Penile erection is a physiological phenomenon where the penis becomes enlarged and firm. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal. Penile erection can also occur due to a full urinary bladder. In some males, erection can occur spontaneously at any time of day, and is known as nocturnal penile tumescence when occurring during REM sleep. Physiology Penile… (More on Erection)