Penis arousal anatomy. Cutaway artwork showing the mechanism in the penis where arousal leads to an erection. In the cross-section at left and down the penis shaft, the two large spongy structures are the corpora cavernosa. A similar structure (corpus spongiosum, bottom left), surrounds the urethra. During arousal, chemical messages are sent to the penis that relax its smooth muscle and dilate arteries (red) that fill the corpora cavernosa with blood, causing the penis to swell and lengthen. As they fill, the corpora cavernosa press against the veins (blue, top), preventing blood from leaving and maintaining an erection.
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