Medical School · Year 1 · Anatomy Thorax Abdomen · includes a quiz and discussion video

Lecture 11: Posterior Abdominal Wall

Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the bony and muscular components of the posterior abdominal wall
2. Identify the major vessels of the posterior abdominal wall (aorta and IVC)
3. Describe the anatomy of the kidneys and suprarenal glands
4. Trace the course of the ureters
5. Describe the lumbar plexus and its branches
6. Explain the lymphatic drainage of the posterior abdominal structures

Bony Framework

The posterior abdominal wall is formed by a combination of bony, muscular, and fascial structures that provide support and protection for the retroperitoneal organs.

The vertebral column forms the central axis of the posterior wall, consisting of the T12 through L5 vertebrae in this region. The lumbar vertebrae are the largest in the vertebral column, reflecting the weight-bearing demands placed upon them. The lumbar curve (lordosis) is convex anteriorly. Important surface landmarks related to the lumbar spine include the L4 vertebra, which corresponds to the highest point of the iliac crest and the level of aortic bifurcation, and the L3 vertebra at the subcostal plane.

Ribs 11 and 12 contribute to the posterior wall in its upper portion. The twelfth rib is particularly important as it overlies the upper pole of the kidney and provides protection to this organ.

The bony pelvis forms the inferior boundary of the posterior wall. The iliac crests, the ala of the sacrum, and the upper portions of the iliac bones all contribute to the framework of the lower posterior wall.

<image>Panel A: Posterior view of lumbar vertebrae T12-L5 in gray forming the central column with lumbar lordosis evident and vertebral bodies and spinous processes visible. Panel B: Ribs 11 and 12 as curved white bones projecting posterolaterally from T11-T12. Panel C: The iliac crests extending laterally with L4 at their highest point and the subcostal plane at L3 marked with dashed lines. Panel D: The sacrum as a triangular bone with the ala of the sacrum visible inferiorly.</image>

Muscles of the Posterior Abdominal Wall

Four muscles contribute to the posterior abdominal wall, providing a muscular foundation for the retroperitoneal structures.

The psoas major is the largest and most medial of these muscles. It originates from the transverse processes of L1-L5 and the lateral surfaces of the vertebral bodies from T12 to L5, including the intervertebral discs. It descends through the pelvis to insert on the lesser trochanter of the femur together with the iliacus. The psoas major is a powerful hip flexor and, when the leg is fixed, flexes the trunk. The lumbar plexus forms within the substance of this muscle, which has important clinical implications.

The psoas minor is present in only approximately 50% of individuals. It lies anterior to the psoas major, originating from T12 and L1 vertebrae and inserting on the pectineal line of the pubis. When present, it is a weak trunk flexor.

The quadratus lumborum lies lateral to the psoas major. It originates from the iliac crest and iliolumbar ligament and inserts on the transverse processes of L1-L4 and the lower border of the twelfth rib. This muscle is important for lateral flexion of the trunk and for fixing the twelfth rib during deep respiration, preventing it from being pulled upward by the diaphragm.

The iliacus fills the iliac fossa of the pelvis. It originates from the iliac fossa and joins the psoas major tendon to insert on the lesser trochanter. Together, the psoas major and iliacus are often referred to as the iliopsoas, the most powerful flexor of the hip.

The transversus abdominis, while primarily an anterolateral abdominal muscle, contributes to the lateral portion of the posterior wall through its posterior attachments to the thoracolumbar fascia.

<image>Panel A: Anterior view of the psoas major as a large elongated red muscle on each side descending along the vertebral bodies with lumbar plexus nerves in yellow emerging from within it. Panel B: The quadratus lumborum as a rectangular red muscle lateral to the psoas attaching to rib 12 superiorly. Panel C: The iliacus as a triangular red muscle filling the iliac fossa and joining the psoas to form the iliopsoas tendon descending to the lesser trochanter. Panel D: The psoas minor as a smaller anterior muscle overlying the psoas major shown present on one side with a 50% prevalence label.</image>

Fasciae of the Posterior Abdominal Wall

Several fascial layers cover the muscles of the posterior wall and provide anatomical compartments.

The psoas fascia covers the psoas major muscle. Superiorly, it thickens to form the medial arcuate ligament, which arches over the psoas and provides an attachment for the diaphragm.

The thoracolumbar fascia is a complex multilayered structure in the lumbar region. The posterior layer covers the erector spinae muscles. The middle layer lies between the erector spinae and the quadratus lumborum. The anterior layer covers the anterior surface of the quadratus lumborum. These layers fuse laterally to provide an attachment point for the transversus abdominis and internal oblique muscles.

The quadratus lumborum fascia covers this muscle anteriorly. It thickens superiorly to form the lateral arcuate ligament, which arches over the quadratus lumborum and provides another attachment for the diaphragm.

Abdominal Aorta

The abdominal aorta is the main arterial trunk supplying the abdomen and lower limbs. It enters the abdomen through the aortic hiatus of the diaphragm at the T12 vertebral level and descends anterior to the vertebral column. At the L4 level (approximately at the umbilicus), it bifurcates into the right and left common iliac arteries.

The abdominal aorta gives off three categories of branches. The anterior unpaired visceral branches supply the gastrointestinal organs. The celiac trunk arises at the T12 level to supply the foregut derivatives. The superior mesenteric artery arises at L1 to supply the midgut derivatives. The inferior mesenteric artery arises at L3 to supply the hindgut derivatives.

The lateral paired visceral branches supply the paired abdominal organs. The middle suprarenal arteries arise at L1 to supply the suprarenal glands. The renal arteries arise at the L1-L2 level to supply the kidneys. The gonadal arteries (testicular in males, ovarian in females) arise at L2 to supply the gonads.

The posterolateral parietal branches supply the body wall. The inferior phrenic arteries supply the diaphragm. Four pairs of lumbar arteries supply the posterior abdominal wall, analogous to the posterior intercostal arteries in the thorax. The median sacral artery is a small midline branch that descends over the sacrum.

The relations of the abdominal aorta are clinically important. Anteriorly lie the pancreas, left renal vein, third part of the duodenum, and coils of small intestine. Posteriorly lie the vertebral bodies. The inferior vena cava and cisterna chyli lie to its right. The left crus of the diaphragm and left sympathetic trunk lie to its left.

<image>Panel A: Anterior view of the abdominal aorta as a large red vessel entering at T12 through the aortic hiatus and bifurcating at L4 into the common iliac arteries. Panel B: The anterior unpaired branches including the celiac trunk at T12, SMA at L1, and IMA at L3 arising from the anterior surface. Panel C: The lateral paired branches including the middle suprarenal arteries, renal arteries at L1-L2, and gonadal arteries at L2 arising from the sides. Panel D: The posterolateral branches including the inferior phrenic arteries superiorly, four pairs of lumbar arteries, and the median sacral artery with the IVC in blue to the right of the aorta.</image>

Inferior Vena Cava

The inferior vena cava is the largest vein in the body, returning blood from the lower limbs, pelvis, and abdomen to the heart. It forms at the L5 vertebral level by the union of the right and left common iliac veins, just to the right of the aortic bifurcation.

The IVC ascends on the right side of the aorta, passes through the caval opening in the central tendon of the diaphragm at the T8 level, and immediately enters the right atrium. The thoracic portion of the IVC is extremely short, only 2-3 centimeters.

The tributaries of the IVC mirror the branches of the aorta, with some important asymmetries. The common iliac veins form the IVC. Four pairs of lumbar veins drain into it. The right gonadal vein drains directly into the IVC, while the left gonadal vein drains into the left renal vein. The renal veins drain into the IVC; importantly, the left renal vein is longer and crosses anterior to the aorta. The right suprarenal vein drains directly into the IVC, while the left suprarenal vein drains into the left renal vein. The inferior phrenic veins have variable drainage. The three hepatic veins drain directly into the IVC just before it passes through the diaphragm.

The relations of the IVC include the liver, first part of the duodenum, pancreatic head, and portal vein anteriorly; the right crus, vertebral bodies, and right renal artery posteriorly; the aorta to its left; and the right kidney and right ureter to its right.

<image>Panel A: Anterior view of the IVC as a large blue vessel forming at L5 from the common iliac veins and ascending to the right of the aorta. Panel B: Paired tributaries including four pairs of lumbar veins and the renal veins with the left renal vein crossing anterior to the aorta. Panel C: Asymmetric drainage patterns with the right gonadal and right suprarenal veins draining directly to the IVC while the left gonadal and left suprarenal veins drain to the left renal vein. Panel D: The three hepatic veins entering the IVC just below the diaphragm with the IVC passing through the caval opening at T8.</image>

Kidneys

The kidneys are paired retroperitoneal organs that perform essential functions in waste excretion, fluid and electrolyte balance, blood pressure regulation, and hormone production. They lie against the posterior abdominal wall, extending from the T12 to L3 vertebral levels. The right kidney is typically positioned slightly lower than the left due to the presence of the liver above it.

Each kidney measures approximately 11 centimeters in length, 6 centimeters in width, and 3 centimeters in thickness, weighing approximately 150 grams. The hilum, located on the medial aspect at approximately the L1 level, is the site where the renal vessels and ureter enter and leave the kidney.

The kidney has a characteristic bean shape with a convex lateral border and a concave medial border containing the hilum. It has distinct superior and inferior poles and anterior and posterior surfaces.

Four layers of tissue surround the kidney from outside to inside. The pararenal fat lies between the renal fascia and the body wall. The renal fascia, also called Gerota's fascia, forms a fibrous envelope that encloses the kidney and suprarenal gland together; superiorly the layers fuse, but inferiorly they remain open around the ureter. The perirenal fat fills the space between the kidney and the renal fascia, providing cushioning. The fibrous capsule is a thin but tough layer that adheres directly to the kidney surface and strips easily from a healthy kidney but may be adherent in chronic kidney disease.

The right kidney relates anteriorly to the liver, second part of the duodenum, and right colic flexure. Its posterior relations include the diaphragm, twelfth rib, psoas major, quadratus lumborum, and transversus abdominis.

The left kidney relates anteriorly to the stomach, spleen, pancreatic tail, jejunum, and left colic flexure. Its posterior relations include the diaphragm, ribs 11 and 12, psoas major, quadratus lumborum, and transversus abdominis.

<image>Panel A: Both kidneys in situ as brown bean-shaped organs on the posterior abdominal wall with the right kidney positioned lower than the left extending from T12 to L3. Panel B: The hilum on the concave medial border containing the renal artery in red, renal vein in blue, and ureter as a white tube arranged from posterior to anterior. Panel C: Rib 12 overlying the upper poles of the kidneys with the convex lateral borders visible. Panel D: Inset cross-section showing the four coverings: pararenal fat as outer yellow layer, renal fascia as a white envelope, perirenal fat as inner yellow layer, and fibrous capsule as a thin gray layer on the kidney surface.</image>

Renal Blood Supply

The kidneys receive a disproportionately large blood supply, approximately 20-25% of cardiac output, reflecting their role in filtering the blood.

The renal arteries arise directly from the abdominal aorta at the L1-L2 level, just below the origin of the superior mesenteric artery. The right renal artery is longer than the left because it must pass behind the IVC to reach the right kidney. The left renal artery is shorter and travels more directly to the left kidney.

Each renal artery divides into five segmental arteries that supply distinct regions of the kidney. These segmental arteries are end arteries, meaning they do not anastomose with each other. Occlusion of a segmental artery therefore causes infarction of the corresponding segment.

Within the kidney, the arterial supply follows a predictable pattern. The segmental arteries divide into interlobar arteries that travel between the renal pyramids. At the corticomedullary junction, these become arcuate arteries that arch along the base of the pyramids. Interlobular arteries ascend into the cortex, giving rise to afferent arterioles that supply the glomeruli.

The renal veins drain into the IVC. The left renal vein is significantly longer than the right, crossing anterior to the aorta (but posterior to the superior mesenteric artery) to reach the IVC. The left renal vein receives the left gonadal vein and left suprarenal vein as tributaries, which is important for surgical planning and understanding patterns of varicocele development.

Nutcracker syndrome occurs when the left renal vein is compressed between the superior mesenteric artery anteriorly and the aorta posteriorly, causing venous hypertension in the left kidney.

<image>Panel A: The renal arteries in red arising from the aorta at L1-L2 with the right renal artery passing behind the IVC shown transparently. Panel B: Each renal artery dividing into five segmental arteries shown as distinct territories in different shades on the right kidney. Panel C: Inset of intrarenal circulation showing segmental arteries dividing into interlobar arteries between pyramids, arcuate arteries at the corticomedullary junction, and interlobular arteries ascending into the cortex. Panel D: The renal veins in blue draining to the IVC with the left renal vein crossing anterior to the aorta and receiving the left gonadal and left suprarenal veins.</image>

Suprarenal Glands

The suprarenal (adrenal) glands are endocrine organs situated superior to the upper poles of the kidneys, enclosed within the renal fascia but separated from the kidneys by a layer of connective tissue. The right suprarenal gland is pyramidal in shape and lies posterior to the inferior vena cava. The left suprarenal gland is crescentic or semilunar in shape and lies close to the aorta, stomach, and spleen.

Unlike most organs, the suprarenal glands receive blood from three arterial sources. The superior suprarenal artery arises from the inferior phrenic artery. The middle suprarenal artery arises directly from the aorta. The inferior suprarenal artery arises from the renal artery. This triple blood supply ensures adequate perfusion of these vital endocrine organs.

In contrast to the multiple arterial sources, each suprarenal gland is drained by a single large suprarenal vein. The right suprarenal vein is short and drains directly into the posterior aspect of the inferior vena cava. The left suprarenal vein is longer and drains into the left renal vein. The short right suprarenal vein is a surgical challenge during right adrenalectomy.

<image>Panel A: The right suprarenal gland as a yellow pyramidal structure on the right kidney's upper pole lying posterior to the IVC. Panel B: The left suprarenal gland as a yellow crescentic structure on the left kidney lying near the aorta. Panel C: The triple arterial supply on each side including the superior suprarenal artery from the inferior phrenic, middle suprarenal artery from the aorta, and inferior suprarenal artery from the renal artery. Panel D: Single venous drainage showing the short right suprarenal vein entering the IVC directly and the longer left suprarenal vein joining the left renal vein.</image>

Ureters

The ureters are muscular tubes that transport urine from the renal pelvis to the urinary bladder, measuring approximately 25 centimeters in length. They are retroperitoneal throughout their course.

Each ureter has two parts. The abdominal portion extends from the renal pelvis to the pelvic brim. The pelvic portion extends from the pelvic brim to the bladder.

Three anatomical constrictions occur along the ureter where the lumen narrows. These are clinically significant as common sites for impaction of renal calculi. The pelviureteric junction, where the renal pelvis narrows to become the ureter, is the first and narrowest constriction. The crossing at the pelvic brim, where the ureter passes over the common or external iliac vessels, is the second constriction. The vesicoureteric junction, where the ureter passes obliquely through the bladder wall, is the third constriction.

In its abdominal course, the ureter descends on the psoas major muscle, crossing anterior to the gonadal vessels. At the pelvic brim, it crosses anterior to the bifurcation of the common iliac artery or the beginning of the external iliac artery.

In the male pelvis, the vas deferens crosses over the ureter near the bladder ("water under the bridge" describes the relationship of ureter to vas). In the female pelvis, the uterine artery crosses over the ureter approximately 2 centimeters lateral to the cervix ("water under the bridge" again, with ureter as water). This relationship is important during hysterectomy to avoid ureteric injury.

The blood supply to the ureter is segmental. The upper ureter receives branches from the renal artery. The middle ureter receives branches from the gonadal arteries, aorta, and common iliac arteries. The lower ureter receives branches from the vesical arteries.

<image>Panel A: The ureters as white muscular tubes (25 cm) descending on the psoas muscle and crossing anterior to the gonadal vessels. Panel B: Three constrictions marked at the pelviureteric junction, the pelvic brim where the ureter crosses the iliac vessels, and the vesicoureteric junction at the bladder. Panel C: Inset showing the pelvic relationship with the vas deferens crossing over the ureter in males and the uterine artery crossing over the ureter near the cervix in females (water under the bridge). Panel D: The segmental blood supply to the ureter from the renal artery superiorly, gonadal and aortic branches in the middle, and vesical arteries inferiorly.</image>

Lumbar Plexus

The lumbar plexus is formed by the ventral rami of spinal nerves L1-L4, with a small contribution from T12. It forms within the substance of the psoas major muscle.

The major branches of the lumbar plexus and their distributions include: the iliohypogastric nerve (T12-L1), which supplies the abdominal wall muscles and skin of the hypogastric region; the ilioinguinal nerve (L1), which supplies the inguinal region and external genitalia; the genitofemoral nerve (L1-L2), which supplies the cremaster muscle (genital branch) and skin of the anterior thigh (femoral branch); the lateral femoral cutaneous nerve (L2-L3), which provides sensory innervation to the lateral thigh; the femoral nerve (L2-L4), the largest branch, which supplies the anterior thigh muscles and provides sensation to the anterior thigh and medial leg; and the obturator nerve (L2-L4), which supplies the medial thigh adductor muscles.

The lumbosacral trunk (L4-L5) descends into the pelvis to join with the sacral plexus.

The branches of the lumbar plexus emerge from the psoas muscle at characteristic locations. The iliohypogastric and ilioinguinal nerves emerge from the lateral border of the psoas and pass between the quadratus lumborum and kidney. The femoral nerve emerges from the lateral border of the psoas to pass beneath the inguinal ligament. The obturator nerve emerges from the medial border of the psoas and passes along the lateral pelvic wall to the obturator foramen.

<image>Panel A: The psoas major as a large semi-transparent red muscle containing the lumbar plexus formed by L1-L4 ventral rami with T12 contribution. Panel B: The iliohypogastric, ilioinguinal, and genitofemoral nerves emerging from the lateral and anterior surfaces of the psoas with their respective spinal levels and courses. Panel C: The lateral femoral cutaneous nerve from the lateral border and the femoral nerve as the largest branch from L2-L4 passing under the inguinal ligament. Panel D: The obturator nerve from the medial border passing to the obturator foramen and the lumbosacral trunk from L4-L5 descending to the sacral plexus.</image>

Lymphatics

The lymphatic drainage of the posterior abdominal wall and retroperitoneal organs converges on the lumbar (para-aortic) lymph nodes.

The lumbar nodes are arranged along the abdominal aorta and inferior vena cava. They receive lymph from the lower limbs, pelvic viscera, posterior abdominal wall, kidneys, suprarenal glands, and gonads.

The cisterna chyli, when present (approximately 50% of individuals), is a dilated lymphatic sac located at the L1-L2 vertebral level, posterior to the aorta. It receives the right and left lumbar trunks from the lumbar nodes and the intestinal trunk from the mesenteric nodes. From the cisterna chyli, or from the confluence of these trunks when the cisterna is absent, the thoracic duct ascends through the aortic hiatus to enter the thorax.

The lymphatic drainage patterns have clinical significance. Testicular cancer, for example, metastasizes to the para-aortic nodes at the level of the renal vessels, reflecting the embryological origin of the testis in the retroperitoneum and its descent into the scrotum. Similarly, ovarian cancer spreads to the lumbar nodes.

<image>Panel A: The lumbar para-aortic lymph nodes as chains of small circles arranged along the aorta and IVC with arrows indicating lymph drainage from the lower limbs, pelvis, kidneys, suprarenal glands, and gonads. Panel B: The cisterna chyli as a dilated green sac at L1-L2 receiving the right and left lumbar trunks and the intestinal trunk. Panel C: The thoracic duct as a green tube ascending from the cisterna through the aortic hiatus into the thorax. Panel D: Inset showing the clinical correlation of testicular cancer metastasizing to para-aortic nodes at the renal vessel level.</image>

Clinical Correlations

Abdominal aortic aneurysm is a focal dilation of the aorta beyond 3 centimeters in diameter. Most occur below the renal arteries (infrarenal) and may extend to involve the iliac arteries. Patients may present with a pulsatile abdominal mass or back pain, though many are asymptomatic until rupture. Rupture typically causes sudden severe abdominal or back pain with hypotension and is a surgical emergency with high mortality.

Renal calculi (kidney stones) commonly become impacted at the three ureteric constrictions. Patients experience severe, colicky flank pain radiating to the groin, following the dermatomal distribution of the ureteric innervation (T10-L1). This pain is often called renal colic despite the ureter being the actual site of obstruction.

Psoas abscess occurs when infection spreads to the psoas muscle, typically from vertebral osteomyelitis (historically tuberculosis) or other retroperitoneal sources. Because of the muscle's attachment to the femur, the abscess can track inferiorly and present as a mass in the groin. Patients often hold the hip in flexion to reduce tension on the inflamed muscle.

Retroperitoneal hemorrhage can occur from trauma, aortic aneurysm rupture, or anticoagulation. Because the retroperitoneal space is large and distensible, massive blood loss can occur without obvious external signs.

Horseshoe kidney is a congenital anomaly in which the inferior poles of the kidneys are fused across the midline by an isthmus of renal tissue. During fetal development, the fused kidney becomes trapped beneath the inferior mesenteric artery and cannot ascend to its normal position, remaining at a lower level (L3-L5). Horseshoe kidney is associated with increased risk of infection and stone formation due to altered urine drainage.

<image>Panel A: Abdominal aortic aneurysm as a dilated red vessel segment exceeding 3 cm in the infrarenal location with the relationship to the renal arteries and iliac bifurcation. Panel B: Renal colic with a stone impacted at the ureteropelvic junction and referred pain pattern from flank to groin following T10-L1 dermatomes. Panel C: Psoas abscess as a green collection within the psoas muscle tracking from a tuberculous vertebral focus toward the groin with the hip in flexion posture. Panel D: Horseshoe kidney with fused inferior poles and isthmus trapped beneath the IMA positioned at L3-L5 level.</image>

Summary

The posterior abdominal wall consists of the lumbar vertebrae, lower ribs, and pelvic bones covered by the psoas major, quadratus lumborum, and iliacus muscles. The lumbar plexus (L1-L4) forms within the psoas major and gives off the femoral and obturator nerves as its major branches.

The abdominal aorta enters at T12, gives off paired and unpaired branches, and bifurcates at L4 into the common iliac arteries. The inferior vena cava forms at L5 from the union of the common iliac veins and ascends to the right of the aorta, receiving paired tributaries. The left renal vein crosses anterior to the aorta and receives the left gonadal and left suprarenal veins.

The kidneys lie at T12-L3, with the right slightly lower than the left. They are enclosed in renal fascia and surrounded by perirenal and pararenal fat. The renal arteries are segmental end arteries. The suprarenal glands have a triple arterial supply but single venous drainage.

The ureters have three constrictions where calculi commonly impact: the pelviureteric junction, the pelvic brim crossing, and the vesicoureteric junction. Lymphatic drainage from the retroperitoneum converges on the para-aortic nodes and drains via the cisterna chyli to the thoracic duct.

Key Terms

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