How it works! From http://www.ncbi.nlm.nih.gov/pmc/articles/

[hoosierphilly]

early sign of peripheral vascular disease? Other Sections▼

AbstractThe Penis and the Endothelial SystemWhy Should We Diagnose ED?PDE-5 Inhibitor Failures and the Need for Follow-upSummaryReferencesThe Penis and the Endothelial SystemThe penis is a sensitive hydraulic organ that is responsive to sympathetic and nervous parasympathetic tone, active vascular flow, and passive veno-occlusion. It is composed of an extensive endothelial surface interlaced with smooth muscle. In order for a man to achieve an erection, there must be a decrease in sympathetic smooth muscle tone, an increase in parasympathetic smooth muscle relaxation, an increase in arterial inflow, a filling of the endothelial spaces (enabled by corporal smooth muscle relaxation), and a passive pinching off of the venules exiting the tunica albuginea.The entire process is exquisitely sensitive to the availability of both neuronal and endothelial nitric oxide, which is in turn dependent on the presence of both neuronal and endothelial nitric oxide synthase (nNOS and eNOS, respectively), the crucial enzyme responsible for the production of nitric oxide gas (Figure 1).6 The penis, richly innervated and with its vast endothelial system, is a natural repository for NOS.7 Consequently, any disease process that reduces the levels of nNOS or eNOS will decrease erectile function. Whereas a reduction in nNOS level is usually iatrogenic, the result of pro-static or pelvic surgery or irradiation, the conditions that reduce the eNOS level are those associated with cardiovascular disease and endothelial dysfunction (Table 1).8 Figure 1
Schematic of the biochemical pathway for erectile function and mode of action of phosphodiesterase-5 (PDE-5) inhibitors. NO, nitric oxide; eNOS, endothelial nitric oxide synthase; nNOS, neuronal nitric oxide synthase; GTP, guanosine triphosphate; GMP, (more ...)