Lesson 9 - Abdomen Wall and Viscera
The abdomen is the next region we will cover. The abdominal cavity is continuous with the thoracic cavity, but separated from it by the diaphragm. As we learned in the last lesson, there are structures, like the inferior vena cava and esophagus, that pierce the diaphragm to reach the abdomen, whereas other structures like the aorta run in close approximation to vertebral bodies posterior to the diaphragm. The abdomen is continuous at its inferior border with the pelvis, and, in fact, many of the organs of the digestive tract sit in the pelvic opening, but will be treated here. In this lesson, you will be learning about the abdominal wall, different abdominal regions, the role of peritoneum in abdominal development, and a little bit about each of the abdominal organs and structures. In lesson #10 you will be learning about where the main and accessory organs of digestion are positioned in the abdominopelvic cavity, abdominal vasculature and innervation, and details about the posterior abdominal region.
Learning Objectives - By the end of this lesson, you should be able to ...
- List and describe the layers of muscle, fascia, and tissue in the abdominal wall
- Articulate the arterial supply and venous drainage of the abdominal wall
- Understand how to break the abdomen into sections using the 4-quadrant system and the 9-quadrant system
- Explain the difference between "retroperitoneal" and "intraperitoneal" structures and organs, and list which abdominopelvic organs belong to which group
- List and briefly describe the main structures and organs in the abdominopelvic cavity, and describe where each is located relative to other structures and to the 9-quadrant system
Layers of the Abdominal Wall
The abdomen is a cylindrical chamber bordered superiorly by the diaphragm and thorax, inferiorly by the pelvis, posteriorly by the lumbar vertebral bodies, and anterolaterally and posterolaterally by the abdominal wall, which is made up of several layers of muscles, fascia, and peritoneum. From outside to in, the posterolateral and anterolateral parts of the abdominal wall are made up of skin, superficial fascia (including two layers inferior to the umbilicus, an outer fatty layer called Camper's fascia and an inner membranous layer called Scarpa's fascia), three muscles (external oblique, internal oblique, transversus abdominis), transversalis fascia, extraperitoneal fascia, and parietal peritoneum. In men, Camper's fascia is continuous with dartos fascia in the scrotum, which contains smooth muscle fibers. Scarpa's fascia continues into the thigh just below the inguinal ligament and fuses with deep fascia of the thigh called fascia lata, and continues into the perineum where it attaches to the ischiopubic rami and posterior margin of the perineal membrane and is called Colles' fascia. Note that Camper's, Scarpa's, and Colles' fascia retain the names of anatomists. As I noted in lesson #1, this is becoming rarer in modern anatomical nomenclature.
Muscles of the anterolateral abdominal wall: Three lateral muscles wrap around the abdomen and terminate in an aponeurosis that invests two muscles that are positioned to either side of the midline. The external oblique muscle is the most superficial of the anterolateral muscles. As noted above, the external oblique muscle is deep to superficial fascia. It originates on the outer surfaces of the lower eight ribs and runs inferomedially to insert onto the iliac crest and an aponeurotic sheath that invests the "midline" muscles. The internal oblique muscle originates on thoracolumbar fascia, the iliac crest, and the lateral 2/3 of the inguinal ligament, and runs largely in an anterosuperior direction to insert onto the inferior borders of the lower three or four ribs, the pubic crest and pectineal line of the pelvis, and the aponeurotic sheath which invests the midline abominal wall muscles. The transversus abdominis muscle originates on thoracolumbar fascia, the iliac crest, lateral 1/3 of the inguinal ligament, and costal cartilages for the lower six ribs, and runs "transversely" to insert onto the pubic crest and pectineal line on the pelvis and the aponeurotic sheath that invests the midline muscles. The three anterolateral muscles are embryological counterparts to the three intercostal muscles - the external intercostal and external oblique muscles both run in an anteroinferior direction (like putting your hands in your pockets) and the internal intercostal and internal oblique muscles both run in a largely anterosuperior direction. The transversus abdominis muscle runs largely transversely to its insertion in an aponeurosis, so it is a little different than the innermost intercostal muscle, which runs in the same orientation as the internal intercostal muscle.
In the midline, the rectus abdomis and pyramidalis muscles run in a vertical direction. Rectus abdominis runs from the pubic crest, pubic tubercle, and pubic symphysis superiorly to insert onto the costal cartilages of ribs 5 to 7 and the xiphoid process, and is split transversely by three to four tendinous intersections (that make up the abdominal "six pack"). The linea alba (in Latin, "white line") separates the two rectus abdominis muscles in the midline. Pyramidalis, when present, runs from the pubic bone and pubic symphysis to insert onto the linea alba.
As noted above, the three anterolateral muscles wrap around the abdomen to insert into a tendinous aponeurotic sheath anteriorly. This sheath invests the rectus abdominis and pyramidalis muscles and is called the rectus sheath. The cross-sectional relationship of the rectus sheath relative to the midline muscles differs along its length. The upper 3/4 of the rectus abdominis muscle is completely invested by the rectus sheath - for this part, the aponeurosis of the internal oblique muscle splits to invest the two rectus muscles, and the aponeuroses of the external oblique and transversus abdominis muscles blend with the the aponeurosis for the internal oblique. Transversalis fascia is in direct contact with the rectus sheath. In the lower 1/4 of the rectus abdominis muscle, the rectus sheath lies anterior to the rectus abdominis muscles, so that transversalis fascia contacts the muscles, not the aponeuroses of the three anterolateral muscles.
The external oblique, internal oblique, transversus abdominis, and rectus abdominis muscles are all innervated by anterior rami of T7 to T12 spinal nerves, and the internal oblique and transversus abdominis muscles receive contribution from L1 spinal nerve. Pyramidalis is innervated by the anterior ramus of T12. All four of the main muscles act to compress abdominal contents (and increase intra-abdominal pressure) during micturation ("peeing"), defecation ("pooping"), and childbirth, and the external and internal obliques additionally flex and laterally flex the trunk. Rectus abdominis additionally flexes the vertebral column and tenses the abdominal wall. "Straight" sit-ups and crunches in the sagittal plane work the rectus abdominis muscle; adding a "twist" or some bending to the motion works the two oblique muscles.
Arterial supply to the anterolateral abdominal wall is via several "old friends" that also supply parts of the thorax and thoracic wall (the musculophrenic, internal thoracic, superior epigastric, and intercostal arteries), and a few "new friends," including the inferior epigastric artery, superficial circumflex iliac artery, deep circumflex iliac artery, and superficial epigastric artery. The "new friends" all derive from the external iliac artery, which is the larger of the two branches of the abdominal aorta after its split at the L4 level.
As noted above, extraperitoneal fascia lies deep to the anterolateral and midline muscles of the abdominal wall. Parietal peritoneum lines the outline of the inner border of the abdomen and is adherent to the inner abdominal wall, whereas visceral peritoneum surrounds the organs within the abdominal cavity. I will discuss the role of peritoneum in the abdomen in the next section.
A final point to note is that the aponeurosis of the external oblique muscle doubles over in the area between the anterior superior iliac spine and pubic tubercle to create the inguinal ligament, an important tunnel for the spermatic cord containing the ductus deferens, ilioinguinal nerve, arteries, and veins that run to the scrotum in males, and the round ligament that runs to the uterus in females. We will consider this region further in the lesson on male and female reproductive organs.
Abdominal Regions - Surface Anatomy
The abdomen can be split into four regions (the "four-quadrant" system) using vertical and horizontal lines through the umbilicus, or nine regions using midclavicular, subcostal, and intertubercular lines. The four-quadrant system is most often used to localize abdominal pain (such as occurs in appendicitis, which can be localized to the right lower quadrant). The nine-region system is more often used to describe the precise topographical location of abdominal organs. The regions defined in this way include the:
- Right hypochondrium: to the right of the right midclavicular line, above the level of the subcostal line. This region is completely surrounded by the rib cage, and houses a large portion of the liver and a portion of the ascending colon including the right colic flexure.
- Epigastric region: above the level of the subcostal line, between the two midclavicular lines. This region houses a large portion of the liver, stomach, duodenum, and most of the transverse colon, in addition to the gall bladder and pancreas.
- Left hypochondrium: to the left of the left midclavicular line above the level of the subcostal plane. Houses a small part of the stomach and the left colic flexure.
- Right flank: between the subcostal line and intertubercular line to the right of the right midclavicular line. Houses the ascending colon.
- Umbilical region: between the right and left midclavicular lines below the subcostal line and above the intertubercular line. Houses a portion of the stomach and small intestines.
- Left flank: to the left of the left midclavicular line between the subcostal line and the intertubercular line. Houses the descending colon and a bit of the small intestines.
- Right groin: below the intertubercular line and to the right of the right midclavicular line. Houses the cecum and the beginning of the ascending colon.
- Pubic region: below the intertubercular line between the right and left midclavicular lines. Houses the appendix and part of the cecum, the small intestines, the bladder, rectum and anal canal, and part of the sigmoid colon.
- Left groin: below the intertubercular line to the left of the left midclavicular line. Houses the inferior part of the descending colon, part of the sigmoid colon, and a portion of the small intestines.
Early during embryological development, the GI tract is oriented longitudinally in the body cavity and suspended from the surrounding walls by a large dorsal mesentary and smaller ventral mesentery (remember from lesson #1 that the terms "ventral" and "dorsal" are used in place of "anterior" and "posterior," respectively, when referring to a quadrupedal organism - these terms are appropriate when referring to embryological development because the embryo has a tail and is not bipedal). In the adult, "ventral mesentery" largely disappears (except for in a few spots like the falciform ligament of the liver and the lesser omentum between the stomach, liver, and gallbladder), whereas "dorsal mesentery" becomes omenta, mesenteries, and ligaments within the abdominal cavity. The set-up of peritoneum is parallel to the set-up of pleura in the thorax - an outer layer, parietal peritoneum, lines the inner abdominal wall (akin to the parietal pleura), and an inner layer, visceral peritoneum, covers the viscera and organs. A potential space, the peritoneal cavity, exists between the two layers of peritoneum. Organs such as the transverse colon, sigmoid colon, stomach, and the bulk of the small intestines are suspended in the peritoneal cavity by folds of peritoneum (called omenta, mesenteries, mesocolon, and ligaments), which are derived from embryonic dorsal mesentery and ventral mesentery. These are called intraperitoneal organs. Other organs like the kidneys, most of the duodenum, ascending and descending colons, and most of the pancreas are outside the peritoneal cavity, so are said to be retroperitoneal. Mesenteries, mesocolons, omenta, and ligaments can be thought of as doubled-over sheets of visceral peritoneum in which blood vessels run. The greater omentum, for example, hangs like an apron from the greater curvature of the stomach and proximal duodenum and drapes over the transverse colon and much of the jejunum and ilium. The right and left gastro-omental vessels run between the doubled-over visceral peritoneum of the greater omentum along the greater curvature of the stomach
There are a number of organs that lie in the abdominal cavity. For our purposes, it is important to understand some basic information about each organ or structure, including its function; its position in the abdomen relative to other structures; its innervation; and its arterial supply and venous drainage. We will leave more detailed information about function, microanatomy and histology to other courses (Physiology, Histology, Embryology).
We can separate intraperitoneal abdominal organs into those through which food passes that are directly involved in digestion (the esophagus, stomach, small and large intestines, rectum and anal canal) and those which perform an accessory role in digestion (liver, gall bladder, pancreas, spleen). We will treat retroperitoneal organs such as the kidneys with the posterior abdominal wall.
- Esophagus: carries food to the stomach. Most of the esophagus lies in the neck and thorax, but a bit appears in the abdominal cavity after running through the right crus of the diaphragm at the level of vertebra T10.
- Stomach: digests food, conducts food between the esophagus and small intestine. The esophagus empties into the stomach through the cardiac sphincter. The fundus of stomach is the part that is above the level of the cardiac sphincter, the body of stomach is the main portion, and the pyloric antrum is the portion just proximal to the pyloric sphincter, which controls the flow of stomach contents into the small intestine. The stomach has a greater curvature and lesser curvature.
- Small intestine: digests food, absorbs nutrients, conducts wastes between the stomach and large intestine. The small intestine can be split into the duodenum, a small "C"-shaped structure adjacent to the head of the pancreas that is hooked around the stomach (and can itself be split into superior, descending, inferior, and ascending parts); the jejunum (the proximal 2/5 of the small intestine which contains abundant plicae circulares); and the ileum (the distal 3/5 of the small intestines). The ileum opens into the large intestine at the ileocecal junction.
- Large intestine: absorbs fluids and salts from wastes and stores undigested nutrients as feces. The small intestine empties into the cecum (L. for "blind sac") of the large intestine, and waste then moves superiorly through about 4 feet of large intestine, including the ascending colon, horizontally through the transverse colon, and inferiorly through the descending colon, sigmoid colon, rectum, and anal canal. The junction between the ascending colon and transverse colon is called the right colic (hepatic) flexure, and the junction between the transverse colon and descending colon is called the left colic (splenic) flexure. The appendix is a blind tube connected to the cecum in close apposition to the ileocecal junction.
- Liver: filters blood, metabolizes carbohydrates, fats, and proteins, and produces bile. The liver is a large organ mostly protected by the rib cage, and it is primarily in the right hypochondrium and epigastric regions, but it does extend into the left hypochondrium. The diaphragmatic surface of the liver is pointed anteriorly, superiorly, and posteriorly, and is largely covered by the diaphragm, whereas the inferiorly-facing visceral surface covers the gallbladder, porta hepatis, and other structures, including the right colic flexure, right anterior part of the stomach, esophagus, lesser omentum, right transverse colon, right kidney, and right adrenal gland.The liver has four lobes - the large right and left lobes are visible on the diaphragmatic surface of the liver, and are separated from one another by the falciform ligament, which is a fold of peritoneum; and the caudate lobe and quadrate lobe are visible only on the visceral side.
- Gallbladder: concentrates and stores bile produced by the liver. Lies on the visceral surface of the liver between the right and quadrate lobes.
- Pancreas: releases digestive enzymes into the duodenum. Situated posterior to the stomach and duodenum, and extends all the way to the spleen on the left. The pancreas is retroperitoneal except for a small part of its tail. The pancreas produces digestive enzymes which are released into the duodenum through the pancreatic duct and accessory pancreatic duct, which run from the tail of pancreas to the head of pancreas, through the body of pancreas and neck of pancreas.
- Spleen: largest lymphatic structure in the body. Connected to the greater curvature of the stomach and left kidney by two ligaments which are parts of the greater omentum.
- Kidneys and adrenal glands: excrete metabolic waste products through urine, maintain electrolyte balance.
- Human Anatomy Lesson 10
This lesson covers abdominal vasculature and innervation and the posterior abdominal region.
- Human Anatomy Lesson 11
This lesson covers the basic anatomy of the pelvis and perineum, focusing on osteology of the pelvis and the muscles of the pelvic floor.
- Human Anatomy Lesson 12
This lesson covers anatomy of the organs in the pelvic cavity, including the rectum, anal canal, bladder, ureters, urethra, and components of the male and female reproductive systems.
© 2015 Robert McCarthy
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