What is erection?
An erection happens when certain structures on your body stiffen due to either increased blood flow, or muscular flexing. But usually, people tend to talk about the penis when they are talking about erection.
When a male is stimulated tactilely (by touch) or mentally (such as with pornographic pictures), blood flow will increase to his penis, filling it with blood. The penis is a tubular organ filled with porous, spongy tissue. Veins constrict to prevent blood leaving, and the arteries dilate to allow more blood to enter, resulting in erection. Erection is controlled by your involuntary nervous system, the same part that controls digestion and relaxation. This is why it's best to be really chilled out if you're trying to get an erection. Too much excitement or anxiety will cause loss of erection.
Women can also get erections. The clitoris is a structure that arises from the same tissue mass as the penis. It, too, is made of spongy tissue that fills up with blood when sexually aroused. Basically, the clitoris is built like and functions like a penis (except the urethra usually doesn't run through it).
Nipples can become erect, too. However, they're built differently from the penis and clitoris. Nipples have small, thin bands of muscle just under the surface. When stimulated by touch, cold temperatures, etc., the muscles under the nipple contract, causing the nipple to perk up. No additional blood flow is involved.
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