Fortnightly Review: Impotence: diagnosis and management of male erectile dysfunction

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Abstract
Normal mechanisms of erection Erection of the penis depends on the adequate filling of the paired corpora cavernosa with blood at systolic pressure or slightly above. Arterial blood enters from the paired cavernosal arteries, which are terminal branches of the internal iliac arteries. Numerous corkscrew shaped helicine arteries branch off each cavernosal artery within the corpora and empty into the lacunar spaces. Erection occurs when the tonically contracted cavernosal and helicine arteries relax, increasing blood flow to the lacunar spaces and resulting in engorgement of the penis. Relaxation of the trabecular smooth muscle of the corpora cavernosa is mediated by acetylcholine released by the parasympathetic nerves. Acetylcholine acts on endothelial cells causing them to release a second non-adrenergic non-cholinergic carrier of the relaxation signal. This carrier is thought to be nitric oxide, possibly of neural origin, but other candidates - for example, vasoactive intestinal polypeptide - have not been conclusively excluded. Nitric oxide may exert a relaxing effect on the trabecular smooth muscle through stimulating guanylate cyclase to produce cyclic guanosine monophosphate (cGMP), which would then function as a second messenger.3 Systemic blood pressure expands the relaxed trabecular walls against the rigid tunica albuginea, compressing the plexus of subtunical venules and restricting venous drainage from the lacunar spaces. Erection is therefore the result of an equilibrium between arterial inflow from the cavernous arteries and the resistance to blood outflow from the lacunar spaces resulting from these veno-occlusive mechanisms (fig 1). Detumescence is accomplished by a reversal of these processes. Increasing sympathetic tone, which is also involved in mediating orgasm and ejaculation, causes the cavernosal and helicine arteries to contract, restricting blood flow into the lacunar spaces. Falling intralacunar pressure then decompresses the subtunical venules, allowing increased venous outflow and restoring the penis to its normal state of flaccidity.4,5 View larger version: In this window In a new window FIG 1 Diagram illustrating distribution of intracavernosal blood in flaccid (top)and erect (bottom) penis