The Internal Iliac Artery: One Trunk, the Whole Pelvis
It is about four centimetres long — shorter than your little finger — and from that stub of vessel come the arteries of the bladder, the rectum, the uterus and vagina or the prostate and seminal vesicles, the whole perineum and its erectile tissue, the buttock, and part of the thigh. No other artery of comparable size carries so much territory. And no other named artery in the body is so unreliable: open two pelvises and the branching order will not match. Surgeons who work here do not memorise a diagram; they memorise the trunk, the two divisions, and the handful of crossings that decide whether an operation ends well.
The delivery went well. Twenty minutes later the midwife's hand, resting on the abdomen, finds a uterus that will not harden — and the blood on the floor is no longer measured in gauze but in litres. Drugs are given, the uterus is rubbed, a balloon is inflated inside it, and still the loss continues. In another hospital a surgeon would now be reaching into the pelvis for the internal iliac arteries. Here, instead, a radiologist threads a catheter up from the groin, round the aortic bifurcation, and down into a vessel four centimetres long. A puff of contrast lights up a delta of branches feeding the bleeding uterus. Small particles go in; the delta fades; the bleeding stops. The uterus survives — it does not die, it does not scar into uselessness — because the pelvis is laced with so many alternative routes that you can close its main supply and the tissue simply borrows blood from elsewhere. That single fact, more than any list of branches, is why this artery is worth knowing properly.
Four centimetres, and where they begin
The aorta ends by splitting twice: once into the common iliacs, then again into a limb artery and a pelvic artery. The abdominal aorta bifurcates at the level of the fourth lumbar vertebra into the two COMMON ILIAC arteries, as traced in the abdominal aorta. Each common iliac runs down and laterally and, at the level of the L5/S1 intervertebral disc — in front of the sacroiliac joint, at the pelvic brim — divides into an EXTERNAL and an INTERNAL iliac artery. The external iliac continues forward under the inguinal ligament to become the femoral artery and belongs to the lower limb. The INTERNAL iliac artery turns downwards and backwards into the true pelvis, and after only about four centimetres, at the upper border of the GREATER SCIATIC FORAMEN, it divides into an ANTERIOR and a POSTERIOR trunk. That is the entire course. Everything difficult about this artery happens after those four centimetres, and almost everything clinically important happens at a crossing rather than along a branch.
The company it keeps
Relations matter here more than in most places, because two of them are how surgeons find their way. IN FRONT of the origin of the internal iliac artery, the URETER crosses the pelvic brim — famously descending over the bifurcation of the common iliac and then running down the lateral pelvic wall. This is the single most reliable way to find a ureter in a difficult pelvis: follow the artery up to the brim and the ureter will be lying on it, adherent to the overlying peritoneum, and it will be seen to contract in a slow worm-like wave when touched. BEHIND the artery lie the internal iliac VEIN and its tributaries, thin-walled and unforgiving, and the LUMBOSACRAL TRUNK (L4–L5) descending to join the sacral plexus. Laterally is the obturator nerve and the wall of the pelvis described in the walls of the pelvis; medially, only peritoneum and the pelvic viscera. A clamp placed carelessly on the artery takes the vein behind it, and bleeding from a torn internal iliac vein in the depths of the pelvis is one of the worst situations in surgery.
The posterior trunk: three branches to the wall
The posterior division is short, constant and entirely PARIETAL — it feeds walls, not organs. Three branches, and they are worth learning as a set because they are the reliable part of a famously unreliable artery. The ILIOLUMBAR artery runs upwards and laterally, back out of the pelvis, to supply iliacus, psoas, quadratus lumborum and the lower lumbar vertebrae — it is, in effect, the internal iliac paying back a debt to the posterior abdominal wall. The LATERAL SACRAL arteries, usually two, descend on the front of the sacrum and send branches into the ANTERIOR SACRAL FORAMINA to supply the sacral canal, its meninges and the roots of the sacral nerves, emerging again through the posterior foramina onto the back of the sacrum. And the SUPERIOR GLUTEAL artery — the largest branch of the whole internal iliac — passes backwards between the lumbosacral trunk and the first sacral nerve and leaves the pelvis through the GREATER SCIATIC FORAMEN ABOVE PIRIFORMIS, the only significant structure to do so. Everything else that uses the greater sciatic foramen passes below the muscle. That single word, ABOVE, is the whole of the superior gluteal artery's identity, and it is asked in every examination.
The anterior trunk: everything else
The anterior division is longer, mostly VISCERAL, and it is where the variability lives. Its branches, in the order most books give them: the UMBILICAL artery, which in fetal life carried blood to the placenta and after birth stays patent only for its first few centimetres — that patent proximal part gives off the SUPERIOR VESICAL arteries to the upper bladder and the ureter, while the distal part obliterates into a fibrous cord, the MEDIAL UMBILICAL LIGAMENT, which raises a fold of peritoneum on the inner surface of the anterior abdominal wall. Then the INFERIOR VESICAL artery in the male, supplying the base of the bladder, the prostate, the seminal vesicles and the lower ureter; its counterpart in the female is the VAGINAL artery. The MIDDLE RECTAL artery supplies the muscular wall of the lower rectum and anastomoses with the superior and inferior rectal arteries. The OBTURATOR artery runs forward along the lateral pelvic wall, below the obturator nerve, and leaves through the OBTURATOR CANAL into the medial compartment of the thigh. The UTERINE artery in the female runs medially in the base of the broad ligament to the side of the cervix. The INTERNAL PUDENDAL artery leaves the pelvis below piriformis, and the INFERIOR GLUTEAL artery — the other large terminal branch — does the same, passing on into the buttock and giving the artery to the sciatic nerve.
Think of the internal iliac as a short municipal main that dives under a city and immediately splits into two contractors. The POSTERIOR contractor takes the structural work — foundations, retaining walls, the buried conduits of the sacrum — and it does the same three jobs in every city, so you can predict it. The ANTERIOR contractor takes the residents: water to the bladder, the rectum, the reproductive organs, and a long service line out to the perineum. Residential work is negotiated case by case, so the anterior contractor's schedule is never the same twice — sometimes two branches share a stem, sometimes one arises from the posterior side, sometimes a branch comes off the parent trunk directly. The surgeon's rule follows from the analogy: never identify a pelvic branch by where you expect it to be. Identify it by following it to what it supplies.
Two crossings that decide operations
Neither is a branch. Both are places where two structures happen to touch. The first is the UTERINE artery and the URETER. The uterine artery runs medially in the base of the broad ligament and, about two centimetres lateral to the cervix, it passes ABOVE and IN FRONT of the ureter, which is running below it towards the bladder. Generations have remembered it as WATER UNDER THE BRIDGE — the water is the ureter, the bridge is the artery. At hysterectomy the uterine artery must be clamped and divided at exactly this point, and it is here, in a bleeding field with the tissues distorted by fibroids or endometriosis, that the ureter is caught in the clamp or the suture. It is the classic ureteric injury of gynaecological surgery and the reason the ureter is deliberately identified and traced before that pedicle is taken. The relation is set out further in the uterus and uterine tubes. The second crossing is the CORONA MORTIS — the crown of death — an anastomotic vessel, arterial or venous or both, running over the superior pubic ramus between the OBTURATOR artery and the INFERIOR EPIGASTRIC or external iliac system. It is present in a substantial minority of pelvises, it is invisible until it is torn, and when it is torn it retracts behind the pubic bone where it cannot easily be caught. It bleeds catastrophically in pubic ramus fractures and it is the reason the surgeon repairing a femoral or inguinal hernia keeps well clear of the lacunar ligament.
The INTERNAL PUDENDAL artery has the strangest journey of any branch, and it is worth tracing finger by finger. It leaves the pelvis through the GREATER sciatic foramen BELOW piriformis, immediately hooks backwards around the ISCHIAL SPINE and the sacrospinous ligament, and re-enters the perineum through the LESSER sciatic foramen — a vessel that leaves a cavity and comes straight back into a different compartment of the same region. It then runs forward in the PUDENDAL CANAL (Alcock's canal) in the lateral wall of the ischioanal fossa, giving the inferior rectal artery to the anal canal, then perineal branches, and finally the arteries of the erectile tissue. It is the artery of the entire perineum, and it travels the whole way beside the nerve that shares its name and its route, described in the pudendal nerve. Two consequences follow. First, the ischial spine is a landmark for both: a needle placed on it blocks the nerve, and a fracture through it threatens the artery. Second, erectile failure of vascular origin is frequently a disease of this arterial pathway or of its parent internal iliac — which is why erectile dysfunction is regarded clinically as an early marker of generalised atherosclerosis rather than a local complaint.
The artery that does NOT come from here
Every list of pelvic arteries has one deliberate omission, and examiners love it. The OVARIAN artery does not arise from the internal iliac at all. It arises from the ABDOMINAL AORTA at the level of L2, high on the posterior abdominal wall, and descends over the pelvic brim in the suspensory ligament of the ovary to reach the gonad. The reason is embryological: the ovary — like the testis — develops on the posterior abdominal wall at upper lumbar level and takes its blood supply with it as it descends. It never acquires a pelvic artery because it was never a pelvic organ. The same logic dictates its venous drainage (right ovarian vein to the inferior vena cava, left to the left renal vein) and its lymphatic drainage (para-aortic nodes at L1–L2, not pelvic nodes) — a point that changes cancer staging and is developed in the ovaries. Note the elegant compromise at the top of the uterus: the ovarian artery, coming down from the aorta, anastomoses freely with the uterine artery, coming up from the internal iliac, along the side of the uterus. Two entirely different vascular territories, meeting and merging in the broad ligament.
- The common iliac artery divides at the L5/S1 disc, in front of the sacroiliac joint, into the external iliac (to the limb) and the internal iliac (to the pelvis).
- The internal iliac is only about 4 cm long and divides at the upper border of the greater sciatic foramen into an ANTERIOR and a POSTERIOR trunk.
- The URETER crosses IN FRONT of its origin at the pelvic brim — the surgeon's landmark for finding the ureter; the internal iliac VEIN and the lumbosacral trunk lie BEHIND.
- POSTERIOR trunk = three PARIETAL branches: iliolumbar, lateral sacral, and superior gluteal — the largest branch of the whole artery, leaving ABOVE piriformis.
- ANTERIOR trunk = umbilical (giving superior vesical, then obliterating as the medial umbilical ligament), inferior vesical (male) or vaginal (female), middle rectal, obturator, uterine, internal pudendal and inferior gluteal.
- The OVARIAN artery is NOT a branch of the internal iliac: it comes from the abdominal aorta at L2, because the gonad developed on the posterior abdominal wall.
Why the pelvis cannot be starved
Tie off the main artery of the pelvis and, remarkably, nothing dies. That is only possible because the pelvis is one of the most richly cross-connected regions in the body. Count the routes. The ILIOLUMBAR and LATERAL SACRAL arteries anastomose with the lumbar arteries and the median sacral artery coming straight down from the aorta. The SUPERIOR RECTAL artery, the terminal branch of the inferior mesenteric and therefore a gut artery, anastomoses in the rectal wall with the middle rectal (internal iliac) and the inferior rectal (internal pudendal). The OVARIAN artery, from the aorta, anastomoses with the UTERINE artery along the side of the uterus. The INFERIOR EPIGASTRIC artery, from the external iliac, anastomoses with the OBTURATOR — the corona mortis again, this time doing something useful. And the SUPERIOR and INFERIOR GLUTEAL arteries anastomose in the buttock with the medial and lateral circumflex femoral and the first perforating branches of the profunda femoris — the cruciate anastomosis. Every one of these connections crosses a territorial boundary, joining internal iliac blood to aortic, mesenteric or external iliac blood. So when the internal iliac artery is LIGATED to control catastrophic pelvic haemorrhage, what happens is not ischaemia but a change of pressure: the ligature converts a high-pressure arterial supply into a low-pressure collateral supply, the pulse pressure downstream falls by around 85 per cent, clot can finally form on a torn vessel — and the organs, still perfused through the collateral web, survive.
In a woman with severe postpartum haemorrhage who has failed drugs, massage and a balloon, the choice is between ligation and embolisation. Bilateral internal iliac LIGATION is the classic open operation: the peritoneum over the pelvic brim is opened, the ureter is lifted medially with it, the artery is identified where the ureter crosses it, and a ligature is passed — carefully, from lateral to medial, so that the needle moves away from the internal iliac vein behind. Fertility is usually preserved. UTERINE ARTERY EMBOLISATION achieves the same haemodynamic result through a groin puncture, with no laparotomy; the same technique, done electively, is a uterine-sparing treatment for symptomatic fibroids, where the particles infarct the fibroid while the myometrium, better collateralised, survives. In blunt PELVIC TRAUMA the bleeding is usually venous rather than arterial — from the presacral and pelvic plexuses torn as the ring springs open — which is exactly why a PELVIC BINDER works: it closes the ring, reduces the volume the pelvis can hold, and tamponades a low-pressure haemorrhage that no clamp could reach. If contrast extravasation on CT shows the bleeding is arterial after all, angiographic embolisation is the answer, and it is the corona mortis and the obturator or superior gluteal arteries that are most often found to be the source.
The veins: valveless plexuses and a road to the spine
The venous side does not mirror the arterial side, and the difference is clinically enormous. Instead of tidy companion veins, the pelvic organs are surrounded by extensive VENOUS PLEXUSES — vesical, prostatic, uterine, vaginal and rectal — thin-walled, wide, freely intercommunicating, and largely VALVELESS. They drain mostly into the internal iliac vein, which joins the external iliac to form the common iliac behind the artery of the same name. Three consequences follow. First, these plexuses hold a large, low-pressure volume of blood: tear them and the bleeding is diffuse, dark and impossible to clamp — which is why pelvic fracture haemorrhage is treated by closing the ring rather than by finding a vessel. Second, because they are valveless and communicate with the SUPERIOR RECTAL vein, part of the pelvic venous blood reaches the PORTAL system — the portosystemic anastomosis of the anorectum. Third, and most important, the pelvic plexuses communicate through the sacral veins with the VERTEBRAL VENOUS PLEXUS of Batson, which runs the whole length of the spine inside the vertebral canal and around the vertebral bodies. That plexus is valveless too, and its pressure rises with every cough and strain. Tumour cells from the prostate, the bladder, the rectum or the uterus can therefore be pushed straight into the vertebral veins and lodge in the lumbar spine, the pelvis or the skull — WITHOUT ever passing through the lungs. It is the anatomical explanation of why prostate carcinoma so characteristically presents with back pain and sclerotic vertebral deposits, and why a spinal metastasis can appear with clear lung fields.
- The UTERINE artery crosses ABOVE the ureter about 2 cm lateral to the cervix — "water under the bridge" — the classic site of ureteric injury at hysterectomy.
- The CORONA MORTIS is an obturator–inferior epigastric anastomosis over the superior pubic ramus: a lethal bleeder in pubic ramus fracture and in hernia surgery.
- The INTERNAL PUDENDAL artery leaves below piriformis, hooks round the ischial spine, re-enters through the lesser sciatic foramen into the pudendal canal, and supplies the whole perineum and its erectile tissue.
- Superior gluteal leaves ABOVE piriformis; internal pudendal and inferior gluteal leave BELOW it — the only exit rule you need for the greater sciatic foramen.
- Collaterals (lumbar and median sacral, superior rectal from the IMA, ovarian, inferior epigastric, and the gluteal–profunda femoris cruciate anastomosis) let the internal iliac be ligated without causing pelvic necrosis.
- The valveless pelvic venous plexuses communicate with Batson's vertebral venous plexus — a route for pelvic tumours to reach the spine, pelvis and skull without passing through the lungs.
- Memorising a fixed branching order. The internal iliac is the most inconstant named artery in the body: branches share stems, swap divisions, or arise from the parent trunk. Identify a pelvic vessel by tracing it to the organ it supplies, never by where the textbook drew it.
- Saying the ureter passes over the uterine artery. It is the reverse: the artery is the BRIDGE and the ureter is the WATER beneath it. Getting this backwards at operation is how a ureter is ligated.
- Assuming the ovarian artery is a pelvic branch because the ovary is a pelvic organ. It arises from the abdominal aorta at L2 — and with it go para-aortic lymph drainage and a right-to-IVC, left-to-left-renal venous pattern that change staging and surgery.
During a difficult hysterectomy the surgeon clamps a vessel running medially in the base of the broad ligament, about two centimetres lateral to the cervix. Post-operatively the patient develops a urinary leak. Which structure was most likely caught in the clamp, and what is its relation to the vessel?
- The common iliac divides at the L5/S1 disc in front of the sacroiliac joint; the internal iliac artery then runs only about 4 cm before dividing at the upper border of the greater sciatic foramen into an anterior and a posterior trunk. The ureter crosses IN FRONT of its origin — the surgeon's landmark — while the internal iliac vein and the lumbosacral trunk lie behind.
- POSTERIOR trunk (constant, parietal): iliolumbar, lateral sacral, and the superior gluteal — the largest branch of all, leaving ABOVE piriformis. ANTERIOR trunk (variable, mostly visceral): umbilical with the superior vesical arteries (distal part = medial umbilical ligament), inferior vesical or vaginal, middle rectal, obturator, uterine, internal pudendal and inferior gluteal, the last two leaving BELOW piriformis.
- Three facts decide operations: the uterine artery crosses ABOVE the ureter 2 cm lateral to the cervix (water under the bridge — the classic hysterectomy injury); the corona mortis links obturator to inferior epigastric over the superior pubic ramus and bleeds lethally in pelvic fracture and hernia surgery; and the ovarian artery is NOT from the internal iliac but from the aorta at L2, with para-aortic lymph drainage to match.
- A dense collateral web (lumbar and median sacral, superior rectal from the inferior mesenteric, ovarian to uterine, inferior epigastric to obturator, gluteal to profunda femoris) means the internal iliac can be ligated — or its branches embolised — to control catastrophic haemorrhage without pelvic necrosis; and the valveless pelvic venous plexuses connect to Batson's vertebral plexus, letting pelvic tumours seed the spine without passing through the lungs.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Pelvis and Perineum: the internal iliac artery and its anterior and posterior divisions.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Vasculature of the pelvis; the ureter and uterine artery; internal iliac ligation.
- Netter FH. Atlas of Human Anatomy — Arteries and veins of the pelvis, male and female.
- Last RJ. Last's Anatomy: Regional and Applied — The internal iliac artery, its branches and pelvic collateral circulation.
- Snell RS. Clinical Anatomy by Regions — The pelvic viscera and their blood supply; corona mortis and pelvic haemorrhage.
- TeachMeAnatomy — The Internal Iliac Artery; Vasculature of the Pelvis.
