erection

An erection is the physiological process by which the penis becomes firm and enlarged due to engorgement with blood. It results from increased arterial inflow into the corpora cavernosa and reduced venous outflow, with the tunica albuginea helping to trap blood within the erectile tissue.

Physiology and pathways: Erection is coordinated by the autonomic nervous system. Sexual arousal triggers the release of nitric oxide, which relaxes smooth muscle in penile arteries and in the trabecular tissue, allowing more blood to flow into the penis and dilate the sinusoids. The resulting pressure compresses venous outflow and maintains rigidity. There are two main neural pathways: reflex erection, triggered by physical stimulation and mediated through the sacral spinal cord (S2–S4); and psychogenic erection, initiated by erotic or cognitive stimuli and involving higher brain centers and spinal pathways to the pelvic nerves.

Maintenance and detumescence: After arousal subsides, smooth muscle contracts, blood flow decreases, and the penis returns to a flaccid state. Hormonal factors, particularly testosterone, influence libido and erectile function. Health conditions such as cardiovascular disease, diabetes, obesity, and certain medications can impair erections, and age can also play a role.

Clinical aspects: Erectile dysfunction is the persistent difficulty in achieving or maintaining an erection sufficient for sexual activity. It has multiple potential causes, including vascular, neurological, hormonal, psychological, and lifestyle factors. Treatments vary by cause and severity and may include lifestyle changes, phosphodiesterase type 5 inhibitors, vacuum erection devices, penile implants, or other interventions. Priapism, a prolonged unwanted erection, is a medical emergency requiring prompt treatment.