Recall from The peritoneal cavity and mesenteries that the peritoneal cavity is divided into supracolic and infracolic regions, using the transverse colon and its mesentery (transverse mesocolon) as the borderline. In Organs in the supracolic region, the supracolic organs were described. In this chapter we discuss organs in the infracolic region = the small and large intestines. We will use the same approach as in the previous chapter by first providing an overview of blood supply, lymphatics, and innervation to the region, then discussing details of the intestines themselves. Note from Ariane: we had chapter 15 broken into two last year (Overview of GI Tract; Supracolic organs), so it’s not the same approach. Let me know how you’d like to edit this!
Overview of blood supply, lymphatics, and innervation
Abdominal esophagus
About an inch long, it enters the abdomen through the esophageal hiatus in the diaphragm. It curves to the left to join the cardia of the stomach. The anterior and posterior vagal trunks are on its anterior and posterior surfaces, respectively. The anterior vagal trunk is composed mainly of fibers from the left vagus nerve; the posterior vagal trunk mainly from right vagal fibers.
Dual: Left gastric vein drains to the portal vein; esophageal veins drain upwardsto the azygos vein. This dual venous drainage is important to consider in the case of portal hypertension (see Infracolic organs: the intestines).
Vagus nerves (parasympathetic—peristalsis, glandular secretion), greater splanchnic nerves (sympathetic—blood vessel constriction, relaxation of muscle). Visceral afferent fibers carrying pain follow the greater splanchnic nerves with cell bodies in dorsal root ganglia T-6 to T-9.
Clinical correlation
The esophageal-gastric junction is marked internally by a wavy line—the so-called “esophageal Z-line.” This can be visualized with endoscopy (image below). It signifies the junction of squamous epithelium of the esophagus with columnar epithelium of the stomach.
Figure 1. Normal esophageal-gastric “Z-line.” The pale tissue is esophagus; the salmon colored region below it is stomach. GRAY’S ATLAS OF ANATOMY, 2ND ED.
Compression of the esophagus by the diaphragm as it passes through the esophageal hiatus is thought to act as a physiological inferior esophageal sphincter. Swallowed foodstuffs pause here briefly before entering the stomach. Failure of this mechanism permits reflux of gastric contents into the lower esophagus (known clinically as GERD (gastro-esophageal reflux disease). Some patients with chronic GERD have columnar cells replacing squamous cells above the normal location of the Z-line. This condition characterized by segments of the distal esophagus being lined by columnar epithelium is called Barrett’s esophagus—it is associated with a higher risk of developing esophageal cancer.
Stomach
This is the expanded portion of the GI tract between the inferior esophageal sphincter above (a physiological sphincter—not anatomical) and the pyloric sphincter distally. Its function is to store and blend (via peristalsis) ingested foods and liquids with stomach secretions, producing a thick liquid called chyme that is transferred in aliquots to the duodenum.
Figure 2. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 4.32.
The cardia is the region that receives the esophagus. The sharp indentation above this junction is the cardiac notch.
The fundus is the rounded part that rises above the cardiac notch. It is in contact with the diaphragm.
Below the level of the cardiac notch, the fundus is continuous with the body of the stomach—the largest part of the organ.
The pyloric part is the distal portion that connects to the duodenum. It has several named subparts. The most distal segment is known simply as the pylorus—it contains a thick ring of smooth muscle called the pyloric sphincter. This regulates the amount and consistency of chyme that is passed on to the duodenum from the stomach.
Figure 3. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 4.32.
An empty stomach has flattened anterior and posterior surfaces, separated by two curvatures. On the right side is the lesser curvature, connecting to the right side of the esophagus and superior border of the duodenum. An indentation in the lesser curvature, the angular incisure, indicates the junction of the body and pyloric part. The greater curvature begins above at the cardiac notch, curves to the left, and ends below and on the right where it merges with the inferior border of the duodenum.
The stomach is located mainly in the left upper quadrant of the abdomen under cover of the rib cage and behind the left lobe of the liver.
The fundus is separated from the left pleural cavity by the diaphragm. A prominent air bubble in a patient’s fundus is often seen superimposed on the left dome of the diaphragm in a chest X-ray.
The spleen is behind the fundus and greater curvature.
The anterior and posterior surfaces are covered with peritoneum. The lesser omentum attaches to the lesser curvature while the greater omentum (gastrocolic and gastrosplenic ligaments) hangs from the greater curvature. The posterior surface of the stomach faces into the lesser sac.
The classic cartoon of the stomach portrays it as J-shaped—its shape, however, can vary. The only two fixed parts of the stomach are the cardia and pylorus. In between these, a normal stomach’s contour depends on age, size of surrounding organs, and body type (habitus) of the individual. Stout individuals with a shorter thorax tend to have higher, more horizontal stomachs (“steer horn” stomach—do you see why?), while those with slender builds have lower, more vertical stomach shapes (See Anatomical classification of the shape and topography of the stomach). Congenital anomalies and pathologic conditions that affect nerve and muscle tone can also alter the shape.
In an empty stomach, the mucosa is raised up into sinuous ridges called gastric folds (gastric rugae). Gastric folds smooth out and become obliterated as the stomach fills and becomes distended.
Via extensive anastomoses between branches of the celiac trunk. The left and right gastric arteries anastomose along the left curvature. Left and right gastro-omental arteries join along the greater curvature. Short gastric arteries branch from the splenic artery and supply the fundus.
Left and right gastric veins are tributaries of the portal vein. The left gastro-omental vein drains to the splenic vein. The right gastro-omental vein usually empties into the superior mesenteric vein. Ultimately all venous blood enters the portal vein.
Lymphatic drainage of the stomach follows the general plan laid out in Overview of GI tract vessels, lymphatics, and nerves: Two sets of nodes filter the lymph = peripheral nodes near the organ receive lymph first, then central nodes near the aorta. Peripheral nodes are situated near the wall of the stomach along the lesser and greater curvatures. Efferent lymph vessels from these nodes transmit the lymph to celiac nodes along the aorta. Celiac nodes drain to intestinal lymph trunks, which merge with the cisterna chyli.
Nerve fibers reach the stomach via the celiac plexus.
Parasympathetic: Branches of anterior and posterior vagal trunks.
Sympathetic: Via greater splanchnic nerves that relay on celiac ganglia.
Visceral afferent fibers transmitting pain travel in greater splanchnic nerves—cell bodies are in T-6 to T-9 dorsal root ganglia (where is pain from the stomach referred to?).
Duodenum
Figure 4. GILROY, ATLAS OF ANATOMY (THIEME MEDICAL PUBLISHERS), 3RD ED., FIGURE 15.8.
The duodenum is the first part of the small intestine, distal to the pyloric sphincter. It is so named because ancient anatomists measured it using the width of a finger and found its length to be twelve finger widths (duodenum: Latin = “twelve”)—about 25 cm long (10 inches). It is the shortest and widest part of the small intestine. The duodenum is an important digestive organ because into it pour pancreatic enzymes (from the main pancreatic duct) and bile (from the common bile duct), which are mixed with its own secretions (intestinal juice) to aid in digestion of foodstuffs within the chyme that has been delivered to the duodenum from the stomach through the pyloric sphincter.
The duodenum is mainly on the right side of the body and is sharply curved (C-shaped), with its concavity aimed to the left. Nestled within its concavity is the head of the pancreas.
Figure 5. Parts of the duodenum. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
The duodenum has four named parts—these are given both names and numbers. From proximal to distal these are:
Crosses left to right over L-1 vertebra and passes upwards slightly after leaving the pylorus. The first few centimeters of part one is referred to by clinicians as the duodenal ampulla or “duodenal cap,” because it fits like a cap over the pylorus. Part one is connected above to the liver by the hepatoduodenal ligament (part of the lesser omentum). Part 1 is therefore intraperitoneal. The anterior surface of part one is often in contact with the gallbladder—thus it maybe stained with green bile in the cadaver. Posterior to part one is the portal vein, common bile duct, and gastroduodenal artery.
Descends along the right side of the first three lumbar vertebrae. Behind it is the hilum of the right kidney. The head of the pancreas abuts its medial border. Parts 2, 3, and 4 of the duodenum areretroperitoneal. Part 2 is important,since in the mucosa of its medial wall is a nipple-shaped elevation called the major duodenal papilla. The papilla is raised up by a muscular chamber embedded in the medial wall of the descending duodenum = the hepatopancreatic ampulla (ampulla of Vater). The common bile duct and main pancreatic duct meet at the ampulla—thus its chamber is filled with both bile and pancreatic enzymes. These enter the duodenum through an opening at the tip of the major duodenal papilla. If the pancreas has an accessory duct, it opens atop a minor duodenal papilla, located superior to the major papilla. Both papillae can be difficult to see in the cadaver if they are obscured by the duodenum’s mucosal circular folds.
Crosses L-3 vertebra and the IVC and aorta, from right to left. The superior mesenteric artery and vein are draped over its anteriorsurface. Thus, part three is sandwiched between vessels—aorta and IVC behind, superior mesenteric vessels in front.
Turns upward from part 3 in front of the aorta. Part 4 is very short; it quickly turns sharply downward at the duodenojejunal flexure to become the jejunum.
The sharp bend at the flexure is thought to be attributed to the attachment of the Ligament of Treitz (suspensory muscle of duodenum) to part 4 of the duodenum. This structure (described by most sources as containing both striated and smooth muscle) arises from the posterior abdominal wall near the celiac trunk and passes behind the pancreas and splenic vein to the superior border of the ascending duodenum. Like the mythical Sasquatch, the ligament of Treitz is often described, but seldom seen in cadaver dissections or imaging of living patients. It is thought to have embryological significance as a retaining band for midgut rotation and clinical significance for surgeons who use it as a landmark (e.g., pancreaticoduodenectomy = “Whipple procedure”).
Clinical correlation
Peptic ulcers are sores that develop in the inner lining of the stomach or duodenum due to mucosal injury. There are many causes, such as bacterial infection (H. pylori), overuse of nonsteroidal anti-inflammatory drugs (NSAIDs), or excessive acid production in the stomach. Peptic ulcers that develop in the duodenum almost always occur in the superior part (duodenal cap).
Clinical correlation
The superior mesenteric artery (SMA) normally branches away from the abdominal aorta at a 45 degree angle across the horizontal part of the duodenum. If this angle is reduced severely, the artery may compress the duodenum, producing a rare condition called superior mesenteric artery syndrome. This can cause distention of the proximal duodenum (and stomach upstream from it) with pain, nausea, and vomiting. The most common cause of SMA syndrome is drastic weight loss, which depletes the mesenteric fat around the SMA that normally supports it.
Since the primary function of the small intestine is absorption, it has many features that produce a huge surface area—several of which are visible grossly:
Ridges called circular folds (plicae circulares) spiral around the interior. Unlike gastric folds in the stomach, circular folds are permanent. These are most prominent in the proximal small intestine,especially in the duodenum.
Intestinal villi are tiny finger-like projections onthe circular folds—these give the mucosa of thesmall intestine a velvety texture (can be seen inthe gross lab).
Arteries to the duodenum are derived from both the celiac trunk and superior mesenteric artery, mainly via an anastomotic network (arcade) of four arteries: the pancreaticoduodenal arteries. The superior pancreaticoduodenal arteries (anterior and posterior) are branches of the gastroduodenal artery—they supply the duodenum proximal to the major duodenal papilla. The inferior pancreaticoduodenal arteries (anterior and posterior) are usually the first branches off the superior mesenteric artery—they supply the duodenum distal to the major duodenal papilla. The basis for this division of blood supply is embryology (of course)! The major duodenal papilla indicates the junction of the foregut and midgut.
A venous network of pancreaticoduodenal veins drains the duodenum and head of the pancreas. These veins drain to both the portal vein directly and to the superior mesenteric vein.
Lymphatic vessels follow the arteries. Lymph first percolates through peripheral nodes near the duodenal wall and pylorus of stomach. From these nodes, efferent vessels transmit lymph to central nodes: celiac nodes above and superior mesenteric nodes below.
Nerve fibers travel in the celiac and superior mesenteric plexuses.
Parasympathetic: Via the vagi. What is the effect of this innervation on the duodenum?
Sympathetic: From greater and lesser splanchnic nerves. Preganglionic fibers synapse in celiac and superior mesenteric ganglia. What are their functions?
Visceral afferent fibers associated with pain travel in greater and lesser splanchnic nerves. Their cells bodies are probably in T-6 to T-10 dorsal root ganglia.
Pancreas
The pancreas is both an exocrine and endocrine gland, functioning in digestion and regulation of blood sugar. It has a lumpy, lobulated texture typical of glandular organs.
Exocrine cells
Endocrine cells
Exocrine cells make up the bulk of the pancreas and these release enzymes (actually “pro-enzymes”) that aid in the digestion of food. Pro-enzymes secreted by exocrine cells are released into a series of ducts within the pancreas, ultimately reaching the lumen of the second part of the duodenum via the main pancreatic duct. Once in the duodenum, pro-enzymes are converted into their active form. The major pancreatic enzymes are proteases, amylase, and pancreatic lipase.
Endocrine cells are grouped into clusters of cells (“islands”) scattered about in the exocrine pancreas called pancreatic islets (islets of Langerhans). Endocrine cells don’t release their products into ducts—instead, they release hormones (mainly insulin and glucagon) directly into blood vessels. These hormones function to control sugar (glucose) levels in the blood.
Figure 6. ELSON S. FLOYD COLLEGE OF MEDICINE.
Being retroperitoneal, the pancreas is glued to the posterior wall of the abdominal cavity. It is posterior to the stomach and lesser sac and conforms to the shape of the structures behind it—curving in front of the vertebral column, aorta, inferior vena cava, and kidneys. The pancreas has a thin, flffy, arched profile when viewed in horizontal section (such as an axial CT scan—see right). The pancreas is about six inches long and passes from right to left across the first two lumbar vertebrae, stretching to the left as far as the spleen.
Figure 7. NETTER, ATLAS OF HUMAN ANATOMY, 7TH ED., PLATE 288.
The pancreas resembles a tadpole—it is widest on the right and tapers down to a thin apex at its left extremity. It has five named parts:
Head: The widest part, it fits snuggly within the concavity created by the four parts of the duodenum.
Neck: Somewhat narrowed part to the left of the head. The superior mesenteric vessels descend posterior to the neck, causing a constriction in the pancreas here.
Body: Continues to the left, becoming progressively narrower as it does so. The left kidney is behind the body.
Tail: Tapered “tip” of the pancreas that usually touches the spleen.
Uncinate process: Technically part of the head, it extends away from the lower part of the head,hooking back behind the neck of the pancreas toward the midline of the body. The superior mesenteric vessels lie on the anterior surface of the uncinate process—thus, they sandwiched between pancreatic tissue: behind the neck above and in front of the uncinate below.
The celiac trunk sprouts from the aorta just above the pancreas. The common hepatic artery passes to the right along the top of the pancreas head. The tortuous splenic artery passes to the left along the superior border of the pancreas body. Parts of the splenic artery are often embedded in pancreatic tissue.
Clinical correlation Because of its deep retroperitoneal location behind other organs, the pancreas is not accessible to direct physical examination. Tumors are rarely palpable, and this is why symptoms of pancreatic cancer do not appear until tumors have grown large enough to impact the function of the pancreas or nearby organs.
Figure 8. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 4.37.
The exocrine secretions of the pancreas are transmitted through a series of ducts that ultimately drain into the second part of the duodenum through two terminal ducts: the main and accessory pancreatic ducts. This reflects the embryology of the pancreas, as it arises from two buds of endoderm, each originally having an independent duct connecting to the duodenum. Fusion of the ducts during development produces the typical configuration:
The dominant main pancreatic duct drains the tail, body, and inferior part of the head and uncinate process—it joins the common bile duct in the hepatopancreatic ampulla and drains into the duodenum via an opening at the top of the major duodenal papilla.
An accessory pancreatic duct drains the superior part of the head—it opens into the duodenum atop a minor duodenal papilla, located above the major papilla.
Figure 9. Extensive blood supply of pancreas and duodenum. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 4.39.
Due to its endocrine function, the pancreas has a rich blood supply.
The head and uncinate process share blood supply with the duodenum, via a rich anastomosis of arteries: the superior pancreaticoduodenal arteries (anterior and posterior) given off from the gastroduodenal artery (a branch of the celiac trunk) and the inferior pancreaticoduodenal arteries (anterior and posterior) from the superior mesenteric artery.
The rest of the pancreas (excluding the head) is supplied by numerous large pancreatic branches of the splenic artery.
Complex. Lymphatic vessels tend to follow the blood vessels. Lymph from the pancreas ultimately filters through up to three sets of the nodes: celiac, superior mesenteric, and a few lumbar lymph nodes(presumably due to the pancreas being closely associated with the posterior body wall).
Clinical correlation The head of the pancreas is the most common site of pancreatic adenocarcinoma. Tumors here can compress the common bile duct, causing jaundice. Inaccessibility to physical examination and complex lymphatic drainage producing variations in metastatic patterns are factors that may lead to late discovery of pancreatic cancer and poor prognosis (five-year survival rates are very low).
Autonomic fibers reach the pancreas through the celiac and superior mesenteric plexuses.
Sympathetic fibers (from greater and lesser splanchnic nerves) probably only function in vasoconstriction of pancreatic blood vessels.
Parasympathetic fibers from the vagus act on pancreatic exocrine and endocrine cells, increasing the secretion of pancreatic proenzymes and insulin.
Visceral affferents from the pancreas enter the spinal cord at T-6 to T-9 levels via greater splanchnic nerves.
Clinical correlation Pain from the pancreas therefore should project to the epigastric region of the abdomen where these dermatoms are located. However, pain from the pancreas can be more widespread, often reported by patients as back pain in the lower thoracic and lumbar regions. This is probably due to irritation of nearby somatic sensory nerves in the posterior abdominal wall.
Liver
The liver is the body’s largest internal organ and largest gland. It is said to account for 2.5% of body weight in the adult, and about twice that in an infant. It has hundreds of functions, a few of which are:
Metabolism of nutrients (carbohydrates, proteins, and fats)
Glycogen storage and adjustment of blood glucose levels
Detoxification of wastes in the blood (from bothendogenous and exogenous sources)
Synthesis of blood plasma proteins
Synthesis and secretion of bile
Production of blood cells (hematopoiesis) during fetal life
Figure 10. Surface projection of liver. Most of the liver is protected by the ribs and costal cartilages. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
The liver is wedge-shaped: taller on the right and tapered down to an apex on the left. Its shape is maintained by a fibrous capsule that completely envelops the organ, which in turn is covered (but not entirely) by visceral peritoneum.
Most of the liver is deep to the ribs and costal cartilages where it is protected (it is a highly vascular organ and can be ruptured). It is molded to the undersurface of the diaphragm, such that its upper surface is shaped like a dome. When projecting the liver to the body’s surface to determine its position, it useful to think of its wedge shape as having upper and lower margins:
Upper margin
On the right, it rises as high as the 5th rib in the supine position.
In the midline, it crosses the junction of the xiphoid process and body of the sternum.
On the left, its superior extent is below the apex of the heart, at about the sixth rib.
Lower margin
On the right, it is just below the costal margin.
In the midline, it crosses the epigastric region (where it may be felt in thin individuals).
On the left. it disappears under the left costal cartilages.
The liver moves with the diaphragm, so it is more accessible to palpation when the diaphragm contracts and descends.
Clinical correlation
A percutaneous (“through the skin”) liver biopsy can be used to help diagnose or confirm liver disease. Knowledge of the liver’s position is vital to the success of this procedure.
Figure 11. Surface location for needle entry: percutaneous liver biopsy. GRAY’S ATLAS OF ANATOMY, 2ND ED.
This is most commonly done with ultrasound guidance. Once the liver’s location is mapped on to the body wall, the point of entry of the biopsy needle depends on the liver’s size. In a classic“transpleural” approach, the needle enters an intercostal space (6th to 9th) along the midaxillary or anterior axillary line, then passes through the pleural cavity and diaphragm (with the patient’s breath held in expiration to protect the lung) to obtain the sample from the liver.
Surfaces
The liver has two named surfaces and one named border.
The diaphragmatic surface is convex and curved to fit under the diaphragm. Peritoneal ligaments (falciform ligament, coronary ligament) attach this surface to the ventral body wall and diaphragm.The inferior vena cava emerges above from the diaphragmatic surface through a deep fissure.
The flat visceral surface faces inferior and posterior. It has a prominent fossa for the gallbladder and several deep fissures that contain blood vessels, bile ducts, and vestigial ligaments (all described later). The visceral surface is in contact with the stomach, duodenum, and right colic (hepatic) flexure of the large intestine. It also relates to the right kidney, separated from it by the hepatorenal recess of the peritoneal cavity. Remember this?
The sharp inferior border separates the diaphragmatic and visceral surfaces on the anterior side of the liver. As mentioned earlier, it can be palpated by a skilled examiner just below the right costal margin.
Figure 15.12. Diaphragmatic surface of liver. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 4.50.
Figure 13. Visceral surface of liver. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
More on the liver’s visceral surface
Two vertical fissures and one horizontal fissure on the visceral surface are arranged like the letter H.
The vertical fissures are the left and right sagittal fissures:
Left sagittal fissure
The left sagittal fissure contains two vestigial ligaments = remnants of fetal development. The inferior part of the left fissure contains the round ligament of the liver (the fibrous remnant of the fetal umbilical vein), while the upper part of the fissure contains the ligamentum venosum (remnant of the fetal ductus venosus, which shunted much of the blood from the umbilical vein around the fetal liver and in to the inferior vena cava).
Right sagittal fissure
The right sagittal fissure is formed by the fossa for the gallbladder and the groove containing the inferior vena cava.
The horizontal fissure on the visceral surface of the liver (the “crossbar” of the letter H) is the deep portahepatis. Much like a hilum in other organs (such as kidneys and spleen), the porta hepatis is the port of entry or exit from the liver. It contains:
Left and right branches of the portal vein
Left and right branches of the hepatic artery proper
Left and right hepatic bile ducts—these merge just outside the porta hepatis to form the common hepatic duct.
Lymphatic vessels and autonomic nerve fibers in the hepatic plexus also traverse the porta hepatis.
Lobes
Anatomists describe four lobes of the liver. These are external landmarks and do not reflect the internal architecture of the liver, which is determined by the branching of bile ducts and blood vessels.
Right lobe
The largest lobe. It is the part of the liver to the right of the gallbladder (on the visceral surface) and the falciform ligament on the diaphragmatic surface.
Left lobe
To the left of the left sagittal fissure and falciform ligament.
Quadrate lobe
Visible only on the visceral surface, it is located between the fossa for the gallbladder, the fissure containing the round ligament, and the porta hepatis.
Caudate lobe (presumably named because it resembles a tail hanging from the liver)
Seen on the visceral surface. Located between the IVC, fissure for the ligamentum venosum, and porta hepatis.
Functional lobes
The liver has two “functional” lobes—actually, two separate functional livers (hemi-livers).
Surgeons of yore who attempted to divide the liver using the falciform ligament as a landmark encountered massive bleeding. Later, anatomists determined that there was a line (Cantlie’s line) extending vertically through the right sagittal fissure along which the right and left hepatic bile ducts as well as the right and left branches of the portal vein and hepatic artery do not cross. Therefore, the liver can be divided into left and right portions along this line with reduced bleeding (the hepatic veins—described soon—do, however, cross this line).
Left functional liver
The left functional liver is to the left of a plane through the right sagittal fissure (includes the quadrate and caudate lobes). Bile is drained by the left hepatic duct and blood supply is via the left branches of the portal vein and hepatic artery proper. The left functional liver is much larger in area than the left anatomic lobe, described earlier.
Right functional liver
The right functional liver is to the right of a vertical plane passed through the liver along the right sagittal fissure (gallbladder and IVC). Bile is drained by the right hepatic duct. Blood supply is via the right branches of the portal vein and hepatic artery proper. Do you see that this functional liver is not quite the same as the right anatomic lobe, described earlier?
Clinical correlation
Cantile's line, extending through the fossae for the gallbladder and IVC (right sagittal fissure), is the plan used by surgeons who perform hepatectomies to divide the liver in two (hemi-livers). Surgical removal of a hemi-liver or even sub-segments of the hemi-liver is done primarily to manage liver tumors.
Each of the two functional livers is further divided into four segments = these are areas of liver tissue supplied by secondary branches of the portal vein and hepatic artery proper. Therefore, based on blood supply, the liver contains eight hepatic segments—these are said to be surgically resectable.
(Also described in
The peritoneal cavity and mesenteries.) So named because of its “sickle” shape (has a curved lower margin, when viewed laterally), it is a double layer of peritoneum that connects the diaphragmatic surface of the liver to the internal surface of the anterior abdominal wall along its midline. It has a curved inferior free edge where the two layers of peritoneum (left and right) merge. Contained within the two layers of this margin is the cord-like round ligament of the liver—the vestigial umbilical vein—that passed from the umbilicus to the liver, supported by the falciform ligament. The falciform ligament develops from the ventral mesentery of the foregut—the lower part of the embryonic septum transversum.
Coronary ligament (anterior and posterior layers)
Attaches the diaphragmatic surface of the liver to the diaphragm above. Presumably named because its layers form a “crown” atop the liver. The anterior layer is formed when the left and right layers of the falciform ligament diverge and pass upwards to attach to the diaphragm. The posterior layer is formed when the visceral peritoneum on the posterior side of the liver’s dome reflects upwards on to the diaphragm. The anterior and posterior layers on the left side of the liver are closely approximated. On the right side, the anterior and posterior layers are separated such that a fair amount of the liver’s diaphragmatic surface is devoid of peritoneum. This part of the liver is called its bare area. It is in contact with the undersurface of the diaphragm.
Triangular ligaments (left and right)
Formed where the anterior and posterior layers of the coronary ligaments join laterally.
Left and right portions of the posterior coronary ligament
The left and right portions of the posterior coronary ligament continue downward on the visceral surface of the lever, attaching along the margins of the fissure for the ligamentum venosum and the porta hepatis. As these layers leave the liver below, they form the lesser omentum (attaching below to the duodenum and stomach). The lesser omentum was described earlier.
Portal triad
Figure 14. Portal triad in cut-away view of hepatoduodenal ligament. NETTER, ATLAS OF HUMAN ANATOMY, PLATE 285.
The portal triad is the trio of structures within the hepatoduodenal ligament that supply blood to and drain bile from the liver:
Hepatic artery proper on the left
Common bile duct on the right
Portal vein posterior, behind both the hepatic artery and bile duct.
Figure 15.15. Formation of portal vein. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
The liver is unique in receiving a double-blood supply: from an artery and a vein!
Arterial supply to the liver is via the hepatic artery proper, a branch of the celiac trunk. 20% of blood entering the liver through the porta hepatis is from the hepatic artery proper. The hepatic artery proper ascends to the liver in the hepatoduodenal ligament (part of the lesser omentum, don’t you know!). Below the porta hepatis, the hepatic artery proper divides into left and right hepatic arteries.
The left hepatic artery enters the porta hepatis, supplying the left functional liver (hemi-liver)
The right hepatic artery passes behind the common bile duct to enter the porta hepatis. It supplies the right functional liver. The right hepatic artery usually gives off a branch to the gallbladder (cystic artery) before entering the liver.
Variations in hepatic arteries are extremely common. Studies have reported up to 30% of bodies with variations. Aberrant hepatic arteries are arteries that enter the liver but are not derived from the hepatic artery proper. For example, aberrant left hepatic arteries have been reported to branch from the left gastric artery, while aberrant right hepatic arteries can branch from the superior mesenteric artery or directly from the aorta.
Figure 15.16. Blood vessels and bile ducts of liver entering/leaving porta hepatis. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 4.57.
The hepatic portal vein carries 80% of the blood entering the liver through the porta hepatis. The portal vein is formed by the union of the splenic and superior mesenteric veins, behind the pancreas. It receives blood from the entire GI tract (except the liver) and from the spleen. This blood is rich in nutrients and vitamins absorbed from the small intestine (and waste products and toxins too). The portal vein ascends in the hepatoduodenal ligament behind the hepatic artery proper.
Below the porta hepatis the portal vein divides into left and right branches, which supply the two hemi-livers (here is one rare case where students are allowed to say that a vein has branches!).
Within the liver, blood from the hepatic artery proper and portal vein mixes within channels called hepatic sinusoids. Here, blood comes into proximity to the amazing cells of the liver’s parenchyma = the hepatocytes. These are the chief functional cells of the liver that perform an astonishing array of duties (metabolic, endocrine, and secretory).
After percolating through the hepatic sinusoids, blood leaves the liver through three hepatic veins.
Note to students: do not confuse the hepatic veins with the hepatic portal vein—they are different structures!
Figure 17. Blood supply and venous drainage of liver. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
The hepatic veins (left, right, and middle) are formed from smaller veins that arise between the hepatic segments (mentioned earlier). These veins in turn are formed by the union of the central veins of the liver (described with the histology of the liver). The three hepatic veins are tributaries of the inferior vena cava—they enter the IVC as it passes through its groove on the back side of the liver, just below the diaphragm.
The liver produces lots of lymph—sources report that 25% or more of the lymph entering the thoracic duct comes from the liver. Lymph vessels are arranged in superficial and deep networks. These vessels drain in two directions (but ultimately all lymph enters the thoracic duct):
Lymph from the upper liver drains toward the bare area and IVC, upwards into posterior mediastinal nodes above the diaphragm.
Lymph from the lower liver drains toward the porta hepatis, then into hepatic nodes just below it. From here, lymph flows to celiac nodes.
Clinical correlation Due to embryology and the resulting close relationship between the liver and the diaphragm, lymphatic drainage of the upper liver travels upwards into nodes within the mediastinum.
Autonomic nerve fibers reach the liver via the hepatic plexus, given off from the celiac plexus. Sympathetic fibers are from the greater splanchnic nerves while parasympathetic fibers are from the vagi.
Sympathetic fibers cause constriction of liver blood vessels. They may also stimulate glucose release from the liver and inhibit glycogen storage.
Visceral afferent fibers from the liver enter T-6 through T-9 levels of the spinal cord (via greater splanchnic nerves), with cell bodies in the corresponding dorsal root ganglia. Some affrent fibers from the upper liver may be part of the phrenic nerves. Pain from liver inflammation therefore may be referred to the epigastric region of the abdomen and also to the shoulder region.
Bile is a yellowish-green fluid continuously produced by the liver. It contains water, electrolytes, bile acids, cholesterol, phospholipids, and bilirubin. It functions to emulsify ingested fats that enter the small intestine—breaking down fats into smaller droplets that can be more easily digested by lipase enzymes.
Bile is manufactured by liver hepatocytes and secreted into tiny passageways adjacent to liver sinusoids called bile canaliculi. Canaliculi join to produce larger and larger ductules that ultimately form left and right hepatic ducts. These leave the liver’s porta hepatis and fuse to form the common hepatic duct.
About one inch below the porta hepatis, the common hepatic duct joins the cystic duct (the duct of the gallbladder) to form the common bile duct (also called simply the “bile duct”). The common bile duct is described in more detail shortly, after we discuss the gallbladder and cystic duct.
The gallbladder is a pear-shaped pouch that stores and concentrates bile, by absorbing water from it. Bile is not manufactured in the gallbladder! It is fused to a depression on the visceral surface of the liver. The gallbladder is surrounded by visceral peritoneum, except on the surface where it fuses to the liver. It has these named parts:
The fundus is the distal, expanded, blind-ended portion that peeks out below the inferior border of the liver. It touches the anterior body wall just below the right ninth costal cartilage, approximately at the midclavicular line.
The body is the central part of the organ. It tapers down to the neck, which bends downward away from the porta hepatis to become continuous with the cystic duct.
The cystic duct is several inches long. It passes downward from the neck, fusing with the common hepatic duct below the porta hepatis. Internally, the cystic duct has a raised ridge of mucosa that spirals around the duct like the threads of a screw. This spiral fold is thought to keep the cystic duct patent so that bile can pass through it.
Keep in mind
Bile flows through the cystic duct in two directions:
Inward when the gallbladder fills with bile
Outward when the gallbladder contracts to eject bile.
Clinical correlation
Anatomists and surgeons love geometrical shapes, don't they? The cystic and common hepatic ducts (and their union) form two sides of a triangle, with the third side being the liver itself. This is the cystohepatic triangle (triangle of Calot)—an important region for surgeons performing a gallbladder removal (cholecystectomy).
Figure 19. Cystohepatic triangle (of Calot)—the most common location for the right hepatic artery. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 4.62.
Within this triangular region, the surgeon can usually locate the right hepatic artery and the cystic artery branching from it—the latter needs to be ligated and divided in order to remove the gallbladder.
Figure 20. Bile duct, main pancreatic duct, and hepatopancreatic ampulla. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
The common bile duct (a.k.a. bile duct; choledochal duct) carries bile from both the liver and gallbladder. Formed from the union of cystic and common hepatic ducts, it descends vertically within the hepatoduodenal ligament, to the right of the hepatic artery proper. Below this, it descends posterior to the first part of the duodenum with the gastroduodenal artery, then passes behind the head of the pancreas. It becomes embedded in the posterior pancreas as it joins the main pancreatic duct. Their union forms a hollow muscular chamber called the hepatopancreatic ampulla (ampulla of Vater). Bile and pancreatic enzymes mix in the ampulla.
The hepatopancreatic ampulla is embedded in the medial wall of the second (descending) part of the duodenum. The presence of the ampulla here causes a bulge in the mucosa of the duodenum (visible in the lab when the descending duodenum is opened) = the major duodenal papilla. Bile and pancreatic enzymes enter the duodenum through an opening at tip of the ampulla internally.
The hepatopancreatic ampulla, as well as the terminal parts of the common bile duct and main pancreatic ducts, are surrounded by smooth muscle sphincters (a total of 3 sphincters). The sphincter of the ampulla is known to clinicians as the sphincter of Oddi. Closure of this sphincter or the sphincter of the common bile duct (choledochal sphincter) causes bile to back up into the cystic duct and fill the gallbladder. All the sphincters are closed until gastric contents enter the duodenum.
When this happens, the hormone cholecystokinin (released from mucosal cells in the first part of the duodenum—and from neurons in the intestinal wall) leaps into action, producing these effects:
Relaxes the three sphincters described above, allowing bile and pancreatic enzymes to flow into the duodenum
Causes smooth muscle in the gallbladder wall to contract, expelling bile into the cystic duct
Clinical correlation
Removal of the gallbladder and ligation of the cystic duct doesn't mean bile won't enter the duodenum anymore. The liver continues to produce it, and it flows down the common hepatic and common bile ducts, but does not enter the gallbladder anymore, so the bile will be more dilute. The remaining ducts will dilate and enlarge a bit to act as a storage reservoir for bile between meals.
Gallstones (cholelithiasis) are solid masses that develop in the gallbladder or bile ducts when normal substances in the bile precipitate out of the liquid, forming crystals. Causes may include too much cholesterol or bilirubin in the bile,too little bile acids, or problems with emptying of the gallbladder, leading to stasis in bile flow. Yellowish gallstones are made from cholesterol, dark gallstones contain bilirubin. Large gallstones can block the cystic ductor ducts downstream, even the hepatopancreatic ampulla. Blocked ducts cause smooth muscle spasm and excruciating pain. Patients with symptoms due to gallstones often have their gallbladders removed.
Chronic obstruction of bile ducts (due to gallstones or compression from nearby tumors) causes bile to back up into the gallbladder and liver, with its pigments being absorbed into the bloodstream. This produces jaundice—yellow discoloration of the skin, mucous membranes, and “whites” of the eyes (scleras) due to excess bilirubin in the blood. As mentioned previously, pancreatic cancer most often affects the head of the pancreas. Its tumors can block the common bile duct here and produce jaundice.
The cystic artery (a branch of the right hepatic artery) supplies the gallbladder, cystic duct, and proximal part of the common bile duct. The distal common bile duct is supplied by the gastroduodenal artery.
Autonomic nerves reach the gallbladder and bile ducts via the hepatic plexus (from the celiac plexus).
Sympathetic fibers are from the greater splanchnic nerves—these cause vasoconstriction.
Parasympathetic fibers are from the vagi. These stimulate contraction of the gallbladder and relaxation of sphincters (sphincter of Oddi and choledochal sphincter).
Visceral afferent nerve fibers enter T-6 to T-9 levels of the spinal cord and some enter the right phrenic nerve. Pain from the gallbladder is referred to the epigastric region of the abdomen and to the right shoulder region. An inflamed gallbladder that touches the body wall may irritate the parietal peritoneum and cause pain directly opposite the fundus of the gallbladder in the right chest wall near the lower costal cartilages.
Spleen
The spleen is not an organ of the GI tract, but we discuss it here since we will encounter it in the supracolic region, it shares blood supply with the other organs in the region, and it is a clinically important structure!
The spleen has several functions:
It removes old blood cells from the circulation as well as blood-borne micro- organisms and cell debris (functions performed internally by regions of the spleen called red pulp).
It contains abundant lymphoid tissue that functions in adaptive immune responses against antigens circulating in the blood (a function that occurs internally in regions called white pulp).
Figure 21. GILROY, ATLAS OF HUMAN ANATOMY, 3RD EDITION, FIGURE 15.32.
The spleen is an oblong organ, shaped like a football with one side flattened, or somewhat like the curved bell of a jellyfish if you prefer. It has a deep red/purple color due to the amount of blood and iron (from breakdown of red blood cells) it contains.
Figure 22. Location of spleen. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
In the left upper quadrant of the abdomen behind the stomach, between it and the diaphragm. When opening the abdomen of the cadaver, the spleen is hidden by the stomach and greater omentum. The spleen is peritoneal, being connected to both the gastrosplenic and splenorenal ligaments. This is a consequence of the spleen developing from a mass of splanchnic mesoderm within the dorsal mesentery of the stomach.
The normal spleen is under the left dome of the diaphragm, entirely protected by the lower rib cage. It is oriented obliquely, paralleling the 10th rib, spanning between ribs 9 and 11. Spleen size varies with age and sex. In a study of college athletes, the mean spleen size was 10 cm long by 5 cm wide, a bit larger than a bar of soap. (Reference: Ultrasound assessment of spleen size in collegiate athletes.) A healthy spleen normally cannot be palpated. It can enlarge up to 10 times its normal size with disease (splenomegaly).
Figure 23. Parts of spleen. GRAY’S ATLAS OF ANATOMY, 2ND EDITION.
The spleen is a soft organ whose shape conforms to the organs around it. It has a convex diaphragmatic surface that faces posterolateral and a flat visceral surface that faces anteromedial against the stomach, tail of the pancreas, and left kidney.
The surfaces are separated by superior and inferior borders—the superior border is normally indented with several notches. The hilum is the fossa on the visceral surface where vessels and nerves enter and leave the spleen. The hilum is situated between the attachments of the gastrosplenic and splenorenal ligaments. The tail of the pancreas often touches the hilum of the spleen. The fibrous capsule surrounding the spleen is fairly weak, allowing the spleen to expand, but also making it vulnerable to rupture and heavy bleeding. Visceral peritoneum surrounds the spleen’s capsule, except at the hilum.
About 10% of individuals may have a small accessory spleen (or spleens). If present, they are usually located near the splenic hilum.
The spleen is highly vascular, receiving its blood from the large splenic artery. Upon reaching the spleen’s hilum in the splenorenal ligament, the splenic artery divides into several terminal branches that enter the spleen’s visceral surface. The short gastric arteries (to the fundus of the stomach) and the left gastro-omental artery (to the greater curvature of the stomach) arise from these terminal branches prior to them penetrating the spleen.
The splenic vein leaves the spleen through the hilum, receives the short gastric veins and left gastro-omental vein, passes through the splenorenal ligament, then becomes retroperitoneal. It courses below the splenic artery and behind the pancreas, rather than along its superior border.Behind the neck of the pancreas the splenic vein joins the superior mesenteric vein to form the portal vein.
Efferent lymph vessels leave the hilum and follow the splenic blood vessels toward the midline of the abdomen, passing first through nodes along the superior border of the pancreas, then through the celiac nodes.
A splenic plexus of nerves (derived from the celiac plexus) follows the vessels to the spleen There is debate on whether the spleen is supplied with both sympathetic and parasympathetic fibers and what their functions might be. Most sources state the spleen only receives sympathetic fibers and these probably supply vessels to regulate blood flow, although there is interesting research suggesting sympathetic innervation may regulate lymphocyte proliferation and circulation and cytokine production, which are important for resistance to disease. (Reference: Innervation of the Human Thymus and Spleen: An Overview.)
An enlarged spleen may refer pain to the epigastric region (T-6 to T-9 dermatomes—because it is a foregut organ). Pain can also be referred to the left shoulder (via the left phrenic nerve) if an enlarged spleen or ruptured spleen irritates the diaphragm (Kehr’s sign).
Clinical correlation
Splenomegaly can make the spleen more vulnerable to injury since enlargement will cause it to bulge below the costal margin, where it is no longer protected by the rib cage. The normal spleen must enlarge 2 to 3 times in size to become palpable in such cases.
Rupture of the spleen can also occur in normal spleens, usually due to fracture of ribs on the left side of the body or penetrating trauma. Because of its vascularity and weak capsule, a ruptured spleen bleeds profusely into the peritoneal cavity, usually necessitating removal of the damaged organ (splenectomy). In these cases, the liver or bone marrow takes over some of the functions of the spleen, although patients without a spleen may be more susceptible to bacterial infections.
