An erection is the firm and enlarged condition of a body organ or part when the erectile tissue surrounding it becomes filled with blood. It is particularly used to refer to such a condition of the penis or, much less often, the clitoris.
The obvious physical effects of an erection are that the penis stiffens, its girth increase, and it rises up to an angle that tends to vary widely between individuals from below horizontal to almost vertical. If the foreskin has not been removed (a circumcision) it will retract back over the head, or glans, of the penis. The testicles are drawn up towards the body as the skin of the scrotum tightens.
The penis contains a pair of structures known as the corpora cavernosa ("cavernous bodies", sing. corpus cavernosum), collections of capillary- and nerve-rich tissue which run down the shaft of the penis. Below them lies the corpus spongiosum ("spongy body"), which contains the urethra. When an appropriate stimulus is felt the corpora cavernosa fill with venous blood, causing the penis to swell. The corpus spongiosum may also become slightly engorged, but not on a significant level. Muscular contraction ensures that the blood does not immediately leave the penis, and the pressure of the engorged corpora cavernosa upon the veins in the penis further increase and prolong the effect. Penile erection most commonly results from sexual stimulation and/or arousal. It may also be caused by a full urinary bladder, or occur spontaneously with no apparent cause. Men are said to experience an erection roughly every 90 minutes during sleep, often waking up with one. Generally, an erection subsides after orgasm and the ejaculation of semen. Some men, either naturally or with training, can withhold ejaculation and thus keep their erection until such time as ejaculation is inevitable.
The clitoris is almost a mirror image of the penis, albeit on a much smaller and less visible scale as most of the clitoris is hidden. It too has a glans and a shaft, a foreskin of sorts (the clitoral hood), and the tissue that develops into the male prostate gland also exists in females. The clitoris also, most importantly, contains erectile tissue made up of corpus cavernosum that behaves just like male erectile tissue. However, in females it is a far more extensive structure than in males. The clitoral body extends upwards and backwards for several centimeters, dividing into two arms called the clitoral crura, then around and into the outer labia.Sexual stimulation and/or arousal causes the erectile tissue to fill with blood. The clitoris becomes harder and more prominent, although it doesn't stiffen altogether like the penis does. With the approach of orgasm, clitoral erection increases even more. This often draws the clitoris upwards into the body so that it appears to shrink.