 minutes in young men.