CLAUS LUNAU / SCIENCE PHOTO LIBRARY CLAUS LUNAU / SCIENCE PHOTO LIBRARY
Penis arousal anatomy. Cutaway artwork (from above) showing the mechanism in the penis where arousal leads to an erection. The two large spongy cylindrical structures within the shaft of the penis are the corpora cavernosa. The urethra (not seen) is below these. 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. The glans (head) of the penis is at left.
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