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©2014 ASRT. All rights reserved. Sectional Anatomy Essentials: Module 7

Sectional Anatomy Essentials – Module 7: The Abdomen

1. The Thorax Welcome to Module 7 of Sectional Anatomy Essentials – The Abdomen. This module was written by Michael A. Manders, BS, R.R.A., R.T. (R), and Jeffrey D. Houston, MD..

2. License Agreement and Disclaimer

3. Objectives After completing this module, you will be able to:

Distinguish the anatomical surface landmarks and regions of the abdomen.

Explain the Addison planes.

Locate the vertebral structures of the abdomen.

Identify the major muscles of the abdomen, locate their insertion points and describe their function.

Locate and identify the lobes of the liver.

Describe the biliary system.

Explain the location and general function of the stomach, gall bladder, pancreas, spleen, adrenal glands and kidneys.

Name and explain the significance of the peritoneal and retroperitoneal spaces.

Locate each anatomical structure on computed tomography (CT), magnetic resonance (MR), and ultrasound images in the transverse axial, coronal, sagittal and orthogonal (oblique) cross-sectional imaging planes.

4. Introduction Throughout the Sectional Anatomy Essentials series, we display most of the cross-sectional anatomy using multidetector computed tomography (CT) and magnetic resonance (MR) images to illustrate the three-dimensional relationship of the structures. Use the slider bar in this animation to scroll through the images. Because you can easily lose your frame of reference when viewing cross-sectional images, the location of the featured slice on many slides will be displayed on adjacent localizer images of the other 2 planes, like the image shown here. Click on the next button when you are ready to proceed.

5. Surface Lines As we begin our discussion of the abdomen, it’s important to mention surface lines. These arbitrary lines are used to reference the location of the different structures inside the abdomen. They include the midsternal line, the bilateral midclavicular lines, the umbilical line, the transpyloric line and the transtubercular line. Surface lines are sometimes used to describe the location of lesions or masses in the abdomen, particularly for surgical planning. In future slides involving the various organs of the abdomen, we’ll refer to surface lines as they correlate to the location of the organs.

6. Surface Lines The midsternal line, or midsagittal line, evenly divides the body into right and left halves. The vertical line that runs through the midpoint of each clavicle is called the midclavicular line, or mammary line. The umbilical line extends horizontally through the umbilicus, or belly button. The transpyloric line extends horizontally at a level midway between the jugular notch of the


sternum and the superior border of the symphysis pubis, the lowest portion of the pelvis. The transpyloric line transects the pylorus of the stomach, costal cartilages of the ninth ribs and the first lumbar vertebra. The transtubercular line extends horizontally through approximately the level of the fifth vertebral body.

7. The 4-Region Method There are 2 accepted ways to divide the abdomen, the 4-region and the 9-region methods, each using different surface lines. The 4-region method uses the midsternal line and the umbilical line to divide the abdomen and pelvis into 4 quadrants: the right upper quadrant, the left upper quadrant, the left lower quadrant and the right lower quadrant. It’s important to remember that each person is a little different internally, so the following descriptions of what organs are typically located in each region may not be exactly the same for everyone.

8. The 4-Region Method The right upper quadrant typically contains the liver, gallbladder, biliary tree, duodenal portion of the small intestine, head of the pancreas, right kidney, right adrenal gland and the hepatic flexure of the colon. The left upper quadrant usually contains the stomach, spleen, left lobe of the liver, body of the pancreas, left kidney, left adrenal gland, the splenic flexure and portions of the transverse and descending colon. The lower quadrants typically include structures in the pelvis, which are discussed in another module.

9. The 9-Region Method The 9-region method uses 4 lines, 2 vertical and 2 horizontal, to divide the inferior chest, abdomen and pelvis into 9 regions. Also known as Addison planes, these regions are useful in clinical medicine because specific disease processes can present as pain and tenderness in specific areas. A clinician who is familiar with these clinical presentations can sometimes spare a patient from the radiation dose and cost associated with diagnostic imaging, particularly computed tomography. In this module, we’ll only focus on the 6 superior regions: the right hypochondric, epigastric, left hypochrondric, left lumbar, umbilicus and right lumbar regions.

10. Hypochondrium and Epigastric Regions The hypochondrium, literally meaning “under cartilage,” represents the upper abdomen. It is surrounded by the inferior chest wall, inferiorly to the level of the transpyloric line. Each hypochondric area extends medially from the lateral chest wall to the inferomedial border of the costal cartilage. The right hypochondric region contains a portion of the liver and the gallbladder while the left hypochondric region contains the spleen as well as portions of the stomach and liver. The region located between the hypochondria is called the epigastric region. The epigastric region typically contains portions of the stomach and liver.

11. Lumbar and Umbilical Regions The right and left lumbar regions are found directly inferior to the right and left hypochondrium regions. They extend medially from the lateral abdominal wall and are bordered superiorly by the transpyloric line and inferiorly by the transtubercular line. Organs located in the right lumbar region include the inferior portion of the liver, the gallbladder, portions of the small intestine and ascending colon, and portions of the right kidney. The left lumbar region contains portions of the small intestine and descending colon, and portions of the left kidney. Located between the right and left lumbar regions is the umbilical


region. The umbilical region typically contains portions of the stomach, the pancreas, portions of the small intestine and the majority of the transverse colon, as well as portions of the right and left kidneys and ureters.

12. Lumbar Spine The skeletal portion of the abdomen consists of the lumbar spine, which typically has 5 lumbar vertebrae. The lumbar vertebrae differ from cervical vertebrae in that they don’t have transverse foramina, and unlike the thoracic vertebrae, they don’t have facets to accommodate the ribs. The lumbar vertebrae have the largest vertebral bodies, gradually increasing in size from superior to inferior, with L5 being the largest vertebral body of the movable portion of the spine. The full weight of the body is transferred from L5 to S1.

13. Muscles of the Posterior Abdomen Of the muscles in the posterior abdomen responsible for movement and stabilization of the spine, we’ll focus on 2: the quadratus lumborum muscles and the psoas muscles.

14. Quadratus Lumborum Muscles The quadratus lumborum muscles are paired quadrilateral-shaped muscles that arise bilaterally from medial portions of the iliac crests and extend superiorly, inserting into the inferior portion of the most inferior rib. The quadratus lumborum also inserts by 4 small tendons into the tips of the most superior 4 transverse processes of the lumbar spine. These muscles are responsible for lateral flexion of the lumbar spine, extension of the lumbar spine, elevating the ilium with ipsilateral contraction and fixating the twelfth rib during forced expiration.

15. Psoas Muscles The psoas muscles are long fusiform muscles located on either side of the lumbar spine. Each psoas muscle is typically divided into deep and superficial portions. The deep portion of the muscle originates from the transverse processes of the 5 lumbar vertebrae; each superficial portion arises from the lateral borders of the twelfth thoracic vertebra and first through fourth lumbar vertebrae, as well as the intervertebral discs. Each psoas muscle joins the ipsilateral iliacus muscle to form the iliopsoas muscle, which then inserts into the lesser trochanter of the femur. The psoas muscles are responsible for lateral flexion of the lumbar spine. In combination with the iliacus muscles, they play a part in the movement of the hip joint.

16. Muscles of the Anterior Abdomen Four pairs of primary muscles make up the anterior abdomen. From superficial to deep, these muscles are rectus abdominis, external oblique, internal oblique and transversus abdominis.

17. Rectus Abdominis Muscles The rectus abdominis muscles are the most superficial of the abdominal muscles and make up the middle anterior abdominal wall. They are divided vertically down the middle by a large tendinous sheath called the linea alba. Three, or in some cases 4, other sheaths, called the tendinous intersections, run horizontally. These intersections form the “6-pack” or “8-pack” look in very fit individuals. The rectus abdominis muscle extends from the inferior pelvis to the fifth


through seventh ribs. It plays roles in flexing the lumbar spine and drawing the ribs to the pelvis, as well as forceful expiration.

18. External Oblique Muscles The external oblique muscles are found bilaterally deep to the rectus abdominis and make up a portion of the anterolateral abdominal wall. Each external oblique muscle arises from the outer inferior borders of the fifth through twelfth ribs. The muscle runs anteroinferiorly toward midline before attaching to the anterior iliac crest and abdominal aponeurosis, a thin tendinous-like sheath, connecting the muscle to the linea alba. In addition to compressing the abdomen, the external oblique muscles are responsible for flexion of the lumbar spine and lateral flexion of the torso.

19. Internal Oblique Muscles The paired internal oblique muscles are found bilaterally, deep to the external oblique muscles. Like the external obliques, they make up a portion of the anterolateral abdominal wall. Each internal oblique muscle arises bilaterally from the anterior iliac crest, the inguinal ligament and the thoracolumbar fascia. The muscle extends anterosuperiorly toward midline before attaching to the costal cartilages of the eighth through twelfth ribs as well as the abdominal aponeurosis, which connects the muscle to the linea alba. The internal oblique muscles are responsible for anterior flexion, lateral flexion and rotation of the lumbar spine, as well as abdominal compression.

20. Transversus Abdominis Muscle The deepest muscles of the anterior abdomen are the paired transversus abdominis muscles, which make up a portion of the anterolateral abdominal wall. Each transversus abdominis muscle extends horizontally, originating from the anterior iliac crest, the lateral half of the inguinal ligament, the thoracolumbar fascia and the costal cartilages of the sixth and twelfth ribs. It attaches to the xiphoid process, the abdominal aponeurosis and the pubic symphysis, which is located in the inferior pelvis. The transversus abdominis muscles are responsible for compressing the abdomen.

21. Abdominal Cavity Deep to the abdominal wall, we find the abdominal cavity, which is located inferior to the thoracic cavity and superior to the pelvic cavity. The abdominal cavity contains the abdominal viscera, otherwise known as the abdominal organs. Dividing the abdominal cavity from the thoracic cavity is the diaphragm.

22. Diaphragm The diaphragm is a thin, sheet-like muscle that extends across the bottom of the rib cage. It originates circumferentially from the costal cartilages and the lumbar vertebrae and inserts into the central tendon of the diaphragm. Small bilateral tendinous structures extend inferiorly from the diaphragm and tether the muscle to the spine to allow for efficient contraction. When the diaphragm contracts, it increases the thoracic volume, allowing air to be pulled into the lungs and making the diaphragm the primary muscle of inspiration. The diaphragm contains


a few hiatuses, or openings, that allow vessels and organs to pass into the abdominal cavity. The 3 large openings are the aortic hiatus, the caval hiatus and the esophageal hiatus.

23. Aortic Hiatus The aortic hiatus is the most inferior of the large openings in the diaphragm and is located at about the level of the T12 vertebral body. Although called a diaphragmatic hiatus, it actually is an opening between the diaphragm and the vertebral column rather than an opening in the diaphragm itself. It allows for the passage of the aorta, the thoracic duct and the azygos vein into the abdominal cavity.

24. Caval Hiatus The caval hiatus is located at about the level of the eighth thoracic vertebral body and transects the central tendon of the diaphragm. This opening allows for the passage of the inferior vena cava, which is responsible for returning deoxygenated blood to the heart. Given the location of the caval hiatus in the central tendon, some believe the hiatus stretches open during inspiration to allow more blood to return to the heart during the decrease in thoracic pressure associated with inspiration. However, some also argue that the caval hiatus actually constricts during inspiration.

25. Esophageal Hiatus The esophageal hiatus is located at about the level of the tenth thoracic vertebral body anterior to the aortic hiatus and to the left of the caval hiatus. The esophageal hiatus allows the esophagus and vagus nerve to pass into the abdominal cavity.

26. Knowledge Check Answer the following question.



29. Peritoneum The peritoneum lines the inside of the abdominal cavity and surrounds the majority of the abdominal viscera. The peritoneum is a large sheet-like serous membrane made up of a layer of mesothelium supported by a layer of connective tissue. Although the peritoneum is technically a single continuous sheet, there are 2 types: parietal and visceral.

30. Parietal and Visceral Peritoneum The parietal peritoneum is the part of the membrane that lines the abdominal and pelvic cavities. The visceral peritoneum is the part that covers most of the abdominal organs, or viscera. You may be wondering how a single continuous membrane can cover both the cavities and the abdominal organs. The easiest way to understand this concept is to think of the parietal peritoneum as an underinflated balloon, although keep in mind that the abdomen isn’t filled with air. If you push your fist into the side of an underinflated balloon, the balloon covers as much of your fist that is


pushed into it. If you advance your fist far enough into the balloon, the balloon eventually covers your wrist. The balloon covering your fist represents the visceral peritoneum.

31. Parietal and Visceral Peritoneum Let’s continue with the comparison. Assume that instead of a fist and wrist, the balloon covers an organ and its blood supply. As the balloon, or peritoneum, covers the blood supply, or wrist, it forms 2 layers with the blood supply between those layers. These double-layered areas are known as the mesentery. Although some linguists argue that mesentery specifically means intestinal, it is typically a generic term used to describe a double-layer extension from any viscera. Sometimes these double-layer extensions are also called peritoneal reflections.

32. Greater and Lesser Omentum Next, let’s talk about 2 specific peritoneal folds: the greater omentum and the lesser omentum. Like the mesentery, the omenta are doubled-layered pockets extending from the lesser and greater curvatures of the stomach. The greater omentum extends inferiorly from the greater curvature of the stomach, passing anterior to the small intestine, before turning superiorly, enveloping the transverse colon and reaching the posterior abdominal wall. The lesser omentum extends inferiorly from the liver to the lesser curvature of the stomach.

33. Peritoneal Spaces The peritoneal reflections create spaces where free intraperitoneal fluid, typically a sign of pathology, can accumulate. These spaces are called the peritoneal spaces. The specific space in which this fluid accumulates can be a clue as to the type of pathology affecting the patient. In the abdomen, the peritoneal spaces can be grouped into the supracolic compartment, the infracolic compartment and the pouch of Morison.

34. Supracolic Compartment The supracolic compartment encompasses the right and left subphrenic and subhepatic spaces. The right and left subphrenic spaces are located between the superior border of the liver and the diaphragm; they are divided by the falciform ligament, a peritoneal reflection that attaches the liver to the anterior abdominal wall. The right and left subhepatic spaces are located along the inferior border of the liver. The right subhepatic space, also known as the pouch of Morison, is located between the right lobe of the liver and the right kidney. The pouch of Morison is important clinically as it is a location where free intraperitoneal fluid, such as ascites or blood from abdominal trauma, will collect. The left subhepatic space is located between the left lobe of the liver and the stomach.

35. Infracolic Compartment The infracolic compartment consists of the right and left paracolic gutters. The right paracolic gutter is located between the ascending colon and the right abdominal wall. The left paracolic gutter is located between the descending colon and the left abdominal wall.

36. Retroperitoneum While the peritoneum covers the majority of the abdominal cavity, there is an area behind the peritoneum called the retroperitoneum. The retroperitoneum contains multiple organs and


vessels we’ll discuss later. Like the peritoneum, the retroperitoneum has some spaces of its own, including the pararenal and perirenal spaces.

37. Renal Fascia Dividing the retroperitoneum is the renal fascia, which is connective tissue that encircles the kidneys. The renal fascia can be further separated into anterior and posterior layers. As their names imply, the anterior layer, commonly known as the fascia of Gerota extends anterior to the kidneys, and the posterior layer, commonly known as the fascia of Zuckerkandl, extends posterior to the kidneys. The area between the 2 layers of fascia is referred to as the perirenal space and contains the kidney and a small amount of surrounding fat. The area anterior to the fascia of Gerota is called the anterior pararenal space, and the area posterior to the fascia of Zuckerkandl is called the posterior pararenal space. The anterior and posterior pararenal spaces typically contain only fat.

38. Viscera Next, we’ll cover the viscera, or organs, of the abdominal cavity. The abdomen contains many of the viscera of the human body. Digestive system organs include the stomach, small intestine, large intestine, liver, gallbladder and biliary tree. The pancreas and adrenal glands are part of the endocrine system. The abdomen contains the spleen and multiple lymph nodes, which are lymphatic system organs. Finally, the kidneys and ureters belong to the excretory system.

39. Stomach The first organ we’ll discuss is the stomach. The stomach is responsible for breaking down food into chyme, which is partially digested food. It is located in the left upper quadrant of the abdominal cavity, directly inferior to the diaphragm. In individuals of average body habitus, or body type, the stomach generally has a curved shape similar to a kidney bean, sloping downward, medially and anteriorly. The extent of this curve changes in individuals with different body habitus. For example, someone with a large, or hyperstenic, body habitus tends to have a more horizontal stomach position whereas people with a very slender, or asthenic, body habitus tend to have a more vertical stomach, sometimes extending into the pelvis.

40. Gastroesophageal Junction and Cardiac Orifice The first structure of the stomach that we’ll mention is the gastroesophageal junction. This intersection is where the tissue of the esophagus stops and the tissue of the stomach begins. Just distal to this point is the cardiac orifice. The esophagus empties food into the stomach through this opening. It is located along the upper inner border of the stomach, an area known as the cardia.

41. Stomach Divisions The stomach is routinely divided into 4 parts: the cardia, the fundus, the body and the pylorus. The cardia is the smallest division of the stomach and is located just distal to the gastroesophageal junction. It is the portion of the stomach that receives food from the esophagus. The fundus is the most superior division of the stomach, located superior to an imaginary line extending horizontally through the cardiac orifice.


42. Stomach Divisions The body of the stomach makes up the majority of the organ, extending from the inferior border of the fundus to another imaginary line drawn inferiorly from the angular notch, perpendicular to the lesser curve of the stomach. The body makes up most of the kidney bean shape that we mentioned earlier. The concave segment of the stomach is known as the lesser curvature of the stomach while the convex segment directly across from it is called the greater curvature.

43. Stomach Divisions The final division of the stomach is the pylorus, which can be further divided into the pyloric antrum and the pyloric canal. The pyloric antrum connects to the body of the stomach while the pyloric canal joins the duodenum, which is the first part of the small intestine. In between the end of the pyloric canal and the beginning of the duodenum is the pyloric sphincter. The pyloric sphincter is a muscular valve that allows chyme to enter the duodenum from the stomach.

44. Gastric Rugae The internal surface of the stomach is covered with multiple folds, called the gastric folds or gastric rugae. The gastric rugae effectively increase the surface area of the stomach. When food enters the stomach, the gastric rugae allow the stomach to expand without causing increased pressure.




48. Small Intestine The gastrointestinal tract continues from the pyloric canal as the small intestine. The majority of the digestion and absorption of minerals takes place in the small intestine. The small intestine is divided into 3 parts: the duodenum, the jejunum and the ileum. Small finger-like projections known as villi arise from the mucosa lining of all 3 segments of the small intestine. Each villus contains many additional finger-like projections called microvilli. The purpose of the villi and microvilli are to increase the internal surface area of the small intestine. For nutrients to be absorbed, they need to touch the internal wall of the small intestine. The villi and microvilli are extensions of this wall, and therefore, allow more complete absorption of nutrients.

49. Duodenum The duodenum is primarily responsible for further breaking down chyme and regulating the rate that chyme leaves the stomach. The duodenum is divided into 4 segments, labeled D1, D2, D3 and D4. The D1 segment, otherwise known as the duodenal bulb, extends laterally to the right, superiorly and posteriorly from the pyloric canal. It is approximately 5 cm long and is the only part of the duodenum that is intraperitoneal.


The duodenum then makes a sharp turn and extends inferiorly. This vertical section is known as the D2 segment. This segment contains the orifice of the pancreatic duct, which releases enzymes into the small intestine to further aid in digestion. At approximately the level of the lower border of the L3 vertebral body, the duodenum again makes a sharp turn, this time extending horizontally across midline. This horizontal segment is known as the D3 segment. The first 3 segments are collectively known as the C-loop of the duodenum.

50. Duodenum The duodenum then turns slightly superiorly before joining the jejunum at the duodenojejunal flexure. This segment is known as the ascending duodenum, or the D4 segment. The position of the duodenojejunal flexure is important. Normally, this flexure should be located at the left pedicle of the L1 vertebra in infants. If the flexure is not at this location, there might be a midgut volvulus, or a twist in the intestine. Throughout the development of the fetal gastrointestinal tract, the midgut undergoes complex rotations in utero. Sometimes, these rotations are not fully completed, leading to an intestinal malrotation. A midgut volvulus occurs when a malrotation causes both intestinal blockage and disruption of the blood supply of the small intestine.

51. Jejunum The middle part of the small intestine is the jejunum, which makes up approximately two-fifths of the small intestine. It usually is located predominantly in the upper left and central abdomen. The jejunum is primarily responsible for the absorption of digested carbohydrates and proteins. Unlike the duodenojenunal flexure that separates the duodenum and jejunum, there is no definite structure that divides the jejunum from the ileum.

52. Ileum The ileum comprises approximately the final three-fifths of the small intestine. The ileum is the longest part of the small intestine and is normally located in the central abdomen and pelvis. It is primarily responsible for the absorption of the vitamin B12 and bile salts, as well as any other nutrients that were not absorbed earlier in the digestive process. The ileum normally terminates in the right lower quadrant of the abdomen at a muscular sphincter called the ileocecal valve. The ileocecal valve is a 1-way valve that stops colonic contents from refluxing back into the small intestine.

53. Large Intestine The final part of the gastrointestinal tract located in the abdomen is the large intestine. The large intestine, or colon, can be divided into 6 parts: the cecum, the ascending colon, the transverse colon, the descending colon, the sigmoid colon and the rectum. We’ll discuss the rectum in the pelvic module. The large intestine absorbs the water from the remaining indigestible material and passes the material from the body.

54. Cecum The cecum is a pouch that marks the beginning of the large intestine. It is separated from the ileum by the ileocecal valve. The vermiform appendix typically originates from the inferiomedial


border of the cecum. “Vermiform” is Latin for “worm-like,” a fitting label since the appendix is a long, tube-like structure that is normally found in the right lower quadrant of the abdomen. There are 3 views as to the function of the appendix. The first of these options suggests that the vermiform appendix is a vestigial structure, meaning that it has lost all or most of its function through human evolution. The second option is that the appendix is part of the lymphatic system, housing many disease-fighting cells. The third option suggests that the appendix is a reservoir for normal intestinal flora, or bacteria. The colon contains many “good” bacteria that are needed to maintain intestinal balance. Some illnesses reduce the number of good bacteria, as does taking antibiotics. When there are a limited number of good bacteria, bad bacteria can proliferate and take over the colon. If this happens, the vermiform appendix holds enough good bacteria that can replicate and return balance to the large intestine.

55. Ascending Colon The ascending colon is slightly smaller in diameter than the cecum. It extends superiorly from the cecum to the inferior border of the liver where it makes a sharp left, or medial, turn. This turn is called the right colic, or more commonly the hepatic, flexure. The colon continues horizontally from this flexure, extending to the left side of the body.

56. Transverse Colon The transverse colon is the most mobile segment of the colon and tends to bow downward while making its way to a point inferior to the spleen. At this location, the transverse colon makes another sharp turn, this time inferiorly. This turn is called the left colic, or more commonly the splenic, flexure.

57. Descending Colon From the splenic flexure, the colon extends inferiorly to approximately the left iliac crest area of the pelvis, forming the descending colon. From the left iliac crest, the colon continues medially and inferiorly, creating the sigmoid colon and the rectum, which we’ll discuss in the pelvic module.



60. Liver The next organ of the digestive system we’ll discuss is the liver. The majority of the liver is located in the right upper quadrant of the abdomen, beneath the right hemidiaphragm. The liver curves anteriorly from the posterolateral chest wall, along the anterior chest wall and into the left hypochondric region. Its function is to detoxify the blood, synthesize proteins, produce coagulation factors and manufacture bile for the emulsification of lipids in the small intestine. It also plays a major role in the metabolism of the body.


61. Blood Vessels of the Liver Many vessels throughout the liver supply oxygenated blood to the organ, deliver nutrient-rich blood for the liver to process and return deoxygenated blood back to the heart. These vessels are the hepatic arteries, the portal vein, and the right, middle and left hepatic veins. Like all living tissue in the body, the liver requires oxygen to survive and do its job. The hepatic arteries supply oxygenated blood to the liver. The portal vein supplies nutrient-rich blood from the gastrointestinal tract and spleen to the liver. The liver then extracts the nutrients from the blood while removing toxins that may have been ingested with the nutrients. The hepatic veins are responsible for returning the clean and deoxygenated blood to the inferior vena cava, which then returns the blood to the heart.

62. Lobes of the Liver Scroll through this animation as we discuss the lobes of the liver. Generally, the liver can be divided into 4 lobes: the right lobe, the left lobe, the caudate lobe and the quadrate lobe. The right lobe is the largest lobe of the liver and is found in the right hypochondric region of the abdomen. The left lobe is located in the epigastric to left hypochondric region of the abdomen. The middle of the liver is made up of the caudate lobe and the quadrate lobe. The caudate lobe is located superior to the quadrate lobe.

63. Couinaud Classification System The hepatic vessels have a secondary function: They are the basis for the division of the liver in the Couinaud classification system. The Couinaud system divides the liver into 8 functional segments, each with its own vascular supply and biliary drainage. Using this system, specific segments of the liver can be resected, and because each segment is a functional unit, the remaining liver can survive and function. In the Couinaud classification system, the portal vein and the hepatic veins mark the borders of each segment. The portal vein divides the liver into upper and lower lobes. The middle hepatic vein divides the liver into right and left lobes, the right hepatic vein further divides the right lobe into anterior and posterior segments and the left hepatic vein divides the left lobe into lateral and medial segments. Some sources say the Couinaud system numbers each segment in a clockwise direction, but the numbering system may also resemble the infinity symbol, or a horizontal figure 8.

64. Couinaud Classification System The first segment, known as segment I, is located medial to the concave border of the crescent and is also known as the caudate lobe. Segment II is located superior to the portal vein and to the left of the left hepatic vein. Segment III is located directly inferior to segment II, inferior to the portal vein and to the left of the left hepatic vein. Segment IV is located between the middle and left hepatic veins, and is further divided into segments IVa and IVb, located superior to and inferior to the portal vein. Segment V is located between the right and middle hepatic veins and inferior to the portal vein. Segment VI is also located inferior to the portal vein and posterior to the right hepatic vein. Segment VII is located superior to the portal vein and posterior to the right hepatic vein, directly above segment VI. Segment VIII is located between the right and middle hepatic veins, superior to the portal vein.


65. Biliary System Next we’ll move to the biliary system, which is also part of the digestive system. As we mentioned previously, the liver produces bile. Bile is a yellowish liquid made up of waste products, cholesterol and bile salts. The biliary system has 2 functions: to eliminate waste from the liver and to deliver bile to the intestine to emulsify fats. The biliary system begins with the right and left hepatic ducts, which receive bile from the liver cells of each respective liver lobe. The right and left hepatic ducts combine to form the common hepatic duct, which then extends inferiorly where it merges with the cystic duct. The cystic duct is a short duct that attaches the gallbladder to the rest of the biliary system.

66. Gallbladder The gallbladder is a hollow organ that stores and concentrates bile from the liver. The gallbladder takes its name from gall, which is a synonym for bile. It consists of 3 parts: the fundus, body and neck. The fundus is located distal to the cystic duct and is contiguous with the body, which is contiguous with the neck. The neck of the gallbladder then tapers before connecting to the cystic duct. Sometimes the gallbladder folds over itself at the junction of the fundus and the body. This fold gives it the appearance of a cone-shaped hat with the tip folded and is called a Phrygian cap. Fatty food entering the small intestine initiates the secretion of cholecystokinin, commonly known as CCK, which causes the gallbladder to contract. This contraction forces approximately 1 to 2 ounces of bile into the small intestine, where the bile emulsifies the fats in the partially digested food.

67. Biliary System The biliary system continues inferiorly from the union of the cystic and common hepatic ducts as the common bile duct, frequently referred to by the abbreviation “CBD.” The common bile duct, joined by the pancreatic duct, empties into the duodenum via a dilated opening called the ampulla of Vater. Surrounding the ampulla of Vater is a smooth muscle sphincter called the hepatopancreatic sphincter, or the sphincter of Oddi. The sphincter of Oddi controls the release of bile and pancreatic enzymes into the duodenum.

68. Pancreas The pancreas is part of both the digestive system and the endocrine system. Histologically, it is composed of 2 types of cells: the pancreatic acini and the islets of Langerhans. The pancreatic acini, or the digestive portion of the pancreas, produce pancreatic juices. Pancreatic juices assist in the digestion of starches, fats and proteins and are transported to the small bowel by the pancreatic duct. The pancreatic juices also neutralize the stomach acid that ends up in the duodenum. The islets of Langerhans, or the endocrine portion of the pancreas, produce multiple important hormones, including insulin. The pancreas is found partially in the epigastrium and in the left hypochondrium. It is made up 4 parts: the head, neck, body and tail. The head of the pancreas is located within the C-shape formed by the duodenum. A thin extension of the pancreatic head called the uncinate process hooks to the left and is insinuated between the superior mesenteric vessels and the aorta. The neck of the pancreas arises from the right upper portion of the pancreatic head and extends superiorly and then to the left, adjacent to the inferoposterior border of the pylorus of the stomach. Continuing to the left is the body of the pancreas, and the tail of the pancreas is located in the left hypochondric region near the middle of the spleen. The pancreatic duct runs


the length of the pancreas, transporting the pancreatic juices to the common bile duct, which then empties into the D2 segment of the duodenum.

69. Adrenal Glands The adrenal glands are another part of the endocrine system found in the abdomen. They are located bilaterally above each kidney and posterior to the peritoneum and are therefore considered to be retroperitoneal structures. The right adrenal gland has a triangular shape while the left adrenal gland is more crescent-shaped. On cross-sectional anatomy, particularly computed tomography, the adrenal glands have a different appearance: x-shaped for the right adrenal gland and y-shaped for the left. The adrenal glands release hormones in response to stress.

70. Spleen Now, we’ll look at the spleen. The spleen is a fist-shaped organ usually about 4 inches long located in the left hypochondric region and is part of the lymphatic system. The spleen filters out and destroys old or defective red blood cells, as well as cellular debris and bacteria. It also stores white blood cells and platelets, releasing them when needed.

71. Excretory System In the abdomen, the excretory system is made up of 2 primary structures: the kidneys and the ureters. These structures are located bilaterally in the retroperitoneum and are responsible for excreting liquid waste from the body.

72. Kidneys Scroll through this series of images as we describe the kidneys. The kidneys are bean-shaped organs located to the right and left of the spine in the retroperitoneum. Although at approximately the level of the second lumbar vertebral body, the right kidney is usually located slightly more inferior and lateral than the left kidney, secondary to the liver also being located in the right upper quadrant of the abdomen. The lateral border of each kidney is convex while the medial borders are concave. Along the medial border of each kidney is a central fissure called the renal hilum. The renal hilum is where the vessels, nerves and ureter enter and leave the kidney.

73. Kidneys Each kidney consists of the cortex and the medulla. The smooth cortex of each kidney is located superficially while the medulla is located internally. The medulla can be further broken down into a number of segments known as the renal pyramids. The majority of urine formation takes place in the renal pyramids. As urine is produced, it flows toward the apex of each pyramid. Multiple apices unite to form a minor calyx, or collecting structure, and then approximately 3 of the minor calyces join to form a major calyx. All the major calyces then unite to form the renal pelvis. After urine flows into the renal pelvis, it exits the kidney via the ureter.

74. Ureters The ureters are tubular structures made up of smooth muscle. They transport urine from the kidneys into the urinary bladder, which we’ll cover in the pelvis module. Each ureter exits the kidney at the hilum and extends inferiorly, maintaining its retroperitoneal location.




77. Arterial System of the Abdomen

The last structures we’ll discuss are the vessels of the abdomen. We’ll introduce these vessels in order of the blood flow. The main vessel that transports oxygenated blood from the heart throughout the abdomen is the abdominal aorta. The abdominal aorta can typically be found anterior and slightly to the left of the spine. It extends inferiorly until bifurcating into the right and left common iliac arteries, which distribute blood to the pelvis and the lower extremities. This bifurcation is normally located at approximately the level of the fourth lumbar vertebral body. As the abdominal aorta extends inferiorly, multiple arteries branch from the vessel to transport blood to the viscera.

78. Phrenic Arteries The first arteries originating from the abdominal aorta are the paired phrenic arteries. These small arteries arise at approximately the level of the twelfth thoracic vertebral body and supply the diaphragm with blood.

79. Celiac Artery The second of the aortic branches is the celiac artery. The celiac artery, commonly known as the celiac trunk, arises from the anterior border of the abdominal aorta at approximately the level of the superior border of the first lumbar vertebral body. As it extends anteriorly, the celiac trunk branches into 3 arteries: the left gastric artery, the common hepatic artery and the splenic artery. The left gastric artery travels along the lesser curvature of the stomach, supplying blood to the stomach as well as to parts of the lower esophagus. It is the middle of the 3 branches.

80. Common Hepatic Artery The rightmost branch of the celiac trunk, the common hepatic artery travels to the right and has 3 branches: the proper hepatic artery, the gastroduodenal artery and the right gastric artery. The proper hepatic artery then bifurcates into the right and left hepatic arteries, which supply blood to the liver. The cystic artery, which most commonly arises from the right hepatic artery, supplies blood to the gallbladder. The gastroduodenal artery travels slightly superiorly and supplies blood to the duodenum, pancreas and greater omentum. The right gastric artery also travels superiorly and supplies blood to the stomach.

81. Splenic Artery The splenic artery is the leftmost branch of the celiac trunk. It travels superiorly to the pancreas where multiple branches supply blood to the pancreas and stomach. The splenic artery then reaches the spleen, which it also supplies with blood.

82. Splenic Artery Take a moment to scroll through the animation on this slide. Review the location of the celiac trunk and its branching arteries as discussed in the previous slides.


83. Suprarenal Arteries As we move inferiorly, the paired suprarenal arteries are next. These arteries arise from the lateral borders of the abdominal aorta at the level of the first lumbar vertebral body. These small vessels supply the adrenal glands with oxygenated blood.

84. Superior Mesenteric Artery The next major artery originating from the abdominal aorta is the superior mesenteric artery, commonly known by its abbreviation “SMA.” The superior mesenteric artery arises from the anterior border of the abdominal aorta at approximately the level of the inferior border of the first lumbar vertebral body. It travels both anteriorly and inferiorly and produces many branches, including the jejunal artery, the ilial artery, the inferior pancreaticoduodenal artery, the right colic artery, the middle colic artery and the ileocolic artery. As the names imply, the jejunal and ilial arteries supply oxygenated blood to the jejunum and ileum, respectively. The inferior pancreaticoduodenal artery transports blood to the pancreas and the duodenum. The right colic artery supplies blood to the ascending colon while the middle colic artery supplies blood to the transverse colon. The ileocolic artery carries blood to the ileum, cecum and appendix. An important relationship to know is that the superior mesenteric artery should be located to the left of the superior mesenteric vein; if this relationship is altered, it can suggest a developmental malrotation of the bowel or abnormal twisting of the mesentery.

85. Renal Arteries The paired renal arteries arise from the abdominal aorta as well. The renal arteries originate at approximately the level of the first and second lumbar vertebral bodies and supply blood to the ipsilateral kidney. Given the large amount of blood that flows to the kidneys to be filtered, it is sometimes possible to have multiple right and left renal arteries instead of the typical single vessels on each side. It also is possible that these additional arteries do not flow into each respective hilum, but rather directly into the superior or inferior poles of the kidney. These vessels are called accessory renal arteries.

86. Gonadal Arteries The gonadal arteries are the next vessels to branch from the abdominal aorta. Specifically called the ovarian arteries in women and the testicular arteries in men, these paired small vessels arise from the abdominal aorta at approximately the level of the second lumbar vertebral body and travel inferiorly, supplying blood to the ovaries or testicles.

87. Lumbar Arteries Arising from the posterior border of the aorta are 4 pairs of arteries called the lumbar arteries. They originate at the levels of the first through fourth lumbar vertebral bodies and supply blood to the spinal cord and the abdominal wall.

88. Inferior Mesenteric Artery The inferior mesenteric artery originates from the anterior border of the abdominal aorta. The inferior mesenteric artery is commonly known by its abbreviation “IMA.” Like the superior mesenteric artery, it has multiple branches that supply the colon with oxygenated blood, including the left colic artery, the sigmoid artery and the superior rectal artery. The left colic artery carries blood to the descending colon, the sigmoid artery supplies blood to the sigmoid colon and the superior rectal artery transports blood to the superior rectum.


89. Median Sacral Artery

The final branch of the abdominal aorta we’ll discuss is the median sacral artery. This single small artery arises just above the aortic bifurcation and supplies the sacrum, the coccyx and the rectum.

90. Spinal Arteries The spinal cord, like many structures of the body, needs a blood supply to perform its function. It receives blood from the anterior and paired posterior spinal arteries. The anterior spinal artery runs along the length of the spinal cord within the anterior median fissure. The paired posterior spinal arteries have a paramedian location along the dorsal spinal cord.

91. Spinal Arteries The anterior spinal artery is formed by 2 small branches originating from the vertebral arteries. It supplies blood to the anterior two-thirds of the spinal cord. The posterior spinal arteries are formed by small branches arising from the vertebral and posterior inferior cerebellar arteries. These arteries carry blood to the posterior one-third of the spinal cord.

92. Radicular Arteries Branching from the vertebral, ascending cervical, deep cervical, posterior intercostal, lumbar and lateral sacral arteries are spinal branches that then split into the anterior and posterior radicular arteries. The anterior and posterior radicular arteries arc around the spinal cord and supply blood to the anterior and posterior spinal arteries.




96. Venous System of the Abdomen The final topic we’ll discuss is the venous system of the abdomen. Just as the abdomen needs oxygenated blood to perform its functions, there also must be a way to return deoxygenated blood from the abdomen to the lungs. Just as we described the blood supply to the abdomen, we’ll systematically follow the blood flow back to the heart, discussing the veins we find along the way.

97. Inferior Vena Cava The primary route that transports deoxygenated blood from the abdomen back to the heart is the inferior vena cava, which is commonly known by its abbreviation “IVC.” The inferior vena cava begins with the combination of the right and left common iliac veins at approximately the level of the fifth lumbar vertebral body. The vessel continues superiorly, receiving multiple veins along the way, until it empties into the right atrium of the heart.


98. Median Sacral Vein The first vein that enters the inferior vena cava at the combination of the common iliac veins is the median sacral vein. The median sacral vein receives blood from the sacrum, the coccyx and the rectum. It enters the inferior vena cava at the combination of the common iliac veins or it can also enter the left common iliac vein before the inferior vena cava.

99. Lumbar Veins The lumbar veins join the inferior vena cava just superior to the common iliac veins. Like the lumbar arteries, the lumbar veins consist of 4 pairs of vessels located at the levels of the first through the fourth lumbar vertebral bodies. They return blood from the lumbar spine.

100. Gonadal Veins The right gonadal vein, whether it is the right ovarian or right testicular vein, meets the inferior vena cava at approximately the level of the second lumbar vertebral body and returns blood from the right side of the genitals. Interestingly, the left gonadal vein extends superiorly to approximately the level of the first lumbar vertebral body and returns blood to the left renal vein, which then flows into the inferior vena cava.

101. Renal Veins The paired renal veins meet the inferior vena cava at approximately the level of the first lumbar vertebral body and return blood from the kidneys. There are 2 interesting facts about the renal veins. First, while the right renal vein drains blood from the right kidney, the left renal vein receives blood from the left gonadal, suprarenal and inferior phrenic veins before draining into the inferior vena cava. The second interesting fact is that sometimes the right renal vein passes posterior to the aorta as opposed to its typical anterior location and is called a retroaortic left renal vein. Sometimes, the left renal vein even bifurcates and encircles the aorta, a condition which is called a circumaortic left renal vein.

102. Phrenic Veins

The suprarenal veins drain blood from the adrenal glands, and the inferior phrenic veins receive blood from the diaphragm. As we discussed earlier, the left suprarenal and inferior phrenic veins return blood to the left renal vein, which eventually flows into the inferior vena cava. The right suprarenal vein meets with the inferior vena cava at approximately the level of the first lumbar vertebral body while the right inferior phrenic veins meet the inferior vena cava at approximately the level of the eighth thoracic vertebral body.

103. Main Portal Vein Next we come to the main portal vein. Although it is not technically a vein because it empties blood into a capillary bed rather than transporting blood back to the heart, this vessel is still called the main portal vein. The main portal vein brings nutrient-rich blood from numerous places to the liver for filtering. The main portal vein is formed by the convergence of the splenic and superior mesenteric veins. The superior mesenteric vein, commonly known by its abbreviation “SMV,” receives blood from the jejunum and ileum and runs along the right side of the superior mesenteric artery.

104. Splenic Vein The splenic vein, while returning blood from the spleen, also receives nutrient-rich blood from the inferior mesenteric vein, which drains the lower intestine. The superior mesenteric vein also


receives nutrient-rich blood from the stomach and upper intestine. Together, these veins transport the blood to the liver, which absorbs and processes the nutrients. After the blood is filtered, the right, middle and left hepatic veins transport it from the liver to the inferior vena cava and back to the heart.

105. Conclusion This concludes Module 7 of Sectional Anatomy Essentials – The Abdomen. You should now be able to:

Distinguish the anatomical surface landmarks and regions of the abdomen.

Explain the Addison planes.

Locate the vertebral structures of the abdomen.

Identify the major muscles of the abdomen, locate their insertion points and describe their functions.

Locate and identify the lobes of the liver.

Describe the biliary system.

Explain the location and general function of the stomach, gallbladder, pancreas, spleen, adrenal glands and kidneys.

Name and explain the significance of the peritoneal and retroperitoneal spaces.

Locate each anatomical structure on computed tomography (CT), magnetic resonance (MR) and ultrasound images in the axial, coronal, sagittal and orthogonal (oblique) cross-sectional imaging planes.

106. Resources 107. Development Team 108. Acknowledgements 109. Module Completion

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