Erection

An erection, clinically: penile erection or penile tumescence is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular, and endocrine factors, and is often associated with sexual arousal or sexual attraction, although erections can also be spontaneous. The shape, angle, and direction of an erection vary considerably between humans. The state of a penis which is partly, but not fully, erect is sometimes known as semi-erection; a penis that is not erect is typically referred to as being flaccid, or soft.

Physiology

Physiologically, an erection is required for a male to effect vaginal penetration or sexual intercourse and is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide to rise in the trabecular arteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis to fill with blood; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood. The erection subsides when parasympathetic activity reduces to baseline.