The term "erection" is most often used to refer to an erection of the penis. It is widely believed that a man's penis needs to be erect in order for him to take part in sexual intercourse and penetrate his partner's vagina and to ejaculate semen, but all are possible without an erection. In a healthy male, an erection will usually occur if the man is sexually aroused (made sexually excited) or stimulated. This can happen if he is touched sexually, or even if he is not touched – for example, if he sees a naked person or thinks about having sex with someone else. Erections can also occur without thinking about sex or being touched. Most boy teenagers have unexpected erections. This is normal. Erections happen several times during the night. This happens to all healthy adult males of all ages. This also happens when a man wakes up. That is normal too. Tissues in the penis called the corpus spongiosum penis and corpora cavernosa penis fill with blood, causing the penis to become longer, thicker, harder and to stand upright. A man's erection usually stops if he is no longer sexually aroused, or after he has had an orgasm and has ejaculated. An erection will usually not be possible immediately following ejaculation, although adolescent males may be able to achieve erections immediately following ejaculation. The time period during which an erection cannot be repeated following ejaculation is called the refractory period.
Erect penises have appeared in erotic (sexually exciting) art for a very long time. Pictures of men with erections appear on ancient objects and in paintings. In the past, the erect penis was also a symbol or sign of health and fertility (the ability to give life). Ancient Egyptians, Greeks and Romans believed in gods that had erect penises. Today, erect penises do not often appear in artworks or movies because many people think that showing a man's penis when it is erect is obscene (not decent). Men with larger penises are often thought to be more handsome, manly and powerful. A lot of people also believe that men with bigger penises are better at making their partners feel good when they are having sex. However, this is not always true.
Like a man's penis, a woman's clitoris can also become erect. This happens in a similar way to an erection of the penis. The clitoris also has a corpora cavernosa, and when a woman is sexually excited this part of the clitoris fill with blood. This causes the clitoris and other parts of the woman's sex organs to become larger, harder, redder in colour, and very sensitive to being touched.
The nipples of men and women can also become erect. Women's nipples become erect when they are breastfeeding (giving milk to their babies from their breasts). Men and women may also have an erection of their nipples when they are sexually excited, or if they feel cold. Nipples do not have any tissue in them that fills with blood. Instead, the body's nervous system makes muscles in the nipples contract (become shorter), causing them to stand up and become slightly harder.
Blood flows out of the corpus spongiosum and corpora cavernosa penis, and the penis goes back to its normal size. A man who often has problems having or keeping an erection of his penis is said to have erectile dysfunction (ED). On the other hand, an erection that lasts too long is also a dangerous condition called priapism. Men who want to get less erections sometimes take anaphrodisiacs.
The term "erection" is most often used to refer to an erection of the penis. A man's penis is usually soft, quite short and hangs down. When the penis is like this, it is said to be flaccid (). In order for the man to take part in sexual intercourse by putting his penis inside a woman's vagina, his penis needs to become longer, thicker, harder. For many men it stands upright. It is okay if it points out or down. When the penis is hard, it is said that the penis is erect or that the man has an erection. The scrotum (the bag containing a man's testicles or "balls") will usually tighten during erection. The penis usually needs to be at least mildly erect before the man can have an orgasm and ejaculate semen.
The scientific name for an erection is "tumescence" (which means "swelling"). Some slang terms for having an erection are "getting hard", "getting wood" and "having a hard-on", and an erect penis is informally called a "boner", "hard-on", "stiffy", "wood" or "woody".
The penis is shaped like a banana or sausage. It has a duct or tube called the urethra running right through it. Semen and urine pass along the urethra to get out of the body. Around the whole urethra is tissue called the corpus spongiosum penis (which is Latin for "body of the penis that is like a sponge"). It is near the lower surface of the penis. Also inside the penis near its upper surface are two cylindrical tissues called the corpora cavernosa penis (Latin for "bodies of the penis that are like caves"). (One of these tissues by itself is called a corpus cavernosum.) The corpus spongiosum penis and corpora cavernosa penis are filled with many blood vessels called arteries which carry blood.
When a man is sexually aroused or stimulated (made sexually excited) by touch – for example, if he masturbates or another person touches him in a sexual way – the touching is picked up by nerves in the arteries of the corpus spongiosum penis and corpora cavernosa penis. The nerves then cause a chemical called nitric oxide to be released into the arteries. This makes the arteries dilate (become wider), and they fill up with blood. Thus, the corpus spongiosum penis and corpora cavernosa penis all fill with blood and become larger and harder. This makes the penis grow longer, thicker and harder. The corpora cavernosa penis press against blood vessels called veins that let blood flow out of the penis. Since less blood can flow out of the penis through the veins, more of it stays in the penis, which remains hard. During an erection, a valve stops urine from entering the urethra so that only semen flows along it. This is why it is very difficult for a man to urinate when he is having an erection.
A man can also become sexually aroused without being touched, for example, if he sees a naked person or thinks about having sex with someone. In this case, his brain sends messages through his spinal cord (which is inside his vertebral column or backbone) to his penis and makes an erection happen. The brain can also stop an erection from happening, even if the man is being touched sexually.
After a man has had an orgasm and has ejaculated, his erection usually ends. How long this takes depends on the length and thickness of his penis. A man can also stop having an erection without having an orgasm and ejaculating, if he is no longer sexually aroused. Nerves in his body cause the arteries in the penis to constrict (become narrower). This forces blood out of the corpus spongiosum penis and corpora cavernosa penis. As they go back to their normal size, they do not press so much on the veins leading out of the penis. This lets blood flow faster out of the corpus spongiosum penis and corpora cavernosa penis. The scientific term for this process is "detumescence".
The angle that an erect penis makes varies from one man to another. If it is imagined that a man is standing upright and there is a horizontal line stretching out from the base of his penis, most men's erect penises point upwards from the line at a certain angle (as shown in the photograph on the left). However, it is common and normal for an erect penis to point nearly vertically upwards, nearly vertically downwards, or even horizontally forward. The angle depends on how long a man's suspensory ligament is. The suspensory ligament is a band of tough, stringy tissue that joins a man's penis to the front of his pelvis.