Christensen (1954) investigated the normal blood flow through the penis of the dog and the mechanical factors involved in initiating and maintaining erection. Special attention was given to the morphologic characteristics and function of the arteries and veins of the glans penis. More recent studies by Ninomiya et al. (1989) confirm most of these findings.
The latter authors add observations based on corrosion casts and histologic sections to explain the hemodynamics. They found many cushions of epithelioid cells in the arterioles of the erectile bodies, which throttle blood flow into the cavernous spaces when the penis is flaccid, but yield to the pressure of increased flow during erection. Pressure changes in the cavernous spaces appear to be the result of contractions of the bulbospongiosus muscle, which compresses the bulb against the pelvic symphysis, and contractions of the external anal sphincter across the surface of the bulb. Aiding tumescence is the slowed egress of blood from the engorged cavernous spaces caused in part by compression of the internal pudendal vein by the contraction of the levator ani, coccygeus, and the internal obturator muscles.
The principal source of blood to the penis is the internal pudendal artery (), a ramification of the internal iliac. This is augmented by the external pudendal artery, which anastomoses with the preputial branch of the dorsal artery of the penis. After giving off the prostatic and urethral arteries, the internal pudendal gives rise to the ventral perineal artery, which supplies the superficial part of the penile root and the perineum. The internal pudendal artery terminates as the artery of the penis, which supplies the penis via three principal arteries: the artery of the bulb, the deep artery of the penis, and the dorsal artery of the penis. Typically, the artery of the bulb arises from the artery of the penis proximal to the deep artery of the penis. At this point, the artery of the penis is continued by the dorsal artery ().
The paired arteries of the bulb diverge into two or three branches, which divide again before entering the corpus spongiosum of the bulbus penis. These supply the spaces and tissue of the corpus spongiosum, the penile urethra, and the pars bulbus glandis (). The principal trunk is partially coiled in the nonerect state. Its branches anastomose with the end branches of the dorsal artery of the penis as well as with branches of the deep artery.
The deep artery of the penis gives off two to five branches and passes through the tunica albuginea to enter the corpus cavernosum. In this cavernous body, the artery again divides into clumps of spiral or looped vessels, the helicine arteries, which open directly into the cavernous spaces. According to Vaerst (1938) and Kiss (1921), helicine arteries retain their spiral shape in the nonerect penis, owing to contracted myoepithelium.
The dorsal artery of the penis runs diagonally distally to the bulbus glandis, anastomosing with the deep artery and artery of the bulb. Proximal to the glans penis, the dorsal artery trifurcates into a preputial branch, a deep branch, and a superficial branch (). The preputial branch runs dorsodistally over the bulbus glandis, supplying the dorsal surface of the pars longa glandis as well as anastomosing with the external pudendal artery in the parietal wall of the prepuce (). The superficial branch runs ventrodistally deep to the epithelium of the skin of the glans, extending almost to the cranial end of the glans. The deep branch of the dorsal artery enters the terminal tunica albuginea and distally reaches the dorsolateral surface of the os penis, deep to the bulbus glandis (). It passes into the pars longa glandis, terminating near the penile tip. Distal to the bulbus glandis, a large anastomotic branch is given off to the corpus spongiosum. The three branches of the dorsal artery of the penis and the external pudendal artery supply blood to the pars longa glandis.
The internal and external pudendal veins drain blood from the penis. The internal pudendal joins the internal iliac vein, and the external pudendal drains into the external iliac. The iliac veins on each side unite to form the common iliac vein, and the two common iliac veins then converge into the caudal vena cava. The intrinsic penile veins partially parallel the arteries at the root of the penis (). The dorsal veins of the penis are united at the ischial arch for a short distance before they diverge into the right and left internal pudendal veins. Unlike the corresponding arteries, the deep vein of the penis and the vein of the bulb unite in a common vein, the vein of the penis, which enters the internal pudendal vein. The ventral perineal vein also empties into this vein of the penis.
The dorsal veins of the penis arise from either side of the bulbus glandis and run along the dorsolateral surface of the penile body as far as the ischial arch. The superficial vein of the glans runs from the dorsal surface of the pars longa glandis to the external pudendal vein ().
The dorsal vein of the penis (), in its course between the bulbus glandis and the ischial arch where it unites with the opposite dorsal vein of the penis, has distinct semilunar valves regularly spaced along its entire length (). The vein of the bulb drains the cavernous spaces of the proximal half of the corpus spongiosum, arising at the junction of its proximal and middle thirds. Typically, two valves are located in the vein of the bulb between its emergence from the cavernous body and its junction with the deep vein of the penis. One to five valves are present between the origin of the vein of the penis and where it is joined by the ventral perineal vein.
The superficial vein of the glans arises from the deep surface of the pars longa glandis, which it helps to drain, and runs dorsocaudally to the fornix of the prepuce, where it bends acutely cranially and drains into the external pudendal vein by two or more connections (). One or more valves are present in each branch of the vein.
The deep vein of the glans () drains blood from the pars longa glandis into the bulbus glandis. It arises on each side of the midline from the middle of the deep surface of the pars longa glandis and runs proximally along the dorsolateral surface of the os penis to enter the bulbus glandis. A semilunar valve prevents blood in the bulbus glandis from going to the pars longa glandis.
There is a double connection on each side of the os penis, between the deep vein of the glans and the dorsal vein of the penis (). A ventral shunt, through irregularly located openings, receives blood from the venous spaces of the bulbus glandis and the corpus spongiosum. The openings are directed proximally and are bordered distally by a lip of endothelium that diverts blood toward the dorsal vein of the penis. There is also a dorsal shunt through the bulbus glandis, which is
narrower in diameter and less clearly defined than the ventral shunt. Numerous branches go from the dorsal shunt into the spaces of the bulbus. Blood going through the dorsal shunt disseminates into cavernous spaces of the bulbus, whereas blood in the ventral shunt is directed into the dorsal vein of the penis without detouring through the venous sinuses of the bulbar erectile tissue.
Nerves leaving the pelvic and sacral plexuses supply the penis. These are the paired pelvic nerve and the paired pudendal nerve respectively. The pelvic plexus lies on the pelvic wall dorsal to the prostate gland, lateral to the rectum. It receives sympathetic postganglionic axons through the hypogastric nerve, which runs caudally from the caudal mesenteric plexus and ganglion. Sympathetic postganglionic axons may also arise from the sacral sympathetic trunk, enter the sacral plexus, and continue into the pudendal nerve to innervate the penis. The pelvic plexus receives parasympathetic preganglionic axons through the pelvic nerve from ventral branches of the first, second, and sometimes the third sacral nerves. Infrequently, only axons from the second sacral nerve go to the plexus. Axons leave the pelvic plexus and go to the bladder, prostate, pelvic urethra, rectum, and the penis. Sensory (afferent) fibers are present in the pelvic nerve, as well as efferent parasympathetic fibers, according to Gruber (1933). The hypogastric nerves (sympathetic) are responsible for ejaculation and prostatic secretion.
The pudendal nerve (mixed) arises from the sacral plexus, which is formed from all three sacral spinal nerve ventral branches. The pudendal nerve gives off superficial and deep perineal nerves and a caudal rectal nerve before continuing as the dorsal nerve of the penis. Superficial perineal nerves supply the skin around the anus and the scrotum (). The caudal rectal nerve supplies the external anal sphincter. Deep perineal nerve branches supply the ischiocavernosus, bulbospongiosus, ischiourethralis, and the retractor penis muscles. The smooth muscle fibers of the retractor penis also receive sympathetic and parasympathetic impulses. The retractor penis muscles receive postganglionic sympathetic axons via the deep perineal nerves (which cause contraction) and postganglionic parasympathetic axons via the pelvic plexus (which cause relaxation). The dorsal nerves of the penis pass cranially, on the dorsolateral surface of the body to the glans penis as sensory nerves of the glans. They give off numerous preputial branches before entering the glans. These branches supply the skin around the preputial orifice ().
Lymph vessels from the penis drain into the superficial inguinal lymph nodes.