This is the smallest celiac branch. The origin of the left gastric artery (LGA) may be in the aorta, close to the celiac trunk, or at the cranial aspect of the celiac trunk, all the way, from the ostium to the bifurcation creating a real trifurcation. It ascends cranially to the left, behind the oriental bursa, to the upper end of the stomach. After giving off distal esophageal branches and branches to the gastric fundus, the artery turns anteroinferiorly into the left gastropancreatic fold, running along the gastric lesser curvature, reaching the pylorus, supplying both gastric walls (anterior and posterior), and ends with an anastomoses with the right gastric artery (Figs. 18.16, 18.17). The left gastric artery may give origin to the left hepatic artery, or an accessory left gastric artery may arise from the left hepatic artery (Figs. 18.18, 18.19. 18.20, 18.21, 18.22). At the gastric fundus, there are anastomoses with the splenic artery through the short gastric arteries (Fig. 18.23).

In the adult, the hepatic artery is smaller than the splenic artery, but larger than the left gastric artery. It originates from the bifurcation of the celiac trunk, directed forward and to the right, toward the porta hepatis, where it divides in the right and left branches to the hepatic lobes (Fig. 18.10). The artery is subdivided in common hepatic artery from the celiac trunk to the origin of the gastroduodenal artery and hepatic artery proper. It is called the hepatic artery proper, from the gastroduodenal artery to the bifurcation into right and left hepatic branches (Fig. 18.12). The common hepatic artery may be extremely short or may not exist at all (Fig. 18.15). The proper hepatic artery may not exist at all (Fig. 18.11) and may be part of a trifurcation of the common hepatic artery. The hepatic artery may arise from the superior mesenteric artery (replaced hepatic artery). The right hepatic branch may arise from the superior mesenteric artery, whereas the left hepatic branch may originate from the left gastric artery. See Variations of the Hepatic Artery at the end of this section.

In the adult individual, the hepatic artery is intermediate in size between the left gastric and splenic arteries. However, in fetal life and early postnatal life it is the largest celiac branch. It has a forward direction and curves to the right, below the epiploic foramen to the upper aspect of the superior part of the duodenum. Crossing anteriorly the portal vein, and roughly parallel, but to the left of the common bile duct, it ascends between layers of the lesser omentum, anterior to the epiploic foramen, to the porta hepatis, where it divides into the right and left branches to the hepatic lobes, accompanying the ramifications of the portal vein and hepatic ducts, within the portal space. The segments of the hepatic artery have been described in the preceding text (Fig. 18.41). Several intra- and extrahepatic branches of the hepatic arteries are identified, including the right gastric artery, the cystic artery, rami to the bile ducts and connections with the supraduodenal artery, and retroduodenal artery.

Redman and Reuter, first demonstrated the existence of interlobar collateral vessels angiographically and described them as preexisting small arterial communications present in the hilum of most cases. Despite the description of interlobar communications by other authors only recently, Tohma et al. described systematically the presence of communicating arcades (CAs) between the right and left hepatic arteries in more than 85% of the cases and proposed a classification (Figs. 18.42, 18.43, 18.44). On the left side, the CAs originated from the segment IV artery (type 1) in 62% of the cases. The CAs originated from the left hepatic artery (type 2) in 38% of the cases (Fig. 18.44). On the right side, the CAs originated from the right anterior hepatic artery (type 1) in 46% of the cases and from the right hepatic artery (type 2) in 15% of the cases (Fig. 18.45). In 38% of the cases, the CAs originated from both the right hepatic artery and the right anterior hepatic artery (type 3). The CAs bifurcated off two branches to the caudate lobe in 18% of the cases and one branch in 45% of the cases. No caudate branches were observed in the remaining 36% of the cases. The CAs are located extrahepatically cranial to the portal vein bifurcation, close to the hilar bile duct (Fig. 18.46).

The biliary and hepatic systems have a complex embryologic origin, which causes a significant number of variations and adaptations to the flow patterns. As the embryologic duodenal diverticulum migrates toward the liver hilum carrying the extrahepatic bile ducts, it is supplied by multiple arteries to maintain adequate perfusion for the developing system. These vessels originate from the aorta, celiac trunk, and superior mesenteric artery. The formation of the intrahepatic bile ducts has been suggested, to be dependent upon contact between the developing embryonic liver mass and the preformed extrahepatic duct system, which carries the arteries with it. As the biliary system matures, many of the vessels are resorbed or incorporated into the main arterial bed. However, the resorption and incorporation is variable and multiple arterial patterns develop. Therefore, only 57% to 61% of the population has what is considered the “standard” hepatic arterial perfusion pattern. The presence of significant variations in the hepatic artery anatomic arterial perfusion creates in later life the so-called replaced arteries, which do not correspond to the classical anatomy and perfuse a complete vascular bed. An accessory artery is a secondary artery that contributes to the perfusion of the vascular arterial bed in addition to the conventional anatomic arteries (see “Variations of the Hepatic Artery” in subsequent text).

The falciform ligament divides the medial and lateral segments of the left lobe of the liver and connects the liver to the diaphragm and the supra-umbilical part of the anterior abdominal wall. Its free edge contains the ligamentum teres and the small para-umbilical veins, and the falciform artery. The falciform artery originates from the middle hepatic artery (when present) and the left hepatic artery. The angiographic incidence of the falciform artery varies from 2% to 25%, but in postmortem dissections, it is identified in as many as 67% of the cases (Fig. 18.47). The terminal branches of the falciform artery are connected to the network of the
phrenic arteries, internal mammary artery, and superior epigastric artery (Fig. 18.48).

The middle hepatic artery or segment IV artery is not considered an anatomic variation, although it may serve as an exclusive supplier for the segment IV of the liver. It may also be the origin of the falciform artery, the right gastric artery, and the accessory left gastric artery. It may arise from the proximal right hepatic artery or from the proper hepatic as an independent artery forming a trifurcation.

The hepatic arteries, particularly the left hepatic artery may develop free communications with the phrenic arterial system. Peripheral hepatic tumors, inflammatory processes of the subphrenic space and base of the lungs, may develop multiple connections, particularly from the bare area of the liver surface. Occasionally a phrenic artery arises directly from the left hepatic artery (Fig. 18.49).

This fissure commences at the right margin of the inferior vena cava and follows the attachment of the right superior coronary ligament to about 3 to 4 cm from the junction of the latter with the right inferior layer. The fissure then curves anteriorly to a point on the inferior margin about midway between the gallbladder fossa and the right margin of the liver. Passing posteriorly, the fissure follows a line that runs parallel to the gallbladder fossa and crosses the caudate process to reach the right side of the inferior vena cava. Lying almost in the coronal plane, the fissure contains the right hepatic vein, with branches passing anteriorly to segments V and VIII and posteriorly to segments VI and VII (Figs. 18.50, 18.51).

This fissure passes from the gallbladder fossa to the left margin of the inferior vena cava. Posteroinferiorly, this fissure is represented by a line from the gallbladder fossa to the main bifurcation of the hepatic pedicle (portal triad) and, then, to the retrohepatic inferior vena cava (Figs. 18.50, 18.51).

This fissure runs from the left side of the inferior vena cava to a point between the dorsal one third and ventral two thirds of the left margin of the liver; it divides the left liver into two sectors: anterior and posterior, separating the segments III and II. It is not the umbilical fissure. Inferiorly, the fissure passes to the commencement of the ligamentum venosum (Figs. 18.50, 18.51).

This fissure is marked superficially by the attachment of the falciform ligament, which contains the ligamentum teres hepatis in its inferior border. Angled less generously than the right fissure, it meets the inferior margin of the liver at an angle of about 50°.

The liver is divided into right and left lobes, separated by the main portal fissure or median fissure (projection of the path of the middle hepatic vein upon the liver surface). The portal fissure runs from the medial side of the inferior vena cava to the middle of the gallbladder bed. The right lobe is vascularized by the right hepatic artery and the left lobe is fed by the left hepatic artery. The hepatic lobes are divided in sectors. The right lobe has an anterior and a posterior sector (also called anteromedial and posterolateral), separated by the right portal fissure (projection of the path of the right hepatic vein upon the liver surface). The left lobe has a lateral (left) and a medial sector (right), separated by the falciform ligament and portoumbilical fissure (partial projection of the path of the involuted umbilical vein, called round ligament) (Figs 18.50, 18.51, 18.52).

The caudate lobe is an independent segment, supplied by the right and left hepatic artery and portal vein. It is called segment I and has venous drainage directly into the inferior vena cava. It is also called lobule of Spiegel. It is considered to be part of the left hepatic lobe (segment I of the left lobe) for surgical purposes. The caudate lobe is connected to the right lobe by a narrow bridge called the caudate process, behind the porta hepatis. Below and to the left, there is a small round appendage called the papillary process, which sometimes covers completely the inferior vena cava bridging the caudate lobe to the right liver lobe (Fig. 18.53A-D).

The left hepatic lobe is further subdivided into three segments. The lateral sector is divided into segment II and segment III by the left portal fissure (projection of the path of the left hepatic vein upon the surface of the left lobe of the liver). Segment II is posterior and superior, and segment III is inferior and anterior (Figs. 18.54, 18.55). The medial sector of the left lobe is segment IV. It has a cuneiform format, with the base turned anteriorly. It may be subdivided into a cranial and a caudal part, also called segments IVa and IVb. Segment IVa is also called the quadrate lobe of the liver (Fig. 18.56).

The right hepatic lobe is subdivided into four segments. The anterior sector is divided in segment VIII and segment V. Segment VIII is superior and segment V is inferior (Figs. 18.57, 18.58). The posterior sector is divided in segment VI and segment VII. Segment VI is inferior and segment VII is superior. The posterior sector is posterior and more lateral than the anterior sector (Figs. 18.59, 18.60).

The segments of the liver, either at the left or the right hepatic lobes, follow a clockwise distribution. The hepatic artery, the bile ducts, and the portal vein branches run in the center of the hepatic segments, whereas the hepatic veins run within the fissures between the segments. The fissures of the liver cannot be seen at the liver surface, except at the umbilical fissure, due to the presence of the falciform ligament and round ligament (ligamentum teres) (Figs. 18.50, 18.51, 18.52).

The classic hepatic lobules are polyhedral structures (hexagonal in histologic sections), about 1 mm in diameter, with a small central vein as a central axis, surrounded by portal triads (Fig. 18.61). Each triad contains a branch of the portal vein, a hepatic artery, a lymphatic vessel, and an interlobular biliary ductule. The portal triad is sheathed by connective tissue, called the portal canal or perivascular fibrous capsule, surrounded by the limiting plate, having in between the space of Mall. In humans, the idea of a functional unit has been proposed instead because the hepatic lobules are not readily perceptible. The functional unit is the portal lobule, consisting of parts of at least three neighboring classic lobules, bile from which drains into a biliary ductule in the portal canal between three such hepatic lobules. Again, the section shows a portal lobule as a polygonal area, centered on a portal triad, with boundaries passing through adjacent central veins. The concept of a portal acinus is more useful considering metabolic organization. The portal acinus is centered on a preterminal branch of a hepatic arteriole and includes the hepatic tissue served by this, and its boundaries are limited by the territory of other acini and by two adjacent central veins (Fig. 18.62). The acinus has been divided into three zones: zone 1 (periportal), zone 2, and zone 3 (close to the central venous drainage). Zone 3 (in the circulatory periphery and close to the central vein) suffers most from injury, developing bridging necrosis. Zone 1, close to the afferent vessels, survives longer and may trigger the regeneration of the liver (Figs. 18.61, 18.62, 18.63).

Portal areas, also called portal triads or portal canals, are located at the corners of the liver lobules. Portal spaces are surrounded by much larger areas with hepatic cords and sinusoids. Each portal space contains three (hence the term portal triad) more-or-less conspicuous tubular structures wrapped together in connective tissue. The tubular structures are branches of the bile duct, the portal vein, and the hepatic artery (Fig. 18.64). There are two less conspicuous structures within the portal space, a nerve and a lymphatic channel, but usually these are not apparent in routine specimens. The portal vein brings much more blood to the liver than does the hepatic artery; each branch of the portal vein is typically much larger than the associated branch of the hepatic artery. The relative sizes of the paired vessels in a portal area thus differ from those of a typical vein–artery pair in other parts of the body, where the artery delivers the same volume of blood that the vein subsequently returns. The portal vein branches, within the portal space arise at right angles to the main axis of the portal vein in a radial fashion. Direct connections with the sinusoids are provided by the inlet venules (Fig. 18.65). The artery and bile ducts occupy the periphery of the portal space, but the artery and the bile duct may be multiple. Between stroma and hepatocytes, there is a small space, called space of Mall—one of sites where lymph originates.

The hepatic artery ramifies parallel with the portal vein branches and bile ducts. Arterioles are released into the lobular parenchyma and terminate at different levels of the lobule, providing arterial blood flow to zone 1 of the acinus through anastomoses with the portal venule inlet (Fig. 18.66). Much like the bronchial arteries that provide bronchial circulation and pulmonary arterial anastomosis in the lungs, in the liver, the main circulatory arterial flow is provided to the periportal area by arterial branches directly to the peribiliary plexus and to the bile duct. Subsequently and to a much lesser extent, the artery provides nutrition to the acinus and to the liver parenchyma itself through the anastomosis of the arterioles with the portal inlet venule. In fact, due to importance of the hepatic artery to the bile ducts, the hepatic artery should be called biliary artery (Fig. 18.67).

According to Lunderquist, there are four types or levels of arterioportal communications: (1) the peribiliary plexus; (2) the terminal arterioloportal anastomosis connecting to the sinusoids; (3) the vasa vasorum on the wall of the portal vein; and (4) the direct arterioportal communications. The vasa vasorum of the wall of the portal vein is provided by direct small branches from the hepatic artery (Fig. 18.68). There are two concentric vascular layers within the wall of the larger bile duct, constituting the peribiliary plexus. The inner layer is a capillary plexus present in the submucosa, which drains into the outer adventitial venous plexus. The latter opens directly into the hepatic sinusoids via the lobular vein or into the portal vein via the interlobular vein (Figs. 18.69, 18.70). The intrahepatic bile duct is supplied by the hepatic arterial branches, which form the peribiliary plexus, whereas the extrahepatic bile duct receives its arterial nutrition from various sources, but most commonly from branches of the gastroduodenal artery.

According to Ekataksin, unlike the portal vein which exclusively supplies sinusoids, the hepatic artery supplies five compartments: (1) peribiliary plexus, (2) portal tract interstitium, (3) portal venous vasa vasorum, (4) fibrous capsule of Glisson, and (5) central sublobular-hepatic venous vasa vasorum, which subsequently either secondarily flows through the lobules or directly drains into the hepatic vein.

Within the portal tract, the arterial bed forms distinct collecting vessels, forming a portal system, the so-called hepatic artery-derived portal system, which anastomoses
with and/or follows or joins the vein–venule to open at the lobular periphery.

Outside the portal tract, the hepatic artery departs from the tract as an “isolated artery” directed toward the two compartments: the Glisson capsule and the central sublobular-hepatic venous vasa vasorum. The capsular arteriole that eventually drains into the local subcapsular lobules is likely to be misinterpreted as evidence of the hepatic artery being a primary feeder of sinusoids. The hepatic venous vasa vasorum that immediately drains into the respective hepatic vein without passing through lobular sinusoids is a hitherto unrecognized, but occasionally suspected, unusual pathway, a “bypass artery.” There are multiple peripheral communications between the hepatic artery and the portal vein radicles, especially in pathologic contexts, such as in liver cirrhosis, when the arterioportal shunts are widely open, contributing therefore to the increase in the portal vein pressure levels and promoting hepatofugal flow in the intrahepatic portal branches (Fig. 18.71). The cirrhotic nodules reduce significantly the number of sinusoids and perinodular shunts develop. The cirrhotic nodule develops a central feeding artery or group of feeding arteries, visible in a central bundle (Fig. 18.72).

To facilitate description, the biliary duct has been divided into three segments, namely hilar (right and left ducts), supraduodenal (common hepatic duct and upper common bile duct), and retropancreatic (lower common bile duct).

The arterial supply of the hilar bile duct (right and left) is provided by direct numerous small branches from the right and left hepatic arteries that form a rich network on the surface of the ducts in continuity with the plexus around the supraduodenal duct (Fig. 18.73).

The arterial supply of the supraduodenal bile duct is essentially axial, and most arteries arise from named arteries related to its upper and lower ends. The main vessels rim along the lateral aspects of the duct. According to Terblanche, these vessels are named 3 o’clock and 9 o’clock arteries and most commonly originate from the posterior pancreaticoduodenal arcade and gastroduodenal artery. Approximately 60% of the vessels supplying the supraduodenal duct run upward from vessels below, whereas 38% run downward from the right hepatic and other arteries. Only 2% of the supply is nonaxial, arising from the common or proper hepatic artery. The 3 o’clock and 9 o’clock arteries give branches to the duct forming the pericholedocal plexus (Figs. 18.35, 18.73, 18.74, 18.75, 18.76, 18.77).

An additional supply to the supraduodenal bile duct may be the retroportal artery, arising from the celiac trunk or superior mesenteric artery close to the origin of these vessels from the aorta. This artery courses to the right, behind the portal vein and to the back of the head of the pancreas, reaching the lower end of the supraduodenal duct. This artery may end, joining the posterior pancreaticoduodenal arcade, close to the distal supraduodenal duct, giving off small branches to the posterior surface of the duct (type I pattern). In one third of cases, the retroportal artery passes upward on the back of the supraduodenal duct, reaching the right hepatic artery (type II pattern). The retroportal artery along its route gives branches to join the pericholedocal plexus (Fig. 18.74).

The retropancreatic bile duct is supplied by direct small branches from the posterior pancreaticoduodenal artery or gastroduodenal artery, forming a mural plexus (Fig. 18.73). More recently (1999), Vellar described the presence of a 12 o’clock marginal artery in a smaller number of individuals, joining the plexus on the posterior surface of the common hepatic duct, in addition to the previously described 3 o’clock and 9 o’clock arteries.

The hilar plexus: The hilar plate is a condensation of connective tissue, which lines the hilum of the liver and forms a roof over the contents of the hilum, separating these structures from the hepatic substance. The right and left hepatic ducts have an intimate relation to the hilar plate. Bridging between the branches of the left hepatic artery and the right hepatic artery are a number of collateral vessels, which are found on the inferior surface of the hilar plate and which form the hilar plexus. The hilar arterial plexus is not only involved in the blood supply of the confluence of the bile ducts and the right and left hepatic ducts and, therefore, in communication with the arterial plexus over the biliary system, but it is the most important collateral between the right hepatic artery and its branches and the left hepatic artery and its branches, and the same communicating arcade described by Tohma et al. in 2005. Branches from the hilar plexus supply the caudate lobe and process. The caudate lobe is an important bridge for collateral blood flow between the right and left sides of the liver. It is suggested in the literature that the caudate lobe takes its blood supply not only from the segment I artery, but also from the communicating arcade. This may be the reason that transcatheter arterial chemoembolization is often not an especially effective treatment for hepatocellular carcinomas in the caudate lobe. According to that description the arterial branches from the hilar plate ascend posterior to the right and left hepatic ducts (Fig. 18.77). It is supposed that the communicating artery may exist as a marginal artery for the hilar bile duct and connect the parabiliary arteries to form the parabiliary arterial network that supplies the biliary tract, as well as the 3 o’clock and 9 o’clock arteries (from the porterior-superior pancreaticoduodenal arcade), which exist as marginal arteries for the common bile duct.

Type 1 is a replacement of the common hepatic artery, arising from the superior mesenteric artery. The replaced common hepatic artery passes through or behind the head of the pancreas (Figs. 18.81, 18.82). In 10% of the population, the hepatic artery is replaced; that is, a portion of the hepatic blood supply is from the superior mesenteric artery (SMA). The entire hepatic arterial supply is from the SMA in 2.5% of the cases and from the aorta in 2% of individuals.

Type 2 is early bifurcation of a short common hepatic artery in right and left hepatic arteries. The right and left hepatic arteries may originate separately from the celiac trunk. The gastroduodenal artery arises from the right hepatic artery (Fig. 18.81).

Type 3 is a replaced right hepatic artery that originates from the SMA and the left hepatic artery taking origin from the celiac trunk (Figs. 18.81, 18.83). This is found in 15% of the population.

Type 4 is a replaced left hepatic artery, which is a tributary of the left gastric artery, whereas the right hepatic artery originates from the celiac trunk (Figs. 18.20, 18.21, 18.22, 18.81). It runs within the ligamentum venosum and frequently sends smaller branches to the stomach and esophagus. In 12% to 23% of individuals either a branch of the left hepatic artery or the entire left hepatic artery arises from the left gastric artery.

The right and left hepatic arteries arise from the celiac trunk, but there is an accessory right hepatic artery from the SMA. The accessory right hepatic artery is in general the first branch of the SMA, and almost always originates the main cystic or accessory cystic artery. The accessory passes through or behind the head of the pancreas (Fig. 18.81). It occurs in 10% to 31% of the population, with 96% of the cases originating from the SMA and 4% from the pancreaticoduodenal trunk. Rare instances of origin from the phrenic artery or gastroduodenal artery have been encountered.

Type 6 is an accessory left hepatic artery arising from the left gastric artery (Fig. 18.81). Found in 8% of the population.

Type 7 is an accessory left hepatic artery arising from the right hepatic artery (Fig. 18.81).

Right hepatic artery passes anteriorly to the common hepatic bile duct, rather than posteriorly (Fig. 18.81).

The head of the pancreas is encircled by two arterial arcades: the posterior (retroduodenal artery) and anterior
pancreaticoduodenal arcades, branches of the gastroduodenal artery. The posterior arcade is proximal on the gastroduodenal artery, whereas the anterior arcade is a terminal branch of the gastroduodenal artery, together with the right gastroepiploic artery. The anterior pancreaticoduodenal arcade joins the posterior arcade, behind the head of the pancreas, forming a common trunk called inferior pancreaticoduodenal artery (or trunk), and ending in an anastomosis with the SMA or the first jejunal artery. In a number of cases there is an intermediary pancreaticoduodenal arcade, and in some cases it is not a real arcade but a terminal artery to the pancreatic head (Fig. 18.26, 18.27, 18.28, 18.29, 18.30)

The main vessel to the neck and proximal body of the pancreas is the dorsal pancreatic artery (Figs. 18.24, 18.87) (also called superior pancreatic artery, colli, suprema, propria, media, isthmi, or dorsalis, and less correctly the arteria pancreatica magna). The dorsal pancreatic artery arises most commonly from the initial splenic artery but also frequently from the celiac trunk bifurcation (Fig. 18.88) or from the common hepatic artery (Fig. 18.89). Less commonly, it originates from the SMA or from a branch of the SMA (Figs. 18.25, 18.90, 18.91). The dorsal pancreatic artery is commonly connected, in the right aspect, with the anterior pancreaticoduodenal arcade or other gastroduodenal artery branch by an anastomotic branch (prepancreatic arcade) and originates the transverse pancreatic artery in the left aspect (arteria transversa pancreatis), which crosses all along the pancreatic body and tail (Figs. 18.87, 18.88). The dorsal pancreatic artery may give rise to omental branches and also an actual or accessory middle colic artery (Fig. 18.92).

The body of the pancreas is supplied by the arteria pancreatica magna (arteria corporis pancreatica or great pancreatic artery) (Fig. 18.93). The arteria pancreatica magna may be single, but it is usually a group of comb-shaped branches, perpendicular to the splenic artery. It anastomoses with the transverse pancreatic artery.

The tail of the pancreas is supplied by multiple branches originated from the splenic artery, right gastroepiploic artery or splenic branches (Fig. 18.94). These branches anastomose with the transverse pancreatic artery and branches of the arteria pancreatica magna.

The short gastric arteries (rami gastrici breves) arise from the splenic artery, divisional branches, polar arteries for the spleen and splenic parenchyma (Figs. 18.23, 18.31). They supply the cranial part of the gastric greater curvature (Fig. 18.17). It varies in number from one to four. Up to nine arteries may be found. Anastomosis occurs with other gastric branches.

The posterior gastric artery arises from the splenic artery and supplies the posterior fundic portion of the stomach, passing through the gastrosplenic ligament. It may provide splenic branches for the upper pole of the spleen.

This artery is also called the left gastro-omental artery. It is most commonly a branch of the splenic artery and arises as a trunk together with the left gastroepiploic artery and the inferior splenic branch. It may, however, arise from the splenic artery isolated. It reaches the greater gastric curvature in the middle and eventually anastomoses with the right gastroepiploic artery (Fig. 18.95). The left omental (epiploic) artery is a major branch of the left gastroepiploic artery and forms the omental arcade in a significant number of cases. Other branches are the omental (or epiploic) arteries and the posterior and anterior short gastric arteries along the gastric body and antrum (Figs. 18.23, 18.31, 18.86).

The splenic branches enter the hilum after division of the main splenic artery in five or more branches (Figs. 18.96, 18.97). The splenic arterial circulation consists of multiple separate segments, with adjacent compartments without arterial interconnections. Within the spleen, the finest arteriolar branches pass out of the trabeculae, their adventitia being replaced by a periarteriolar sheath. These sheaths constitute the white pulp with splenic lymphatic follicles. The arterioles divide into a series of straight vessels termed penicillary arterioles. These pass through the marginal zones of the white pulp, developing the ellipsoids by thickening of the sheath by aggregation of macrophages and fibroblasts. Beyond the ellipsoids, each vessel continues as a fine arteriole or divides in two. Eventually the blood passes into the red pulp and the venous sinusoids, venules, and small veins. The small veins form the larger veins and exit through the hilum.

In more than 80% of cases, there are only two segments, the superior and the inferior. The superior segment is larger and heavier than the inferior segment in more than 65% of cases. In fewer cases, there are three and four splenic segments (superior, intermediate, and inferior).