An erection of the penis occurs when engorgement of venous blood in two tubular structures at the bottom of the penis, the corpora cavernosa, results from a variety of stimuli. The corpus spongiosum is a single tubular structure located just above the corpora carvernonosa, and contains the male urethra, through which urine and semen pass during urination and ejaculation, respectively. This may also become slightly engorged with blood, but less so than the corpora cavernosa. Penile erection usually results from sexual stimulation and/or arousal, but can also occur by such causes as a full urinary bladder or spontaneously, most commonly during erotic or wet dreams. An erection results in swelling and enlargement of the penis or the swelling of the female counterpart to the penis, clitoris. Erection enables sexual intercourse and other sexual activities, though it is not essential for all sexual activities.
In addition to sexual arousal, erection in males can be caused by mechanical stimulation, or by the pressure of the filled urinary bladder. Erections when waking up are common, most likely due to a full bladder. They sometimes already occur in infant boys, and in utero. Recently some scientists have begun to doubt this very much (sources: see below). The question why most men have erections when waking up ("morning wood") is still not settled.
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