6.0 Abdominal Vasculature and Posterior Abdominal Wall (Exam 3)

• The splenic artery is often embedded in the superior aspect of the pancreas, near the 2nd part of the duodenum

Ascending/Descending Colon

• These regions are retroperitoneal, generally held against the posterior abdominal wall by the peritoneum.
• Ascending/descending colon exhibits varying degrees of mesenterization, up to and including that exhibited by the sigmoid colon (sigmoid mesocolon).

Aorta and its Branches

• The celiac trunk is a wide mid-line vessel from the anterior surface of the aorta at T12, surrounded by the celiac plexus, an autonomic plexus of nerve fibers and postganglionic neurons.
• The celiac trunk typically branches immediately into 3 major arteries.
• The splenic artery supplies: the spleen, short gastric and left epiploic arteries to the greater curvature of the stomach, as well as a number of pancreatic branches due to its close relationship with the superior aspect of the pancreas.
• The left gastric artery ascends to the esophagus, gives off esophageal branches, and then descends to the lesser curvature of the stomach to anastomose with the right gastric artery.
• The common hepatic artery is the most complex, its first branch being the gastroduodenal artery which descends behind the duodenum to branch into the superior pancreaticoduodenal and right gastroepiploic arteries.
• The next branch of the common hepatic artery is the right gastric artery that supplies the greater curvature.
• The continuation of the common hepatic artery becomes the proper hepatic artery which divides into the right and left hepatic branches near the liver.
• The cystic artery, which supplies the gallbladder, usually comes from the right hepatic branch.
• The small and large intestines are supplied by the superior and inferior mesenteric arteries, both single midline branches of the abdominal aorta.
• The superior mesenteric artery supplies the region from the mid duodenum through to the mid transverse colon.
• The superior mesenteric arises just below (about 1 cm) the celiac trunk at about the level of L1 under cover of the pancreas.
• The superior mesenteric autonomic plexus surrounds the root of the vessel and the fibers follow its numerous branches to become distributed throughout the small and large intestinal walls.
• The superior mesenteric artery emerges from behind the head of the pancreas and crosses the uncinate process and horizontal (3rd) part of the duodenum.
• It descends in the root of the mesentery toward the terminal portion of the ileum.
• The first branch of the superior mesenteric artery is the inferior pancreatico-duodenal artery that anastomoses with its superior counterpart off the common hepatic branch of the celiac trunk.
• Off the left side of the superior mesenteric vessel arise numerous jejunal and ileal branches.
• The second major branch of the superior mesenteric artery is the middle colic artery supplying the mid transverse colon.
• The right colic artery passes toward the ascending colon.
• The ileocolic artery extends toward the cecum and forms an anastomotic loop with the terminal portion of the superior mesenteric.
• The anterior and posterior cecal arteries are branches from the ileocolic artery, the latter usually giving rise to the appendicular artery.
• The ileocolic, right and middle colic arteries anastomose along the margin of the colon to form the marginal artery (of Drummond).
• The inferior mesenteric artery supplies the remainder of the transverse colon, artery of the embryonic hindgut.
• It arises about 3-4 cm above the bi-furcation of the abdominal aorta.
• The first branch of the inferior mesenteric artery is the left colic artery which runs upward to supply the last part of the transverse colon by contributing to the continuation of the marginal artery.
• A descending branch off the left colic artery supplies The descending colon.
• A number of sigmoid arteries supply the sigmoid colon.
• The final branch of the inferior mesenteric is the superior rectal artery which supplies the upper rectal area, the remainder of the rectum is supplied by middle and inferior rectal arteries, arising from the internal iliac and internal pudendal arteries respectively.
• The sigmoid and left colic arteries also contribute to the formation of the marginal artery.

Venous Drainage of the Intestine

• Venous intestinal blood is enriched with nutrients absorbed from the lumen and is carried directly to the liver for storage, metabolism and detoxification.
• The hepatic portal system is the system where venous intestinal blood is carried directly to the liver.
• Following its filtering through the liver, the venous blood is then dumped into the systemic venous system (inferior vena cava) by the hepatic vein.
• The hepatic portal vein is formed by the union of the superior mesenteric vein with the splenic vein, which has already accepted the inferior mesenteric vein.
• This union occurs posterior to the head/neck of the pancreas.
• Return of blood through the liver may become obstructed which results in an increase in hydrostatic pressure in the venous portal system.
• Alternate pathways for the return of this blood are found at sites where the venous drainage is shared by both the hepatic portal and systemic venous systems, referred to as portocaval (portosystemic) anastomoses.
• Portosystemic anastomoses are found at the following sites: lower end of the esophagus, the anal canal, at the umbilicus, retroperitoneal regions of the intestine.
• Esophageal veins drain via the left gastric vein to the portal vein OR the azygous vein in the thorax (esophageal varices).
• Rectal veins drain via the inferior mesenteric vein to the portal vein OR to the internal iliac veins via the middle and inferior rectal veins at the anal canal.
• Distended anal veins are hemorrhoids; but portal obstruction is a rare cause for these.
• The region at the umbilicus may be drained by small veins travelling in the falciform ligament to the portal vein OR by systemic veins draining the anterior abdominal wall (caput medusae).
• Veins in the wall of the intestine may drain via portal tributaries OR to veins of the adjacent abdominal wall (areas of the duodenum, ascending/descending colon) in the retroperitoneal regions of the intestine.

Abdominal Aorta

• The abdominal aorta lies just to the left of midline.
• It exhibits three main types of branches: parietal, paired visceral, and unpaired visceral. Located in the abdominal cavity with the abdominal cavity.
• The peritoneum is removed and retroperitoneal organs are identified the various organs directly against the posterior abdominal wall as well as its structure can be studied.

Abdominal Autonomic Plexuses

• There is an extensive prevertebral autonomic plexus on the anterior surface of the abdominal aorta and around the branches of vessels originating there.
• This plexus is a continuation of that seen in the thorax and pelvis with subdivide parts named after vascular structures to which they are related.
• The celiac plexus is located surrounding the root of the celiac trunk, with a celiac ganglion on either side.
• The thoracic splanchnic nerves bring predominantly pre-ganglionic sympathetic innervation.
• The ant/post vagus nerves bring pre-ganglionic parasympathetic into the plexus.
• Only those sympathetic pre-ganglionic fibers that have not already synapsed in the paravertebral (sympathetic) ganglia of the sympathetic trunk will synapse in these prevertebral ganglia.
• Vagal fibers do not synapse in these ganglia but rather in the wall of the organs they innervate.
• There are a number of subsidiary plexuses extending out along the various vessels in and around the celiac trunk: hepatic, gastric, splenic, suprarenal, renal, and gonadal.
• The superior mesenteric plexus lies around the base of the superior mesenteric artery, also receives sympathetic innervation from the thorax via the splanchnic nerves.
• The intermesenteric plexus lies between the two mesenteric arteries, with sympathetic input inferiorly via the lumbar splanchnic nerves.
• The inferior mesenteric plexus receives its parasympathetic input not from the vagus but from the pelvic parasympathetic outflow via the pelvic splanchnic nerves.
• Continuing down into the pelvis are the superior and inferior hypogastric plexuses.
• Both vagal and sympathetic nerves carry afferent (sensory) fibers back to the CNS.
• These fibers do not synapse in the various autonomic ganglia are and responsible for the sensation of pain from abdominal viscera.

Kidneys

• At birth the kidney exhibits fetal lobulation which is normally lost by age 5 or 6.
• Kidneys lie on the posterior abdominal wall in the paravertebral gutter around the level of the upper 3 lumbar vertebra - the left a little higher that the right with its upper pole overlapping the diaphragm.
• The mid region of the concave border is hollowed out to form the renal sinus where the vessels and ureter enter and leave the organ.
• Each kidney is covered by a fibrous capsule.
• A sagittal section through the kidney reveals its component regions.
• A lighter cortex region of the kidney lies immediately under the capsule and extends inward as renal columns between darker, roughly pyramidal regions (renal pyramids which comprise the medulla).
• The apex of each renal pyramid is the renal papilla surrounded by a small funnel-shaped minor calyx.
• Minor calyces are "branches" of larger major calyces which in turn are branches of the renal pelvis.
• The renal pelvis narrows into the ureter proper - a muscular tube traversing the posterior abdominal wall into the pelvis and penetrating the walls of the urinary bladder.
• Renal arteries carry about 20% of the cardiac output, with about 90% of this is delivered to the cortex for filtration.
• At the hilus each renal artery divides into a number of branches to supply the five segments of each kidney.
• While there is a "typical" pattern, it is highly variable and some lobes - particularly the lower ones may receive arterial branches directly from the aorta.
• Ureters are the continuation of the renal pelvis.
• There are three points of constriction where kidney stones may become stuck and cause pain: narrowing of pelvis to ureter proper, where ureter crosses the common iliac artery bifurcation before bending down into pelvis, penetration of bladder wall.
• The ureters are retroperitoneal, and cross anterior to the bifurcation of the common iliac artery into external and internal iliac arteries.
• Blood supply is derived "segmentally" from the renal and vesical arteries.

Suprarenal Glands

• Essential endocrine glands capping the apical lobe of each kidney and separated from it by perirenal fat.
• The left one is semilunar in shape while the right one is more pyramidal.
• They are richly supplied by blood (endocrine function) by branches from the inferior phrenic, renal arteries, and the aorta.
• Venous drainage differs in that the left drains to the the renal vein while the right drains to the inferior vena cava.
• Preganglionic sympathetic fibers penetrate the suprarenals and synapse in the medulla to stimulate release of epinephrine and norepinephrine, it an exception to the pattern of sympathetic innervation.

Muscles of the Posterior Abdominal Wall

• Psoas Major is located in the paravertebral gutter adjacent to the lumbar vertebral bodies.
• It passes out under the inguinal ligament with the iliacus muscle to a common tendinous attachment on the lesser trochanter of the femur.
• The psoas major is innervated by ventral rami of L1,2,3.
• The iliacus arises from internal aspect of the ala of the iliac bone, innervated by the femoral nerve.
• The iliacus and psoas are covered by iliac fascia.
• The "iliopsoas" combined is a flexor of the hip on the trunk.
• A psoas minor may occur on the anterior surface of the major attaches distally to the pectineal line.
• Quadratus Lumborum the flat muscle immediately lateral to the upper part of psoas, extends from R12 and the tips of the lumbar transverse processes to the iliolumbar ligament (between L5 and the iliac crest).
• Quadratus Lumborum stabilizes R12 in inspiration/laterally flexing the trunk, innervated by the ventral rami of T12, L1,2,3.
• A thickening of the of the fascia over the surface of the psoas major forms the medial arcuate ligament of the diaphragm.
• Thickening of fascia over the quadratus lumborum forms the lateral arcuate ligament of the diaphragm.
• Both the medial and lateral arcuate ligaments give rise to the vertebral (lumbar) portions of the diaphragm.
• The Diaphragm is a fibromuscular partition filling the thoracic outlet and separating thoracic and abdominal cavities.
• The muscle fibers of the diaphragm arise from the inner aspect of the thoracic outlet and arise to insert into a trefoil central tendon.
• The muscular component can be said to arise from these sources: sternal part , costal part, and lumbar (vertebral) part.
• The sternal part has two slips from the deep surface of the xiphoid process and descends to the central tendon.
• Small gap on each lateral aspect is the sternocostal triangle and transmit the superior epigastric arteries. and lymphatics, a potential site for diaphragmatic herniae.
• The costal part is from the inner aspect of lower 6 ribs and costal cartilages and forms left and right domes of the diaphragm.
• The costal part inserts into the anterolateral aspects of the central tendon.
• Inconsistent lumbocostal triangles may exist between the costal musculature and the lateral arcuate ligament, they relate to the posterosuperior aspects of the kidneys and are separated from them only by pleura.
• The lumbar (vertebral) part arises from the lateral, medial and median arcuate ligaments and as crura from the upper lumbar vertebrae.
• Fibrous Arches: medial arcuate ligament is a thickening of the fascia over the upper psoas major spanning the body of L1 to its transverse process; the lateral arcuate ligament is a thickening of the fascia over the upper quadratus lumborum spanning the transverse process of L1 to R12
• Muscle fibers from both diaphragmatic ligaments extend up to the central tendon
• The Crura Diaphragm arise from the lumbar vertebrae in the form of two muscular crura that ascend to the central tendon.
• The right crus of the diaphragm is from L1-3/4, and the left crus arises from L1-2 only
• The crura unit in front of the aorta in a fibrosus arch - the median arcuate ligament - that forms the aortic hiatus of the diaphragm.
• The diaphragmatic right crus splits to surround the esophagus in a sling-like loop
• The diaphragmatic left crus goes to the left of the esophagus but may assist in formation of the esophageal hiatus
• The Openings of the diaphragm include: aortic, esophageal, caval
• The diaphragmatic aortic opening is anterior to T12, between crura and transmits the aorta, thoracic duct and azygous vein and greater splanchnic nerve
• The diaphragmatic esophageal opening is at level of T10 and transmits the esophagus/right and left vagus nerves
• The diaphragmatic caval opening is at level of T8 and transmits the inferior vena cava as well as the right phrenic nerve and lymphatics from the liver
• In general:, the splanchnic nerves pierce the crura; the sympathetic trunk passes against the vertebral bodies posterior to the medial arcuate ligament; the left phrenic n. pierces the diaphragm to the left of the pericardial sac

Nerves of the Posterior Abdominal Wall

• Ventral rami of T1-T12 represent "segmental" nerves - each ventral ramus remaining separate and responsible for innervating an individual dermatome.
• Ventral ramus of T12 remains separate and is referred to as the subcostal nerve. Ventral rami of L1-4, however, give rise to the lumbar plexus - eventually anastomose to form nerves largely concerned with innervation of the extensor and adductor compartments of the thigh.
• Part of the ventral rami L4 and all of L5 contribute to the sacral plexus that will also supply structures of the lower limbs.
• The subcostal nerve appears beneath the lateral arcuate ligament and crosses the quadratus lumborum below R12, and supplies the lower part of rectus abdominis, the anterolateral abdominal muscles and the strip of overlying skin.
• Iliohypogastric and ilioinguinal nerves are both derived from L1, where the iliohypogastric supplies skin over the upper lateral surface of the thigh and lower abdomen above the pubis. = Ilioinguinal passes forward with the spermatic cord and is distributed to the skin of external genitalia and adjacent region of the thigh.
• Lateral femoral cutaneous nerve (L2,3) emerges from the lateral border of the psoas and passes into the thigh beneath the lateral end of the inguinal ligament, may become entrapped/cause pain.
• Superficial below the ligament supplies skin on the lateral aspect of the thigh.
• Femoral nerve is the largest of this plexus, formed from L2,3,4, lies in the groove between the psoas and iliacus, under the inguinal ligament and supplies muscles of the anterior compartment of the thigh as well as overlying skin.
• Genitofemoral nerve (L1,2) appears on the surface of the psoas, and divides: genital (supplying skin of external genitalia + cremaster muscle in the male) )femoral branch supplies a small patch of skin just below the inguinal ligament) branch.
• Obturator nerve has similar root values to the femoral (L2,3,4), appears at the medial border of the psoas and passes through the pelvis before exiting into the thigh - it supplies motor innervation to the medial (adductor) compartment of the thigh and overlying skin.
• Lumbosacral trunk is a portion of L4 that joins with L5 to form a stout trunk that enters the pelvis to join in the formation of the sacral plexus. Psoas major muscle plays an important role as an anatomical landmark for the identification of the various branches of the lumbar plexus

Questions about the anatomy of the abdomen, kidneys, and related structures. Topics covered include muscles, nerves, arteries, and venous drainage of the abdomen and kidneys. Also includes references to the diaphragm and renal system.