Figure 37-3

Figure 37-3. The mechanism of penile erection. In the flaccid state (A), the arteries, arterioles, and sinusoids are contracted. The intersinusoidal and subtunical venular plexuses are wide open, with free flow to the emissary veins. In the erect state (B), the muscles of the sinusoidal wall and the arterioles relax, allowing maximal flow to the compliant sinusoidal spaces. Most of the venules are compressed between the expanding sinusoids. Even the larger intermediary venules are sandwiched and flattened by distended sinusoids and the noncompliant tunica albuginea. This effectively reduces the venous capacity to a minimum.