What happens to cause the penis to get stiff during an erection?

Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
As you would guess, erections happen when guys are aroused. The specific cause of the fireworks could be a titillating touch or word, or an image that a guy finds stimulating. When aroused, the brain tells the body to increase blood flow down under. When the blood flows in to a guy's penis, it fills spongy chambers inside the organ and  stays put. The end result? Like a balloon being blown full of air, the penis grows bigger and stiffer.
Dr. Michael Roizen, MD
Internal Medicine
A man can get aroused when he is touched, smells perfume, looks at sexy pictures, thinks about sex, or sees an attractive woman or man, depending on his sexual orientation. So, what happens to make the penis stiff during an erection? Sex starts in the brain. The brain sends a signal that causes blood to flood into your penis like Class V whitewater rapids. The muscles around the penis arteries relax so blood can flow in, where they are absorbed into a sponge-like structure called the corpus cavernosum. All that blood squeezes the veins, so the blood can’t get out. The blood engorges the penis, and it gets hard. At the same time, another structure on the underside of the penis stays pliable so semen can come out when you ejaculate.

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