Abdominal Wall, Peritoneum and Intestines
Written Learning Objectives
1. Identify the four major quadrants of the abdominal region, and describe basic contents of these quadrants.
The abdominal region is generally divided into quadrants to better visualize and describe the abdominal organs or specific pathologies. These help guide in understanding where to palpate and/or auscultate during a physical exam.
2 planes (imaginary lines) are used to create the quadrants:
Median plane: longitudinal and divides into right & left portions
Transumbilical plane: transverse and extends laterally from umbilicus (~ intervertebral/IV disc L3/L4)
The quadrants are named:
Right upper quadrant (RUQ)
Left upper quadrant (LUQ)
Right lower quadrant (RLQ)
Left lower quadrant (LLQ)
Using the image & chart below begin to familiarize yourself with the basic contents of each quadrant. There is no need to memorize this at this point, but you can use this as a reference as we discuss each organ.
Be aware that there are 9 further subdivisions/regions of the abdomen that may be referenced in clinical lectures. These regions are based on 2 sagittal planes & 2 transverse planes. The specific sagittal & transverse planes utilized may differ according to clinical specializations.
2. Describe the layers and neurovasculature of the abdominal wall, and identify the umbilical folds on the deep surface of the anterior abdominal wall.
To avoid injury to neurovasculature and close an incision with minimal disruption, abdominal wall anatomy should be explored. There are 2 major walls: anterolateral & anterior abdominal walls.
Anterolateral Abdominal Wall
From superficial to deep the layers of the anterolateral abdominal wall are organized in the following manner:
Skin
Superficial fascia
3 anterolateral abdominal wall muscles: colloquially referred to as ‘the obliques’
Muscle fibers of these 3 muscles are organized in different directions to be provide support maintenance of tone of abdomen and allows lateral flexion of the trunk (like in an ‘oblique crunch’)
The tendons of these muscles (called aponeuroses because of their sheet-like appearance) extend anteriorly to form the rectus sheath
Transversalis fascia
Deep fascia on inner surface of deepest anterolateral abdominal muscle, transversus abdominis m.
Extraperitoneal fat
Smaller amounts anterolaterally with increasing amounts posteriorly
Peritoneum
A serous membrane lining the abdominopelvic cavity (will discuss in more detail in upcoming learning objective)
Anterior Abdominal Wall
There is a similar organization as that discussed with the anterolateral abdominal wall, but with a different organization of muscle and a very prominent tendon sheath.
Rectus Sheath: tendons (flattened tendons = aponeuroses) of the 3 anterolateral abdominal muscles, which encloses the rectus abdominis m.
Has an anterior and posterior portion (incomplete posteroinferiorly)
The linea alba (the ‘white line’ of the median rectus sheath) spans between the xiphoid process and pubic symphysis, and is the medial-most attachment point of the 3 anterolateral abdominal muscles to their contralateral counterparts
Rectus abdominis mm.: straight/vertical muscles that span from the inferior rib cage to the pubic bone
Actions: trunk flexion (like in a sit up or crunch) and compression of the abdominal viscera
Tendinous intersections are dispersed throughout the muscle and attach to the anterior rectus sheath
When these muscles are tensed or hypertrophied, the muscle bulges around the intersections creating a ‘six-pack’ appearance, if a smaller superficial fascial layer
The deep surface of the anterior abdominal wall is also lined with peritoneum and in certain areas will overlie structures creating peritoneal folds.
Median umbilical fold: unpaired & in midline
Created by peritoneum covering: median umbilical ligament, a remnant of the fetal urachus - a connection of the apex of the bladder to the umbilicus
Medial umbilical folds: paired & lateral to the median umbilical fold
Created by peritoneum covering: medial umbilical ligaments, which are formed by the obliterated/closed portions of the umbilical arteries
Lateral umbilical folds: paired & lateral to the medial umbilical folds
Created by peritoneum covering: inferior epigastric vessels
The spaces/fossa medial and lateral to this fold are important in differentiating between direct and indirect inguinal hernias. We will discuss this in more detail in an upcoming learning objective.
Neurovascular Supply of Abdominal Wall
Innervation
The innervation of the abdominal wall is regular and segmental (no plexus formation) similar to the thoracic wall. The nerves are derived from ventral primary rami (VPRs) of T7-T12.
The VPR of L1 divides into 2 named nerves: iliohypogastric n. (superior; just superior to the pubic symphysis) and ilio-inguinal n. (inferior; inguinal canal region).
Vasculature
Superficial vasculature
As in most areas, there is superficial vasculature. Of clinical importance, the superficial epigastric v. is a tributary of the femoral v. These veins often anastomose with patent paraumbilical vv. (hepatic portal system) and may become distended with portal hypertension (discussed in more detail later). This is known as caput medusae.
Major vasculature
The main arterial supply and venous drainage of the abdominal wall are the superior & inferior epigastric aa. & vv. These vessels anastomose deep to the rectus abdominis m.
Superior epigastric a. is the continuation of the internal thoracic a.
Inferior epigastric a. is a branch of the external iliac a.
3. Describe the basic structure and contents of the inguinal canal.
The inguinal canal is an oblique passageway for structures in the inferior portion of the anterolateral abdominal wall/superior ‘groin’ region. The canal extends between the deep inguinal ring and superficial inguinal ring, and is approximately 4 cm long.
Deep inguinal ring: entrance or internal aperture of inguinal canal
This is formed by an invagination of transversalis fascia lateral to the inferior epigastric vasculature or lateral umbilical fold
The inferior epigastric vasculature is medial to the deep inguinal ring
Superficial inguinal ring: exit or medial aperture of inguinal canal
This is formed by the external oblique aponeurosis splitting and forming medial & lateral crura.
Transmits structures from the inguinal canal
The walls of the inguinal canal are formed by aponeuroses of the anterolateral abdominal muscles and transversalis fascia. The floor of the canal is formed by the inguinal ligament, which is the inferiormost extent of the external oblique muscle aponeurosis. It extends from the anterior superior iliac spine (ASIS; hip point) to the pubic tubercle.
An important clinical landmark lies in the posterior wall of a portion of the inguinal canal. This is the inguinal triangle (of Hesselbach). Boundaries of this triangle are:
Superolaterally: Inferior epigastric vasculature
Medial: Rectus abdominis m., lateral border
Inferior: Inguinal ligament
We will discuss this triangle in more detail when discussing direct inguinal hernias.
The inguinal canal allows for the descent of the testes during development and the passage of the following structures:
Round ligament of uterus: one of two remnants of the embryonic gubernaculum associated with the ovaries, which allows for the descent of the ovaries and connection of the ovaries and uterus to the labium majus
Other gubernacular remnant is the ovarian ligament located between the uterus and ovary and does not allow descent of the ovaries into the inguinal region
The round ligament of the uterus connects the uterus to the labium majus and presents as a tubular collection of adipose tissue
Spermatic cord: collection of structures (ductus deferens, vasculature, muscle, etc.) between the deep inguinal ring and the scrotum at the testis
Ilio-inguinal n. (L1): typically located superficial to the round ligament of uterus or spermatic cord
Vasculature and lymphatics
4. Differentiate between indirect and direct inguinal hernias.
An inguinal hernia involves the herniation/protrusion of viscera through the inguinal region. These types of hernias are the most common type of abdominal hernia.
There are two types of inguinal hernias: direct and indirect. It is often difficult to differentiate between a direct and indirect inguinal hernia through a physical exam. Both types of inguinal hernias are more common in individuals assigned male at birth due to the relatively larger inguinal canal region due to the descent of the testes.
There are 5 major ways that the types of inguinal hernias differ:
Frequency:
Indirect inguinal hernias are more common than direct inguinal hernias
Does it enter the inguinal canal region medial or lateral to the lateral umbilical fold? Recall that the lateral umbilical fold is formed by peritoneum covering the inferior epigastric vasculature
A direct inguinal hernia enters the inguinal canal medial to the lateral umbilical fold
An indirect inguinal hernia enters the inguinal canal lateral to the lateral umbilical fold
How does the herniation enter the inguinal canal?
A direct inguinal hernia typically enters through the weak fascia associated with the inguinal (Hesselbach’s) triangle
An indirect inguinal hernia typically enters through the deep inguinal ring and often traverses the entire inguinal canal
Both types tend to exit through the superficial inguinal ring
Likelihood to enter the scrotum?
Direct inguinal hernias rarely enter the scrotum, whereas an indirect inguinal hernia may enter the scrotum particularly in cases with a persistent patent processus vaginalis
The processus vaginalis is an extension of peritoneum that traverses the inguinal canal during development which precedes the descent of the testes. The majority of this process will fuse around the time of birth.
The path of an indirect inguinal hernia is often similar to the pathway followed during the descent of the testes.
Acquired v. congenital
A direct inguinal hernia is almost always acquired, often caused by weakness of abdominal musculature.
An indirect inguinal hernia can be either congenital or acquired.
A congenital indirect inguinal hernia is typically due to a patent processus vaginalis.
An acquired indirect hernia is often due to heavy, unstable lifting.
5. Diagram the pathway & divisions of the gastrointestinal (GI; gut) tube/tract.
The pathway of the gastrointestinal (GI; gut) tube from proximal to distal:
Pharynx → Esophagus → Stomach → Duodenum → Jejunum → Ileum → Cecum → Ascending colon → R. colic (hepatic) flexure → Transverse colon → L. colic (splenic) flexure → Descending colon → Sigmoid colon → Rectum → Anal canal
6. Describe the basic organization of the perironeum and peritoneal cavity, and define terms such as mesenteries, omenta, and peritoneal ligaments.
The peritoneum, a serous membrane lining the abdominopelvic cavity, consists of two layers:
Parietal peritoneum: lines the wall of the abdominal cavity
Visceral peritoneum: covers viscera suspended within the abdominal cavity
Between the parietal & visceral peritoneum is a dynamic potential space, the peritoneal cavity. This cavity typically contains a small amount of peritoneal fluid. The peritoneal cavity is typically divided into the greater sac (largest cavity) and the lesser sac or omental bursa (located posterior to the stomach). These parts of the peritoneal cavity are connected via the epiploic foramen (of Winslow).
There are numerous terms used for peritoneal layers, which are typically unique only in terms of which organ it attaches. Here are some generalizations regarding these terms to help you navigate more detailed discussions to come:
Mesentery: a double layer of peritoneum that serves to attach intraperitoneal viscera to the posterior body wall
Specific mesenteries are often given names based on location and typically start with ‘meso,’ such as mesoappendix, transverse mesocolon, etc.
‘The’ mesentery (of jejunum & ileum) anchors the majority of the small intestine to the posterior abdominal wall
Mesentery will contain neurovasculature and lymphatics
Omenta: folds/layers of peritoneum similar to mesentery but with some connection to the stomach & proximal duodenum
Lesser omentum: a double layer of peritoneum that attaches the lesser curvature of the stomach and proximal duodenum to the liver
Often divided into 2 peritoneal ‘ligaments’: hepatogastric & hepatoduodenal ligaments
Greater omentum: some portions are 4 layers of peritoneum (the double layer folds again to equal 4 layers) that hangs inferiorly like an apron (‘omental apron’ covering portions of the small intestine) from the greater curvature of the stomach and proximal duodenum before looping back to the transverse colon
Peritoneal ligament: a double layer of peritoneum, connecting viscera with another organ OR viscera to the abdominal wall
The names of the ligaments are typically very helpful in terms of understanding what structures are connected
For example, hepatoduodenal ligament connects liver (hepato-) and duodenum (duodenal)
Portions of the omenta or mesentery are further divided into these peritoneal ligaments
Peritoneal fold: a reflection of parietal peritoneum on an abdominal wall, causing a visible, raised structure
These are often caused by fetal, obliterated vasculature or canals or patent vasculature and ducts
The umbilical folds discussed in LO2 are examples of peritoneal folds
7. Define the terms intraperitoneal, primarily retroperitoneal, secondarily retroperitoneal, and subperitoneal.
Organs are typically described as intraperitoneal, retroperitoneal, or subperitoneal in the abdominopelvic cavity.
Intraperitoneal viscera: almost completely covered with visceral peritoneum
Suspended within but not inside the peritoneal cavity
Examples: stomach, liver, most of small intestine, cecum, appendix, transverse colon, sigmoid colon, gallbladder, spleen
Retroperitoneal viscera: lie posterior to the peritoneum; typically have peritoneum covering anterior surface
Primary retroperitoneal viscera: develop and remains retroperitoneal (i.e. always retroperitoneal)
Examples: Kidneys, inferior vena cava, proximal rectum, ureters, suprarenal glands
Secondarily retroperitoneal viscera: begin development intraperitoneal, but are eventually drawn retroperitoneal
Examples: portions of the duodenum, ascending and descending colon, most of pancreas
Fusion fascia: double layer of connective tissue formed by secondarily retroperitoneal structures colliding with the posterior abdominal wall
Clinically important because they allow for greater facility in mobilizing and accessing a structure & are devoid of neurovasculature
How structures become secondarily retroperitoneal:
Subperitoneal viscera: lie inferior to the peritoneum; may have peritoneum covering superior surface
Typically pelvic viscera
Examples: bladder & inferior portion of rectum
8. Define the terms foregut, midgut, & hindgut, and list the basic blood supply, innervation, and structures of each.
The embryological gut tube is divided into 3 subdivisions: foregut, midgut, and hindgut. These regions have distinct arterial supply and parasympathetic innervation (although there are certain small areas of overlap).
We tend to discuss structures in the adult as being part of the foregut, midgut, or hindgut as a means of compartmentalizing and understanding basic neurovascular divisions.
The table below shows basic patterns. Again note that there are small areas of arterial supply overlap.
9. Compare and contrast the small and large intestine structures, and describe the neurovasculature supply to both.
Small Intestine
The small intestine consists (proximally to distally) of the duodenum, jejunum, and ileum. We will discuss the duodenum in more detail in S11 based on its close proximity to the pancreas. The jejunum and ileum are often collectively referred to as the ‘small bowel.’
The small intestine mesentery (aka ‘THE’ mesentery) anchors the jejunum and ileum to the posterior abdominal wall, and allows for mobility of the viscera and transmission of vasculature from retroperitoneal sources to the viscera.
The mucosa of the jejunum (mostly in LUQ) and ileum (mostly in RLQ) are characterized by circular folds, except in the terminal portions of the ileum and most prominent in the proximal jejunum. Functionally, these folds serve to increase surface area for absorption and segmentalize the intestine.
There is no clear boundary or demarcation indicating when the jejunum transitions into the ileum. There are some notable differences between the two, but are typically most noticeable in the proximal jejunum and distal ileum.
Arterial supply: as these are both midgut structures, the jejunum and ileum are solely supplied by a series of branches of the superior mesenteric a. (SMA), specifically the jejunal & ileal aa. (intestinal brs.)
Intestinal branches branch from the SMA on the left side, form elaborate anastomotic arcades, and relatively long straight arteries (arteriae rectae) deliver blood to the jejunum and ileum.
Venous drainage: superior mesenteric v., which when joined with the splenic v. forms the hepatic portal vein
Innervation: predominantly innervated by the superior mesenteric plexus, which is an autonomic plexus composed of sympathetic & parasympathetic fibers
Sympathetic fibers: derived from greater & lesser thoracic splanchnic nn. that synapse in the celiac and superior mesenteric ganglia
Function: vasoconstriction & inhibitory to smooth muscle
Parasympathetic fibers: derived from the vagus n. (CN X)
Function: motor to smooth muscle; secretomotor to mucosa
Embedded in the walls of the viscera of the gastrointestinal tract is the Enteric Nervous System (ENS), a complex series of nerve fibers and ganglia located between the longitudinal and circular layers of the muscular wall (myenteric/Auerbach’s plexus), or within the submucosa (submucosal/Meissner’s plexus). The ENS is part of the autonomic nervous system, and it may also function independently. The function of these nervous structures are similar to those described for sympathetics and parasympathetics.
Large Intestine (Colon)
The large intestine is comprised of several regions (proximal to distal): cecum & attached appendix, ascending colon, R. colic (hepatic) flexure, transverse colon, L. colic (splenic) flexure, descending colon, sigmoid colon, and rectum.
The large intestine is derived from both the midgut and hindgut. The transition between midgut and hindgut is 2/3rds of the way on the transverse colon.
The intraperitoneal components of the large intestine have specific mesenteries: such as transverse mesocolon (transverse colon), mesoappendix (appendix), and sigmoid mesocolon (sigmoid colon).
There are four notable structures unique to the colon:
Taeniae coli: the outer longitudinal layer of the muscularis of the colon condenses into 3 bands of muscle (the taeniae coli)
Deviations of fibers from the free taeniae coli form circumferential bands around the large intestine at regular intervals
Haustra (haustrations): the circumferential bands described above give the large intestine a sacculated appearance, and these sacs are known as hausta
Omental appendages: appendages of the large intestine
Fat-filled outpouchings from the peritoneum
Semilunar folds: caused by the haustra of the colon
Unlike the circular folds of the small intestine, the semilunar folds do not form complete rings
Arterial supply: supplied by the superior mesenteric a. and inferior mesenteric a.
Branches of the SMA that supply the large intestine (the portions that are midgut derivatives):
Ileocolic a. (supplies cecum)
Appendicular a. (supplies appendix)
R. colic a. (ascending colon)
Middle colic a. (transverse colon)
Branches of the IMA that supply the large intestine (the portions that are hindgut derivatives):
L. colic a. (descending colon)
Sigmoid aa. (sigmoid colon)
Superior rectal a. (superior rectum)
The more inferior portions of the rectum are supplied by the middle rectal a. (a branch of the internal iliac a.) and the inferior rectal a. (a branch of the internal pudendal a.).
Most branches of the SMA and IMA which serve the colon do so by means of the marginal a., an important anastomotic arcade of branches from the SMA & IMA. This artery runs along the portions of the border of the inferior large intestine. Relatively short straight aa. (arteriae rectae) bridge the distance from the marginal a. to the large intestine
Venous drainage: tributaries of the superior mesenteric vein (SMV) and inferior mesenteric vein (IMV). Thus, both are draining into the portal venous system.
Innervation: The innervation of the large intestine includes contributions from the enteric nervous system and other contributions of the autonomic nervous system.
Sympathetic fibers: significant variety of sources including: superior mesenteric nerve plexus & lumbar splanchnic nn.
Function: vasoconstriction & inhibitory to smooth muscle
Parasympathetic fibers: derived from the vagus n. (CN X) for midgut portions & pelvic splanchnic nn. via the inferior hypogastric plexus for hindgut structures
Function: motor to smooth muscle; secretomotor to mucosa
