Iliac Veins

The iliac veins are major systemic veins of the pelvis that collect venous blood from the lower limbs, pelvic walls, pelvic organs, perineum, and portions of the lower anterior abdominal wall. They form an important connection between the venous systems of the lower body and the inferior vena cava.

On each side, the external iliac vein and internal iliac vein unite to form a common iliac vein. The right and left common iliac veins then join in the lower abdomen to form the inferior vena cava.

The iliac venous system contains extensive communications between superficial, deep, pelvic, vertebral, and abdominal venous networks. These connections are clinically important in venous obstruction, thrombosis, pelvic surgery, and the development of collateral circulation.

Major Divisions

The iliac venous system consists of three principal paired veins:

• External iliac veins
• Internal iliac veins
• Common iliac veins

General Drainage Pattern

The external iliac veins primarily receive venous blood from the lower limbs and lower anterior abdominal wall. The internal iliac veins drain most pelvic organs, pelvic walls, gluteal regions, and much of the perineum.

These vessels unite on each side to form the common iliac veins, which carry blood toward the inferior vena cava.

External Iliac Vein

The external iliac vein is the proximal continuation of the femoral vein. It begins posterior to the inguinal ligament and ascends along the pelvic brim toward the sacroiliac joint.

It joins the internal iliac vein to form the common iliac vein.

Formation of the External Iliac Vein

The femoral vein becomes the external iliac vein as it passes deep to the inguinal ligament.

This transition provides continuity between the major deep venous drainage of the lower limb and the pelvic venous system.

Course of the External Iliac Vein

The external iliac vein ascends along the medial side of the external iliac artery.

As it approaches the common iliac region, its relationship with the artery changes slightly because of the arrangement of the pelvic vessels.

Relationship to the External Iliac Artery

The external iliac vein generally lies medial to the corresponding external iliac artery.

This relationship is an important landmark during pelvic surgery and vascular procedures.

External Iliac Vein Tributaries

The external iliac vein receives several tributaries, including vessels corresponding broadly to branches of the external iliac artery.

Important tributaries include the inferior epigastric and deep circumflex iliac veins.

Inferior Epigastric Vein

The inferior epigastric vein drains the lower anterior abdominal wall.

It accompanies the inferior epigastric artery and enters the external iliac vein near the inguinal region.

Deep Circumflex Iliac Vein

The deep circumflex iliac vein accompanies the corresponding artery along the iliac crest and drains into the external iliac vein.

Pubic Venous Connections

Venous channels near the superior pubic ramus can connect the external iliac or inferior epigastric venous system with the obturator venous system.

These communications vary and are important during surgery near the superior pubic ramus.

Internal Iliac Vein

The internal iliac vein is the major venous drainage pathway for the pelvis.

It receives numerous tributaries from the pelvic walls, pelvic viscera, gluteal region, and perineum.

Formation of the Internal Iliac Vein

The internal iliac vein is formed by the convergence of multiple venous tributaries within the pelvis.

Unlike many major veins, it does not have a single simple distal continuation comparable to the external iliac vein.

Course of the Internal Iliac Vein

The internal iliac vein ascends from the pelvic cavity toward the sacroiliac joint.

It lies posterior and medial to the internal iliac artery for much of its course and joins the external iliac vein to form the common iliac vein.

Internal Iliac Tributaries

The internal iliac vein receives numerous tributaries that generally correspond to branches of the internal iliac artery.

These tributaries form extensive plexuses around pelvic organs before converging into larger veins.

Major Internal Iliac Tributaries

Tributary	Major Drainage Territory
Superior gluteal vein	Gluteal region
Inferior gluteal vein	Gluteal region and posterior thigh connections
Internal pudendal vein	Perineum and external genital structures
Obturator vein	Medial thigh and pelvic region
Lateral sacral veins	Sacral and vertebral regions
Middle rectal veins	Rectum
Vesical veins	Urinary bladder
Uterine and vaginal veins	Female pelvic reproductive organs

Superior Gluteal Veins

The superior gluteal veins accompany the superior gluteal artery through the greater sciatic foramen.

They drain structures of the gluteal region and terminate in the internal iliac venous system.

Inferior Gluteal Veins

The inferior gluteal veins drain the inferior gluteal region and communicate with venous channels of the posterior thigh.

They pass through the greater sciatic foramen and drain into the internal iliac vein.

Internal Pudendal Vein

The internal pudendal vein drains deep structures of the perineum and external genital region.

It accompanies the internal pudendal artery and ultimately enters the internal iliac venous system.

Obturator Vein

The obturator vein accompanies the obturator artery and nerve through the obturator canal.

It drains the medial thigh and communicates with pelvic venous channels before entering the internal iliac vein.

Lateral Sacral Veins

The lateral sacral veins drain structures around the sacrum and communicate with the vertebral venous plexuses.

These connections provide pathways between pelvic and vertebral venous systems.

Pelvic Venous Plexuses

Many pelvic organs are surrounded by interconnected venous plexuses rather than being drained solely by isolated veins.

These plexuses communicate extensively with one another and with tributaries of the internal iliac veins.

Vesical Venous Plexus

The vesical venous plexus surrounds the urinary bladder, particularly near its base.

Venous blood from this plexus drains through vesical veins into the internal iliac veins.

Prostatic Venous Plexus

In males, the prostatic venous plexus surrounds the prostate and communicates with vesical veins and the internal vertebral venous plexus.

These valveless communications are clinically important because they provide potential pathways for the spread of disease.

Uterine Venous Plexus

In females, veins surrounding the uterus form a uterine venous plexus.

Uterine veins drain this plexus toward the internal iliac veins.

Vaginal Venous Plexus

The vaginal venous plexus surrounds the vagina and communicates with uterine, vesical, and rectal venous networks.

Its drainage ultimately reaches the internal iliac veins.

Rectal Venous Drainage

The rectum has venous drainage into both portal and systemic venous systems.

The superior rectal vein drains toward the inferior mesenteric vein and portal circulation, while middle and inferior rectal veins communicate with systemic pathways involving the internal iliac venous system.

Portosystemic Connections

Connections between rectal venous channels contribute to a site of communication between portal and systemic venous circulations.

These connections are part of a broader network rather than a single direct vein-to-vein junction.

Common Iliac Veins

The common iliac veins are formed by the union of the external and internal iliac veins on each side.

They ascend toward the lower lumbar region and unite to form the inferior vena cava.

Right Common Iliac Vein

The right common iliac vein is generally shorter and follows a relatively direct course toward the inferior vena cava.

It lies posterior and then lateral to the corresponding artery as the vessels approach the caval confluence.

Left Common Iliac Vein

The left common iliac vein is longer and follows a more oblique course toward the right side of the vertebral column.

It must cross toward the right because the inferior vena cava lies to the right of the midline.

Formation of the Inferior Vena Cava

The right and left common iliac veins unite to form the inferior vena cava, usually near the level of the fifth lumbar vertebra.

The inferior vena cava then ascends on the right side of the vertebral column.

Comparison of the Common Iliac Veins

Feature	Right Common Iliac Vein	Left Common Iliac Vein
Relative length	Shorter	Longer
Course	More direct	More oblique
Midline relationship	Near right-sided IVC	Crosses toward the right
Termination	Joins left vein to form IVC	Joins right vein to form IVC

Relationship of the Left Common Iliac Vein to the Right Common Iliac Artery

The left common iliac vein passes posterior to the right common iliac artery near the lumbosacral region.

This crossing is clinically important because the overlying artery can compress the vein against the vertebral column.

May-Thurner Syndrome

May-Thurner syndrome, also called iliac vein compression syndrome, involves compression of the left common iliac vein by the overlying right common iliac artery against the lumbar vertebral column.

This anatomical relationship can contribute to impaired left lower limb venous drainage and may increase the risk of deep venous thrombosis in susceptible individuals.

Common Iliac Tributaries

The common iliac veins receive tributaries in addition to the external and internal iliac veins.

These may include iliolumbar veins and, particularly on the left, the median sacral vein in variable configurations.

Iliolumbar Veins

Iliolumbar veins drain the iliac and lower lumbar regions and communicate with lumbar and vertebral venous networks.

They commonly terminate in the common iliac or internal iliac venous system.

Median Sacral Vein

The median sacral vein accompanies the median sacral artery and drains structures near the midline of the sacrum.

Its termination is variable and may involve the left common iliac vein or the junction of the common iliac veins.

Relationship to Iliac Arteries

The iliac veins lie close to the corresponding arteries, but their positions are not perfectly symmetrical.

Venous structures are generally more posterior and medial, and arterial crossings can influence venous flow.

Relationship to the Lumbosacral Trunk

The lumbosacral trunk passes through the posterior pelvic region near the iliac vessels.

Knowledge of the relationship between the veins, arteries, and nerves is important during pelvic and vascular surgery.

Relationship to the Ureter

The ureters descend into the pelvis near the bifurcation of the common iliac arteries or adjacent iliac vessels.

This region contains closely packed vascular and urinary structures that are important surgical landmarks.

Relationship to Pelvic Lymph Nodes

External iliac, internal iliac, and common iliac lymph nodes are distributed along the corresponding iliac vessels.

The veins therefore have close relationships with major lymphatic drainage pathways of the pelvis and lower limb.

Function

The iliac veins return systemic venous blood from the lower limbs and pelvis toward the heart.

They also provide extensive collateral connections between pelvic, abdominal, vertebral, and lower limb venous systems.

Lower Limb Venous Return

Deep venous blood from the lower limb passes through the femoral vein, which becomes the external iliac vein after passing deep to the inguinal ligament.

The external iliac vein then joins the internal iliac vein to form the common iliac vein.

Pelvic Venous Return

Blood from pelvic organs and walls drains primarily through tributaries and plexuses of the internal iliac veins.

This blood then enters the common iliac veins and inferior vena cava.

Overall Drainage Pathway

A simplified pathway is:

Lower limb and pelvic veins → External and internal iliac veins → Common iliac veins → Inferior vena cava → Right atrium

Valves

Valves are less consistently present in the large pelvic veins than in many peripheral veins of the lower limb.

Many pelvic venous plexuses and communicating channels are valveless, allowing pressure changes to influence flow between interconnected venous territories.

Collateral Circulation

The iliac veins communicate with numerous collateral pathways through abdominal wall, pelvic, lumbar, vertebral, and superficial venous networks.

These connections can enlarge when a major iliac vein or the inferior vena cava becomes obstructed.

Vertebral Venous Connections

Pelvic veins communicate with the internal and external vertebral venous plexuses through sacral and other connecting veins.

Because these plexuses are largely valveless, blood can move in different directions according to pressure gradients.

Deep Vein Thrombosis

Deep vein thrombosis can extend from lower limb veins into the external and common iliac veins or arise within the iliac venous system itself.

Proximal iliac thrombosis can substantially impair venous drainage from the affected lower limb.

Iliofemoral Deep Vein Thrombosis

Thrombosis involving the iliac and femoral venous segments is commonly described as iliofemoral deep vein thrombosis.

Because these veins carry a large proportion of lower limb venous return, obstruction can produce extensive swelling and venous congestion.

Pulmonary Embolism

Thrombi within the iliac or lower limb deep veins can detach and travel through the inferior vena cava and right side of the heart into the pulmonary arterial circulation.

This is an important mechanism of pulmonary embolism.

Pelvic Venous Congestion

Dilation and impaired drainage of pelvic veins can contribute to pelvic venous congestion.

The pelvic venous plexuses have extensive communications, and abnormalities in gonadal or iliac venous flow can influence these networks.

Iliac Vein Compression

Iliac veins may be compressed by arteries, masses, enlarged lymph nodes, pregnancy-related changes, or other pelvic structures.

Persistent compression can alter venous flow and promote development of collateral pathways.

Central Venous Catheters

The external iliac and common iliac veins may be traversed by catheters introduced through femoral venous access.

From the femoral vein, a catheter passes into the external iliac vein, common iliac vein, and inferior vena cava.

Femoral Venous Access

Femoral venous access provides a route to the right side of the heart and central venous circulation.

Knowledge of iliac venous anatomy is important when advancing wires, catheters, and other devices through this pathway.

Iliac Vein Stenting

Selected cases of clinically significant iliac venous obstruction or compression may be treated with endovascular stenting.

Imaging is used to define the anatomy and location of the obstructed venous segment.

Inferior Vena Cava Filters

Devices placed within the inferior vena cava are commonly introduced through jugular or femoral venous approaches.

A femoral approach requires passage through the external and common iliac veins before reaching the inferior vena cava.

Imaging

Ultrasound, CT, MRI, and contrast venography can be used to evaluate the iliac veins.

The deep pelvic location of these vessels can make direct ultrasound assessment more difficult than examination of veins in the thigh and leg.

CT Venography

CT venography can demonstrate iliac venous anatomy, thrombosis, compression, collateral vessels, and relationships with adjacent arteries and pelvic structures.

MR Venography

MR venography can evaluate the iliac veins and pelvic venous system without ionizing radiation.

It can provide multiplanar visualization of central venous anatomy and flow-related abnormalities.

Contrast Venography

Catheter venography provides direct visualization of venous channels using intravascular contrast.

It can be combined with endovascular treatment when clinically indicated.

Pelvic Surgery

The iliac veins are important structures during pelvic, vascular, urologic, gynecologic, and oncologic surgery.

Their thin walls and close relationships with arteries, lymph nodes, nerves, and pelvic organs make careful identification essential.

Lymph Node Dissection

Pelvic lymph nodes lie along the external, internal, and common iliac vessels.

Lymph node dissection in these regions therefore requires careful preservation or controlled management of adjacent veins and their tributaries.

Embryological Development

The iliac veins develop through remodeling of embryonic cardinal and subcardinal venous channels and their connections.

Developmental variation can alter the configuration of the iliac veins and inferior vena cava.

Venous Variations

Variations may include duplication, unusual communications, altered tributary patterns, and anomalies associated with the inferior vena cava.

Such variants can affect imaging interpretation and surgical or endovascular procedures.

External, Internal and Common Iliac Veins Compared

Vein	Formation or Origin	Major Drainage	Termination
External iliac	Continuation of femoral vein	Lower limb and lower abdominal wall	Joins internal iliac vein
Internal iliac	Convergence of pelvic tributaries	Pelvic organs, walls, gluteal region and perineum	Joins external iliac vein
Common iliac	External + internal iliac veins	Lower limb and pelvis	Joins opposite common iliac vein to form IVC

Key Features of the Iliac Veins

Feature	Key Point
External iliac vein	Continuation of femoral vein above inguinal ligament
Internal iliac vein	Major venous drainage of pelvis
Common iliac vein	Formed by external and internal iliac veins
IVC formation	Right and left common iliac veins unite near L5
Important arterial crossing	Right common iliac artery crosses anterior to left common iliac vein
Major clinical association	May-Thurner syndrome and iliofemoral DVT
Pelvic venous organization	Extensive interconnected venous plexuses

Anatomical and Clinical Importance

The iliac veins form the major venous outflow pathway from the lower limbs and pelvis. The external iliac veins continue the deep venous system of the lower limbs, while the internal iliac veins collect blood from extensive pelvic and perineal venous networks. Their union creates the common iliac veins, which then form the inferior vena cava.

The asymmetrical course of the common iliac veins is clinically important. In particular, the left common iliac vein passes posterior to the right common iliac artery, creating an anatomical site where venous compression can occur.

The iliac veins are central to the anatomy of deep vein thrombosis, collateral venous circulation, pelvic surgery, femoral venous access, and endovascular intervention. Their extensive communications with pelvic and vertebral venous plexuses also provide important alternative pathways for venous flow when normal drainage is obstructed.