The penis and the urethra are part of a man’s urinary and reproductive system. The scrotum, testes (testicles), vas deferens, seminal vesicles, and prostate gland comprise the rest of the male reproductive system. During sexual arousal, the penis becomes erect, enabling penetration during sexual intercourse. An erection results from a complex interaction of neurologic, vascular, hormonal, and psychologic stimuli.
It is not clear whether aging itself or the disorders associated with aging cause the gradual changes that occur in men's sexual functioning. The frequency, duration, and rigidity of erections gradually decline as men age. Levels of the male sex hormone (testosterone) tend to decrease, reducing sex drive (libido). Blood flow to the penis decreases.