A man’s liver is his carburetor.
Anonymous
FIG. 7.1 Coronal CT reformat of abdomen with large heterogeneous left renal mass (white arrow).
The abdominal cavity houses many structures that have a large array of functions. It is for this reason that cross-sectional imaging of the abdomen is so essential in visualizing these various organs and body systems (Fig. 7.1).
OBJECTIVES
• List the structures of the abdominal cavity, and differentiate among those that are contained within the peritoneum and those that are contained within the retroperitoneum.
• Describe the peritoneal and retroperitoneal spaces.
• Describe the lobes, segments, and vasculature of the liver.
• Define the structures of the biliary system.
• State the functions and location of the pancreas and spleen.
• Identify the structures of the urinary system.
• List and identify the structures of the stomach and intestines.
• Identify the branches of the abdominal aorta and the structures they supply.
• Identify the tributaries of the inferior vena cava and the structures they drain.
• List the muscles of the abdomen, and describe their functions.
• List the vessels that form the portal vein, and describe the flow of blood through the portal hepatic system.
ABDOMINAL CAVITY
The abdominal cavity is the region located between the diaphragm and sacral promontory (Fig. 7.2). Together, the abdominal and pelvic cavities are commonly divided into four quadrants or nine distinct regions (see Chapter 1). Contents of the abdominal cavity include the liver, gallbladder and biliary system, pancreas, spleen, adrenal glands, kidneys, ureters, stomach, intestines, and vascular structures.
Peritoneum
The walls of the abdominal cavity are lined by a thin serous membrane called the peritoneum. This membrane is divided into two layers: the parietal peritoneum, which lines the abdominal walls, and the visceral peritoneum, which covers the organs (Fig. 7.3). The two layers of peritoneum are separated by a thin film of serous fluid for lubrication that allows organs to move against each other without friction. The peritoneum forms a cavity that encloses the following organs of the abdomen: liver (except for the bare area), gallbladder, spleen, stomach, ovaries, and majority of intestines (Figs. 7.4 and 7.5). In males, the peritoneal cavity is a closed cavity, but in females, it communicates with the exterior through the uterine tubes, uterus, and vagina (Fig. 7.6A and B). The peritoneal cavity includes the greater sac and lesser sac (omental bursae). The greater sac is located between the inner surface of the anterior abdominal wall and the outer surface of the abdominal viscera. It is bounded by the parietal and visceral peritoneum, and it communicates with the lesser sac through the epiploic foramen (of Winslow) (Fig. 7.3). The lesser sac is located primarily between the posterior surface of the stomach and the posterior abdominal wall (Figs. 7.3 and 7.7-7.9).
Numerous folds of peritoneum extend between organs, serving to hold them in position and at the same time enclose the vessels and nerves proceeding to each structure. These folds or double layers of peritoneum are termed mesentery, omenta, and peritoneal ligaments. The mesentery is a double layer of peritoneum that encloses the intestine and attaches it to the abdominal wall. The mesentery serves as a route for blood vessels, lymphatics, and nerves to reach the small intestine. An omentum is a mesentery or double layer of peritoneum that is attached to the stomach. The normal omentum is usually imperceptible on routine scans, visible only when fluid is present (Figs. 7.7 and 7.10). The greater omentum is a fat-laden fold of peritoneum that drapes down from the greater curvature of the stomach and connects the stomach with the spleen and transverse colon, whereas the lesser omentum attaches the duodenum and lesser curvature of the stomach to the liver (Figs. 7.10-7.13).
Numerous peritoneal ligaments serve to connect an organ with another organ or abdominal wall. These peritoneal ligaments are not ligaments in the classic sense but are distinct regions of mesentery connecting the structures for which they are named. Three regions of the greater omentum that are characterized as peritoneal ligaments are the gastrocolic, gastrosplenic, and gastrophrenic. These ligaments attach the greater omentum to the transverse colon, hilum of the spleen, greater curvature and fundus of the stomach, diaphragm, and esophagus (Figs. 7.3, 7.11, and 7.12). Ligaments of the lesser omentum include the hepatogastric and hepatoduodenal, which serve to connect the stomach and duodenum to the liver (Fig. 7.10). Ligaments associated specifically with the liver are the round ligament (ligamentum teres), falciform ligament, and coronary ligaments. The round ligament is a remnant of the left umbilical vein of the fetus and runs within the free inferior margin of the falciform ligament to the umbilicus. The falciform ligament extends from the liver to the anterior abdominal wall and diaphragm, forming a plane that divides the liver anatomically into right and left lobes. The falciform ligament provides the structural support that attaches the upper surfaces of the liver to the diaphragm and upper abdominal wall (Figs. 7.14 and 7.15). The coronary ligaments surround the superior pole of the liver and attach the liver to the diaphragm, forming the margins of the bare area (Figs. 7.7 and 7.16). Additional peritoneal ligaments are described in Table 7.1.
Inflammation of the peritoneum or peritoneal cavity is termed peritonitis. Acute peritonitis is most commonly caused by the leakage of infection through a perforation in the bowel.
TABLE 7.1 Peritoneal Ligaments
|
Ligaments |
Location |
|
Gastrocolic ligament |
Apron portion of the greater omentum attached to the transverse colon |
|
Gastrosplenic ligament (gastrolienal ligament) |
Left portion of the greater omentum that connects the hilum of the spleen to the greater curvature and fundus of the stomach |
|
Splenorenal (lienorenal) ligament |
Connects the spleen and kidney |
|
Gastrophrenic ligament |
Superior portion of greater omentum attached to the diaphragm and posterior aspect of the fundus and esophagus |
|
Hepatorenal ligament |
Connects the liver and kidney |
|
Hepatoesophageal ligament |
Connects the liver and esophagus |
|
Hepatogastric ligament Hepatoduodenal ligament |
Connects the liver to the lesser curvature of the stomach Connects the superior region of the duodenum to the liver |
|
Falciform ligament |
Extends from the liver to the anterior abdominal wall and diaphragm |
|
Round ligament (ligamentum teres) |
Remnant of the fetal umbilical vein, lying in the free edge of the falciform ligament |
|
Coronary ligaments |
Reflections of the peritoneum that surround the bare area of the liver |
|
Triangular (left and right) |
Where the layers of the coronary ligament meet to the left and right, respectively |
|
Phrenocolic ligament |
Attaches the left flexure of the colon to the diaphragm |
Peritoneal Spaces
The peritoneal cavity contains potential spaces resulting from folds of peritoneum that extend from the viscera to the abdominal wall. These spaces can be divided into the supracolic and infracolic compartments (Fig. 7.17). The supracolic compartment is located above the transverse colon and contains the right and left subphrenic spaces and right and left subhepatic spaces. The subphrenic spaces are located between the diaphragm and the anterior portion of the liver. They are divided into right and left compartments by the falciform ligament (Figs. 7.18 and 7.19). The subhepatic spaces are located posterior and inferior between the liver and the abdominal viscera. The right subhepatic space, located between the liver and kidney, contains Morison’s pouch, which is the deepest point of the abdominal cavity in a supine patient and a common site for collection of fluid (Figs. 7.20 and 7.21).
Below the transverse colon is the infracolic compartment, which consists of the right and left infracolic spaces and the paracolic gutters. The right and left infracolic spaces are divided by the mesentery of the small intestine. The right and left paracolic gutters are troughlike spaces located lateral to the ascending and descending colon (Figs. 7.17 and 7.22 and Table 7.2). The deeper right gutter is a common site for free fluid collection. The paracolic gutters communicate with the spaces of the pelvis.
TABLE 7.2 Peritoneal and Retroperitoneal Spaces
|
Space |
Location |
|
Peritoneal Spaces Supracolic Compartment |
Above transverse colon |
|
Subphrenic Spaces |
Between diaphragm and anterior liver |
|
Right and Left |
Right and left spaces divided by falciform ligament |
|
Subhepatic Spaces |
Posterior and inferior to liver |
|
Right |
Between right lobe of liver and kidney; contains Morison's pouch |
|
Left |
Between left lobe of liver and kidney; includes lesser omentum |
|
Infracolic Compartment Infracolic Spaces |
Below transverse colon |
|
Right and Left Paracolic Gutters |
Divided by mesentery of small intestine |
|
Right |
Between ascending colon and right abdominal wall |
|
Left |
Between descending colon and left abdominal wall |
|
Retroperitoneal Spaces Pararenal Spaces Anterior |
Between anterior renal (Gerota's) fascia and posterior surface of peritoneum |
|
Posterior |
Between posterior renal (Gerota's) fascia and muscles of posterior abdominal wall |
|
Perirenal Space Right and Left |
Around kidney and adrenal glands; completely enclosed by renal (Gerota's) fascia |
Retroperitoneum
Structures located posterior to the peritoneum, yet lined by it anteriorly, are considered to be in the retroperitoneum and include abdominal and pelvic structures, such as the kidneys, ureters, adrenal glands, pancreas, duodenum, aorta, inferior vena cava, bladder, uterus, and prostate gland. In addition, the ascending and descending colon and most of the duodenum are situated in the retroperitoneum (Figs. 7.3-7.5).
Retroperitoneal Spaces
The retroperitoneum can be divided into compartments or spaces that include the anterior and posterior pararenal spaces and left and right perirenal spaces (Fig. 7.23). The anterior pararenal space is located between the anterior surface of the renal fascia (Gerota’s fascia) and the posterior peritoneum. It contains the retroperitoneal portions of the ascending and descending colon, the pancreas, and the duodenum. The posterior pararenal space is located between the posterior renal fascia and the muscles of the posterior abdominal wall. There are no solid organs located in this space, just fat and vessels (Figs. 7.24 and 7.25). The left and right perirenal spaces are the areas located directly around the kidneys and are completely enclosed by renal fascia. The perirenal spaces contain the kidneys, adrenal glands, lymph nodes, blood vessels, and perirenal fat. The perirenal fat separates the adrenal glands from the kidneys and provides cushioning for the kidney (Fig. 7.26 and Table 7.2).
LIVER
The liver is a large, complex organ with numerous functions, which include metabolic and hematologic regulation and bile production. It is the largest organ of the abdomen, occupying a major portion of the right hypochondriac and epigastric regions, sometimes extending into the left hypochondriac and umbilical regions. The liver is bordered superiorly, laterally, and anteriorly by the right hemidiaphragm (Fig. 7.27). The medial surface is bordered by the stomach, duodenum, and transverse colon; the inferior surface is bordered by the hepatic flexure of the colon; and the posterior surface is bordered by the right kidney (Figs. 7.28 and 7.29). The liver is surrounded by a strong connective tissue capsule (Glis- son’s capsule) that gives shape and stability to the soft hepatic tissue. It is also entirely covered by peritoneum except for the gallbladder fossa, the surface apposed to the inferior vena cava (IVC), and the bare area, which is the liver surface between the superior and inferior coronary ligaments. The liver is attached to the diaphragm via the right and left triangular ligaments, which are extensions of the coronary ligaments (Figs. 7.7, 7.16, and 7.28).
Surface Anatomy
The liver can be divided into lobes according to surface anatomy or into segments according to vascular supply. The four lobes commonly used for reference based on surface anatomy are the left, right, caudate, and quadrate. The left lobe is the most anterior of the liver lobes, extending across the midline. The right lobe is the largest of the four lobes and is separated from the left lobe by the interlobar fissure. The smallest lobe is the caudate lobe, which is located on the inferior and posterior liver surface, sandwiched between the IVC and the ligamentum venosum. The ligamentum venosum is a fibrous remnant of the ductus venosum of the fetal circulation. The quadrate lobe is located on the anteroinferior surface of the left lobe between the gallbladder and the round ligament. The hilum of the liver, the porta hepatis, is located on the inferomedial border of the liver. It is the central location for vessels to enter and exit the liver (Figs. 7.27-7.34).
Within the liver there are several main grooves or fissures that are useful in defining the lobes and boundaries of the hepatic segments. The fissure for the round ligament divides the left hepatic lobe into medial and lateral segments. The fissure for the ligamentum veno- sum separates the caudate lobe from the left lobe, and the transverse fissure (portal) contains the horizontal portions of the right and left portal veins. The interlobar fissure (main lobar fissure), an imaginary line drawn through the gallbladder fossa and the middle hepatic vein to the IVC, divides the right from the left lobes of the liver (Fig. 7.30).
Segmental Anatomy
Current practice favors the division of the liver into eight segments, according to its vascular supply, which can aid in surgical resection. According to the French anatomist Couinaud, the liver can be divided into segments based on the branching of the portal and hepatic veins. The three main hepatic veins divide the liver longitudinally into four sections (Fig. 7.35). The middle hepatic vein divides the liver into right and left lobes. The right lobe is divided into anterior and posterior sections by the right hepatic vein, and the left lobe is divided into medial and lateral sections by the left hepatic vein. Each section is then subdivided transversely by the right and left portal veins, creating eight segments. Each segment can be considered functionally independent; each has its own branch of the hepatic artery, portal vein, and bile duct and is drained by a branch of the hepatic veins (Figs. 7.36-7.49).
FIG. 7.38 Axial view of liver segments.
FIG. 7.44 Axial view of liver segments.
Portal Hepatic System
The liver receives nutrient-rich blood from the gastrointestinal tract via the portal hepatic system (Figs. 7.35, 7.36, and 7.50). The major vessel of this system is the portal vein, which is formed in the retroperitoneum by the union of the superior mesenteric and splenic veins, posterior to the neck of the pancreas at the portal splenic confluence (Figs. 7.50-7.55). It passes obliquely to the right, posterior to the hepatic artery within the lesser omentum, and enters the liver at the porta hepatis (Figs. 7.28, 7.56, and 7.57). At the porta hepatis, the portal vein branches into right and left main portal veins that then follow the course of the right and left hepatic arteries. The right main portal vein first sends branches to the caudate lobe (segment I) and then divides into anterior and posterior branches that subdivide into superior and inferior branches to supply the right lobe of the liver (segments V, VI, VII and VIII). The left main portal vein initially courses to the left, then turns medially toward the ligamentum teres. It branches to supply the lateral segments (segments II and III) of the left lobe and the superior and inferior sections of segment IV (Figs. 7.35 and 7.41-7.49).
Portal hypertension is caused by obstruction of blood flow in the portal hepatic system. This condition can lead to splenomegaly and ascites. The most common cause of portal hypertension is cirrhosis of the liver.
FIG. 7.50 Anterior view of portal hepatic system.
FIG. 7.51 CT MIP of portal vein.
FIG. 7.56 Coronal MR venogram of portal hepatic system.
FIG. 7.57 Coronal CT reformat of portal hepatic system.
Vasculature
The liver is unusual in that it has a dual blood supply, receiving arterial blood (20%-25%) from the common hepatic artery and nutrient-rich venous blood (75%- 80%) from the portal vein. The common hepatic artery usually arises as one of three branches off the celiac trunk, coursing to the right to enter the lesser omentum anterior to the portal vein (Figs. 7.58-7.61). It branches into the proper hepatic and gastroduodenal arteries just above the duodenum. While within or just before entering the porta hepatis, the proper hepatic artery divides into right and left hepatic arteries, which continue to branch and supply the lobes of the liver. The right hepatic artery is larger than the left and supplies the majority of the right lobe of the liver. It passes posterior to the uncinate process of the pancreas and runs along the posterior wall of the common bile duct into the right hepatic lobe. The left hepatic artery approaches the liver in the lesser omentum and branches to supply the caudate, quadrate, and medial and lateral segments of the left lobe of the liver (Fig. 7.62).
The venous drainage of the liver occurs via the small interlobar and intersegmental hepatic vessels that merge into the three major hepatic veins, emptying directly into the IVC, just below the diaphragm (Figs. 7.50 and 7.63). The right hepatic vein, the largest, lies between the right anterior and posterior hepatic segments; drains segments V, VI, and VII; and enters the IVC at the right lateral aspect. The middle hepatic vein lies in the interlobar fissure; drains segments IV, V, and VIII; and then enters the IVC at the anterior or right anterior surface. The smallest hepatic vein, the left hepatic vein, courses between the medial and lateral segments of the left lobe, drains segments II and III, then enters the left anterior surface of the IVC (Figs. 7.35-7.49, 7.64, and 7.65). Segment I drains directly into the IVC through smaller hepatic veins. Frequently, the middle and left hepatic veins converge to form a common trunk before emptying into the IVC just below the diaphragm. The IVC lies in a groove along the posterior wall of the liver and ascends into the thoracic cavity through the caval hiatus of the diaphragm and enters the right atrium of the heart (Figs. 7.64-7.67).
GALLBLADDER AND BILIARY SYSTEM
The biliary system is composed of the gallbladder and bile ducts (both intrahepatic and extrahepatic), which serve to drain the liver of bile and store it until it is transported to the duodenum to aid in digestion (Fig. 7.68). The hollow, pear-shaped gallbladder is located in the gallbladder fossa on the anteroinferior portion of the right lobe of the liver, closely associated with the interlobar fissure. It functions as the reservoir for storing and concentrating bile before it is transported to the duodenum. The gallbladder can be divided into a fundus, body, and neck (Figs. 7.69-7.74). The fundus is the rounded distal portion of the gallbladder sac that is frequently in contact with the anterior abdominal wall.
FIG. 7.68 Anterior view of intrahepatic biliary system.
FIG. 7.69 Anterior view of extrahepatic biliary system.
The widest portion, the body, gently tapers superiorly into the neck. The narrow neck lies to the right of the porta hepatis and continues as the cystic duct. The neck contains circular muscles that create spiral folds within the mucosa called the spiral valves of Heister (Fig. 7.70). These valves are particularly prominent at the bend formed by the neck and cystic duct, a common area for gallbladder impaction during acute or chronic cholecystitis. The gallbladder has a muscular wall that contracts when stimulated by the hormone cholecystokinin forcing bile through the extrahepatic biliary system into the duodenum. Cholecystokinin is secreted by cells in the duodenum as a response to ingestion of fat and protein into the stomach or duodenum. Bile is an alkaline fluid formed within the liver and stored in the gallbladder to be discharged into the duodenum for assistance in the digestion and absorption of fats and elimination of cholesterol and bilirubin from the body. It is collected for transport to the gallbladder by the intrahepatic bile ducts. The intrahepatic bile ducts run beside the hepatic arteries and portal veins throughout the liver parenchyma. The intrahepatic ducts merge into successively larger ducts as they follow a course from the periphery to the central portion of the liver, eventually forming the right and left hepatic ducts (Figs. 7.68-7.71). The right and left hepatic ducts unite at the porta hepatis to form the proximal portion of the common hepatic duct (CHD), which marks the beginning of the extrahepatic biliary system (Fig. 7.69).
The CHD is located anterior to the portal vein and lateral to the hepatic artery in its caudal descent from the porta hepatis. As the CHD descends in the free border of the lesser omentum, it is joined from the right by the cystic duct to form the common bile duct (CBD). The CBD continues a caudal descent along with the hepatic artery and portal vein within the hepatoduodenal ligament (Fig. 7.68). It curves slightly to the right, away from the portal vein, then courses posterior and medial to the first part of the duodenum behind the head of the pancreas (Figs. 7.71, 7.72, and 7.75-7.79). The CBD follows a groove on the posterior surface of the pancreatic head, then pierces the medial wall of the second part of the duodenum along with the main pancreatic duct (duct of Wirsung) through the ampulla of Vater (Fig. 7.69). The ends of both ducts are surrounded by the circular muscle fibers of the sphincter of Oddi (Fig. 7.70).
PANCREAS
The pancreas is a long, narrow retroperitoneal organ that lies posterior to the stomach and extends transversely at an oblique angle between the duodenum and splenic hilum. The pancreas can be divided into the head, uncinate process, neck, body, and tail (Fig. 7.80). The broad, flat head of the pancreas lies inferior and to the right of the body and tail, nestled in the curve of the second portion of the duodenum at approximately the level of L2-L3. The head is anterior to the IVC and renal veins (Figs. 7.80 and 7.81). Two vessels can be commonly seen running through the head: the CBD in the right posterior aspect and the gastroduodenal artery in the anterior aspect (Figs. 7.79, 7.80, 7.82, and 7.83). The uncinate process is a medial and posterior extension of the head, lying between the superior mesenteric vein and IVC (Figs. 7.80-7.83). The neck, the constricted portion of the gland, is located between the pancreatic head and body. Located just posterior to the neck is the portal splenic confluence, where the portal vein is formed by the merging of the superior mesenteric and splenic veins (Figs. 7.51, 7.55, 7.57, and 7.80). The body is the largest and most anterior portion of the pancreas, extending transversely to the left, anterior to the aorta and superior mesenteric artery (Figs. 7.84 and 7.85). The splenic vein runs along the posterior surface of the body on its route to the portal splenic confluence. The body tapers superiorly and posteriorly into the pancreatic tail. The tail extends into the left anterior pararenal space, anterior to the left kidney, to end at the splenic hilum (Figs. 7.80 and 7.84-7.86).
The pancreas has both an endocrine (insulin, glucagon) and exocrine (digestive enzymes) function. It delivers its endocrine hormones into the draining venous system and its enzymes into the small intestines. The endocrine hormones help control plasma glucose concentration. Insulin’s chief role is to regulate cellular absorption and utilization of glucose, thereby affecting carbohydrate, protein, and lipid metabolism in body tissues. Glucagon, acting in opposition to insulin, tends to raise plasma sugar levels by increasing the rate of glycogen breakdown and glucose synthesis in the liver. Pancreatic digestive enzymes include amylase for the digestion of starch, lipase for the digestion of lipids, peptidases for protein digestion, and sodium bicarbonate to neutralize gastric acid. The pancreatic enzymes are carried to the duodenum via a system of pancreatic ducts. The main pancreatic duct (duct of Wirsung) begins in the tail and runs the length of the gland to the ampulla of Vater, where it empties, together with the CBD, into the duodenum through the sphincter of Oddi (Figs. 7.69-7.71 and 7.86). The arterial supply of the pancreas comes from branches of the celiac and superior mesenteric arteries. Venous blood drains from the pancreas into the portal vein via the superior mesenteric or splenic vein. The pancreas is unencapsulated and has a distinct lobulated appearance, making identification easy in cross-section.
Acute pancreatitis can lead to the leakage of powerful digestive enzymes. Pancreatic necrosis results when the enzymes "digest" the surrounding tissue.
SPLEEN
The spleen, the largest lymph organ in the body, is composed of vascular and lymphoid tissue. The cellular components of the spleen create a highly vascular, spongy parenchyma called red and white pulp. The red pulp contains large quantities of blood, and the white pulp contains lymphoid tissue and white blood cells. The spleen is an intraperitoneal organ that is covered entirely by peritoneum except at its small bare area at the splenic hilum. It is located posterior to the stomach in the left upper quadrant of the abdomen, protected by the 9th through 11th ribs (Figs. 7.37 and 7.87-7.89). The spleen is bordered on its medial side by the left kidney, splenic flexure of the colon, and pancreatic tail. The posterior border of the spleen is in contact with the diaphragm, pleura, left lung, and ribs. The spleen is attached to the greater curvature of the stomach and the left kidney by the gastrosplenic and lienorenal ligaments, respectively (Fig. 7.87). The spleen receives its arterial blood from the splenic artery and is drained via the splenic vein. The splenic artery and vein enter and exit the spleen at the splenic hilum between the gastric and renal depressions (Figs. 7.80, 7.88, and 7.89). The spleen is a highly vascular organ that functions to produce white blood cells, filter abnormal blood cells from the blood, store iron from red blood cells (RBC), and initiate the immune response. Normal splenic parenchyma is homogeneous; however, immediately after intravenous contrast injection, the spleen can have a heterogeneous appearance on early arterial phase images (Fig. 7.89).
ADRENAL GLANDS
The paired adrenal (suprarenal) glands are retroperitoneal structures located superior to each kidney (Figs. 7.90 and 7.91). They are separated from the superior surface of the kidneys by perirenal fat and are enclosed, along with the kidneys, by renal fascia (Gerota’s fascia) (Figs. 7.26 and 7.92). The right adrenal gland is located just posterior to the IVC, medial to the posterior segment of the right hepatic lobe, and lateral to the right crus of the diaphragm. It is generally lower and more medial than the left adrenal gland and commonly appears as an inverted V in cross-section (Figs. 7.93-7.95). The left adrenal gland lies anteromedial to the upper pole of the left kidney. It is located in a triangle formed by the aorta, pancreatic tail, and left kidney (Fig. 7.96). It commonly appears as a triangular or Y-shaped configuration (Figs. 7.97 and 7.98). The posterior surfaces of both the right and left glands border the crus of the diaphragm. Each adrenal gland has an outer cortex and an inner medulla, which function independently (Fig. 7.90).
FIG. 7.90 Anterior view of adrenal glands.
The adrenal cortex produces more than two dozen steroids, collectively called adrenocortical steroids or just corticosteroids. The corticosteroids are broken into three main categories: glucocorticoids, which affect glucose metabolism; mineralocorticoids, which regulate sodium and potassium levels; and androgens and estrogens, which are responsible for promoting normal development of bone and reproductive organs. The adrenal medulla produces the hormones epinephrine and norepinephrine, which accelerate metabolism and increase energy and are responsible for the body’s “fight-or-flight” response. The adrenal glands receive arterial blood from the superior, middle, and inferior suprarenal arteries. The drainage of the right gland is via a short suprarenal vein that empties directly into the IVC. The left gland is drained by the left suprarenal vein, which empties into the left renal vein. See abdominal aorta and branches and IVC and tributaries at the end of this chapter.
FIG. 7.93 Axial view of right adrenal gland.
FIG. 7.96 Axial view of left adrenal gland.
URINARY SYSTEM
The structures of the urinary system include the kidneys, ureters, bladder, and urethra. Those that are located within the abdomen are the kidneys and ureters (Fig. 7.99). The bladder and urethra are located in the pelvis and are discussed in Chapter 8. The kidneys are retroperitoneal bean-shaped organs that lie against the posterior abdominal wall on either side of the vertebral column (Figs. 7.98-7.100). They lie at an oblique orientation, with the upper poles more medial and posterior than the lower poles. They are located on each side of the spine between T12 and L4 and are embedded in perirenal fat (Figs. 7.99-7.101). The right kidney is usually slightly lower due to displacement by the liver (Figs. 7.99, 7.102, and 7.103). The kidneys function to excrete waste (end products of metabolism) from the blood, form urine, and balance body fluids. The kidneys also have endocrine functions that include production and release of erythropoietin, which stimulates the bones to make red blood cells; renin, which aids in regulating blood pressure; and the active form of vitamin D, which helps maintain calcium absorption and mineral metabolism. Each kidney is composed of an outer cortex and an inner medulla. The renal cortex comprises the outer one-third of the renal tissue and has extensions between the renal pyramids of the medulla. The cortex contains the functional subunit of the kidney, the nephron, which consists of the glomerulus and convoluted tubules and is responsible for filtration of urine (Fig. 7.104). The renal medulla consists of segments called renal pyramids that radiate from the renal sinus, the fat-filled cavity surrounding the renal pelvis, to the outer surface of the kidney (Figs. 7.101 and 7.104). The striated-appearing pyramids contain the loops of Henle and collecting tubules and function as the beginning of the collecting system. Arising from the renal papilla are the cup-shaped minor calyces (Fig. 7.104). Each kidney has 7 to 14 minor calyces that merge into 2 or 3 major calyces. The major calyces join to form the renal pelvis, which is the largest dilated portion of the collecting system and is continuous with the ureters (Fig. 7.104).
Surrounding the kidneys and perirenal fat is another protective layer called the renal fascia (Gerota’s fascia). The renal fascia functions to anchor the kidneys to surrounding structures in an attempt to prevent bumps and jolts to the body from injuring the kidneys. In addition, the renal fascia acts as a barrier, limiting the spread of infection that may arise from the kidneys (Fig. 7.101). The medial indentation in the kidney is called the hilum; it allows the renal artery and vein and ureters to enter and exit the kidney (Figs. 7.90, 7.99, and 7.105-7.108).
The kidneys can be divided into five segments according to their vascular supply: apical, anterosuperior (upper anterior), anteroinferior (middle inferior), inferior, and posterior (Fig. 7.109). The segmental classification helps with surgical planning for partial nephrectomies.
The ureters are paired muscular tubes approximately 10 to 12 inches in length that transport urine to the urinary bladder. The upper half of the ureters are within the abdomen and the lower half within the pelvis. Each ureter originates at the renal pelvis and courses slightly anteriorly and medially and then descends the abdomen just anterior to the psoas muscles. (Figs. 7.108-7.111).
The ureters then enter the posterior wall of the bladder at an oblique angle (Fig. 7.111). Urine is excreted from the bladder through the urethra.
Renal agenesis is the failure of kidney formation during fetal development, resulting in the absence of one or both kidneys. Unilateral renal agenesis may be asymptomatic and is often incidentally diagnosed by abdominal computed tomography (CT) or ultrasound. Bilateral renal agenesis is invariably fatal.
FIG. 7.109 Segments of right kidney.
STOMACH
The stomach is the dilated portion of the digestive system that acts as a food reservoir and is responsible for the early stages of digestion. It has four major functions: (1) storage of food, (2) mechanical breakdown of food, (3) dissolution of chemical bonds via acids and enzymes, and (4) production of intrinsic factor, which is necessary for the absorption of vitamin B12. The stomach is located under the left dome of the diaphragm, with the superior portion joining the esophagus at the cardiac orifice and cardiac sphincter, creating the gastroesophageal junction (Figs. 7.112 and 7.115). The stomach has two borders called the lesser and greater curvatures. Between the two curvatures is the largest portion of the stomach, termed the body (Figs. 7.112, 7.113, and 7.116-7.118). On the superior surface of the body is a rounded surface called the fundus (Figs. 7.114 and 7.115). The inferior portion, the pyloric antrum, empties into the duodenum through the pyloric sphincter (Figs. 7.118 and 7.119). The anterior surface is in contact with the diaphragm, anterior abdominal wall, and left lobe of the liver. Located posterior to the stomach are the spleen, the left adrenal gland and kidney, and the body and tail of the pancreas. When empty, the inner surface of the stomach creates prominent folds called rugae, which allow the stomach to expand with the ingestion of food (Figs. 7.112, 7.113, and 7.115). The stomach is one of the most vascular organs within the body. The arterial blood is supplied by branches of the gastric, splenic, and gastroduodenal arteries (Fig. 7.58). Venous drainage corresponds to the arterial supply. The gastric veins usually drain directly into the portal vein or into the superior mesenteric vein.
The average adult produces 2 to 3 liters per day of gastric juices, which contain mucus, hydrochloric acid, intrinsic factor, and the digestive enzymes pepsinogen and lipase. The stomach can hold up to 3 liters of food, which is mixed with digestive juices to form a semifluid mass called chyme.
INTESTINES
The small intestine (small bowel) is located between the pylorus and ileocecal valve and consists of loops of bowel averaging 6 to 7 meters in length. It can be subdivided into the duodenum, jejunum, and the ileum (Figs. 7.120 and 7.121). The proximal portion of the small intestine is the duodenum, which begins at the gastric pylorus and curves around the head of the pancreas, forming the letter C (Figs. 7.118-7.122). The duodenum is mostly retroperitoneal, making it less mobile than the rest of the small intestine. Although quite short, the duodenum is divided into four portions. The first (superior) portion, located in the anterior pararenal space, is formed by the first 2 inches of the duodenum, the conical-shaped duodenal bulb. It is suspended in the abdomen by the hepatoduodenal ligament and is the most common site for peptic ulcer formation. The second (descending) portion is formed by the next 4 inches of the duodenum that descends along the right side of the vertebral column just anterior to the right renal hilum; it contains the ampulla of Vater and receives pancreatic and biliary drainage. The third (horizontal) portion is about 10 cm long and runs horizontally in front of the third lumbar vertebra. In its horizontal course from right to left, the third portion of the duodenum runs anterior to the IVC, aorta, and inferior mesenteric artery, and posterior to the superior mesenteric artery (Figs. 7.123 and 7.124). The fourth (ascending) portion is about 2.5 cm in length and ascends on the left side of the aorta to the level of the L2 vertebra, where it meets up with the jejunum at the duodenojejunal flexure. The duodenojejunal flexure is fixed in place by the ligament of Treitz, a suspensory ligament created from the connective tissue located around the celiac trunk and left crus of the diaphragm (Fig. 7.122). This location marks the entry of the small bowel into the peritoneal cavity. The remainder of the small intestine, the jejunum and ileum, is suspended from the posterior abdominal wall by a fan-shaped mesentery. The jejunum is approximately 2.5 m long (about 40% of the small bowel) and occupies the left upper abdomen or umbilical region of the abdomen (Figs. 7.120-7.124). This section of small bowel is where the bulk of chemical digestion and nutrient absorption occurs. The jejunum contains numerous circular folds that give it a feathery appearance on diagnostic imaging examinations. It also has a thicker and more vascular wall than the ileum. The ileum is the longest portion of the small intestine, averaging 3.5 m in length (about 60% of the small bowel), and is located in the right lower abdomen (Figs. 7.120, 7.121, and 7.124-7.126). It is in the ileum that intrinsic factor from the stomach combines with vitamin B12 for absorption in the terminal ileum. Vitamin B12 is essential for normal red blood cell formation and nervous system function. The loops of ileum terminate at the ileocecal valve, a sphincter that controls the flow of material from the ileum into the cecum of the large intestine (Figs. 7.120, 7.121, and 7.127). The segments of the small intestine receive blood entirely from branches of the superior mesenteric artery and are drained by branches of the superior mesenteric vein.
The large intestine (large bowel) lies inferior to the stomach and liver and almost completely frames the small intestine (Figs. 7.120 and 7.128). The large intestine has a larger diameter and thinner walls than the small intestine and is approximately 1.5 m long, starting at the ileocecal junction and ending at the anus. The outer, longitudinal muscle of the large intestine forms three thickened bands called taeniae coli, which gather the cecum and colon into a series of pouchlike folds called haustra. On the outer surface of the large intestine are small fat-filled sacs of omentum called the epiploic appendages. The three main divisions of the large intestine are the cecum, colon, and rectum (Fig. 7.128). The cecum is a pouchlike section of the proximal portion of the large intestine and is about 7 cm in length. This is the location of the ileocecal valve and the slender vermiform appendix, which attaches to the posteromedial surface of the cecum (Figs. 7.128-7.130). The colon is the longest portion of the large intestine and can be subdivided into four distinct portions: ascending, transverse, descending, and sigmoid (Figs. 7.131 and 7.132). The ascending colon is retroperitoneal and commences at the cecum, ascending the right lateral wall of the abdomen to the level of the liver. It then curves sharply to the left, creating the hepatic flexure (Figs. 7.128, 7.133, and 7.134). The hepatic flexure marks the beginning of the transverse colon. The transverse colon travels horizontally across the anterior abdomen toward the spleen, where it bends sharply downward, creating the splenic flexure and the beginning of the descending colon (Figs. 7.135 and 7.136). The transverse colon is located within the peritoneal cavity and is the largest and most mobile portion of the large intestine, making its position quite variable in the patient. The descending colon is retroperitoneal and continues inferiorly along the left lateral abdominal wall to the iliac fossa, where it curves to become the S-shaped sigmoid colon posterior to the bladder (Fig. 7.128). The sigmoid colon joins the rectum, which is the terminal portion of the colon (Figs. 7.128, 7.132, 7.137, and 7.138). The rectum is considered a pelvic organ and is covered in greater detail in Chapter 8. The major functions of the large intestine include reabsorption of water and the storage and elimination of fecal material. The superior and inferior mesenteric arteries and veins supply and drain blood from the large intestine (Fig. 7.128).
When the epiploic appendages become inflamed due to torsion or ischemia, it results in a condition called epiploic appendagitis. The condition commonly presents with acute lower quadrant pain, which can simulate appendicitis or diverticulitis. Epiploic appendagitis can occur at any age but is more common in the third through fifth decades. Treatment is somewhat controversial, but conservative therapy is generally favored because it is typically a self-limiting condition.
ABDOMINAL AORTA AND BRANCHES
The abdominal aorta is a retroperitoneal structure beginning, as an extension of the thoracic aorta, at the aortic hiatus of the diaphragm. The abdominal aorta gradually diminishes in diameter as it descends the abdomen just left of the midline next to the vertebral bodies. It delivers blood to all the abdominopelvic organs and structures. At approximately the level of L4, the abdominal aorta bifurcates into the right and left common iliac arteries. The branches of the abdominal aorta can be divided into the paired branches, including the inferior phrenic, lumbar, suprarenal, renal, and gonadal arteries; and unpaired branches, which include the celiac trunk, splenic, superior mesenteric, and inferior mesenteric arteries (Figs. 7.139-7.142). Each of these branches has a typical configuration that is described in this text; however, many normal variations of these vessels may occur.
Paired Branches
Inferior Phrenic Arteries. The paired inferior phrenic
arteries are the first to branch from the lateral surface of the abdominal aorta just as it descends through the aortic hiatus. The right inferior phrenic artery passes upward on the right side behind the IVC, and the left inferior phrenic artery passes behind the stomach and the abdominal part of the esophagus (Figs. 7.139, 7.141, and 7.143). The inferior phrenic arteries extend to supply the inferior surface of the diaphragm.
Lumbar Arteries. Four pairs of lumbar arteries arise from the posterior wall of the abdominal aorta at the level of L1-L4 (Figs. 7.139 and 7.141). The lumbar arteries supply the posterior abdominal wall, lumbar vertebrae, and the intervertebral disks.
Suprarenal Arteries. These arteries course laterally and slightly superiorly to supply the adrenal glands. The middle suprarenal arteries exit the lateral walls of the aorta near the base of the superior mesenteric artery. The superior suprarenal arteries are branches of the inferior phrenic arteries, and the inferior suprarenal arteries extend from the renal arteries (Figs. 7.90 and 7.143).
Renal Arteries. The two large renal arteries arise from the lateral walls of the aorta just below the superior mesenteric artery. Each vessel travels horizontally to the hilum of the corresponding kidney (Figs. 7.1397.141 and 7.143-7.148). Because of the position of the aorta on the left side of the vertebral column, the right renal artery is slightly longer than the left renal artery.
The right renal artery passes posterior to the IVC and right renal vein on its course to the right kidney (Fig. 7.144). Typically, the left kidney is higher than the right kidney, which means the left renal artery is generally slightly superior to the right (Fig. 7.145). As each renal artery reaches the renal hilum, it typically divides into anterior and posterior branches and then into five segmental arteries—apical, upper, middle, lower, and posterior—(Figs. 7.146 and 7.148). Each segmental artery further divides into interlobar arteries, one for each pyramid and adjoining cortex. As the interlobar arteries curve over the renal pyramids, they become the arcuate arteries from which the interlobular arteries arise to supply the renal cortex (Fig. 7.146).
a Renal artery stenosis causes renal ischemia and can result in secondary hypertension.
FIG. 7.145 Axial CT of abdomen with renal arteries and veins.
FIG. 7.146 Anterior view of renal vasculature.
Gonadal Arteries. The gonadal arteries originate from the anterior wall of the aorta just inferior to the renal arteries. They descend along the psoas muscles to reach their respective organs (Figs. 7.139, 7.149, and 7.150). In the male, the gonadal arteries are termed the testicular arteries, which supply the testes and scrotum, whereas the gonadal arteries in the female are termed the ovarian arteries, which supply the ovaries, uterine tubes, and uterus.
Unpaired Branches
Celiac Trunk. The celiac trunk is a very short vessel that leaves the anterior wall of the aorta just after the aorta passes through the diaphragm. The short celiac trunk divides into three branches: left gastric, common hepatic, and splenic arteries (Figs. 7.151-7.153). Variations of the celiac trunk are not rare; occasionally, the common hepatic artery will branch directly from the superior mesenteric artery.
The left gastric artery courses superiorly and toward the left within the lesser omentum to supply the cardiac region of the stomach, then passes along the lesser curvature toward the pylorus, giving off esophageal and gastric branches to supply the abdominal esophagus and adjacent anterior and posterior walls of the body of the stomach. The left gastric artery continues toward the right to anastomose with the right gastric artery (Figs. 7.151, 7.154, 7.155, and 7.157).
The common hepatic artery crosses to the right toward the superior aspect of the duodenum and divides into the proper hepatic artery and the gastroduodenal artery (Figs. 7.151-7.153, 7.156, and 7.157). The proper hepatic artery ascends obliquely to the right in the hepatoduodenal ligament, adjacent to the portal vein and CBD, divides near the porta hepatis into the right and left hepatic branches, and usually gives off the right gastric artery (Figs. 7.151, 7.156, and 7.157). The right hepatic branch dispatches the cystic artery to the gallbladder and divides into the anterior and posterior segmental arteries to supply the segments of the right and caudate lobes of the liver. The left hepatic branch also gives off an artery to the caudate lobe, as well as medial and lateral segmental arteries to supply the segments of the left lobe and the intermediate branch to the quadrate lobe. The right gastric artery, which can also arise from the common hepatic or gastroduodenal arteries, supplies the lower part of the lesser curvature of the stomach and anastomoses with the left gastric artery within the lesser curvature of the stomach (Figs. 7.151 and 7.157). The gastroduodenal artery descends behind the pylorus to give off many branches, including the anterior and posterior superior pancreaticoduodenal arteries, which supply the superior part of the duodenum and head of the pancreas, and the right gastroepiploic (gastro-omental) artery. The right gastroepiploic artery passes through the greater omentum, anastomoses with the left gastroepiploic artery on the inferior surface of the greater curvature, and dispatches numerous gastric branches to the anterior and posterior walls of the pyloric and body portions of the stomach (Figs. 7.151 and 7.156-7.158).
The splenic (lienal) artery is the largest branch of the celiac trunk and passes to the left behind the stomach and along the upper border of the pancreas, within the splenorenal ligament, to the hilum of the spleen. At the point where the splenic artery courses near the border of the pancreas, it gives off numerous pancreatic branches including the dorsal, great, and caudal pancreatic arteries that supply the body and tail of the pancreas (Fig. 7.159). Just before the splenic artery terminates into numerous splenic branches, it gives rise to the left gastroepiploic (gastro-omental) artery, which gives off epiploic and gastric branches to the greater omentum and anterior and posterior walls of the fundus of the stomach (Figs. 7.151 and 7.156-7.161).
Superior Mesenteric Artery. The large superior mesenteric artery (SMA) emerges just below the celiac trunk at approximately the level of L1 (Figs. 7.139, 7.162, and 7.163). It descends behind the body of the pancreas, then over the horizontal portion of the duodenum to course in the mesentery to the ileum (Figs. 7.158, 7.162, and 7.164-7.167). The artery supplies the head of the pancreas and the majority of the small and large intestines. Branches of the superior mesenteric artery include the inferior pancreaticoduodenal artery, jejunal arteries, ileal arteries, middle colic artery, right colic artery, and ileocolic artery.
The inferior pancreaticoduodenal artery extends to the head of the pancreas and duodenum, and then divides into the posterior ramus, which anastomoses with the posterior superior pancreaticoduodenal artery, and the anterior ramus, which anastomoses with the anterior superior pancreaticoduodenal artery. The jejunal and ileal arteries extend to supply the jejunum and ileum, except the end segment near the cecum. The middle colic artery reaches the transverse colon, and the right colic artery passes to the ascending colon. The ileocolic artery courses behind the peritoneum across the right ureter into the right iliac fossa and divides to supply a portion of the ascending colon, cecum, vermiform appendix, and terminal portion of the ileum (Figs. 7.166 and 7.168).
Inferior Mesenteric Artery. The inferior mesenteric artery (IMA) arises 3 to 4 cm above the bifurcation of the aorta at approximately the level of L3-L4. It descends in front of the abdominal aorta and then to the left, where it gives off the left colic artery, sigmoid arteries, and the superior rectal artery (Figs. 7.164, 7.165, and 7.169). The left colic artery is a retroperitoneal structure that passes along the anterior surface of the left psoas and quadratus lumborum muscles. It bifurcates into ascending and descending branches that supply the walls of the left third of the transverse colon and the entire descending colon. The sigmoid branches (2 or 3) course within the mesentery to supply branches to the terminal descending colon and to the sigmoid colon. The superior rectal artery crosses the common iliac artery and vein as it descends to branch and supply the rectum (Figs. 7.169-7.171).
INFERIOR VENA CAVA
AND TRIBUTARIES
The inferior vena cava (IVC) is the largest vein of the body (Fig. 7.172). It carries blood to the heart from the lower limbs, pelvic organs and the abdominal viscera, and abdominal wall. The IVC is formed by the union of the common iliac veins at approximately the level of L5. It courses superiorly through the retroperitoneum along the anterior aspect of the vertebral column and to the right of the aorta (Figs. 7.164 and 7.165). As the IVC ascends the abdominal cavity, it passes along the posterior surface of the liver and enters the thorax at the caval hiatus to enter the right atrium of the heart. The IVC receives many tributaries, including the inferior phrenic, lumbar, right gonadal, renal, right suprarenal, and hepatic veins, throughout its course in the abdomen (Fig. 7.172).
Inferior Phrenic Veins
The inferior phrenic veins extend from the inferior surface of the diaphragm. The left inferior phrenic vein is often doubled and drains into either the left suprarenal vein, left renal vein, or the IVC. The right inferior phrenic vein drains directly into the IVC (Fig. 7.172).
Lumbar Veins
The lumbar veins consist of four pairs of vessels that collect blood from the posterior abdominal wall at the L1-L4 levels (Figs. 7.173-7.175). They receive veins from the vertebral plexuses and then travel horizontally along the transverse processes of the vertebrae deep to the psoas muscles. The lumbar veins on the left are typically longer than those on the right because they must cross over the vertebral column to drain into the IVC. The arrangement of these veins varies, with some entering the lateral walls of the IVC and others emptying into the common iliac vein or are united on each side by a vertical connecting vein termed the ascending lumbar vein. Typically, the right ascending lumbar vein continues as the azygos vein and the left ascending lumbar vein continues as the hemiazygos vein. Additionally, a diminutive median sacral vein may accompany the median sacral artery. It typically drains into the left common iliac vein but may also drain into the junction of the common iliac veins (Figs. 7.172 and 7.173).
FIG. 7.173 Anterior view of IVC and lumbar veins.
Gonadal Veins
The gonadal veins, ovarian in females and testicular in males, ascend the abdomen along the psoas muscle, anterior to the ureters. The right gonadal vein enters the anterolateral wall of the IVC just below the opening for the right renal vein, whereas the left gonadal vein typically empties directly into the left renal vein (Figs. 7.149, 7.150, 7.172, and 7.173).
Renal Veins
Blood leaves the kidney by way of interlobular veins that carry blood from the renal cortex to the arcuate veins, which carry blood from the medulla to the interlobar veins. The interlobar veins drain into the segmental veins. The five segmental veins correspond to the respective segmental arteries and merge to form the renal vein (Fig. 7.146). The renal veins pass anterior to the renal arteries to empty into the IVC at about the level of L2. The left renal vein passes posterior to the superior mesenteric artery and anterior to the aorta on its route from the left kidney to enter the left lateral wall of the IVC. It receives the left gonadal vein, left inferior phrenic vein, and generally the left suprarenal vein. The shorter right renal vein is typically lower than the left renal vein as it travels its short course to enter the right lateral wall of the IVC (Figs. 7.172, 7.173, 7.176, and 7.177).
Suprarenal Veins
The right suprarenal vein courses from the medial side of the right adrenal gland to empty directly into the IVC. The left suprarenal vein courses from the inferior pole of the left adrenal gland to empty directly into the left renal vein or left inferior phrenic vein (Figs. 7.172 and 7.173).
Hepatic Veins
The three short hepatic veins (right, middle, left) begin as smaller vessels that collect blood from the liver parenchyma. The hepatic veins course from the inferior aspect of the liver to the superior aspect of the liver, where they empty into the IVC just below the diaphragm. In general, the right and left hepatic veins drain the right and left lobes of the liver, respectively, whereas the middle hepatic vein drains the medial segment of the left lobe and the anterior portions of the right lobe (see liver section, Figs. 7.63-7.68 and 7.172).
LYMPH NODES
Many lymph nodes exist within the abdominal cavity. Abdominal lymph nodes occur in chains along the main branches of the arteries of the intestine and abdominal aorta. Most abdominal lymph nodes appear as small oblong soft tissue masses oriented parallel to their accompanying vessels and may be difficult to visualize in cross-section unless they are enlarged as a result of an abnormality. Typically, lymph nodes are considered enlarged if their short axis diameter is greater than 1 cm. Abdominal nodal groups surround the aorta and IVC and organs of the abdomen. Lymph from the abdominal cavity empties into the lumbar trunks, which drain lymph from the legs, lower abdominal wall, and the pelvic organs; and the intestinal trunks, which drain organs located within the abdominal cavity. These trunks then join the thoracic duct and ultimately enter the venous system (see Chapter 6, Figs. 6.34-6.36, and Figs. 7.178-7.180).
MUSCLES OF THE ABDOMINAL WALL
The abdominal wall is formed superiorly by the diaphragm and is inferiorly continuous with the pelvic cavity at the pelvic inlet. Posteriorly, the abdominal wall is formed by the five lumbar vertebrae, the 12th pair of ribs, the upper portion of the pelvis, quadratus lumborum muscles, and psoas muscles (Fig. 7.181). The quadratus lumborum muscle forms a large portion of the posterior abdominal wall. It extends from the iliac crest to the inferior border of the 12th rib and transverse processes of the lumbar vertebrae to aid in lateral flexion of the vertebral column. The large psoas muscles extend along the lateral surfaces of the lumbar vertebrae to insert on the lesser trochanter of the femur and act to flex the thigh and trunk (Figs. 7.182-7.184). Anteriorly, the abdominal wall is formed by the lower portion of the thoracic cage and by layers of muscles that include the rectus abdominis, external oblique, internal oblique, and transversus abdominis (Figs. 7.185 and 7.186). The paired rectus abdominis muscles, visualized on the anterior surface of the abdomen and pelvis, originate from the pubic symphysis and extend vertically to the xiphoid process and costal cartilage of the fifth, sixth, and seventh ribs. They function to flex the lumbar vertebrae and support the abdomen (Figs. 7.182 and 7.184). The anterior surface of the rectus abdominis muscle is crossed by three tendinous intersections that course transversely, forming individual muscle bellies that can contract separately (Fig. 7.185). A longitudinal band of fibers that forms a central anterior attachment for the muscle layers of the abdomen is the linea alba, which extends from the xiphoid process of the sternum to the pubic symphysis.
FIG. 7.182 Axial view of the abdominal wall.
The linea alba is formed, at the midline, by the interlacing of fibers from the rectus abdominis and oblique muscles (Figs. 7.184- 7.186). The external and internal oblique muscles are located on the outer lateral portion of the abdomen and extend from the cartilages of the lower ribs to the level of the iliac crest (Figs. 7.182, 7.184, 7.186, and 7.187).
The oblique muscles work together to flex and rotate the vertebral column and compress the abdominal viscera. The external oblique is the most extensive of the three broad abdominal muscles and contains a triangular opening, the superficial inguinal ring, that allows for the passage of the spermatic cord or round ligament of the uterus (Fig. 7.185). The inguinal ligament is a fibrous band formed by the thickened inferior border of the aponeurosis of the external oblique muscle. It extends from the anterior superior iliac spine to the pubic tubercle and gives rise to the lowermost fibers of the internal oblique and transversus abdominis muscles (Fig. 7.186). The transversus abdominis muscle lies deep to the internal oblique muscles. Its fibers extend transversely across the abdomen to provide maximum support for the abdominal viscera. The transversus abdominis muscle extends from the lower six costal cartilages, lumbar fascia, iliac crest, and inguinal ligament to insert into the xiphoid process, linea alba, and pubic symphysis (Figs. 7.182, 7.184, 7.186, and 7.187 and Table 7.3).
TABLE 7.3 Abdominal Muscles
|
Muscle |
Origin |
Insertion |
Function |
|
Rectus abdominis |
Pubic bone near symphysis |
Costal cartilage of fifth, sixth, and seventh ribs; xiphoid process of sternum |
Flexes trunk |
|
External oblique |
Lower eight ribs |
Linea alba and iliac crest |
Compresses abdominal viscera, flexes and rotates spine |
|
Internal oblique |
Iliac crest, lumbodorsal fascia, and inguinal ligament |
Lower three ribs, linea alba |
Compresses abdominal viscera, flexes and rotates spine |
|
Transversus abdominis |
Lower six ribs, iliac crest, and lumbodorsal fascia |
Pubic bone and linea alba |
Compresses abdominal viscera |
|
Quadratus lumborum |
Iliac crest |
Twelfth rib and transverse processes of lumbar vertebrae |
Flexes spine laterally |
|
Psoas |
Vertebral bodies, intervertebral disks, and transverse processes of T12—L5 |
Lesser trochanter of femur |
Lateral flexion of the trunk and flexor of the hip |
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