In some circumstances, release of nitric oxide precedes relaxation of muscles in the corpora cavernosa and<unk>  spongiosum, in a process similar to female arousal. The spongy tissue fills with blood, from arteries down the length of the penis. A little blood enters the<unk>  spongiosum; the remainder engorges the corpora cavernosa, which expand to hold 90% of the blood involved in an erection, increasing both in length and in diameter. The function of the<unk>  spongiosum is to prevent compression of the urethra during erection. Blood can leave the erectile tissue only through a drainage system of veins around the outside wall of the<unk><unk> . The expanding spongy tissue presses against a surrounding dense tissue (tunica albuginea) constricting these veins, preventing blood from leaving. The penis becomes rigid as a result. The glans penis, the expanded cap of the<unk>  spongiosum, remains more malleable during erection because its tunica albuginea is much thinner than elsewhere in the penis.
Corpus cavernosum penis
Vas deferens
Iris sphincter muscle
Penis captivus
Meibomian gland
Penis
Subfornical organ
Palatal obturator