BIOLOGY 03048, ANATOMY: WEEK 7+: ABDOMEN 10/12/99
W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, www.hvcc.edu/academ/faculty/crone/index.html)
Readings: Moore and Dalley, Ch. 2
possible web site: http://www.vesalius.com/graphics/cf_storyboards/ant_anat/cfsb_ant_anat1.html
(anatomy of inguinal hernia)
possible web site: http://www.mindspring.com/~dmmmd
(atlas of GI endoscopy--lots of views of structures and pathologies)
ANTERIOR ABDOMINAL WALL: The anterior abdominal wall muscles consist of a) three layers of flat muscles, the tendons or aponeuroses of which converge on the midline or linea alba and b) a vertically oriented pair of muscles, which are incompletely sheathed by those aponeuroses (rectus sheath).
The flat muscles (external oblique, internal oblique, and transversus abdominis) arise from the lateral aspect of the torso. This muscle wall protects the abdominal viscera. These three muscles act to compress the abdominal contents (i.e., raise intrabdominal pressure) during expiration, urination, defecation, and assist in maintaining pressure on the curve of the low back, resisting lordosis (sway back).5
layers of the abdominal body wal, from superficial to deep:5,6
The anterior rectus abdominis muscle is segmented, and is a flexor of the vertebral column. The rectus sheath is derived from the aponeuroses of the three muscle layers, and varies in compostion along its length, e.g., arcuate line. The linea alba is a favored midline for incisions: why? The inferior epigastric arteries off of the external iliac arteries enter the rectus sheath at the level of the arcuate line. The superior epigastric arteries, the inferior continuation of the internal thoracic arteries, anastomose with their counterparts.
In the superficial fascia above the abdominal muscles, one can find superficial veins which can be distended around the umbilicus (with inferior vena cava blockage) in a clinical picture called caput medusae.5,6
The ventral rami of the lower 6 thoracic nerves help supply the abdominal wall. T10 is around the umbilicus. T7-9 superior to the umbilicus, and T11-L1 to the region inferior to umbilicus, as seen in dermatome diagrams. The ventral ramus of spinal nerve L1 has a main branch, the iliohypogastric nerve, and an inferior collateral branch, the ilioinguinal nerve, which is sensory for the scrotal pouch (or the female equivalent, the labia majora).6
INGUINAL CANAL: The inguinal region is the lower lateral portion of the abdominal wall. The inguinal ligament is a folded-over aponeurosis (tendon sheet) of the external oblique muscle between the anterior superior iliac spine and the pubic tubercle. The inguinal canal exists with inner (deep inguinal ring) and outer (superficial inguinal ring) openings.
During development, the testes and spermatic cord"push" through layers of abdominal wall, so that each layer contributes to the inguinal canal. From superficial to deep:
From the extenal oblique aponeurosis: external spermatic fascia and superficial inguinal ring.
From the internal oblique: cremasteric fascia and loops of cremaster muscle.
From the transversalis fascia: internal spermatic fascia and deep inguinal ring.
The inguinal canal contains the spermatic cord (ductus deferens, testicular vessels and nerves) in a male and the round ligament (supports the medial wall of the ovary) in the female.
INGUINAL HERNIAS: With the two openings as sites of weakness, there can be protrusions of intraabdominal fat and intestines which are called either:5
indirect hernia: when the protrusion occurs, lateral to the inferior epigastric artery, through the deep inguinal ring into the inguinal canal toward the superficial inguinal ring (more common),
direct hernia: when the protrusion occurs, medial to the inferior epigastric artery, through the wall near the superficial ring, covered by peritoneum and transversalis fascia
SCROTUM: In order to mature, mammalian sperm requires lower than normal body temperature; hence the location of the testes in the scrotum off of the anterior abdominal wall. The temperature of the scrotum may be slightly adjusted by the wrinkling affects of the smooth muscle contractions of the dartos muscle in the superficial fascia (membranous layer). Sperm mature in the highly convuluted epididymis, and then with contractions of smooth muscle in the ductal wall, these sperm move out through in the ductus deferens (vas deferens). The testicular artery, vein, and lymphatics join with the ductus deferens just before entering the deep ring of the inguinal canal. These collectively make up the spermatic cord (so, during a vasectomy one wants to isolate the ductus deferens). The cremaster muscle reflexively draws the scrotum up through the motor innervation of L1-L2, the genitofemoral nerve.
DEVELOPMENT OF ABDOMINAL DIGESTIVE TRACT: During development, in the region of the future stomach and duodenum, we have not only the typical dorsal mesentery, but also a ventral mesentery (ventral mesogastrium). The liver develops as an endodermal outgrowth of the duodenum in the ventral mesentery.5 In fetal circulation, the umbilical vein (adult, ligamentum teres) travels through the ventral mesenteric sheet which the liver splits into the falciform ligament between liver and umbilicus, and lesser omentum between liver and stomach. The umbilical vein bypasses the portal vein via the ductus venosus (adult, ligamentum venosum) to the IVC (inferior vena cava).
Since the liver is involved with the IVC, it shifts to the right of midline, and the stomach shifts to the left, thereby folding the mesogastrium between the two.5 The structures of portal vein, hepatic artery, and common bile duct run in the hepatodoudenal ligament edge of the lesser omentum. Furthermore, this liver/stomach rotation leads to coelomic space behind the lesser omentum: the omental bursa with an omental foramen leading to it. Hence, an ulcer of the posterior wall of the stomach may perforate and drain into here, as vs. into the general peritoneal cavity.
While all this is going on, the rest of the gut is elongating, forming a primary intestinal loop or region that is nourished by the superior mesenteric artery. Elongations and rotations then occur to form current pattern of small and large intestine. A Meckel's (ileal) diverticulum is the remnant of the yolk stalk involvement of this primary loop formation.5 From the left colic flexure on down, this developing digestive tract is nourished by the inferior mesenteric artery. The stomach, duodenum and the other viscera (liver, pancreas, spleen) that develop in the mesogastrium were not part of the primary loop, so they are nourished by branches off of the celiac trunk. The spleen is not part of the gut tube, but instead is a vascular organ formed in the dorsal mesogastrium, splitting that into splenorenal and gastrosplenic ligaments.
Later in development, there is fusion of much of the dorsal mesentery of the abdominal gut to the parietal peritoneum of the dorsal wall, so that many structures, e.g., pancreas, duodenum, ascending and descending colon, are known as"retroperitoneal."2 In contrast, the free dorsal mesogastrium along the stomach's greater curvature expands out to become the greater omentum, fusing secondarily with the transverse colon, so that the transverse colon is posterior to it (and helps support its weight). Since the greater omentum often wraps around inflamed sites, one can call it the"policeman" of the abdomen, with its rich vasculature and lymphatics (but also fat storage in obese patients!).5
ORGANS OF THE ABDOMINAL DIGESTIVE TRACT:
The stomach is the most dilated portion of the tract. The small intestine consists of three parts of duodenum, jejunum, and ileum. The duodenum is thick-walled and its mucous membrane contains large digestive glands. Furthermore, there is an orifice for bile and pancreatic secretion at the duodenal papilla.
Duodenal ulcers are a common form of peptic ulcer (ulceration of the mucous membrane of stomach or duodenum, with infection of Helicobacter pylori typically involved).3 The jejunum and ileum make up the remaining 6 meters of mobile small intestine (40%, 60%, respectively). The jejunum has circular folds in its mucosal surface known as plicae circulares. The ileum has patches (Peyer's patches) of lymphoid tissue. Between the duodenojejunal junction and ileocecal junction, the root of the small intestine mesentery is anchored, but the rest of the mesentery is free, anterior and mobile.
The ileum ends into the medial side of the cecum via the ileocecal junction. The cecum contains the vermiform appendix. The appendix is attached to the ileum by a small mesentery and has a highly variable position. Lots of lymphoid tissue and little musculature makes up the wall of the appendix. Hence, with a small lumen that is easily obstructed, the appendix can become readily inflammed and swollen in appendicitis.
The large intestine (colon) is divided by the left and right colic flexures into ascending, transverse, descending colons, (and sigmoid colon, rectum inferiorly). The longitudinal muscles of the colon do not surround it but are in 3 muscular bands of the teniae coli, so especially in the sigmoid colon, weakenings or diverticula (diverticulosis) can be seen. Colon cancer is most often seen (75%) in the rectum or descending/sigmoid colons.3
The liver is the largest abdominal organ, with four lobes: right, left, caudate, and quadrate (caudate and quadrate are functionally part of left lobe). The liver is arranged into functional units of acini, supplied by a branch of the hepatic artery, a branch of the portal vein, and a branch of the bile duct, and drained by a central vein.2 (NB: the porta hepatis on the visceral side of the liver is the entranceway of these vessels into liver). Central veins coalesce to hepatic veins, which drain into IVC (inferior vena cava).
The gall bladder is in a depression at the visceral edge of the liver. With a thin muscular coat and thick mucous membrane, it concentrates the bile. Gallstones have a dietary component (seen most in Western countries with high cholesterol diets), and usually consist of mixed stones, with cholesterol and bile pigment contributions.3 The cystic duct joins the common hepatic duct to form the common bile duct. The bile duct is in the right free margin of the lesser omentum and is accompanied by portal veins and the hepatic artery. Just before it enters the duodenum it is joined by the duct of the pancreatic duct, the common entrance of which is guarded by the sphincter of Oddi, which opens at meal time.
The pancreas, whose head in surrounded by the duodenum, has dual exocrine (buffers, digestive enzymes) and endocrine (insulin, glucagon) functions. Given its location, pancreatic disease processes may spread to the omental bursa or to the retroperitoneum.
The spleen has an elastic covering containing fibers that divide the spleen into compartments for clearing damaged red blood cells and other lymphoid functions. With this soft capsule, the spleen is likely to bleed profusely when ruptured.
CIRCULATORY ISSUES: The abdominal aorta is posterior, midline, and seen at L1-L4. At L4, it bifurcates into the right and left common iliac arteries. Basically, the abdominal aorta supplies 3 major areas:
1) branches to walls of abdomen (lumbar arteries)
2) branches to three paired glands: renal arteries are large, short stems off at right angles; suprarenal arteries are smaller; testicular/ovarian arteries are long migrating vessels, following the embryology of the gonads.
3) branches to digestive tract (and unpaired organs), with three important unpaired arteries that arise from the front of the abdominal aorta: A) celiac artery (trunk) B) superior mesenteric artery C) inferior mesenteric artery
The celiac artery is a short trunk with three branches: the left gastric artery, the splenic artery, and the common hepatic artery. Additional arteries allow for an anastomosis around the stomach: right gastric artery, and right and left gastroepiploic (gastro-omental) arteries.
The superior mesenteric artery arises immediately inferior to the celiac trunk and supplies (almost) all of the small intestine and the right half of the large intestine. The inferior mesenteric artery arises just above the bifurcation of the abdominal aorta, and supplies the left side of the colon.
The inferior vena cava (IVC) is the largest blood vessel in the body, and drains the lower body. The IVC DOES NOT receive blood directly from the digestive tract, pancreas, or spleen. Blood from the spleen and pancreas in the splenic vein join up with the superior mesenteric vein behind the neck of the pancreas, resulting in formation of the portal vein. One can bypass the portal circulation with anastomoses to the systemic circulation when blood does not flow through the liver, e.g., cirrhosis. The increased flow through anastomoses can appear as varices, hemorrhoids, (one form of) splenomegaly, etc.2,5 The lymph collected by intestinal villi drains into the cisterna chyli (L2-L3), from which extends the thoracic duct.
ABDOMINAL NERVE SUPPLY: The smooth muscles and glands of the digestive tract are under dual innervation. Parasympathetic nerves maintain and coordinate gut function, e.g., secretion of digestive fluids and control of peristalsis, while sympathetic nerves do the opposite. Sympathetic gut innervation reaches the preaortic ganglia via splanchnic nerves. Knowing about splanchnic nerve origins will be helpful to interpret referred pain patterns:5
|
greater: |
from T5-9, goes to celiac ganglion; postganglionic fibers from there cover range of celiac artery, adrenal glands, and intestine to transverse colon |
|
lesser, least: |
from T10-12, goes to superior mesenteric ganglion; postganglionic fibers from there cover proximal colon |
|
lumbar: |
from L1-2/3, to inferior mesenteric ganglion, covers descending colon on down, kidney, pelvic structures |
Parasympathetic fibers of the vagus nerve supply the general region of the digestive tract supplied by the celiac trunk and superior mesenteric artery. In contrast, the general region supplied by the inferior mesenteric artery is supplied by sacral parasympathetics from the ventral roots of S2-S4.
KIDNEYS: Kidneys are bean-shaped organs buried in fat in the upper abdominal cavity and behind the peritoneum. Kidneys contain an outer cortex and an inner medulla, with Bowman's capsule and convoluted tubules in the cortex and the loops of Henle and collecting ducts in the medulla. The ureters taper from the renal pelvis and descend along the parietal peritoneum, anterior to psoas major muscle.
SUPRARENAL (ADRENAL) GLANDS: The suprarenal glands contain an outer cortex producing glucocorticoids (aldosterone, cortisol) and an inner medulla that is developmentally associated with sympathetic ganglia and so produces epinephrine and norepinephrine.
POSTERIOR ABDOMINAL WALL: The deep posterior abdominal wall muscles include the iliopsoas and the quadratus lumborum. The tendons of iliacus and psoas major converge into a single insertion. This makes iliopsoas a strong flexor of the thigh. The quadratus lumborum affects the vertebral column as an extensor and lateral flexor. Immediately anterior to these muscles is the"retroperitoneal" space of kidney, etc. In a psoas abcess, the psoas fascia (continuous with the transversalis fascia) can"trap" the drainage from a diseased thoracic vertebra (e.g., TB) and hence harm the muscle and important nerves/plexuses with it.5
THE ACUTE ABDOMEN: Pain sensations from the abdomen are carried by spinal nerves (parietal peritoneum) and autonomic nervous system (abdominal organs and visceral peritoneum). Referred pain is that pain appearing in areas of the body surface that send sensory impulses to the same segments of the spinal cord that receive the visceral sensory impulses from the diseased organ.
As disease spreads to involve the parietal peritoneum and the spinal nerves, localization improves. A general rule about pain is that a previously well patient that has severe abdominal pain for six hours may need surgery.1
Types of pain:1,7
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cramping or colicky pain: |
characteristic of obstruction of hollow structures, e.g., intestine |
|
burning pain: |
active peptic ulceration |
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tearing pain: |
rupture of esophagus, dissecting aneurysm |
Common acute abdomen categories:1,7
|
acute appendicitis: |
initial umbilical pain; later, tenderness in the right lower quadrant (inflammed parietal peritoneum sensitive to stretching, hence rebound tenderness). |
|
perforated peptic ulcer: |
sudden, severe, midepigastric in location that soon involves the entire abdomen. Boardlike abdominal rigidity |
|
acute cholecystitis: |
severe right upper quadrant pain, associated with nausea/vomiting. |
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acute pancreatitis: |
deep, unrelenting epigastric pain, radiating to the back |
|
acute small intestinal obstruction: |
peristaltic pain, epigastric and umbilical regions |
|
acute diverticulitis: |
intermittant left lower quadrant pain (cramping) |
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large bowel obstruction: |
gradual onset of lower abdominal cramping pain, abdominal distention |
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renal colic: |
rhythmic flank pain that radiates towards the groin, following the course of the ureter |
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ruptured tubal pregnancy: |
lower abdominal and pelvic pain, often unilateral. Can be associated with shock if patient is hemorrhaging |
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Please send comments and questions to: cronewil@hvcc.edu
Copyright 1999 by Wilson Crone
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This page updated on October 25, 1999