The tunica of the corpora cavernosa is a bilayered structure with multiple sublayers. The inner circular bundles support and contain the cavernous tissue. From this inner layer radiate intracavernosal pillars that act as struts to augment the septum; both of these structures provide essential support to the erectile tissue (_{Hsu et al, 1992}). The outer-layer bundles are oriented longitudinally and extend from the glans penis to the proximal crura. These fibers insert into the inferior pubic ramus but are absent between the 5 and 7 o’clock positions. In contrast, the corpus spongiosum lacks an outer layer or intracorporeal struts, ensuring a lower-pressure structure during erection. The tunica is composed of elastic fibers forming a network on which the collagen fibers rest. Emissary veins run between the inner and outer layers for a short distance, often piercing the outer bundles obliquely. Branches of the dorsal artery take a more direct perpendicular route, however, and are surrounded by a periarterial fibrous sheath.

The paired internal pudendal artery is the major carrier of the blood supply to the penis. The terminal portion of this artery divides into 3 branches: the bulbourethral artery, the dorsal artery, and the cavernous artery (deep artery). The cavernous artery supplies the corpora cavernosa; the dorsal artery, the skin, subcutaneous tissue, and the glans penis; and the bulbourethral artery, the corpus spongiosum. Accessory pudendal arteries from external iliac or obturator arteries may supply a major portion of the penis in some cases. Collaterals among the 3 branches are often observed (_{Breza et al, 1989}). The venous drainage of the glans is mainly through the deep dorsal vein. The corpus spongiosum is drained via the circumflex, urethral, and bulbar veins, but the drainage of the corpora cavernosa is more complex: The mid and distal shafts are drained by the deep dorsal vein to the preprostatic plexus; the proximal portion is drained by the cavernous and the crural veins to the preprostatic plexus and internal pudendal vein. The drainage of all 3 corpora originates in the subtunical venules, which unite to form emissary veins that pierce the tunica albuginea. The glans penis possesses numerous large and small veins that communicate freely with the dorsal veins. The penile skin and subcutaneous tissue are drained by superficial dorsal veins, which then empty into the saphenous veins.

Activation of the autonomic nerve produces a full erection, that is, filling and trapping of blood in the cavernous bodies. After full erection is achieved, contraction of the ischiocavernosus muscle (from activation of the somatic nerves) compresses the proximal corpora and raises the pressure in the entire corpora well above the systolic blood pressure, resulting in rigid erection (

Table 37-1) This rigid phase occurs naturally during masturbation or sexual intercourse but can also occur from slight bending of the penis, without muscular action. The erection process can be divided into phases, as shown in Table 37-1 and

Figure 37-2. The hemodynamic of the glans penis is somewhat different. Arterial flow increases in the glans in a manner similar to that in the shaft. Because it lacks the tunica albuginea, however, the glans functions as an arteriovenous fistula during the full erection phase. Nevertheless, during rigid erection, most of the venous channels are temporarily compressed, and further engorgement of the glans can be observed (_{Lue, 2000}).