How does Erection happens?

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.
An erection of the penis can also happen by itself when a man is asleep, often when he is dreaming. The scientific term for this is nocturnal penile tumescence (which means "swelling of the penis at night"). It is also common for a man to wake up after a night's sleep to find that he has an erection. This is often caused by his bladder being full of urine, which presses on the tissues in the penis. Such an erection is sometimes informally called a morning glory or morning wood.
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.

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