Abdomen, Abdominal Wall, & Plexes

This section describes the abdomen, abdominal cavities, abdominal wall, and plexes and their structures.

The abdomen is the largest cavity in the body, extending from underneath the diaphragm down to the groins. Bounded at the back of the body by the spine, and round its upper sides by the ribs, the front of the abdomen is covered by a thick sheet of muscle which can be felt just by “pulling it in”. There are a great number of organs, often called the viscera, in the abdomen. Nearly all the alimentary canal lies inside the abdomen, starting with the stomach sited just under the diaphragm and ending with the rectum, which empties out via the anus. The alimentary canal is the body's food processing and excretory system – it breaks down food into substances that can be absorbed into the blood to be carried to all parts of the body, and ejects indigestible wastes. Backing up the alimentary canal are important abdominal glands such as the liver and pancreas, plus the spleen, which is part of the lymph-vascular system. A huge network of blood vessels serves all the abdominal organs and nerves.

Behind the alimentary canal lie the kidneys, each joined by a tube, called a ureter, to the bladder, which is in the lower part of the abdomen and in which urine is stored before its release. Closely connected to the urinary system is the reproductive system. In women, nearly all the sex organs are inside the abdomen, but in men part of the sex organs descend to their permanent position outside the body before birth.

It might seem impossible for so many vital organs to be squeezed into such a comparatively small space, but the 33 ft. or so of gut are coiled and twisted to fit inside the abdomen. To keep everything in place, the abdomen is lined with a membrane called the peritoneum and the organs are attached to it by sheets or strings of tissue known as mesenteries.

The peritoneum covers all the organs contained within the abdomen. Thus, the liver, the stomach and the intestines are covered with peritoneum, as are the spleen, gall bladder, pancreas, uterus and appendix. The function of the peritoneum is to allow the various structures inside the abdomen to move about freely. While the peritoneum covers organs such as the stomach, it also lines the abdominal cavity. The former is known as the visceral peritoneum, and the latter as the parietal peritoneum. The parietal peritoneum has an extremely sensitive nerve supply, so that any injury or inflammation occurring in this layer is felt as an acute localized pain. The visceral peritoneum is not so sensitive and pain is only experienced if, for example, the intestine is stretched or distended.

The abdomen can be divided into quadrants or nine abdominal regions. Pain felt in these regions may be considered to be direct or referred. The midline in the sagittal plane is the linea alba . The lateral edge of the rectus sheath is the linea semilunaris. The lower costal margin, the iliac crest and public tubercle can be palpated.

The abdominal wall is divided into four quadrants by a vertical and a horizontal line bisecting the umbilicus. An older more complicated scheme divided the abdomen into nine regions. Although the old system generally is not used, some regional names persist, such as epigastric for the area between the costal margins, umbilical for the area around the umbilicus, and hypogastric or suprapubic for the area above the pubic bone.

RIGHT UPPER QUADRANT (RUQ )

Liver

Gallbladder

Duodenum

Head of pancreas

Right kidney and adrenal

Hepatic flexure of colon

Part of ascending and transverse colon

LEFT UPPER QUADRANT (LUQ)

Stomach

Spleen

Left lobe of liver

Body of pancreas

Left kidney and adrenal

Splenic flexure of colon

Part of transverse and descending colon

RIGHT LOWER QUADRANT (RLQ)

Cecum

Appendix

Right ovary and tube

Right ureter

Right spermatic cord

LEFT LOWER QUADRANT (LLQ)

Part of descending colon

Sigmoid colon

Left ovary and tube

Left ureter

Left spermatic cord

Below the skin the superficial fascia is divided into a superficial fatty layer, Camper's fascia, and a deeper fibrous layer, Scarpa's fascia. The deep fascia lies on the abdominal muscles. Inferiorly Scarpa's fascia blends with the deep fascia of the thigh. This arrangement forms a plane between Scarpa's fascia and the deep abdominal fascia extending from the top of the thigh to the upper abdomen. Below the innermost layer of muscle, the transverses abdominis muscle, lies the transversalis fascia. The transversalis fascia is separated from the parietal peritoneum by a variable layer of fat. Fascias are sheets of connective tissue covering or binding together body structures.

External spermatic fascia: Outermost covering of the spermatic cord that is derived from a layer of the abdominal wall.

Iliacus fascia: Part of the transversalis fascia that covers the iliacus muscle.

Psoas fascia: Part of the transversalis fascia that covers the psoas minor muscle.

Quadratus lumborum fascia: Part of the transversalis fascia that covers the quadratus lumborum muscle.

Renal fascia: A fascial pouch derived from extra peritoneal connective tissue that contains the kidneys, the suprarenal glands, the renal vessels and perirenal fat

Transversalis fascia: The deep fascia lining the inner surface of the abdominal wall.

Muscles of the anterior abdominal wall act to increase intra-abdominal pressure in forced respiration, urination, defecation and parturition; also play a role in flexing the vertebral column and in bending from side to side.

The rectus muscle extends from the xiphoid process of the sternum and 5,6,7th costal cartilages to the pubic symphysis and pubic crest. The muscle is enclosed within the rectus sheath formed by the aponeurosis of the lateral abdominal muscles. Along the length of this strap muscle there are three fibrous intersections separating the muscle into four segments. The fibrous intersections are attached to the anterior surface of the rectus sheath, but not to the posterior surface. This allows the superior and inferior epigastric vessels to pass along the posterior surface of the muscle without encountering a barrier.

The lateral muscles arise from the lower part of the rib cage, the lumbar fascia and the iliac crest. External Oblique muscle arises from the lower eight ribs. The fibers run downwards and forwards to form an aponeurosis (a sheetlike fibrous membrane, resembling a flattened tendon, that serves as a fascia to bind muscles together or as a means of connecting muscle to bone) anteriorly. The aponeurosis passes anteriorly to the rectus muscle to insert into the aponeurosis from the other side of the linea alba. Inferiorly the aponeurosis inserts into the anterior superior iliac spine and stretches over tubercle, forming the inguinal ligament. The nervous innervation is from the anterior primary rami (T7-L2) and the action of these muscles is to support the abdominal wall, assist in forced expiration, aid in raising intraabdominal pressure and, with muscles of the opposite side, abducts and rotate trunk.

Internal Oblique muscle arises from the lumbar fascia, the iliac crest and the lateral two-thirds of the inquinal ligament and runs upwards and forwards to form an aponeurosis. Above the arcuate line the aponeurosis splits to enclose the rectus muscle. Below the arcuate line the aponeurosis passes anterior to the rectus muscle. The inferior part of the aponeurosis inserts into the symphysis pubis. At this insertion the aponeurosis is fused with the aponeurosis of the transverses abdominis muscle to form the conjoint tendon. The nervous innervation is from the anterior primary rami (T7-T12) (conjoint tendon ilioinguinal N (L1)) and the action of these muscles is to support the abdominal wall, assist in forced respiration, aid in raising intraabdominal pressure and, with muscles of other side, abduct and rotate the trunk. The conjoint tendon supports the posterior wall of the inguinal canal.

Transverse Abdominis muscle arises from the lower six costal cartilages, the lumbar fascia and the iliac crest. The fibers run forwards to form an aponeurosis. Superiorly the aponeurosis passes behind the rectus muscle. Below the arcuate line the aponeurosis passes anterior to the muscle. The inferior fibers of the aponeurosis are fused with those of the internal oblique to form the conjoint tendon. The nervous innervation is from the anterior primary rami (T7-T12). Conjoint tendon ilioinguinal nerve (L1). The action of these muscles is to support the abdominal wall, aid in forced expiration and raising intraabdominal pressure. Conjoint tendon supports post wall of inguinal canal.

Rectus abdominis muscle arises from the pubic crest and public symphysis. Insertion is from 5, 6, 7 costal cartilages, med inferior costal margin and post aspect of the xiphoid. Nervous innervation is provided by the anterior primary rami (T7-L2). The action of this muscle is to flex the trunk, aid in forced expiration and raise intraabdominal pressure.

The Inguinal Ligament is formed by the aponeurotic fibres of the external oblique muscle. The ligament stretches from the anterior superior iliac spine to the pubic tubercle. At the medial end of the inguinal ligament, fibers are reflected backwards to insert into the superior ramus of the pubis, forming the lacunar ligament. The iliopsoas muscles, the femoral vein artery and nerve, all pass below the inguinal ligament . The inguinal canal passes obliquely through the abdominal wall above the ligament.

The inguinal canal transmits the vas deferens in the male and the round ligament in the female. The deep ring is the entrance to the inguinal canal on the inside of the abdominal wall. The deep ring is formed in the transversalis fascia. As the canal passes through the abdominal wall it receives a layer of muscle from the internal oblique, the cremaster muscle. At the superficial ring the inguinal canal passes through the external oblique aponeurosis and receives a layer from the aponeurosis, the external spermatic fascia in the male. The deep inguinal ring lies lateral to the inferior epigastric vessels. The superficial ring lies above and medial to the pubic tubercle.

A direct inguinal hernia occurs when a loop of gut pushes peritoneum and conjoint tendon through the superficial ring. An indirect hernia occurs when a loop of gut pushes peritoneum through the deep ring into the inguinal canal.

Superficial Epigastric and Superficial Circumflex Iliac (from femoral artery)

The posterior abdominal wall extends from the twelfth rib to the pelvic brim. Vertebrae T12 to L5 are located in the midline posteriorly and the iliolumbar and sacroiliac ligaments anchor the ilium of the hip bone to the 5 th lumbar vertebra and sacrum. Muscles located in the posterior abdominal wall include diaphragm, psoas major and minor (if present), quadratus lumborum, iliacus, and transverses abdominis.

The kidneys, ureters and suprarenal glands are deeply embedded in and supported by sub serous fat and fascia on the posterior wall between peritoneum and the transversalis fascia lining the abdominal cavity. The aorta, inferior vena cava and other vessels supplying gonads, kidneys, adrenal glands and body wall lie in subserous fascia on the posterior wall. The inferior vena cava lies to the right of the aorta. Both vessels bifurcate to form common iliac vessels. These again bifurcate into external and internal iliac vessels before leaving the posterior abdominal wall. The bed of the posterior abdominal wall is made up of three bony and four muscular structures; The bones are the bodies of the lumbar vertebrae, the sacrum, and the wings of the ileum. The muscles are the diaphragm – posterior portion, the quadratus lumborum, the psoas major, and the iliacus.

The muscles of the posterior abdominal wall are the psoas, quadratus lumborum and the iliacus. The Psoas muscle arises from the sides of the upper lumbar vertebrae and the intervertebral discs. The muscle runs downwards into the pelvis and out again under the inguinal ligament. It inserts into the lesser trochanter of the femur in common with the iliacus muscle. The psoas is innervated by the L2,3,4 lumbar nerves. The psoas is enclosed within the psoas fascia, a compartment which may limit the spread of a psoas abscess. The psoas muscle flexes the hip, or flexes the lumbar spine. Several structures such as the kidney and ureter, gonadal vessels, appendix and lumbar nerves have a close relationship to the muscle. Patients attempt to immobilize the psoas muscle when there is pain from many of these structures. This is accomplished by drawing the knees upward passively.

The Quadratus Lumborum muscle arises from the medial half of the twelfth rib and inserts into the iliac crest. It forms a bed for the kidney. It is innervated by the T12 and lumbar nerves. Its action is to fix the twelfth rib during inspiration. The Iliacus muscle arises from the iliac fossa in the pelvis. It runs below the inguinal ligament to insert together with psoas into the lesser trochanter. It is innervated by the femoral nerve.

The aorta passes into the abdomen from the thorax in the midline lying on the vertebral bodies. The crura of the diaphragm form an opening so that the aorta passes behind the diaphragm under the median arcuate ligament. The aorta gives off four pairs of lumbar arteries that supply the abdominal wall (similar to the intercostals arteries of the thorax). Four other pairs are also given off: The inferior phrenic arteries supplying the diaphragm; the middle suprarenal arteries; the renal arteries; the gonadal arteries . There are three unpaired arteries which arise from the anterior aorta: the celiac trunk; the superior mesenteric artery; the inferior mesenteric artery. At the lower border of the L4 lumbar vertebra the aorta bifurcates into the common iliac arteries.

Most of the GI tract and abdominal accessory digestive organs are positions within the abdominal cavity. These organs are not firmly embedded in solid tissue but are supported and covered by serous membranes. A serous membrane is an epithilial membrane that lines the thoracic and abdominal cavities and covers the organs that lie within these cavities. A serous membrane has a parietal portion lining the body wall and a visceral portion covering the internal organs. The serous membranes associated with the lungs are called pleurae. The serous membranes of the abdominal cavity are called peritoneal membranes, or peritoneum. The peritoneum is the largest serous membrane of the body. It is composed of simple squamous epithelium with portions reinforced with connective tissue.

The parietal peritoneum lines the wall of the abdominal cavity. Along the dorsal, or posterior, aspect of the abdominal cavity the parietal peritoneum comes together to form a double-layered peritoneal fold called the mesentery, which supports the GI tract. The dorsal mesentery gives the pendulous small intestine freedom for peristaltic movement and provides a structure through which intestinal nerves and vessels traverse. The mesocolon is a specific portion of the mesentery that supports the large intestine. The peritoneal covering continues around the intestinal viscera as the visceral peritoneum . The peritoneal cavity is the space between the parietal and visceral portions of the peritoneum.

Extensions of the parietal peritoneum, located in the peritoneal cavity, serve specific functions. The falciform ligament, a serous membrane reinforced with connective tissue, attaches the liver to the diaphragm and anterior abdominal wall.

These tissues hold the organs of the digestive tract in position and convey nerves, blood vessels, and lymphatic ducts to the viscera. The space between the visceral and parietal membranes contains a watery fluid that permits the abdominal organs to slide freely against the abdominal wall. A ruptured appendix can lead to inflammation of the peritoneum, a condition known as peritonitis.

The omentum is a highly vascular, fatty tissue approximately 14 inches in length and 10 inches wide that hangs like an apron over the intestines and lower abdomen area. The greater omentum passes from the greater curvature of the stomach to the transverse colon, hanging like an apron in front of the intestines. The lesser omentum passes from the margins of the porta hepatis and the bottom of the fissure of the ductus venosus to the lesser curvature of the stomach and to the the upper border of the duodenum for a distance of about 2 cm beyond the gastroduodenal pylorus. Although the omentum has been viewed as an inert tissue bereft of significant biological function, scientists are now discovering that it is a physiologically dynamic tissue with a considerable body of research that supports its therapeutic potential. The omentum contains angiogenic factors that stimulate the growth of new blood vessels into whatever tissue it is surgically placed next to, including the brain and spinal cord. The omentum is rich in lymphatic vessels and tissue that are critical in removing metabolic waste and excess fluid, destroying toxic substances, and fighting disease. Omental areas called “milky spots” are capable of generating specialized immune cells that facilitate healing. Some scientists believe that migration of omental immune cells, called macrophages, can help repair injured spinal cords. The omentum's lymphatic system has an enormous capacity to absorb edema fluid, including that associated with spinal cord swelling.

The visceral peritoneum contains no afferent nerve supply. Pain from diseased viscera is due to muscle spasm, ischemia or subsequent involvement of the parietal peritoneum. The parietal peritoneum is innervated segmentally by spinal nerves that innervate the overlying muscles. The diaphragmatic peritoneum is innervated by C4 centrally and IC nerves peripherally. The remainder of the peritoneum by IC and Lumber nerves. The pelvic peritoneum is mainly innervated by the obturator nerve.

The blood flow to the splanchnic organs is derived from three main arterial trunks: the celiac, the superior mesenteric artery and the inferior mesenteric artery. The celiac artery supplies blood to the foregut and surrounding tissues, the superior mesenteric artery supplies blood to the midgut, and the inferior mesenteric artery is responsible for blood to the hindgut. Each of these three trunks supplies blood flow to its specific section of the gastrointestinal tract through a vast arcade network. This arcade system is an effective collateral circulation and is generally protective against ischemia, since blood can reach a specific segment of gut via more than one route.

The great plexuses of the sympathetic nervous system are aggregations of nerves and ganglia, situated in the thoracic, abdominal, and pelvic cavities, and named the cardiac, celiac, and hypogastric plexuses. They consist not only of sympathetic fibers derived from the ganglia, but of fibers from the medulla spinalis, which are conveyed through the white rami communicantes. From the plexuses branches are given to the thoracic, abdominal, and pelvic viscera.

A dense cluster of nerve cells and supporting tissue, located behind the stomach in the region of the celiac artery just below the diaphragm. It is also known as the celiac plexus. The largest of the three sympathetic plexuses, it is situated at the level of the upper part of the first lumbar vertebra and is composed of two large ganglia, the celiac ganglia, and a dense network of nerve fibers uniting them together. It surrounds the celiac artery and the root of the superior mesenteric artery. Rich in ganglia and interconnected neurons, the solar plexus is the largest autonomic nerve center in the abdominal cavity . Through branches it controls many vital functions such as adrenal secretion and intestinal contraction. Popularly, the term “solar plexus” may refer to the pit of the stomach. A blow to that area, if it penetrates to the true solar plexus, not only causes great pain but may also temporarily halt visceral functioning. It carries pain “messages” from the pancreas, liver, kidney, gall bladder, spleen and bowels. The plexus and the ganglia receive the greater and lesser splanchnic nerves of both sides and some filaments from the right vagus, and give off numerous secondary plexuses along the neighboring arteries.

The phrenic plexus accompanies the inferior phrenic artery to the diaphragm, some filaments passing to the suprarenal gland. It arises from the upper part of the celiac ganglion, and is larger on the right than on the left side. It receives one or two branches from the phrenic nerve. At the point of junction of the right phrenic plexus with the phrenic nerve is a small ganglion. This plexus distributes branches to the inferior vena cava, and to the suprarenal and hepatic plexuses.

The hepatic plexus, the largest offset from the celiac plexus, receives filaments from the left vagus and right phrenic nerves. It accompanies the hepatic artery, ramifying upon its branches, and upon those of the portal vein in the stance of the liver. Branches from this plexus accompany all the divisions of the hepatic artery. The superior gastric plexus accompanies the left gastric artery along the lesser curvature of the stomach, and joins with branches from the left vagus.

The renal plexus is formed by filaments from the celiac plexus, the aorticorenal ganglion, and the aortic plexus. It is joined also by the smallest splanchnic nerve. The nerves from these sources, fifteen or twenty in number, have a few ganglia developed upon them. They accompany the branches of the renal artery into the kidney. Sympathetic and parasympathetic smooth muscle of the blood vessels supplying the kidney, renal pelvis and upper ureter.

The superior mesenteric plexus is a continuation of the lower part of the celiac plexus, receiving a branch from the junction of the right vagus nerve with the plexus. It surrounds the superior mesentery artery, accompanies it into the mesentery, and divides the number of secondary plexuses, which are distributed to all the parts supplied by the artery; pancreatic, small intestine, ileocolic and colic branches. The nerves composing this plexus are white in color and firm in texture; in the upper part of the plexus close to the origin of the superior mesenteric artery is a ganglion. Sympathetic smooth muscle of vessels supplying the lower pancreas, lower duodenum, jejunum, ileum, cecum, ascending colon and most of the transverse colon. Parasympathetic smooth muscle in the gut wall of the same distribution area.

The abdominal aortic plexus is formed by branches derived, on either side, from the celiac plexus and ganglia, and receives filaments from some of the lumbar ganglia. It is situated upon the sides and front of the aorta, between the origins of the superior and inferior mesenteric arteries.

The Inferior mesenteric plexus is derived chiefly from the aortic plexus. It surrounds the inferior mesentery artery, and divides into a number of secondary plexuses, which are distributed to all the parts supplied by the artery, the left colic, and sigmoid plexuses and superior hemorrhoidal plexus. Sympathetic smooth muscle of the vessels supplying the descending colon, sigmoid colon and rectum.

The hypogastric plexus is situated in front of the last lumbar vertebra and the promontory of the sacrum, between the two common iliac arteries, and is formed by the union of numerous filaments. It divides into two lateral portions which are named the pelvic plexuses. Sympathetic and parasympathetic vascular smooth muscle of the pelvic viscera.

The pelvic plexuses supply the viscera of the pelvic cavity, and are situated at the sides of the rectum in the male, and at the sides of the rectum and vagina in the female.

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Edited: December 2003 by Richard Hennessey, March 2004 by Laura Palmer