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1. Describe the basic structure, features, and orientation of the duodenum.
1. Describe the basic structure, features, and orientation of the duodenum.
The duodenum is the proximal part of the small intestine, which receives chyme from the stomach and secretions from the pancreas and liver/gallbladder. Owing to its relationships to the stomach, pancreas, and liver/gallbladder, the duodenum is the most anatomically constant part of the small intestine. The pancreas and the proximal third of the duodenum are derivatives of the embryonic foregut, whereas the distal two-thirds of the duodenum is a midgut derivative.
The duodenum consists of four parts, described by their relative positions. They include the:
Superior (1st) part – horizontal, lies at L1 level ~ transpyloric plane;
Descending (2nd) part – vertical, descends from L1 to L3 (anterior to the right kidney), contain the major & minor duodenal papillae;
Inferior (3rd) part – horizontal , crosses L3 vertebra, is crossed by the SMA & SMV; and
Ascending (4th) part – vertical, ascends from L3 to L2.
The proximal three parts of the duodenum form a ‘C’-shape, which cradles the head of the pancreas. It is this region which shares a common blood supply with the pancreas (described below).
The superior (1st) part is a small portion of the duodenum (about 5 cm), and receives chyme from the pylorus. The superior part is unique from the remainder of the duodenum in that it is smooth-walled in appearance, whereas the other parts of the duodenum have circular folds, permanent infoldings of the mucosa to increase surface area and slow the movement of materials.
The descending (2nd) part receives secretions of the pancreas and liver/gallbladder to mix with chyme from the stomach. The major duodenal papilla (MDP), an eminence of mucosa, transmits the hepatopancreatic ampulla, the distal dilation of the typical junction of the pancreatic and (common) bile ducts. The major duodenal papilla is the landmark differentiating the foregut (proximal to the MDP) from the midgut (distal to the MDP). When an accessory pancreatic duct is present, this duct typically drains into the descending part at the minor duodenal papilla, proximal (superior) to the MDP. The descending part is secondarily retroperitoneal.
The inferior/horizontal (3rd) part of the duodenum is transmitted between the abdominal aorta and the SMA, which in the presence of an aneurysm, makes the inferior part of the duodenum susceptible to superior mesenteric artery syndrome. The inferior part is secondarily retroperitoneal.
The ascending (4th) part is the smallest (about 2.5 cm) of the duodenum, and is contiguous with the jejunum at the duodenojejunal flexure. This flexure is sometimes supported by the suspensory ligament of (Treitz) the duodenum. The ascending part is secondarily retroperitoneal. Beyond the duodenojejunal flexure, the jejunum is intraperitoneal.
The duodenum is supplied by blood by the gastroduodenal a., inferior pancreaticoduodenal a., and brs. of jejunal aa.
The duodenum is innervated by branches from the celiac and superior mesenteric autonomic plexuses. Parasympathetic stimulation increases bowel motility, mucosal secretion, and vasodilation. Sympathetic stimulation slows bowel motility and mucosal secretion, and also increases vasoconstriction. Referred pain from the duodenum presents to the epigastric region.
2. Describe the basic structure, features, and orientation of the pancreas.
2. Describe the basic structure, features, and orientation of the pancreas.
The pancreas is an important endocrine and exocrine (e.g. accessory digestive) gland of the foregut. The endocrine portions of the pancreas (pancreatic islets) regulate blood glucose, and secrete into capillaries. The exocrine portions of the pancreas are acinar, and secrete into ducts coalescing into a pancreatic duct, and sometimes an accessory pancreatic duct.
The pancreas is derived from the embryonic foregut, and is topographically divided into four parts.
Head - cradled by the duodenum,
Uncinate process - located posterior to the SMA and SMV,
Neck - posterior to the pylorus of stomach, anterior to the SMA & SMV (adjacent to the origin of the hepatic portal v.),
Body - posterior to the stomach, anterior left renal vessels & splenic a.
Tail - anterior to left kidney, oriented towards the spleen, intraperitoneal.
The (main) pancreatic duct (of Wirsung) runs the length of the pancreas (from tail-to-head), and terminates in the lumen of the descending duodenum. Most typically, it joins with the (common) bile duct to form the hepatopancreatic ampulla (of Vater), which is transmitted through the major duodenal papilla. The accessory pancreatic duct (of Santorini), when present, drains the superior portion of the head of the pancreas, and is transmitted into the descending duodenum through the minor duodenal papilla. As both pancreatic ducts and the (common) bile duct are transmitted through the head of the pancreas, pressure exerted upon the head (from edema, neoplasm, etc.) may interfere with exocrine pancreatic function as well as the drainage of bile. Variants exist with each duct draining independently into the duodenum. The entirety of this region is often called the pancreaticobiliary junction by clinicians.
The pancreas is supplied with blood by branches of the gastroduodenal a., splenic a., and inferior pancreaticoduodenal a. The details of blood supply to the pancreas will be discussed in a subsequent learning objective.
The pancreas is primarily innervated by the celiac plexus, but is also supplied by branches from the superior mesenteric plexus. Parasympathetic stimulation enhances exocrine secretion and vasodilation. Sympathetic stimulation inhibits exocrine secretion and enhances vasoconstriction. Pain from the pancreas is referred to the epigastric region.
The pancreas is secondarily retroperitoneal, except for the tail.
3. Diagram the vascular supply to the duodenum and pancreas, and describe the collateral blood flow between the celiac and superior mesenteric arteries.
3. Diagram the vascular supply to the duodenum and pancreas, and describe the collateral blood flow between the celiac and superior mesenteric arteries.
The blood supply to the pancreas and duodenum is sourced from both the celiac trunk and the superior mesenteric a. When considering the regions served, it may be easier to consider the blood supply of the duodenum and head & neck of the pancreas separately from the supply to the body and tail of the pancreas.
The duodenum and head & neck of the pancreas are served:
Superiorly, by branches of the celiac trunk:
Gastroduodenal a. (& brs.)
Anterior superior pancreaticoduodenal a., &
Posterior superior pancreaticoduodenal a., and
Inferiorly, by branches of the SMA:
Inferior pancreaticoduodenal a.
Anterior br.,
Posterior br.
The gastroduodenal a. branches from the common hepatic a. and descends toward the head of the pancreas posterior to the retroperitoneal portion of the superior (1st) part of the duodenum. A perforating duodenal ulcer may jeopardize the gastroduodenal a., the (common) bile duct, or the hepatic portal v.
The body and tail of the pancreas are predominantly served by the celiac trunk (via the splenic a.). These branches include:
Splenic a.
Dorsal pancreatic a. → inferior pancreatic a.
Greater pancreatic a.
Artery to the tail of pancreas
Very frequently, anastomoses may be found between the dorsal pancreatic a. and the pancreaticoduodenal arteries.
4. Describe the branches of the abdominal aorta and their targets.
4. Describe the branches of the abdominal aorta and their targets.
The thoracic aorta becomes the abdominal aorta at the aortic hiatus (of the diaphragm), approximately at the T12 vertebral level. The abdominal aorta descends in the retroperitoneum, left lateral to the inferior vena cava, and anterior to the bodies of the vertebrae, sending branches to viscera and the body wall along its length, until bifurcating (approximately at L4) into the common iliac aa.
The branches of the abdominal aorta may be conceptualized as: 1) anteriorly oriented, unpaired branches to viscera, 2) laterally oriented, paired branches to viscera, and 3) posterolaterally oriented, paired branches to the body wall.
Anteriorly oriented, unpaired brs. to viscera:
Celiac (~T12) - serves the foregut (abdominal esophagus - proximal duodenum @ Major duodenal papilla)
Superior mesenteric (~L1) - serves the midgut (distal duodenum @ Major duodenal papilla - proximal two-thirds of transverse colon)
Inferior mesenteric (~L3) - serves hindgut (distal third of transverse colon - rectum)
Laterally oriented, paired brs. to viscera:
Middle Suprarenal (~L1) - serve the suprarenal (adrenal) glands
Renal (~L1) - serve the kidneys
Inferior suprarenal
Gonadal (~L2) - serve the gonads
Posterolaterally oriented, paired brs. to body wall:
Inferior phrenic (~T12)
Superior suprarenal
Lumbar aa. (~L1-L4)
Recall that anastomoses among abdominal arteries are quite common, and that generally there will be anastomoses between some branches of the celiac trunk and the SMA, and between some branches of the SMA with the IMA (e.g. via the marginal a.).
5. Describe the tributaries of the inferior vena cava (IVC).
5. Describe the tributaries of the inferior vena cava (IVC).
The inferior vena cava (IVC) is the major vein which returns blood from the back, abdominal wall, pelvis, and abdominopelvic viscera. The IVC begins at the union of the common iliac veins (~L5), is transmitted through the caval opening of the diaphragm (~T8), and ends in the right atrium of the heart. The IVC is also: found to the right of the abdominal aorta, is valveless, and typically has tributaries that mirror the branches of the abdominal aorta, for example:
Right inferior phrenic v.
Renal vv.
Right gonadal v.,
Lumbar vv. (typically 1st and 2nd pairs of lumbar vv. drain into the azygos system, whereas 3rd and 4th pairs of lumbar vv. drain into the IVC).
Some exceptions of venous return mirroring abdominal aortic brs. exist, e.g. the hepatic portal system, hepatic vv., and the unique tributaries of the left renal v. (L. inferior phrenic v., L. suprarenal v., & L. gonadal v.).
All blood returning through the SVC and IVC is part of the caval system. Blood returning from the abdominopelvic GI viscera is part of the hepatic portal system. This blood is delivered to the liver via the hepatic portal v., is processed in the liver, and returns to the caval system via a variable number of hepatic vv. (typically 3).
6. Describe the autonomics of the abdominal portion of the gastrointestinal system.
6. Describe the autonomics of the abdominal portion of the gastrointestinal system.
Enteric Nervous System:
Embedded within the wall of the gut tube is the Enteric Nervous System (ENS), a complex series of nerve fibers and ganglia located either 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 (ANS), and it may also function independently.
Autonomic control of the gut tube function may be loosely divided into functions that are typically parasympathetic (stimulates gut motility, stimulates glandular secretion, vasodilation, & sphincter relaxation), and functions that are typically sympathetic (inhibits gut motility, inhibits glandular secretion, vasoconstriction, & sphincter contraction).
Perivascular Plexuses:
The fibers supplying the ENS typically follow arteries to the viscera. These fibers contribute to what are known as perivascular plexuses, networks of both sympathetic and parasympathetic fibers sometimes named for the vessels they travel along and/or originate near. Perivascular plexuses give input to the ENS and also control the smooth muscle of the tunica media of arterial vessels, especially at the level of the arterioles.
Perivascular plexuses of and about the aorta are contiguous elements of the ANS; fibers are shared between and among names plexuses, and there is not a clear, specific pattern of innervation from any particular thoracic splanchnic n. to any particular organ. Many named plexuses share a close proximity to similarly named ganglia (where presynaptic sympathetic fibers synapse, and where presynaptic parasympathetic fibers travel through). The existence and relative locations of individual ganglia may vary. Some major perivascular plexuses and the regions they innervate include:
celiac plexus - all viscera served by the celiac trunk & branches (foregut)
superior mesenteric plexus - all viscera served by the SMA & branches (midgut)
renal plexus - kidneys and associated vasculature
intermesenteric/aortic plexus - indistinct plexus of autonomic fibers between the celiac & superior mesenteric plexuses (superiorly) and the inferior mesenteric & superior hypogastric plexuses (inferiorly)
inferior mesenteric plexus - all viscera served by the IMA (hindgut)
superior hypogastric plexus - fibers communicate between aortic plexus (abdomen) and the inferior hypogastric plexuses (pelvis); communication to ureteric, gonadal, and iliac perivascular plexuses
Sympathetic Influx:
The preganglionic sympathetic fibers that serve abdominal viscera and local perivascular plexuses come from the thoracic and lumbar splanchnic nn. These fibers synapse in pre-aortic (pre-vertebral) ganglia, which are often named for their associated perivascular plexus.
Thoracic splanchnic nn.
Greater thoracic splanchnic nn. (T5-9)
Lesser thoracic splanchnic nn. (T9/10-11)
Least thoracic splanchnic nn. (T11/12)
Lumbar splanchnic nn.
L1 -L2: Abdominal plexuses
L3 - L4: Superior hypogastric plexus & Hypogastric nn.
Parasympathetic Influx:
The preganglionic parasympathetic fibers that serve abdominal viscera and local perivascular plexuses come from the vagal trunks (from vagus nn.) and the pelvic splanchnic nn. Recall that parasympathetic preganglionic fibers are relatively long and synapse in the walls of their target organs (i.e. either in the walls of arterioles, or in ganglia of the myenteric or submucosal plexuses).
Vagal trunks: The vagal trunks are formed by the L. & R. vagus nn. The anterior vagal trunk predominantly consists of fibers from L. vagus n. (with some contributions from the R. vagus n.), and the posterior vagal trunk predominantly consists of fibers from the R. vagus n. (and some fibers from the left vagus n.). Both trunks enter the abdominopelvic cavity via the esophageal hiatus of the diaphragm (~T10). The vagal trunks both innervate the thoracic & abdominal parts of the esophagus and the stomach. The ultimate targets of vagal trunks differ beyond the stomach.
Anterior vagal trunk: divides shortly thereafter into hepatic, gastric, and pyloric brs. The hepatic br. serves the liver, gallbladder, and extrahepatic biliary tree. The gastric and pyloric brs. serve the anterior aspects of the stomach.
Posterior vagal trunk: fibers contributing to the celiac plexus (and plexuses beyond),but they do not synapse. Preganglionic parasympathetic fibers of the posterior vagal trunk serve the GIT and associated organs of the foregut and midgut and the perivascular plexuses associated with these regions.
Pelvic splanchnic nn.: The pelvic splanchnic nn. originate from the ventral primary rami of S2-4 as they exit the anterior sacral foramina. The pelvic splanchnic nn. conduct preganglionic parasympathetic fibers to the perivascular plexuses and viscera of the pelvis and hindgut. The majority of the fibers will directly enter the inferior hypogastric plexuses (the major autonomic organizational plexuses of the pelvis) or they may take an independent pathway over the pelvic brim to the inferior mesenteric plexus (to the hindgut). These fibers may bypass the IHP plexus altogether. The pelvic splanchnic nn. as well as the sacral splanchnic nn. (pelvic sympathetic outflow) will be more thoroughly discussed in the reproduction sessions.
Referred Pain:
General visceral afferent (GVA) fibers conduct sensory information from viscera and perivascular plexuses to the CNS. This information may relate to reflexes, distention, traction, or ischemia. GVA fibers that relate to smooth muscle reflexes are conducted with parasympathetics, whereas GVA fibers that relate to distention, traction, and ischemia (those loosely categorized as ‘pain’) travel with sympathetic fibers. While these are not true pain fibers, the brain will often interpret these signals as difficult to describe, poorly localized, and midline sensations of pain. Referred pain of abdominal viscera is localized to three regions:
Epigastric - viscera and perivascular plexuses of the foregut
Peri-umbilical - viscera and perivascular plexuses of the midgut
Suprapubic - viscera and perivascular plexuses of the hindgut
Below the levels of the peritoneum (typically, within the pelvis), GVA fibers that conduct ‘pain’ travel with parasympathetic fibers. Thus, the inferior-most extent of peritoneum roughly delineates the ‘pelvic pain line,’ above which ‘pain’ follows sympathetic fibers, and below which, ‘pain’ follows parasympathetic fibers. This will be discussed in greater detail in the reproduction sessions.
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