Pelvic vessels and nerves
Pelvic Vessels, Nerves and Lymph Drainage: Internal Iliac, Ureter and Plexuses
Pelvic vessels and nerves for FCPS Part 1: internal iliac divisions and branches, the course and danger points of the pelvic ureter, venous plexuses, autonomic nerves and lymph nodes. Covers haemorrhage control, ureteric injury and pelvic pain blocks.
Common and internal iliac arteries
- The aorta bifurcates at L4; each common iliac divides in front of the sacroiliac joint (L5/S1) into external and internal iliac arteries.
- The internal iliac descends into the pelvis behind the ureter and divides at the upper border of the greater sciatic foramen into two divisions.
| Division | Branches | Where they go |
|---|---|---|
| Posterior (parietal) | Iliolumbar, lateral sacral, superior gluteal (largest branch, leaves above piriformis) | Iliac fossa, sacral canal, buttock |
| Anterior (mainly visceral) | Umbilical (gives superior vesical; distal part becomes the medial umbilical ligament), obturator, inferior vesical (men) or uterine and vaginal (women), middle rectal, internal pudendal, inferior gluteal | Pelvic viscera, perineum, thigh, buttock |
- Branches that leave the pelvis: superior and inferior gluteal, internal pudendal (both through the greater sciatic foramen), obturator. Branches that stay in the true pelvis: vesical, uterine, vaginal, middle rectal.
- Abnormal obturator artery from the inferior epigastric (about a quarter of people) runs near the femoral ring and can be cut in femoral hernia repair.
- The inferior gluteal artery contributes to the cruciate anastomosis at the back of the hip with the medial and lateral circumflex femoral and first perforating arteries.
Controlling pelvic bleeding
- Internal iliac ligation (intractable postpartum haemorrhage, pelvic surgery) is placed about 2.5–3 cm beyond its origin, i.e. distal to the posterior division, so the buttock blood supply (superior gluteal) is spared and gluteal necrosis avoided. It lowers pulse pressure in the uterine arteries; collaterals (lumbar, iliolumbar, median sacral, superior rectal) keep the organs alive.
- The ureter crosses in front of the internal iliac artery and must be identified before the tie.
- Post-delivery bleeding into the broad ligament, or postpartum haemorrhage found at laparotomy, usually comes from the uterine artery.
- Aorto-iliac occlusive disease (Leriche syndrome): claudication of buttocks and thighs, weak femoral pulses and erectile dysfunction (poor internal iliac and internal pudendal flow). Calf claudication alone suggests femoropopliteal disease.
Ureter in the pelvis
- Crosses the pelvic brim at the bifurcation of the common iliac artery, in front of the sacroiliac joint, then runs down the lateral wall in front of the internal iliac vessels, behind the ovary (forming the posterior boundary of the ovarian fossa), and turns medially at the ischial spine.
| Danger point | Structure tied | Operation |
|---|---|---|
| Pelvic brim | Ovarian vessels in the infundibulopelvic ligament | Oophorectomy, hysterectomy |
| Base of broad ligament, 1.5–2 cm lateral to cervix | Uterine artery passes above it | Hysterectomy |
| Lateral vaginal fornix | Close to the vaginal vault | Vaginal hysterectomy |
| Side wall | Internal iliac artery | Internal iliac ligation |
- In men the vas deferens crosses above the ureter near the bladder.
- Narrowings where stones lodge: pelviureteric junction, pelvic brim, vesicoureteric junction (narrowest).
- Advanced cervical cancer spreading into the parametrium encases both ureters: bilateral hydronephrosis and renal failure.
- A ligated ureter causes loin pain and fever within a day or two of surgery.
Pelvic veins
- Vesical, prostatic, uterine, vaginal and rectal venous plexuses drain to the internal iliac veins.
- The dorsal venous complex (Santorini plexus) in front of the prostate under the pubic arch bleeds heavily in radical prostatectomy; the erectile neurovascular bundles lie posterolaterally.
- Pelvic plexuses communicate with the valveless vertebral venous plexus (Batson): prostate and other pelvic cancers reach the lumbar spine and pelvis.
Somatic nerves
| Nerve | Origin | Course and importance |
|---|---|---|
| Obturator | L2–L4 | Along the lateral pelvic wall below the brim to the obturator canal; at risk in pelvic node dissection; ovarian disease can refer pain to the medial thigh |
| Lumbosacral trunk | L4–L5 | Crosses the pelvic brim to join the sacral plexus; injured in sacral fractures |
| Sacral plexus | L4–S4 | On piriformis: sciatic, superior and inferior gluteal, pudendal, nerve to obturator internus (L5–S2), nerve to quadratus femoris |
| Nerve to levator ani | S4 | Pelvic floor from above |
Autonomic nerves
| Nerve | Type | Function |
|---|---|---|
| Pelvic splanchnic nerves (nervi erigentes) | Parasympathetic only, S2–S4 | Detrusor contraction, defaecation, erection, hindgut motility from the splenic flexure down |
| Superior hypogastric plexus | Sympathetic (T10–L2), in front of L5 below the aortic bifurcation | Emission of semen, bladder neck closure; damaged in aortic surgery (dry orgasm) |
| Inferior hypogastric (pelvic) plexus | Mixed | Distributes both to pelvic organs; injury in rectal surgery causes bladder and sexual dysfunction |
| Ganglion impar | Sympathetic; the single lowest ganglion where both trunks meet in front of the sacrococcygeal junction or coccyx | Block for intractable perineal visceral and sympathetic pain |
- Visceral pain from organs below the pelvic pain line (below the peritoneum) travels with the parasympathetic S2–S4 fibres; above it, with sympathetic fibres. Somatic perineal pain is relieved by pudendal block, sympathetic and visceral perineal pain by ganglion impar block.
Lymph drainage
| Nodes | Drain |
|---|---|
| External iliac | Bladder, upper vagina, cervix, body of uterus, prostate (part), lower limb via deep inguinal |
| Internal iliac | Rectum, cervix, prostate, middle vagina, deep perineum |
| Obturator | Cervix, bladder; first nodes sampled in cervical cancer |
| Sacral | Rectum, posterior cervix |
| Common iliac then para-aortic | All pelvic chains eventually |
| Para-aortic directly | Gonads, uterine tube, fundus |
| Superficial inguinal | Anal canal below pectinate line, vulva, lower vagina, scrotal skin, round ligament region of the uterus |
- Cervical cancer spreads first to the obturator, internal and external iliac nodes, then common iliac and para-aortic.
Remember Tie the internal iliac beyond the posterior division and look for the ureter first.
Exam trap The superior gluteal artery is the largest branch of the posterior division.
Exam trap Pelvic splanchnic nerves are parasympathetic; the ganglion impar is sympathetic.
How FCPS asks about this
- Why tie the internal iliac 2.5–3 cm from its origin: to spare the posterior division and buttock supply.
- Branch of the posterior division: iliolumbar, lateral sacral, superior gluteal.
- Ureter injured when ligating the ovarian artery: at the pelvic brim, near the common iliac bifurcation.
- Ureter relation in hysterectomy: below the uterine artery about 2 cm lateral to the cervix.
- Block in front of the sacrococcygeal junction: ganglion impar for perineal visceral pain.
- Nerves causing erection: pelvic splanchnics S2–S4 (parasympathetic).
- Buttock claudication with impotence: aorto-iliac disease.
Test yourself on this topic
5 of the 33 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A squamous carcinoma arises at the anal verge, below the dentate line. Which of the following lymph node groups is it most likely to reach first?
Answer: Horizontal group of superficial inguinal nodes
Below the dentate (pectinate) line, the anal canal develops from ectoderm and drains like skin, to the superficial inguinal nodes, specifically the medial nodes of the horizontal group, which also drain the perineum, buttock and lower abdominal wall. Above the dentate line, lymph follows the arteries to the internal iliac and inferior mesenteric nodes. Enlarged inguinal nodes in anal cancer therefore indicate spread from the lower canal or verge.
Dentate line drainage
- Below: superficial inguinal (horizontal group)
- Above: internal iliac and inferior mesenteric
- Below: somatic pain; above: painless
- Below: squamous epithelium
Remember Below the dentate line, think like skin.
Exam trap Examine the groins in every patient with anal canal cancer.
Why the other options are wrong
- Vertical group of superficial inguinal nodes Drains the lower limb along the great saphenous vein.
- Internal iliac nodes Drains the anal canal above the dentate line.
- Inferior mesenteric nodes Drains the rectum and upper anal canal.
- Para-aortic nodes Drain the gonads and upper abdominal organs.
During a vaginal hysterectomy, a clamp is placed far out on the paracervical tissue at the level of the lateral vaginal fornix. Which structure, lying about 1.5–2 cm lateral to the supravaginal cervix, is most at risk?
Answer: Ureter
The ureter runs forwards and medially in the base of the broad ligament, passing below the uterine artery about 1.5–2 cm lateral to the supravaginal cervix, just above the lateral fornix, before entering the bladder. Clamps placed too laterally on the cardinal ligament can crush, tie or cut it. Surgeons therefore keep clamps close to the cervix and push the bladder up to lift the ureters away. The ureter can also be felt through the lateral fornix in some patients.
Ureter beside the cervix
- Runs under the uterine artery
- About 1.5–2 cm lateral to the cervix
- Just above the lateral fornix
- Keep clamps close to the cervix
Remember Water under the bridge, beside the cervix.
Exam trap Ureteric injury may present days later with loin pain, fever or urine leaking from the vagina.
Why the other options are wrong
- Ovarian artery Runs in the infundibulopelvic ligament, well above this level.
- Obturator nerve Lies on the lateral pelvic wall, far lateral.
- Internal iliac vein Lies on the pelvic side wall, not beside the cervix.
- Round ligament Runs from the uterine cornu to the inguinal ring, higher up.
At laparotomy for intractable postpartum haemorrhage, the surgeon ligates the internal iliac artery 2.5–3 cm beyond its origin rather than at the origin itself. Why is the tie placed at this distance?
Answer: To spare the posterior division to the buttock
The internal iliac artery divides into anterior and posterior divisions a short distance from its origin. The posterior division gives the iliolumbar, lateral sacral and superior gluteal arteries, which supply the pelvic wall and gluteal muscles. Tying the artery beyond this point blocks the anterior division, including the uterine artery, while sparing gluteal supply and reducing the risk of buttock ischaemia. Ligation works mainly by lowering pulse pressure in the pelvis, allowing clots to form; collaterals keep the uterus alive.
Internal iliac ligation
- Tie distal to the posterior division
- Spares superior gluteal supply
- Reduces pulse pressure in the pelvis
- Identify the ureter first
Remember Tie the anterior division, spare the buttock.
Exam trap The ureter crosses the front of the internal iliac artery, so it must be seen before tying.
Why the other options are wrong
- To avoid tying the ovarian artery with it The ovarian artery arises from the aorta, not here.
- To preserve flow in the uterine artery The aim is to reduce uterine flow, not keep it.
- To keep the obturator nerve away from the tie Not the reason for the site of ligation.
- To protect the external iliac vein beneath The vein lies behind the artery, but this is not why the tie is placed distally.
During radical prostatectomy, heavy bleeding comes from veins in front of the prostate under the pubic arch. Which of the following are these veins?
Answer: Dorsal venous complex
The deep dorsal vein of the penis passes under the pubic arch and divides into a plexus of large, thin-walled veins on the front and sides of the prostate, the dorsal venous complex or Santorini plexus, which drains into the vesical and internal iliac veins. It must be controlled before dividing the urethra at the prostatic apex in radical prostatectomy, otherwise bleeding is heavy. The same plexus links with the vertebral venous plexus, explaining spinal metastases from prostate cancer.
Prostatic venous plexus
- From the deep dorsal vein of the penis
- Lies anterior to the prostate
- Control before dividing the apex
- Links to vertebral (Batson) plexus
Remember Front of the prostate: Santorini's veins.
Exam trap The neurovascular bundles for erection run posterolaterally, not in this anterior complex.
Why the other options are wrong
- Internal iliac veins Lie on the pelvic side wall, away from the front of the prostate.
- Obturator veins Run along the side wall to the obturator canal.
- Inferior vesical venous plexus Lie at the bladder base, posterolaterally.
- Internal pudendal veins Lie in the pudendal canal below the pelvic floor.
Why are internal haemorrhoids painless while a thrombosed external haemorrhoid or anal fissure is very painful?
Answer: Visceral above, somatic below
The dentate (pectinate) line marks the junction of endoderm and ectoderm. Above it: columnar epithelium, visceral innervation (insensitive to cutting and pricking), arterial supply from the superior rectal artery, venous drainage to the portal system (superior rectal vein) and lymph drainage to internal iliac and inferior mesenteric nodes; internal haemorrhoids arise here and cause painless bright red bleeding. Below it: stratified squamous anoderm with somatic sensation (inferior rectal nerve from pudendal), inferior rectal vessels draining to systemic veins and lymph to superficial inguinal nodes; lesions here—fissures, thrombosed external piles, perianal abscess—are painful. Anal fissures usually lie in the posterior midline.
Dentate line
- Above: visceral nerves (painless), superior rectal vessels → portal, lymph → internal iliac
- Below: somatic (inferior rectal/pudendal) → painful; systemic veins; lymph → superficial inguinal
- Internal piles: painless bleeding
- Fissure: posterior midline, very painful
Remember Above the line, no pain; below the line, real pain.
Why the other options are wrong
- No blood supply to internal piles They are vascular cushions.
- Keratinised skin starts there Keratinised skin starts further down at the anal verge.
- Splanchnic pain fibres below Pelvic splanchnics are visceral, above the line.
- Internal piles always thrombose Not correct.
Where this sits in the CPSP syllabus
- Paper 1: Pelvis and perineum, Pelvic vessels, nerves and lymph drainage
- Surgery & Allied, Paper 2: Pelvis and perineum, Pelvic vessels, nerves and lymph drainage