Male sexual arousal. Computer artwork showing a flaccid (unaroused) penis (top) and an erect (aroused) penis (bottom). At centre left is a cross-section through the penis. In the cross- section veins are blue, arteries are red and the urethra is yellow. The two large structures surrounding the arteries at centre right and left are the corpora cavernosa. The corpus spongiosum surrounds the urethra. During arousal chemical messages are sent to the penis that relax its smooth muscle and dilates the arteries. The corpora cavernosa fill with blood, causing the penis to swell and lengthen. As they fill the corpora cavernosa press against the veins, preventing blood from leaving the penis and maintaining an erection.