Veins, Lymphatics and Nerves of the Pelvis
The pelvic venous system is responsible for taking blood from the pelvic walls and viscera back to the main circulation. Like the arterial analogues, the external iliac vein primarily drains the lower limbs, while the internal iliac vein drains the pelvic viscera, walls, gluteal region and perineum. In most instances, the major veins are mirror images of their arterial counterparts. However, the smaller vessels can vary from one individual to another. The lymphatic system, however, is relatively simple and follows the major vessels of the pelvis.
The goal of this article is to complete the discussion on the neurovascular supply and lymphatic drainage of the pelvis. No emphasis will be placed on the innervation of the region as most of these structures have been discussed in previous articles.
The inferior epigastric, deep circumflex iliac and pubic veins are all pelvic tributaries of the external iliac vein. The external iliac vein is a cranial continuation of the femoral vein. The nomenclature of the vessel changes at the mid inguinal point, posterior to the inguinal ligament. The deep circumflex iliac vein is a product of the venae comitantes of the eponymous artery. It crosses the anterior surface of the external iliac artery before entering the external iliac vein. Inferior to the entry point of the deep circumflex iliac vein, the inferior epigastric vein enters the external iliac vein cranial to the inguinal ligament. The pubic vein forms a bridge between the obturator vein and the external iliac vein. On the left hand side, the external iliac vein is always medial to its corresponding artery. However, on the right, it starts out in a medial position and gradually becomes posterior as it gets closer to the point of fusion.
The internal iliac vein receives the middle rectal, obturator, lateral sacral, inferior gluteal and superior gluteal veins as tributaries. The obturator vein enters the pelvis by way of the obturator foramen, where it takes a posterosuperior route along the lateral pelvic wall, deep to its artery. In some instances, the vessel is replaced by an enlarged pubic vein, which then terminates in the external iliac vein. The superior and inferior gluteal veins are venae comitantes of their corresponding arteries. The tributaries of the superior gluteal veins are named after the branches of the corresponding artery. They pass above piriformis and enter the pelvis via the greater sciatic foramen before terminating in the internal iliac vein as a single branch. The inferior gluteal veins form anastomoses with the first perforating vein and medial circumflex femoral vein before entering the pelvis via the greater sciatic foramen. The middle rectal vein is a product of the rectal venous plexus that drains the mesorectum and the rectum. It also receives tributaries from the bladder, as well as gender specific tributaries from the prostate and seminal vesicle or the posterior wall of the vagina. It terminates in the internal iliac vein after travelling along the pelvic part of levator ani. Finally, the lateral sacral veins travel with their arteries before entering the internal iliac vein.
The internal and external iliac veins unite at the sacroiliac joint, on the right hand side of the fifth lumbar vertebra, to form the common iliac vein. The right common iliac vein is almost vertical and shorter than the left common iliac vein, which takes a more oblique course. The right obturator nerve crosses the right common iliac vein posteriorly, while the sigmoid mesocolon and superior rectal vessels cross the left common iliac vein anteriorly. The internal pudendal vein drains to the internal iliac vein, while the median sacral veins drain into the common iliac vessels directly. The median sacral veins unite into a single vessel before entering the left common iliac vein. The internal pudendal veins (venae comitantes of their corresponding artery) receive inferior rectal veins and either clitoral and labial or penile bulb and scrotal veins before joining the common iliac vein.
The pelvic lymph nodes are named after the vessels they aggregate around. The three general groups that drain the pelvis and its contents are the internal, external and common iliac nodes. The internal iliac nodes drain the majority of the contents of the pelvic viscera, with the exception of the gonads and most of the rectum. They are arranged around the branches of the internal iliac vessels and also drain the deep perineum, gluteal region and muscles of the posterior thigh. The external iliac nodes are arranged into medial, anterior and lateral clusters. The major drainage channel is via the medial subgroup of nodes. It receives lymph from the inguinal nodes (and by extension, the lower limb), adductor regions, fundus of the bladder and membranous urethra. It also drains the clitoris or glans penis, membranous urethra, upper vagina and uterine cervix and prostate. Both the internal and external iliac nodes drain to the common iliac nodes. These are also subdivided into anterior, medial and lateral groups.
The lumbosacral trunk (L4, L5, S1, S2, S3), sacral plexus and coccygeal plexus all pass through the pelvic cavity. Fibers of the pelvic splanchnic (parasympathetic), sacral splanchnic (sympathetic) and the inferior hypogastric plexuses also have fibers supplying the viscera of the pelvis. The left and right paravertebral sympathetic ganglion chains terminate in the pelvis, by fusing anterior to the pelvic surface of the coccyx to form the ganglion impar. The sacral ventral rami provide visceral and muscular innervation to the contents of the pelvis. More specifically, the second to fourth ventral rami innervate the viscera, while the fourth ventral ramus and its branches supplies levator ani, ischiococcygeus, iliococcygeus, the external anal sphincter and the skin between the coccyx and anus.