Erection

An erection, clinically termed penile erection or penile tumescence, is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. It results from interactions among psychological, neural, vascular, and endocrine factors and may be associated with sexual arousal or occur spontaneously. Parasympathetic activity and nitric oxide contribute to increased penile blood flow, while restricted venous outflow helps maintain the erection. Erections during sleep or on waking are called nocturnal penile tumescence.

Clinical terms

Penile erection or penile tumescence

Physiological mechanism

Parasympathetic activity raises nitric oxide levels, dilating arteries and increasing blood flow.

Sleep-related term

Nocturnal penile tumescence (NPT)