Chapter 28: The liver, billiary passages, pancreas and spleen
The ductal systems of the liver and pancreas develop in the embryo as outgrowths of the alimentary canal at the junction of the foregut and midgut, i.e., at the middle of the future duodenum.
The liver (Gk, hepar; hence the adjective hepatic) is a large, soft, reddish organ and the largest gland in the body. The exocrine secretion of the liver is termed bile. Many products of the hepatic cells are discharged directly into the blood stream and may be considered the endocrine secretion of the liver.
The liver lies mostly under cover of the thoracic bony cage (see fig. 26-1) and is covered by the diaphragm. It can be demonstrated in vivo by the uptake of radio-active isotopes (fig. 28-1). The liver, which is relatively large at birth, has diaphragmatic and visceral surfaces.
The diaphragmatic surface, smooth and convex, is separated from the visceral surface by the sharp inferior border. The visceral surface faces inferiorward, posteriorward, and to the left. It is related to the right colic flexure, the right kidney and to the duodenum and stomach. The visceral surface has quadrate and caudate lobes (see fig. 28-3C) defined by an H-shaped set of grooves.
The cross-bar of the "H" is the porta hepatis, or hilus of the liver, which contains the hepatic ducts and the branches of the portal vein and hepatic artery. The structures (vessels and ducts) at the porta constitute the hepatic pedicle and have many variations that are of surgical importance. The limbs of the "H" (see fig. 28-3C) are (1) the fissure containing the ligamentum teres (obliterated left umbilical vein), (2) the fissure containing the ligamentum venosum (obliterated ductus venosus), (3) the fossa containing the gallbladder, and (4) the sulcus in which the inferior vena cava is lodged.
The liver can be divided into right and left anatomical lobes along the left-hand limb of the "H" (fig. 28-2) and by the attachment of the falciform ligament on the diaphragmatic surface (fig. 28-3). The caudate and quadrate lobes are classified with the right anatomical lobe of the liver. However, the right and left functional lobes, as defined by ductal and vascular anatomy, would be separated by a plane through the right hand limb of the "H" from the gallbladder to the sulcus of the vena cava (see fig. 28-2). A number of portal segments have been described within these functional lobes (fig. 28-4). Hepatic segments, unlike the bronchopulmonary segments of the lung, are not demarcated by connective tissue septa. The segmental anatomy of the liver is of diagnostic and surgical importance.
Peritoneal Relations (see fig. 28-3).
The liver is nearly surrounded by peritoneum, which (as the ventral mesogastrium of the embryo) attaches it to the body wall (falciform ligament) and to the stomach (lesser omentum) (see fig. 26-4). The falciform ligament connects the anterior abdominal wall and diaphragm to the liver. The free edge of the falciform ligament meets the inferior border of the liver at a notch for the ligamentum teres, where that ligament (obliterated left umbilical vein) is conveyed to the porta. As the two layers of the falciform ligament reach the liver, the left layer becomes continuous superiorly with the left triangular ligament and the right layer with the upper layer of the coronary ligament. The upper and lower layers of the coronary ligament, which meet at the right as the right triangular ligament, diverge toward the left and enclose the triangular "bare area" of the liver, which is in direct contact with the diaphragm. The junction of the left triangular and coronary ligaments leads to the lesser omentum, which has an L-shaped attachment to the liver. The vertical limb is at the fissure for the ligamentum venosum, and it extends to the lesser curvature of the stomach (gastrohepatic ligament). The horizontal limb of the L is at the porta hepatis and extends to the first part of the duodenum (hepatoduodenal ligament). The right free border of the lesser omentum contains the bile duct, portal vein, and hepatic artery.
The liver lies mostly under cover of the thoracic bony cage and is covered by the diaphragm. On the right side, it extends superior to the inferior border of the lung and causes dullness there on percussion. The liver lies largely, and in some people entirely, to the right of the median plane. Its sloping, lower margin may reach to, or even be inferior to, the right iliac crest. The radio-opacity of the liver is largely responsible for the outline of the diaphragm on radiographs. The liver moves with respiration, and its position varies with the diaphragm and with body type. Hepatic tissue may be obtained for biopsy by puncture through a lower intercostal space.
Blood Supply (fig. 28-5).
The liver has a double blood supply: from the hepatic artery and from the portal vein. About 2/3 to 3/4 of the blood in the liver comes from the portal vein. However, because of higher oxygen tension in the hepatic artery, the artery supplies about 2/3 to 3/4 of the liver oxygen under normal circumstances. Despite this, the hepatic artery can usually be ligated without liver necrosis. Once the portal vein and hepatic artery blood has traversed the liver lobule via hepatic sinusoids, it is returned to the vena cava by the hepatic veins (at least 3: right, middle and left).
Subperitoneal networks drain to the internal thoracic nodes, and some vessels accompany the ligamentum teres to the umbilicus. Lymphatics accompany the blood vessels in the lesser omentum to reach the celiac nodes. Metastases can reach the liver from the thorax, breast, or any region drained by the portal vein.
The hepatic plexus is derived from the celiac plexus (see fig. 32-6). Pain fibers from the biliary passages are significant.
The biliary passages are important surgically because of the frequency of inflammation and of gallstones. The passages are supplied chiefly by the cystic artery (a branch of the right hepatic artery). Pain fibers are carried by the splanchnic nerves, and pain from distension or spasm may be severe.
The right and left hepatic ducts emerge from the liver and unite to form the common hepatic duct. This receives the cystic duct from the gallbladder and becomes the common bile duct (or choledochal duct; from Gk, chole, "bile"), which opens into the second part of the duodenum in common with or at least beside the pancreatic duct (fig. 28-6).
The gallbladder stores and concentrates bile. It lies in a fossa on the visceral surface of the liver, partially covered by peritoneum. The gallbladder has a fundus (at or inferior to the lower border of the liver), body, and neck, which continues as the cystic duct. The region of the neck is frequently "S" shaped and may present an abnormal pouch, and its mucosa presents spiral folds. The gallbladder lies between the liver, the first or second part (or both) of the duodenum, and the anterior abdominal wall. The gallbladder is located in the angle between the right costal margin and the linea semilunaris, usually on the transpyloric plane, but it may be as low as the iliac crest. The gallbladder can be made radio-opaque (cholecystography, fig. 28-7). Biliary constituents may crystalize and form gallstones (cholelithiasis).
The common bile duct runs in the free edge of the lesser omentum, then posterior to the first part of the duodenum and through (or at least enfolded by) the head of the pancreas, ending in the second part of the duodenum. As the bile duct traverses the duodenal wall, its wall acquires the choledochal sphincter (of Oddi) and its lumen becomes narrowed. It is usually united with the pancreatic duct, and the two ducts frequently empty into a common channel (hepatopancreatic ampulla), which enters the duodenum at the greater duodenal papilla (see fig. 28-6).
Pancreas (fig. 28-8)
The pancreas, an exocrine and endocrine gland, has a head, neck, body, and tail. The portal vein is formed posterior to the neck of the pancreas by the union of the superior mesenteric and splenic veins.
The head lies within the curve of the duodenum and usually surrounds the lower part of the bile duct. The head is prolonged inferiorly and to the left as the uncinate process, which hooks posterior to the superior mesenteric vessels. The body and tail extend to the left, across the vertebral column. The tail (within the lienorenal ligament) ends at the spleen. The body lies superior to the duodenojejunal flexure and presents anterior, inferior, and posterior surfaces.
The pancreas lies posterior to the stomach and anterior to the inferior vena cava, aorta, left crus of the diaphragm, left suprarenal gland, and left kidney (see figs. 26-6 and 29-2). The splenic artery runs tortuously along the superior border of the pancreas, whereas the splenic vein lies on the posterior surface of the organ.
Aberrant pancreatic tissue may be found in the stomach, small intestine, gallbladder, or spleen. Rarely, the pancreas is annular, and the ring may constrict or obstruct the duodenum.
The pancreas (except for its tail) is retroperitoneal. The root of the transverse mesocolon is attached to the anterior border of the body (see fig. 26-6), and the anterior surface of the organ is related to the lesser sac, whereas the inferior surface is associated with the greater sac (see fig. 26-4).
The pancreatic duct begins in the tail, runs to the right, and, meeting the bile duct, empties into the second part of the duodenum at the greater duodenal papilla. A part of the head is drained by an accessory pancreatic duct, which frequently empties separately into the duodenum about 2 cm more proxial at the lesser duodenal papilla (see fig. 28-6B).
The pancreas develops from the gut as dorsal and ventral diverticula that soon fuse, and their ductal systems also unite. The body of the adult organ is drained by the dorsal duct (the original termination of which may open on the lesser duodenal papilla), whereas the head is drained by both dorsal and ventral ducts (which, by their union, empty on the greater duodenal papilla).
The head is supplied by the anterior and posterior superior pancreaticoduodenal arteries (from the hepatic via the gastroduodenal artery) and by the anterior and posterior inferior pancreaticoduodenal arteries (from the superior mesenteric) (see fig. 27-4). The remainder of the organ is supplied by branches of the splenic artery. The lymphatic drainage is to the adjacent nodes. Pain fibers are carried by the splanchnic nerves.
The spleen (Gk, splen; L., lien; hence the adjectives splenic and lienal) is a soft, vascular organ surrounded by a fibrous capsule and part of the immune system. The spleen lies posterolaterally against the diaphragm and ribs 9 to 11 on the left-hand side of the body. Usually it is palpable only when enlarged.
It is convenient to speak of diaphragmatic and visceral surfaces, superior and inferior borders, and anterior and posterior ends of the spleen. The diaphragmatic surface is related to the costal part of the diaphragm. The visceral surface is related to the stomach, left colic flexure, and left kidney and presents a fissure, the hilus, for blood vessels. The superior border is notched.
Accessory splenic tissue may be found in any part of the abdominal cavity, but chiefly in the tail of the pancreas.
The spleen (except at the hilus) is covered by visceral peritoneum. It is connected to the stomach (gastrolienal, or gastrosplenic, ligament) and to the left kidney (phrenicolienal and lienorenal ligaments). The lienorenal ligament (see fig. 26-7) transmits the splenic vessels and contains the tail of the pancreas.
The spleen is supplied by the splenic artery, usually a branch of the celiac trunk. It runs tortuously to the left along the superior border of the pancreas (see fig. 27-4). The splenic vein runs from the spleen to the right on the posterior surface of the body of the pancreas and unites with the superior mesenteric vein to form the portal vein (fig. 28-8).
28-1 How is the liver divided into right and left anatomical lobes?
28-2 How is the liver divided into right and left functional lobes?
28-3 What is the ligamentum teres?
28-4 How is the liver maintained in position?
28-5 What and where is the "bare area" of the liver?
28-6 What is contained in the right free border of the lesser omentum?
28-7 What is the Greek word for bile?
28-8 Where is the gallbladder in terms of surface anatomy?
28-9 What are the main sphincters found in the biliary region?
28-10 Where is the portal vein formed?
28-11 What is an annular pancreas?
28-12 What is the developmental basis of the main and accessory pancreatic ducts?
28-13 On which side is the spleen?
28-14 Is the spleen palpable?
28-15 Where may accessory splenic tissue commonly be found?
Figure 28-1 Scintigram of the liver produced by the uptake of a radio-isotope. The fainter area at the right of the scan is the spleen. (From Deland, F. H., and Wagner, H. N., Atlas of Nuclear Medicine, vol. 3, W. B. Saunders Company, Philadelphia, 1972.)
Figure 28-2 The lines separating the lobes of the liver, projected on the visceral surface. line A separates the left and right anatomical lobes, the caudate (C.) and quadrate (Q.) lobes belonging to the right anatomical lobe. line F separates the left and right functional lobes. CP., caudate process.
Figure 28-3 The liver and its peritoneal relations. The stippled areas represent surfaces not covered by peritoneum. A, Anterior view. B, Superior view. C, The visceral surface of the liver, viewed from posteriorly.
Figure 28-4 The vascular segments of the liver from anterior and as projected onto the visceral surface. AI, anteroinferior; AS, anterosuperior; L, left; U, latero-inferior; LS, laterosuperior; MI, medio-inferior; MS, mediosuperior; PI, postero-inferior; PS, posterosuperior; R, right.
Figure 28-5 The blood supply of the liver and the portal circulation, projected onto the visceral surface. In prenatal life, oxygenated blood from the placenta is carried by the left umbilical vein and ductus venosus to the inferior vena cava and right atrium. Postnatally, part of this pathway becomes the ligamentum teres and the ligamentum venosum. The alimentary canal is supplied by blood from the aorta (Ao.) and is drained by the portal vein (P.). The portal blood and that from the hepatic artery (H.) provide a double blood supply to the liver. The blood is distributed by sinusoids within the liver and is drained by the hepatic veins into the inferior vena cava.
Figure 28-6 A, One pattern of the hepatic pedicle, showing structures important in cholecystectomy. B, An enlarged view to show the bile and pancreatic ducts, together with their sphincters (indicated in red).
Figure 28-7 Gallbladder. A, A cholecystogram showing the right, left, and common hepatic ducts, cystic duct, gallbladder (the fundus is at the level of L2 vertebra here), and bile duct. Ct. fig. 28-6. B, The position of the gallbladder is shown by radio-opaque, multifaceted gallstones. The fundus is at the level of the upper portion of the L4 vertebra here. (A, Courtesy of John Pepe, M.D., Brooklyn, New York.)
Figure 28-8 The pancreas and portal vein. A, Anterior view showing the head of the pancreas within the cavity of the duodenum. B, Posterior view showing the formation of the portal vein posterior to the neck of the pancreas. The pancreas develops by the fusion of a dorsal with a ventral component, hence the frequency of two ducts (main and accessory) that open into the duodenum. 1 to 4, parts of the duodenum; G.B., gallbladder.