An understanding of the basic physiology of erection (that is, how an erection works) will allow patients to understand not only the causes of erectile dysfunction (ED) but will also lay the foundation for the understanding of ED treatments. The penis is an organ with paired erection chambers (corpora cavernosa), which are filled with spongy erectile tissue (corporal sinusoids) composed predominantly of muscle. Erection and loss of erection are related primarily to blood flow events regulated by the relaxation and contraction, respectively, of the smooth muscle in the penile arteries and the erectile bodies themselves. Erection is a hydraulic event, regulated by hormones and nerves, which allow increased blood flow into and storage of blood within the erectile bodies leading to an increase in pressure and the development of rigidity (hardness). Penile erection is triggered by one of two main mechanisms: direct stimulation of the genitalia or through stimuli coming from the brain (fantasy, smell, etc).
Upon stimulation, chemicals are released in the brain that cause signals to pass down the spinal cord and outward through special nerves (nervi erigentes) into the penis. These nerves release another chemical (Nitric Oxide) that causes the aforementioned smooth muscle to relax and blood rushes into the erectile bodies, causing erection. Anxiety or fear can prevent the brain signals from reaching the level required to induce erection. Medical conditions can block the erection arteries or cause scarring of the spongy erection tissue and prevent proper blood flow or trapping of blood and, therefore, limit the erection. Thus, the erection mechanism is much like a tire; a firm tire is dependent upon a hose that can deliver air in adequate amounts in a speedy fashion and a valve mechanism that holds the air in place. In the penis the hose is represented by the erection arteries, which rapidly carry blood into the erectile bodies and the valve mechanism, while complicated in its structure, ensures that the blood is trapped inside the erectile bodies until ejaculation occurs or the sexual stimulus has passed.