Erectile dysfunction (ED) refers to the inability to achieve or maintain an erection that is sufficient for satisfactory intercourse. It is multidimensional and can arise from organic, relational, and/or psychogenic causes.1 The Massachusetts Male Aging Study (MMAS) reported a 52% prevalence of mild to moderate ED in men aged 40 to 70, and ED was found to be associated with age, health status, and emotional function.2,3 ED has detrimental effects on partner satisfaction and the couple’s quality of life. It was previously believed that most ED was psychogenic; however, more recent evidence shows that roughly 80% of ED is organic.3 ED may be a manifestation of endothelial dysfunction. Cardiovascular disease, poor general health status, diabetes, smoking, medications, and socioeconomic status are well-established risk factors for ED. Identification of these comorbid conditions offers the men’s health provider an opportunity to diagnose and intervene on conditions that may otherwise be asymptomatic. This chapter includes a brief discussion of the physiology of erection as it relates to a more complex discussion of the pathophysiology of ED.
PHYSIOLOGY OF ERECTILE FUNCTION
Key Points: Physiology of Erection
Erections are the result of neural activation, cavernosal smooth muscle relaxation, increased penile blood flow, and venous occlusion.
Nitric oxide (NO) activates guanylate cyclase to generate cyclic guanosine monophosphate (cGMP), leading to sequestration of calcium and smooth muscle relaxation
Phosphodiesterase type 5 (PDE5) inhibitors act by inhibiting PDE5 and increasing cGMP concentrations
The penis is composed of 3 cylindrical bodies, the paired communicating corpora cavernosa dorsally, and the corpus spongiosum ventrally. The corpus spongiosum surrounds the urethra and is continuous with the glans penis. These cylinders are encased by the dense, bilayered tunica albuginea, which is relaxed in the flaccid state and stretched in the erect state. All 3 corporal bodies are surrounded by Bucks (deep) and Dartos (superficial) fascia. The bulbospongiosus and ischiocavernosus muscles surround the base of the penis and contribute to erection and ejaculation (Fig. 2-1).4
Pelvic and penile anatomy. Top: Relations of the bladder, prostate, seminal vesicles, penis, urethra, and scrotal contents. Lower left: Transverse section through the penis. The paired upper structures are the corpora cavernosa. The single lower body surrounding the urethra is the corpus spongiosum. Lower right: Fascial planes of the lower genitourinary tract.
The arterial supply to the penis is derived from the internal pudendal artery, which is a branch of the internal iliac artery. The internal pudendal artery traverses Alcock canal before dividing into the bulbourethral, dorsal, and cavernosal arteries. The venous drainage of the penis is via the deep and superficial veins.
The penis is basally in a flaccid state, which is mediated by smooth muscle contraction as a result of sympathetic stimulation, myogenic control, and endothelium-derived contracting factors such as ...