The common iliac vein is a large venous structure that is formed by the union of the internal and external iliac veins at the level of the pelvis. It is located on each side of the body and begins near the sacroiliac joint. The left and right common iliac veins ascend and converge at approximately the level of the fifth lumbar vertebra, forming the inferior vena cava, which carries deoxygenated blood toward the heart. The common iliac veins are located posterior and slightly medial to the common iliac arteries and are situated near important structures, such as the ureters and lumbosacral trunk.
Structure and Anatomy
Origin
The common iliac vein is formed by the union of the internal iliac vein and the external iliac vein on each side of the body. This junction typically occurs at the level of the sacroiliac joint, near the pelvic brim. The internal iliac vein drains blood from the pelvic organs and structures, while the external iliac vein continues from the femoral vein, draining the lower limb. Together, they form the common iliac vein, which carries venous blood from the lower body toward the inferior vena cava.
Course
The common iliac veins run superiorly and posteriorly from their origin at the sacroiliac joint.
- Right Common Iliac Vein: The right common iliac vein ascends slightly shorter than the left and runs along the right side of the fifth lumbar vertebra. It is positioned more vertically compared to the left and follows a shorter path before merging with the left common iliac vein to form the inferior vena cava.
- Left Common Iliac Vein: The left common iliac vein has a longer and more oblique course, running upward and to the right. It passes behind the right common iliac artery at approximately the level of the fifth lumbar vertebra before joining the right common iliac vein to form the inferior vena cava.
Relationship with Surrounding Structures
The common iliac veins are located in close proximity to several key anatomical structures:
- Common Iliac Arteries: The common iliac veins lie posterior and slightly medial to the common iliac arteries. The left common iliac vein, in particular, passes behind the right common iliac artery, which is an important relationship to consider in clinical settings.
- Ureters: The ureters, which carry urine from the kidneys to the bladder, cross anteriorly over the common iliac veins. This relationship is important in surgical procedures involving the pelvis, as injury to the veins or ureters can occur if not carefully navigated.
- Lumbosacral Trunk: This nerve structure is located near the common iliac vein and contributes to the sacral plexus, which innervates the lower limb. The proximity of the lumbosacral trunk to the common iliac veins is relevant during pelvic surgeries.
- Psoas Major Muscle: The common iliac veins are situated medial to the psoas major muscle, which runs along the posterior abdominal wall. The muscle provides a landmark for identifying the veins during surgical or diagnostic procedures.
Tributaries
The common iliac vein receives blood from several important tributaries:
- Iliolumbar Vein: This vein drains blood from the lower lumbar region and the iliac fossa, feeding into the common iliac vein.
- Median Sacral Vein: Draining blood from the sacrum and coccyx, the median sacral vein typically joins the left common iliac vein or at the junction of the right and left common iliac veins.
- Venous Plexuses: The common iliac veins also collect blood from pelvic venous plexuses, including those associated with the rectum, bladder, prostate (in males), and uterus (in females).
Termination
The right and left common iliac veins converge at approximately the level of the fifth lumbar vertebra to form the inferior vena cava. This convergence typically occurs just below the bifurcation of the abdominal aorta. The inferior vena cava continues superiorly to carry deoxygenated blood from the lower body toward the heart. The exact location of this convergence can vary slightly among individuals but generally occurs near the lumbosacral junction.
Variations
There can be anatomical variations in the size, course, and termination of the common iliac veins. In some individuals, the veins may be asymmetrical in length or diameter, with the left vein often being longer and slightly more angled than the right. Additionally, variations in the number and course of tributaries, such as the iliolumbar or median sacral veins, can occur. These variations are generally rare but are important to recognize in imaging studies or surgical planning.
Function
The primary function of the common iliac vein is to collect and transport deoxygenated blood from the lower limbs, pelvis, and abdominal walls toward the heart. Below is a detailed explanation of its various functions.
Venous Drainage of the Lower Limbs
The external iliac vein, which contributes to the formation of the common iliac vein, is responsible for draining blood from the lower limbs. It continues from the femoral vein and collects deoxygenated blood from structures such as the:
- Thigh and Leg: The superficial and deep veins of the leg, including the great and small saphenous veins, drain into the femoral vein, which becomes the external iliac vein. This blood is then funneled into the common iliac vein, ensuring proper circulation from the lower limbs back toward the heart.
- Lower Abdominal Wall: The external iliac vein also drains venous blood from the lower abdominal wall, particularly from the inferior epigastric vein, which is important for maintaining circulation in the abdominal muscles and connective tissues.
Venous Drainage of the Pelvic Organs and Structures
The internal iliac vein, which joins the external iliac vein to form the common iliac vein, drains the pelvic organs and surrounding structures. The common iliac vein thus plays an essential role in transporting blood from these regions. The internal iliac vein collects blood from:
- Pelvic Organs: The bladder, rectum, prostate (in males), uterus (in females), and other pelvic organs. Venous blood from these structures flows into the internal iliac vein, contributing to the venous drainage system.
- Gluteal Region: The superior and inferior gluteal veins drain the gluteal muscles, including the gluteus maximus, medius, and minimus, feeding blood into the internal iliac vein. This blood is eventually carried by the common iliac vein toward the heart.
- Perineum and External Genitalia: In males, the internal pudendal vein drains the external genitalia and perineal muscles. In females, venous blood from the external genitalia and perineum is also collected by tributaries of the internal pudendal vein, which drains into the internal iliac vein.
Transport of Deoxygenated Blood to the Inferior Vena Cava
The common iliac vein is the final conduit for venous blood from the lower body before it enters the inferior vena cava. Both the left and right common iliac veins merge at the level of the fifth lumbar vertebra to form the inferior vena cava, which transports deoxygenated blood to the right atrium of the heart. This step is crucial in maintaining systemic circulation, as it ensures that blood from the lower limbs, pelvis, and abdominal regions reaches the heart to be re-oxygenated in the lungs.
Drainage from Lumbar and Sacral Regions
The common iliac vein also receives blood from tributaries that drain the lumbar and sacral regions, including the:
- Iliolumbar Vein: The iliolumbar vein drains blood from the posterior abdominal wall, lower lumbar region, and iliac fossa. It feeds into the common iliac vein, ensuring proper venous drainage from the lower spine and adjacent structures.
- Median Sacral Vein: This vein drains blood from the sacrum and coccyx, contributing to the venous return from the lower spine and pelvic bones. The median sacral vein typically joins the left common iliac vein or at the junction of the common iliac veins.
Facilitating Venous Return and Preventing Venous Stasis
The common iliac veins play a vital role in facilitating venous return by carrying deoxygenated blood back toward the heart. By ensuring efficient drainage from the lower body, the common iliac veins prevent venous stasis (the pooling of blood), which could otherwise lead to complications such as edema, varicose veins, or deep vein thrombosis (DVT). The veins are also large enough to accommodate the significant volume of blood returning from the lower body during physical activity or periods of rest.
Supporting Collateral Circulation
In cases of venous obstruction, such as compression or thrombosis, the common iliac veins can participate in collateral circulation, where blood is rerouted through alternate venous pathways. This adaptive function helps maintain blood flow from the lower body even when the primary venous routes are compromised. Collateral pathways may involve smaller pelvic veins or the vertebral venous system, ensuring that venous return to the heart is sustained.
Clinical Significance
The common iliac vein is clinically significant due to its role in draining venous blood from the lower limbs, pelvis, and abdominal regions to the inferior vena cava. Its anatomical location and proximity to key structures, such as the common iliac artery and ureters, make it important in various clinical situations:
Deep Vein Thrombosis (DVT)
The common iliac vein is a common site for deep vein thrombosis, where blood clots can form, particularly in patients with prolonged immobility, pregnancy, or surgery. A thrombus in this vein can obstruct blood flow and may lead to life-threatening conditions like pulmonary embolism if the clot dislodges and travels to the lungs.
May-Thurner Syndrome
In May-Thurner syndrome, the left common iliac vein is compressed by the overlying right common iliac artery, leading to an increased risk of DVT in the left leg. This condition causes venous stasis, pain, and swelling in the affected limb, requiring medical or surgical intervention to restore proper blood flow.
Surgical Considerations
During surgeries involving the pelvis, lower spine, or vascular system, the common iliac vein is vulnerable to injury due to its proximity to major arteries and nerves. Accidental damage to the vein can result in significant blood loss or the formation of hematomas, requiring careful management.
