Penis

The penis is the primary male external reproductive organ in humans and many other vertebrates. It serves for urination as part of the urinary tract and, in sexual reproduction, for delivering semen into the female reproductive tract. In humans it consists of a root attached to the pelvic region and a shaft ending in the glans penis. Most males have a foreskin (prepuce) that may be removed in circumcision. The penis contains three erectile tissues: two corpora cavernosa on the dorsal aspect and a ventral corpus spongiosum that surrounds the urethra. During erection, nerves trigger the release of nitric oxide, causing relaxation of smooth muscle and engorgement of the corpora cavernosa with blood; venous outflow is constricted by the tunica albuginea, producing rigidity.

The dorsal nerves and the pudendal nerve provide sensation, while autonomic fibers regulate erection and ejaculation. Blood supply comes from arteries including the dorsal and deep arteries of the penis, with drainage via penile veins. The glans is highly innervated and sensitive, contributing to sexual sensation.

Developmentally, the penis forms from the genital tubercle in the embryo; in males, exposure to androgens leads to growth and differentiation. The penis is homologous to the clitoris in females. Common anatomical variants include the foreskin; circumcision is a surgical option in some populations. Conditions such as phimosis (tight foreskin), balanitis (inflammation of the glans), and hypospadias (urethral opening on the underside) are noted.

In clinical contexts, erectile dysfunction refers to persistent difficulty achieving an erection; priapism is a prolonged erection. Penile cancer is rare but possible. The penis also plays a role in reproduction by delivering semen during ejaculation, which is facilitated by coordinated muscular contractions of the pelvic region.