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Case 9

LO 3 Outline the blood and lymphatic supply and innervations of the GIT

Arterial Supply The gastrointestinal system (GIS) and its derivatives are supplied by 3 major arteries, all of which branch from the anterior surface of the abdominal aorta (NOTE that the abdominal aorta also has posterior and lateral branches). They also pass through derivatives of the dorsal and ventral mesenteries. Therefore structures they supply include: • GIT • Liver • Pancreas • Gallbladder • Spleen • Lymph nodes Artery

Level of branching from aorta

Areas supplied

celiac artery superior mesenteric artery Inferior mesenteric artery

Upper border of vertebra LI Lower border of LI Vertebral level LIII

Foregut Midgut HIndgut

The abdominal aorta: Begins: at aortic hiatus of diaphragm (lower border of T12) Ends: level with vertebra L4; slightly to left of midline. Terminal branches: the 2 common iliac arteries. Definitions of foregut. midgut and hindgut Foregut (Celiac trunk): Begins: anbdominal oesophagus Ends: just inferior to the major duodenal papilla (midway along descending part of duodenum). Includes: abdominal oesophagus, stomach, duodenum (superior to major papilla), liver, pancreas and gall bladder. Midgut (superior mesenteric artery): Starts: slightly inferior to the major duodenal papilla in the descending part of the duodenum. Ends: At the junction between the proximal 2/3 and the distal 1/3 of the transverse colon. Includes: duodenum (inferior to the duodenal papilla), jejunum, ileum, cecum, appendix, ascending colon and the right (proximal) 2/3 of the transverse colon. Hindgut (inferior mesenteric artery): Starts: just before the left colic flexure (the junction between the proximal 2/3 and the distal 1/3 of the transverse colon) Ends: midway through the anal canal. Includes: left (distal) 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and upper part of the anal canal. Celiac Trunk Arises: from abdominal aorta, immediately below abdominal hiatus of diaphragm, anterior to upper part of vertebra LI. Branches: Immediately branches into 3 arteries:

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Left gastric artery Smallest branch of celiac trunk. It ascends to the cardioespphageal junction. Sends oesophageal branches upwards to the abdominal part of the oesophagus. After ascending, the left gastric artery turns to the right and descends along the lesser curvature of the stomach. It supplies both surfaces of the stomach (in this area only). It anastamoses with the right gastric artery. Splenic artery Largest branch of the celiac trunk. Takes a course to the left along the superior border of the pancreas. This artery is very tortuous. It travels in the splenorenal ligament. Divides into many branches which enter the hilum of the spleen. The splenic artery also supplies the pancreas: as it passes along the superior border of the pancreas, it gives off many small branches to supply the neck, body and tail of the pancreas. Also gives off: Short gastric arteries (pass through gastrosplenic ligament; supplies fundus of stomach). Left gastro-omental artery (passes along greater curvature of stomach; anastomoses with right gastroomental artery). Common hepatic artery Medium-sized branch of the celiac trunk. Runs to right. Divides into 2 terminal branches: Hepatic artery proper: ascends towards liver in free edge of lesser omentum. Runs to the left of the bile duct and anterior to the portal vein. Divides into right and left hepatic arteries. The right hepatic artery gives off the cystic artery to the gallbladder. Gastroduodenal artery: May give off the supraduodenal artery; it then descends posterior to the superior part of the duodenum. At the lower border of the superior part of the duodenum it divides into its 2 terminal branches: right gastro-omental artery Superior pancreaticoduodenal artery The right gastro-omental artery anastomoses with the left gastroomental artery from the splenic artery and sends branches to both surfaces of the stomach and also into the greater omentum. The superior pancreatico-duodenal artery divides into anterior and Posterior branches and supplies the head of the pancreas and the duodenum. These vessels anastomose with branches of the inferior pancreatico-duodenal artery. Superior Mesenteric Artery Arises from the abdominal aorta immediately below the celiac artery, anterior to the lower part of vertebra LI. Branches: The superior mesenteric artery gives off many branches: Inferior Pancreaticoduodenal artery This is the first branch of the superior mesenteric artery.

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It divides immediately into anterior and posterior branches with ascend along the anterior and posterior surfaces of the head of the pancreas respectively. These branches then anastamose with the corresponding branches of the superior pancreaticoduodenal artery. The network described above supplies the head and uncinate process of the pancreas and the duodenum. Jejunal and Ileal Arteries On the left of the superior mesenteric artery, distal to the inferior pancreaticoduodenal artery, jejunal and ileal branches are given off which supply the jejunum and most of the ileum. These arteries pass between the two layers of the mesentry. These branches form anastamosing arcades (arches) and extending from the terminal arcade are vasa recta (straight arteries) which provide the final link so the walls of the small intestine. Middle colic artery The first of the 3 branches (middle colic, right colic and ileocolic arteries) from the right side of the superior mesenteric artery. This artery arises as the superior mesenteric artery emerges from beneath the pancreas. This artery divides into right and left branches before entering the transverse mesocolon. The right branch anastomoses with right colic artery (also a branch of the superior mesenteric) The left branch anastomoses with the left colic artery (branch of the inferior mesenteric artery). Right colic artery This is the second of the 3 branches from the right side fo the superior mesenteric artery. Passes to the right in a retroperitoneal position. Supplies the ascending colon. It divides into 2 branches as it nears the colon: a descending branch ( which anastomoses with the ileocolic artery) and an ascending branch ( which anastomoses with the middle colic artery). Ileocolic artery This is the last branch arising from the right side of the superior mesenteric artery. This artery passes downwards and to the right (towards the right iliac fossa). It divides into 2 branches: Superior branch (anastomoses with the right colic artery0 Inferior branch (continues towards the ileocolic junction and divides into 4 branches: colic, caecal, appendicular and ileal). The middle colic, right colic and ileocolic arteries supply the terminal ileum, cecum, ascending colon, & 2/3 of the transverse colon. Inferior Mesenteric Artery This is the smallest of the 3 anterior branches of the abdominal aorta. Arises anterior to the body of vertebra LIII. Passes (descends) anteriorly to the aorta and then deviates to the left and continues to descend. Branches: Left colic artery This is the first branch of the inferior mesenteric artery. It ascends retroperitoneally. Divides into 2 branches: Ascending branch: supplies upper part of descending colon & distal part of transverse colon. Descending branch: supplies lower part of descending colon. Sigmoid arteries

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The sigmoid arteries consist of 2 – 4 branches. These descend to the left in the sigmoid mesocolon. They supply the lowest part of the desceding colon and the sigmoid colon. Superior rectal artery This is the terminal branch of the inferior mesenteric artery. This artery descends into the pelvic cavity in the sigmoid mesocolon. As it descends, it crosses the left common iliac vessels. The superior rectal artery divides opposite vertebra SIII into 2 terminal branches which descend on either side of the rectum.

Venous Drainage Venous drainage for the following structures is through the portal system of veins, which deliver blood from these structures to the liver: • Spleen • Pancreas • Gallbladder • Abdominal part of the GIT, except for the inferior part of the rectum Blood then passes through hepatic sinusoids and subsequently through progressively larger veins, eventually entering the hepatic veins. The hepatic veins return venous blood to the IVC just inferior to the diaphragm. Portal Vein This is the final common pathway of blood from the organs mentioned above. This vein is formed when 2 veins join together posterior to the neck of the pancreas at the level of LII: the splenic vein and the superior mesenteric vein. The portal vein ascends towards the liver. It passes posteriorly to the superior part of the duodenum. It enters the right margin of the lesser omentum. Before entering the liver, the portal vein divides into right and left branches, these then enter the liver parenchyma. The portal vein has many tributaries, including: Right and left gastric veins Cystic veins Para-umbilical veins Splenic Vein Large, straight vein, formed from many smaller vessels leaving the hilum of the speen. Passes to the right. Passes through the splenorenal ligament with the splenic artery and the tail of the pancreas. Continues to right, in contact with the body of the pancreas as it passes to the posterior abdominal wall. The splenic vein joins the superior mesenteric vein to form the portal vein; this occurs posterior to the neck of the pancreas. Tributaries to the splenic vein: Short gastric veins Seft gasto-omental vein Pancreatic veins Inferior mesenteric vein (usually)

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Superior Mesenteric Vein Drains blood from the: • Small intestine • Cecum • Ascending colon • Transverse colon Begins in the right iliac fossa and ascends in the mesentery to the right of the superior mesenteric artery. Posterior to the neck of the pancreas, the superior mesenteric vein joins the splenic vein to form the portal vein. A corresponding vein accompanies each branch of the superior mesenteric artery, the superior menenteric vein has the following tributaries: Jejunal veins Ileal veins Ileocolic vein Right colic vein Middle colic vein As well as: Right gastro-omental vein Anterior and posterior inferior pancreaticoduodenal veins Inferior Mesenteric vein Drain blood from the following structures: • Rectum • Sigmoid colon • Descending colon • Splenic flexure It starts as the superior rectal vein and ascends. Receives tributaries from the: sigmoid veins Left colic veins The inferior mesenteric vein continues to ascend and joins the splenic vein (usually), posterior to the body of the pancreas. HOWEVER, sometimes it ends at the junction of the splenic and superior mesenteric veins or joins the superior mesenteric vein.

Lymphatics Lymphatic drainage of the:

spleen Pancreas Gall bladder Liver Abdominal part of the GIT (as low as inferior part of the rectum)

Is through vessels and nodes that eventually end in large collections of lymph nodes. These lymph nodes, known as pre-aortic lymph nodes, are found at the origins of the 3 anterior branches of the abdominal aorta which supply these structures. These collections are therefore called:

Case 9 • • •

LO 3 Celiac group of pre-aortic lymph nodes Superior mesenteric group of pre-aortic lymph nodes Inferior mesenteric group of pre-aortic lymph nodes

Lymph from the viscera is supplied by the following: (NB check on this!) Celiac trunk (i.e. structures that are part of foregut) Drains to celiac group of pre-aortic lymph nodes. These celiac nodes also receive lymph from the superior mesenteric and inferior mesenteric groups of nodes. Lymph from the celiac nodes enters the cisterna chili. Superior mesenteric artery (i.e. structures that are part of midgut) Drain to superior mesenteric group of pre-aortic lymph nodes. These superior mesenteric nodes also receive lymph form the inferior mesenteric groups of nodes. Lymph form the superior mesenteric nodes drains to the celiac nodes. Inferior mesenteric artery (i.e. structures that are part of hindgut) Drains to the inferior mesenteric group of pre-aortic lymph nodes. Lymph from the inferior mesenteric nodes drains to the superior mesenteric nodes.

Innervation The abdominal viscera are innervated by both extrinsic and intrinsic components of the nervous system. Extrinsic innervation: receives motor impulses from, and sends sensory information to, the CNS. Abdominal viscera receiving extrinsic innervation include: abdominal part of GIT Spleen Pancreas Gall bladder liver

Intrinsic innervation: This is known as the enteric nervous system and is the gut’s own selfsufficient network of sensory and motor fibres that regulates the digestive tract activities. Consists of motor and sensory neurons in two interconnected plexuses in the walls of the GIT. The function of these neurons: control the coordinated contraction and relaxation of intestinal smooth muscle and regulate gastric secretion and blood flow.

Sympathetic trunks Two parallel nerve cords extending on either side of the vertebral column. They run from the base of the skull to the coccyx. These two trunks come together anterior to the coccyx to form the ganglion impar. Small raised areas are visible throughout the sympathetic trunk. These are collections of neuronal cell bodies outside the CNS and are known as the paravertebral sympathetic ganglia. These occur in varying numbers in each region of the sympathetic trunks (labeled according to the regions of the spinal cord): Cervical region: 3 ganglia

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Thoracic region: 11 or 12 ganglia Lumbar region: 4 ganglia Sacral region: 4 or 5 ganglia Anterior to the coccyx: the ganglion impar The ganglia are therefore separated by trunks (i.e. parts of the sympathetic trunk). These are both connected to adjacent spinal nerves by grey rami communiantes in all regions of the trunk except in the thoracic and lumbar region of the trunk, where white rami communicantes are found. Neuronal fibres found in the sympathetic trunks: • Preganglionic and postganglionic sympathetic fibres • Visceral afferent fibres Splanchnic nerves Important in the innervation of the abdominal viscera. They pass from the sympathetic trunk or sympathetic ganglia to the Prevertebral plexus and ganglia anterior to the abdominal aorta. Splanchnic nerves carry visceral efferent fibres, there are 2 types of slpanchnic nerves: Thoracic, lumbar & sacral splanchnic nerves: Carry preganglionic sympathetic fibres From the sympathetic trunk to ganglia in the prevertebral plexus and also visceral afferent fibres. Pelvic splanchnic nerves (parasympathetic root): Carry preganglionic parasympathetic fibres From the S2 to S4 spinal nervesto an extension of the prevertebral plexus in the pelvis (the inferior hypogastric plexus or pelvic plexus). Thoracic splanchnic nerves There are 3 thoracic splanchnic nerves: Greater splanchnic nerve: arises from 5 – 9/10 thoracic ganglia and travels to celiac ganglia. Lesser splanchnic nerve: arises from 9 & 10 or 10 & 11 thoracic ganglia & travels to the adrenocotical region. Least splanchnic nerve: Arises from the 12th thoracic ganglion and travels to the renal plexus. Lumbar and sacral splanchnic nerves Lumbar splanchnic nerves: Sacral splanchnic nerves:

2–4 Pass from lumbar part of sympathetic trunk or associated ganglia Enter the prevertebral plexus pass from the sacral part of sympathetic trunk or associated ganglia enter the inferior hypogastric plexus ( an extension of the prevertebral plexus into the pelvis).

Pelvic splanchnic nerves These are unique as they are the only splanchnic nerves which carry parasympathetic fibres, hence they are also called the parasympathetic root. Preganglionic parasympathetic fibres originate in the sacral spinal cord. They pass from S2 – S4 spinal nerves to the inferior hypogastric plexus.

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Once in the plexus, some of these fibres pass upward into the abdominal pevertebral plexus and are distributed with the arteries supplying the hindgut. Therefore, the preganglionic parasympathetic fibres innervate the: • distal 1/3 of the transverse colon • descending colon • sigmoid colon Abdominal Prevertebral Plexus and Ganglia Definition: a collection of nerve fibres that surrounds the abdominal aorta and is continueous onto its major branches. Scattered throughout the length of this plexus are cell bodies of postganglionic sympathetic fibres, some of which are organized into ganglia, while others are distributed randomly. Those which form ganglia are usually associated with specific branches of the abdominal aorta and named accordingly. There are 3 major divisions of the abdominal prevertebral plexus and associated ganglia: • Celiac plexus Nerve fibres and ganglia associated with the roots of the celiac trunk and the superior mesenteric artery. They include: 2 celiac ganglia 1 superior mesenteric ganglion 2 aorticorenal ganglia • Aoritc plexus Nerve fibres and ganglia on the anterior and lateral surfaces of the abdominal aorta extending from just below the origin of the superior mesenteric artery to the bifurcation of the abdominal aorta into its terminal branches, the two common iliac arteries. Major ganglion here: Inferior mesenteric ganglion (at root of inferior mesenteric artery) • Superior hypogastric plexus This is the final part of the abdominal prevertebral plexus before it continues into the pelvic cavity. It contains numerous small ganglia. Inferiorly, it divides into the hypogastric nerves which descend into the pelvis and contribute to the formation of the inferior hypogastric (pelvic) plexus. Each of these plexuses gives rise to many secondary plexuses, which may also contain ganglia, and are named after the vessels with which they are associated, e.g. external iliac plexus. The abdominal prevertebral plexus receives: • Preganglionic parasympathetic and visceral afferent fibres from the vagus nerve. • Perganglionic sympathetic and visceral afferent fibres from the thoracic and lumbar splanchnic nerves. • Preganglionic parasympathetic fibres from the pelvic splanchnic nerves.

Parasympathetic Innervation Parasympathetic innervation of the following structures is from 2 sources, namely the vagus nerves [X] and the pelvic splanchnic nerves: Abdominal part of the GIT Spleen Pancreas

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Gall bladder Liver Vagus nerves Enter the abdomen associated with the oesophagus as the oesophagus passes through the diaphragm; they enter as the anterior and posterior vagal trunks. Provide parasympathetic innervation to the foregut and midgut. They send branches to the abdominal prevertebral plexus, which contain preganglionic parasympathetiv fibres and visceral afferent fibres. Pelvic Splanchnic Nerves These fibres carry preganglionic parasympathetic fibres from S2 to S4 levels. They enter the inferior hypogastric plexus in the pelvis. Some of the branches pass upward into the inferior mesenteric part of the abdominal prevertebral plexus and are distributed with the branches of the inferior mesenteric artery. Therefore, it follows that these branches provide parasympathetic innervation to the hindgut.

Enteric System This is a division of the visceral part of the nervous system. This is a local neuronal circuit in the wall of the GIT. It consists of motor and sensory neurons organized into 2 interconnected plexuses: • Myenteric plexus • Submucosal plexus These plexuses are found between the layers of the GIT wall. The enteric system also includes the associated nerve fibres that pass: -between the plexuses, and -form the plexuses to the adjacent tissue. Function: Regulates and coordinated many GIT activities: • Gastric secretory activity • Gastrointestinal blood flow • Peristalsis (contraction and relaxation cycles of smooth muscle) The enteric system is generally independent of the CNS, its activity is modified by input from the: • Postganglionic sympathetic neurons • Preganglionic parasympathetic neurons Reference: Gray’s Anatomy for Students, p. 293 - 311