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The two corpora cavernosa and corpus spongiosum (also known as the ''corpus cavernosum urethrae'' in older texts and in the adjacent diagram) are three expandable erectile tissues along the length of the [[penis]], which fill with [[blood]] during penile [[erection]]. The two corpora cavernosa lie along the penis shaft, from the pubic bones to the head of the [[penis]], where they join. These formations are made of a sponge-like tissue containing trabeculae, irregular blood-filled spaces lined by endothelium and separated by septum of the penis.<ref>{{REFweb

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]].