Erectile Hydraulics: Maximizing Inflow While Minimizing Outflow

Abstract

Penile rigidity depends on maximizing inflow while minimizing outflow. The aim of this review is to describe the principal factors and mechanisms involved. Erectile quality is the main outcome measure. Data from the pertinent literature were examined to inform our conclusions. Nitric oxide (NO) is the principal factor increasing blood flow into the penis. Penile engorgement and the pelvic floor muscles maintain an adequate erection by impeding outflow of blood by exerting pressure on the penile veins from within and from outside of the penile tunica. Extrinsic pressure by the pelvic floor muscles further raises intracavernosal pressure above maximum inflow pressure to achieve full penile rigidity. Aging and poor lifestyle choices are associated with metabolic impediments to NO production. Aging is also associated with fewer smooth muscle cells and increased fibrosis within the corpora cavernosa, preventing adequate penile engorgement and pressure on the penile veins. Those same penile structural changes occur rapidly following the penile nerve injury that accompanies even "nerve-sparing" radical prostatectomy and are largely prevented in animal models by early chronic use of a phosphodiesterase type 5 (PDE5) inhibitor. Pelvic floor muscles may also decrease in tone and bulk with age, and pelvic floor muscle exercises have been shown to improve erectile function to a similar degree compared with a PDE5 inhibitor in men with erectile dysfunction (ED). Because NO is critical for vascular health and ED is strongly associated with cardiovascular disease, maximal attention should be focused on measures known to increase vascular NO production, including the use of PDE5 inhibitors. Attention should also be paid to early, regular use of PDE5 inhibition to reduce the incidence of ED following penile nerve injury and to assuring normal function of the pelvic floor muscles. These approaches to maximizing erectile function are complementary rather than competitive, as they operate on entirely different aspects of erectile hydraulics.