Anatomy 534 - Abdominal Vasculature and Posterior Abdominal Wall PDF

Anatomy 534 Retroperitoneal Organs 158 the 2nd part of the duodenum. The splenic artery is often embedded in the superior aspect of the pancreas. Ascending/Descending Colon While these are generally held against the posterior abdominal w...

Anatomy 534 Retroperitoneal Organs 158 the 2nd part of the duodenum. The splenic artery is often embedded in the superior aspect of the pancreas. Ascending/Descending Colon While these are generally held against the posterior abdominal wall by the peritoneum (retroperitoneal), these regions can exhibits varying degrees of mesenterization - up to and including that exhibited by the sigmoid colon (sigmoid mesocolon). The Aorta and its Branches Developmentally Anatomy 534 Abdominal Vasculature 159 The Celiac Trunk 1 - aorta 2 - celiac trunk 3 - common hepatic. 4 - splenic a. 5 - left gastric a. 6 - gastroduodenal a. 7 - heaptic a. proper 8 - cystic a. 9 - right gastric a. 10 - superior pancreatico- duoenal a. 11- rt. gastroepiploic 12- lt. gastroepiploic 13 - short gastric aa. 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. It typically branches immediately into 3 major arteries: - splenic artery that supplies the spleen as well as short gastric and left epiploic aa. to the greater curvature of the stomach. It also gives off a number of pancreatic br. due to its close relationship with the superior aspect of the pancreas. - left gastric artery that ascends to the esophagus where it gives off esophageal br. and then descends to the lesser curvature of the stomach to anastomose with the right gastric a. - common hepatic artery is the most complex. Its first branch is the gastroduodenal a. that descends behind the duodenum to branch into the superior pancreaticoduodenal and right gastroepiploic aa. The next branch is the right gastric a. that supplies the greater curvature. The continuation becomes the proper hepatic a. (hepatic artery proper) that divides into the right and left hepatic br. near the liver. The cystic a. that supplies the gallbladder usually arises from the right hepatic br. Anatomy 534 Abdominal Vasculature 160 The Superior Mesenteric Artery The small and large intestine are supplied by the superior and inferior mesenteric aa., both single midline branches of the abdominal aorta. The superior mesenteric 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 auto- nomic plexus surrounds the root of the vessel and the fibers follow its numerous branches to become distributed through- out the small and large intestinal walls. The superior mesenteric 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 is the inferior pancreatico-duodenal a. that anastomoses with its superior counterpart off the common hepatic br. of the celiac trunk. Off the left side of the vessel arise numerous jejunal and ileal branches. The second major branch is the middle colic a. supplying the mid transverse colon. The right colic a. passes toward the ascending colon. The ileocolic a. extends toward the cecum and forms an anastomotic loop with the terminal portion of the superior mesenteric. The anterior and posterior cecal aa. are branches from this, the latter usually giving rise to the appendicular a. The ileocolic, right and middle colic aa. anastomose along the margin of the colon to form the marginal a. Anatomy 534 Abdominal Vasculature 161 The Inferior Mesenteric Artery Artery of the embryonic hindgut, supplies the remainder of the transverse colon. It arises about 3-4 cm above the bi- furcation of the abdominal aorta. The first branch is the left colic a. It runs upward to supply the last part of the transverse colon by contributing to the continuation of the marginal a. A descending br. off the left colic supplies The descending colon. A number of sigmoid aa. supply the sigmoid colon. The final branch of the inferior mesenteric is the superior rectal a. This vessel supplies the upper rectal area. The remainder of the rectum is supplied by middle and inferior rectal aa. arising from the internal iliac and internal pudendal aa. respectively. The sigmoid and left colic aa. also contribute to the formation of the marginal a. 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. This system is referred to as the hepatic portal system. 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 v. with the splenic vein (which has already accepted the inferior mesenteric v.). This union occurs posterior to the head /neck of the pancreas. Anatomy 534 Abdominal Vasculature 162 Hepatic Portal System Return of blood through the liver may become obstructed for a number of reasons resulting 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. These alternative pathways are referred to as portocaval (portosystemic) anastomoses. They are found at the following sites: - lower end of the esophagus where esophageal veins drain via the left gastric vein tothe portal vein OR the azygous v in the thorax (esophageal varices). - at the anal canal where rectal vv. drain via the inferior mesenteric v. to the portal vein OR to the internal iliac vv. via the middle and inferior rectal vv. Distended anal veins are hemorrhoids - but portal obstruction is a rare cause for these. - at the umbilicus where the region may be drained by small veins travelling in the falciform ligament to the portal vein OR by systemic veins draining the ant. abdominal wall. The pattern of distended veins radiating from the umbilicus = caput medusae. - at retroperitoneal regions of the intestine where 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. Anatomy 534 Abdominal Vasculature 163 The Abdominal Aorta With the abdominal cavity eviscerated and the peritoneum removed and retroperitoneal organs identified the various organs directly against the posterior abdominal wall as well as its structure can be studied. The abdominal aorta lies just to the left of midline and exhibits three main types of branches as shown in the accompanying diagram Anatomy 534 Abdominal Vasculature 164 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 except that it has been subdivided into a number of parts named after vascular structures to which they are related. The celiac plexus is located surrounding the root of the celiac trunk. There is a celiac ganglion on either side. The thoracic splanchnic nn (greater) bring predominantly pre-ganglionic sympathetic innervation while the ant/post vagus nn. 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. The main ones are the hepatic, gastric, splenic, suprarenal, renal, and gonadal. The superior mesenteric plexus lies around the base of the superior mesenteric artery and also receives sympathetic innervation from the thorax via the splanchnic nn. The intermesenteric plexus lies between the two mesenteric arteries. From this point inferiorly, the sympathetic input is via the lumbar splanchnic nn. The inferior mesenteric plexus receives its parasympathetic input NOT from the vagus but from the pelvic parasympathetic outflow via the pelvic splanchnic nn. 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 and are responsible for the sensation of pain from abdominal viscera. Anatomy 534 Posterior Abdominal Wall 165 The 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 lies immediately under the capsule and extends inward as renal columns between darker, roughly pyramidal regions - the renal pyramids which comprise the medulla. The apex of each pyramid is the renal papilla and is 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. Blood Supply Renal arteries carry about 20% of the cardiac output. 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. Each of these "segmental" Anatomy 534 Posterior Abdominal Wall 166 arteries can be considered an "end artery". If ligated there develops an obvious ischemic area within the kidney parenchyma. Ureters 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 Note that the ureters are retroperitoneal and that they cross anterior to the bifurcation of the common iliac artery into external and internal iliac aa. Blood supply is derived "segmentally" from the renal and vesical aa. 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 aa. 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. This is an exception to the pattern of sympathetic innervation. Anatomy 534 Posterior Abdominal Wall 167 Muscles of the Posterior Abdominal Wall Psoas Major 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 attach- ment on the lesser trochanter of the femur. It is innervated by ventral rami of L1,2,3 Iliacus Arises from internal aspect of the ala of the iliac bone. Both it and the psoas are covered by iliac fascia. The iliacus is innervated by the femoral n. The combined "iliopsoas" in a flexor of the hip on the trunk. A psoas minor may occur on the anterior surface of the major and it attaches distally to the pectineal line. Quadratus Lumborum A flat muscle immediately lateral to the upper part of psoas. It extends from R12 and the tips of the lumbar transverse processes to the iliolumbar ligament (between L5 and the iliac crest). It stabilizes R12 in inspiration as well as laterally flexing the trunk and is 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 give rise to the vertebral (lumbar) portions of the diaphragm. Anatomy 534 Posterior Abdominal Wall 168 The Diaphragm A fibromuscular partition filling the thoracic outlet and separating thoracic and abdominal cavities. The majority of the muscle fibers 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 (two slips from the deep surface of the xiphoid process) - descends to the central tendon. - small gap on each lateral aspect is the sternocostal triangle and transmit the superior epigastric aa. and lymphatics. A potential site for diaphragmatic herniae. - costal part (inner aspect of lower 6 ribs and costal cartilages) - form left and right domes of the diaphragm. Insert into the anterolateral aspects of the central tendon. - inconsistent lumbocostal triangles may exist between the costal musculature and the lateral arcuate ligament. They have a relationship with the posterosuperior aspects of the kidneys and are separated from them only by pleura. - lumbar (vertebral) part (from the lateral, medial and median arcuate ligaments and as crura from the upper lumbar vertebrae). Anatomy 534 Posterior Abdominal Wall 169 - fibrous arches: - the 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 ligaments extend up to the central tendon - crura: - arise from the lumbar vertebrae in the form of two muscular crura that ascend to the central tendon. The right crus is from L1-3/4 whereas the left crus arises from L1-2 only. - they unit in front of the aorta in a fibrosus arch - the median arcuate ligament - that forms the aortic hiatus of the diaphragm. - the right crus splits to surround the esophagus in a sling-like loop (sphincter?) - the left crus goes to the left of the esophagus but may assist in formation of the esophageal hiatus Openings of the diaphragm include: - aortic: anterior to T12, between crura and transmits the aorta, thoracic duct and azygous vein and greater splanchnic nerve - esophageal: at level of T10 and transmits the esophagus and right and left vagus nn. - caval: 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 Anatomy 534 Posterior Abdominal Wall 170 Nerves of the Posterior Abdominal Wall The ventral rami of T1-T12 represent "segmental" nerves - each ventral ramus remaining separate and responsible for innervating an individual dermatome. The ventral ramus of T12 remains separate and is referred to as the subcostal nerve. The ventral rami of L1-4 however, give rise to the lumbar plexus - a branching array of ventral that 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. It supplies the lower part of rectus abdominis, the anterolateral abdominal muscles and the strip of overlying skin. The iliohypogastric and ilioinguinal nerves are both derived from L1. The iliohypo- gastric supplies skin over the upper lateral surface of the thigh and lower abdomen above the pubis. The ilioinguinal passes forward with the spermatic cord and is distributed to the skin of external genitalia and adjacent region of the thigh. The 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 where it may become entrapped and cause pain. Becoming superficial below the ligament it supplies skin on the lateral aspect of the thigh. The femoral nerve is the largest of this plexus, formed from L2,3,4. It lies in the groove between the psoas and iliacus, passes under the inguinal ligament and supplies muscles of the anterior compartment of the thigh as well as overlying skin. The genitofemoral nerve (L1,2) appears on the surface of the psoas. It divides into a genital (supplying skin of external genitalia + cremaster m. in the male) and femoral (supplies a small patch of skin just below the inguinal ligament) branch. The obturator nerve has similar root values to the femoral (L2,3,4). It appears at the medial border of the psoas and passes through the pelvis before exiting into the thigh. It supplies motor Anatomy 534 Posterior Abdominal Wall 171 innervation to the medial (adductor) compartment of the thigh and overlying skin. The 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. The psoas major muscle plays an important role as an anatomical landmark for the identification of the various branches of the lumbar plexus (see diagram to the right).