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==Physiology==
In some circumstances, release of nitric oxide precedes relaxation of muscles in the corpora cavernosa and corpus 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 corpus 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 corpus 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 corpus cavernosum. 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 corpus spongiosum, remains more malleable during [[erection]] because its tunica albuginea is much thinner than elsewhere in the penis.