CHAPTER 7

A man’s liver is his carburetor.

ANONYMOUS

ABDOMINAL CAVITY

The abdominal cavity is the region located between the diaphragm and sacral promontory (Figure 7.2). 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.

LIVER

The liver is a large, complex organ with numerous functions that include metabolic regulation, 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 (Figures 7.27 and 7.28). 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 (Figure 7.29). The liver is surrounded by a strong connective tissue capsule (Glisson’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 (liver surface between the superior and inferior coronary ligaments) (Figures 7.7, 7.8, 7.19, and 7.28).

Within the liver there are several main grooves or fissures that are useful in defining the lobes and boundaries of the hepatic segments. The umbilical fissure (fissure for ligamentum teres) divides the left hepatic lobe into medial and lateral segments. The fissure for the ligamentum venosum 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), also called the fissure for the gallbladder, divides the right from left lobes of the liver (Figure 7.30). 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 (Figure 7.28).

GALLBLADDER AND BILIARY SYSTEM

The biliary system is composed of the gallbladder and bile ducts (both intrahepatic and extrahepatic) that serve to drain the liver of bile and then store it until it is transported to the duodenum to aid in digestion (Figure 7.65). 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 main lobar fissure. It functions as the reservoir for storing and concentrating bile before being transported to the duodenum. The gallbladder can be divided into a fundus, body, and neck (Figures 7.66 to 7.71). The fundus is the rounded distal portion of the gallbladder sac that is frequently in contact with the anterior abdominal wall. 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 that are called the spiral valves of Heister (Figure 7.67). These valves are especially 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 cholecystokinin, forcing bile through the extrahepatic biliary system into the duodenum. Bile, formed within the liver, 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 (Figures 7.65 to 7.68). 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 (Figure 7.66). 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 (Figure 7.65). 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 (Figures 7.69 through 7.76). 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 (Figure 7.66). The ends of both ducts are surrounded by the circular muscle fibers of the sphincter of Oddi (Figure 7.67).

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 (Figure 7.77). 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. Two vessels can be commonly seen running through the head: the common bile duct in the right posterior aspect and the gastroduodenal artery in the anterior aspect (Figures 7.78 and 7.79). The uncinate process is a medial and posterior extension of the head, lying between the superior mesenteric vein and inferior vena cava (Figure 7.77). 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 (Figure 7.77). 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 (Figures 7.80 and 7.81). 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 to the left anterior pararenal space, anterior to the left kidney, to end at the splenic hilum (Figures 7.77 and 7.80 through 7.82). 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 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 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 common bile duct, into the duodenum through the sphincter of Oddi (Figures 7.66 and 7.82). 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.

SPLEEN

The spleen is the largest lymph organ in the body composed of 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 with 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 ninth through eleventh ribs. 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 (Figure 7.83). 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 (Figures 7.84 and 7.85). 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, 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 (Figure 7.85).

ADRENAL GLANDS

The paired adrenal (suprarenal) glands are retroperitoneal organs located superior to each kidney (Figures 7.86 and 7.87). They are separated from the superior surface of the kidneys by perirenal fat and are enclosed, along with the kidneys, by Gerota’s fascia (Figure 7.88). 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 on cross section (Figures 7.89 through 7.91). 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 (Figure 7.92). It commonly appears as a triangular or Y-shaped configuration (Figures 7.93 and 7.94). 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 that function independently (Figure 7.86). 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 energy and are responsible for the body’s “fight or flight” response. The adrenal glands receive arterial blood from the superior, middle, and inferior adrenal 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.

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 (Figure 7.95). The bladder and urethra are located in the pelvis and are discussed in Chapter 8. The kidneys are retroperitoneal bean-shaped organs that lie in the paravertebral gutters against the posterior abdominal wall (Figure 7.96). 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 (Figure 7.97). The right kidney is usually slightly lower due to displacement by the liver (Figures 7.98 and 7.99). 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 (Figure 7.100). The renal medulla consists of segments called renal pyramids that radiate from the renal sinus to the outer surface of the kidney. The striated-appearing pyramids contain the loops of Henle and collecting tubules and function as the beginning of the collecting system. Arising from the apices of the pyramids are the cup-shaped minor calyces. 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 (Figure 7.100). The fat-filled cavity surrounding the renal pelvis is called the renal sinus.

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. 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 (Figures 7.97 and 7.101 through 7.104). The kidneys can be divided into five segments according to their vascular supply: apical, anterosuperior (upper anterior), anteroinferior (middle inferior), inferior, and posterior (Figure 7.105). The segmental classification helps with surgical planning for partial nephrectomies. The ureters are paired muscular tubes that transport urine to the urinary bladder. Each ureter originates at the renal pelvis and descends anteriorly and medially to the psoas muscles, just anterior to the transverse processes of the lumbar spine (Figures 7.104 through 7.106). The ureters then enter the posterior wall of the bladder at an oblique angle (Figure 7.107). The primary function of the urinary system is to filter blood, produce and excrete urine, and help maintain normal body physiology.

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. The stomach is located under the left dome of the diaphragm, with the superior portion joining the esophagus at the cardiac orifice (cardiac sphincter), creating the esophagogastric junction (Figures 7.108 through 7.110). 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 (Figures 7.111 to 7.113). On the superior surface of the body is a rounded surface called the fundus (Figure 7.110). The inferior portion (pyloric antrum) empties into the duodenum through the pyloric sphincter (Figure 7.114). The anterior surface is in contact with the diaphragm, anterior abdominal wall, and left lobe of the liver. Located posterior to the stomach is the gastric portion of 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 that allow the stomach to expand with the ingestion of food (Figures 7.108 and 7.110). The average adult produces 2 to 3 L per day of gastric juices that contain mucus, hydrochloric acid, intrinsic factor, and the digestive enzymes of pepsinogen and lipase. The stomach can hold up to 3 L of food, which it mixes with digestive juices to form chyme. 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 (Figure 7.55). Venous drainage corresponds to the arterial supply. The gastric veins usually drain directly into the portal vein or into the superior mesenteric vein.

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 (Figures 7.115 and 7.116). 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 (Figures 7.113 through 7.117). 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 is formed by the first 2 inches of the duodenum, the conical-shaped duodenal bulb. It is the most common site for peptic ulcer formation. The second (descending) portion is formed by the next 4 inches of duodenum that descends along the right side of the vertebral column; 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 superior vena cava (SVC), aorta, and inferior mesenteric artery and posterior to the superior mesenteric artery. 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 around the celiac axis and left crus of the diaphragm (Figure 7.117). 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 meters long and occupies the left upper abdomen or umbilical region of the abdomen (Figures 7.115 through 7.119). 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 barium or computed tomography (CT) examinations. The lower three fifths of the small intestine, the ileum, is the longest portion of the small intestine, averaging 3.5 meters long and located in the right lower abdomen (Figures 7.115, 7.116, and 7.119 through 7.121). 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 (RBC) 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 (Figures 7.118, 7.122, and 7.123). The mesentery serves as a route for blood vessels, lymphatics, and nerves to reach the small intestine. The segments of the small intestine receive blood 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 (Figures 7.115 and 7.123). The large intestine has a larger diameter and thinner walls than the small intestine and is approximately 1.5 meters long, starting at the ileocecal junction and ending at the anus. The outer, longitudinal muscle of the large intestine forms three thickened bands called taenia coli that 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 (Figure 7.123). The cecum is a pouchlike section of the proximal portion of the large intestine located at the ileocecal valve. The slender vermiform appendix attaches to the posteromedial surface of the cecum (Figures 7.123 through 7.125). The colon is the longest portion of the large intestine and can be subdivided into four distinct portions: ascending, transverse, descending, and sigmoid (Figures 7.126 and 7.127). 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 (Figures 7.123, 7.128, and 7.129). 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 (Figures 7.130 and 7.131). 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 (Figure 7.123). The sigmoid colon joins the rectum, which is the terminal portion of the colon (Figures 7.132 and 7.133). The rectum is considered a pelvic organ and is covered in greater detail in Chapter 8. The superior and inferior mesenteric arteries and veins supply and drain blood from the large intestine. The major functions of the large intestine include the reabsorption of water and the storage and elimination of fecal material.

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 has a gradual diminishment of its 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 (dorsal) parietal branches, including the inferior phrenic and lumbar arteries; paired visceral branches, including the suprarenal, renal, and gonadal arteries; and unpaired (ventral) visceral branches that include the celiac trunk, splenic, superior mesenteric, and inferior mesenteric arteries (Figures 7.134 through 7.136). Each of these branches has a typical configuration that is described within this text; however, many normal variations of these vessels may occur.

INFERIOR VENA CAVA AND TRIBUTARIES

The IVC is the largest vein of the body (Figure 7.167). 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 (Figures 7.159 and 7.160). As the IVC ascends the abdominal cavity, it passes the posterior surface of the liver and pierces the diaphragm at the caval hiatus to enter the right atrium of the heart. The IVC receives many tributaries throughout its course in the abdomen that include the inferior phrenic, lumbar, right gonadal, renal, right suprarenal, and hepatic veins (Figure 7.167).

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. Abdominoaortic nodal groups surround the aorta and IVC and visceral lymph nodes drain adjacent organs that include the mesenteric hepatic, splenic, and pancreaticoduodenal nodal groups. Lymph from the abdominal cavity empties into the lumbar trunk, which drains lymph from the legs, lower abdominal wall, and the pelvic organs; and the intestinal trunk, which drains organs located within the abdominal cavity. These trunks then join the thoracic duct and ultimately enter the venous system (Figures 7.173 through 7.175).

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, twelfth pair of ribs, upper portion of the pelvis, quadratus lumborum muscles, and psoas muscles (Figure 7.176). 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 twelfth 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 (Figures 7.177 through 7.180). 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 (Figures 7.181 and 7.182). 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 (Figures 7.177 and 7.179). The anterior surface of the rectus muscle is crossed by three tendinous intersections that course transversely, forming individual muscle bellies that can contract separately (Figure 7.181). 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. The linea alba is formed, at the midline, by the interlacing of fibers from the rectus abdominis and oblique muscles (Figures 7.178, 7.179, and 7.182). 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 (Figures 7.177, 7.179, and 7.181 through 7.183). 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 (Figure 7.181). 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 origin to the lowermost fibers of the internal oblique and transversus abdominis muscles (Figure 7.182). 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 (Figures 7.177, 7.179, and 7.183 and Table 7.3).