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Thanks to our partners!Anatomy of Lower GI KEAGAN FERNANDES (4 Year Medical Student)Objectives tract and key landmarks:e anatomy and key features of the lower GI • Small intestine • Appendix • Caecum • Colon (ascending, transverse, descending, sigmoid) • Rectum • Anal canal • Understand and describe the arterial supply and venous drainage of the lower GI tractObjectives (secondary) Describe the anatomy of the abdominal areas, including: • Abdominal cavity • Calot’s triangle • Peritoneum & mesenteries • Inguinal Canal • Hesselbach’s Triangle • Anterolateral abdominal wall • Posterior abdominal wall Interesting factoid GI tract is conventionally divided into • upper (mouth to ileum) • lower (cecum to anus) GI bleeding:- Demarcation between the upper and lower GI tract is the duodenojejunal (DJ) flexure (ligament of Treitz); • bleeding above the DJ flexure is called upper GI bleeding • below the DJ flexure is called lower GI bleeding. Small intestine • Organ located within the GI Tract. • Average person’s small intestine length = 6.5m • Assists in digestion and Absorption of ingested food • It extends from the duodenal bulb (1 part of duodemun) to the ileocecal junction, where it meets the large intestine at the ileocecal valve. Anatomically, the small bowel can be divided into three parts:- • Duodenum • Jejunum • Ileum • Anatomy of the small intestine • Structure of the small intestine, • Neurovascular supply of small intestine Small intestine (duodenum) • Proximal portion of the small intestine is the duodenum. • Derived from the Latin ‘duodenum digitorum’, meaning twelve fingers length. • It runs from the pylorus of the stomach to the duodenojejunal junction. • The duodenum can be divided into four parts: ➢superior ➢descending ➢inferior ➢ascending • Together these parts form a ‘C’ shape, that is around 25cm long, and which wraps around the head of the pancreas. Internally, the descending duodenum is marked by the major duodenal papilla – the opening at which bile and D1- Superior (Spinal Level L1) pancreatic secretions 1 section: - “the cap” to enter from the ampulla of Vater (hepatopancre Ascends upward from the pylorus atic ampulla). of the stomach D2 – Descending (L1-L3) Connected to the liver by the Descending portion curves inferiorly around hepatoduodenal ligament. the head of the pancreas This area is the most common site Lies posteriorly to the transverse colon of duodenal ulceration Lies anterior to the right kidneyD3 – Inferior (L3) The inferior duodenum travels: laterally to the left, crossing over the inferior vena cava and aorta. It is located inferiorly to the D4- Ascending (L3-L2) pancreas After the duodenum crosses the posteriorly to the superior aorta, it ascends and curves mesenteric arteryand vein. anteriorly to join the jejunum at a sharp turn known as the duodenojejunal flexure Small intestine(duodenum) (histology) • Duodenum has a villous mucosa. • Distinct from the mucosa of the pylorus, which directly joins to the duodenum. • Like other structures of the gastrointestinal tract, the duodenum has a mucosa, submucosa, muscularis externa, and adventitia. • Glands line the duodenum, known as Brunner's glands, which secrete mucus and bicarbonate to neutralize stomach acids. • These are distinct glands not found in the ileum or jejunum, the other parts of the small intestine.Jejunum and Ileum • Distal two parts of the small intestine • Completely intraperitoneal • Attached to posterior abdominal wall by mesentery (double layer of peritoneum). • No clear external demarcation jejunum and ileum (can be identified by different mesenteric vascular arcade pattern) • Jejunum and ileum are macroscopically different • Ileum ends at the ileocecal valve.Characteristic features of Jejunum and Ileum Vasculature and Lymphatics (Duodenum) Arterial supply of the duodenum is derived from two sources: Proximal to the major duodenal papilla – Gastroduodenal artery (branch of the common hepatic artery from the coeliac trunk). Distal to the major duodenal papilla – Inferior pancreaticoduodenal artery(branch of the superior mesenteric artery) Lymphatic drainage is to the pancreatoduodenal and superior mesenteric nodes.c portal vein.Vasculature and Lymphatics (Jejunum)(Ileum) • Arterial supply to Jejunum and Ileum is from the Superior Mesenteric Artery (SMA). • Superior Mesenteric Artery arises from the aorta at the level of L1 vertebrae (IMMEDIATELY INFERIOR to the COELIAC TRUNK). • It moves in between layers of mesentery , splitting into approximately 20 branches. • These branches anastomose to form loops, called arcades. From the arcades, long and straight arteries arise, called VASA RECTA Clinical Relevance: Ileocecal valve • The ileocecal valve represents the separation between the small and large intestine. • Its main function is to prevent the reflux of enteric fluid from the colon into the small intestine. • It is also used as a landmark during colonoscopy, indicating that the limit of the colon has been reached and that a complete colonoscopy has been performed.Clinical Relevance: Ileocecal valve • The ileocaecal valve is also important in the setting of large bowel obstruction. • Should the ileocaecal valve be competent, a closed loop obstruction can occur and cause bowel perforation. • Should the ileocaecal valve be incompetent (allow backflow of enteric contents into the small bowel) then the situation is less emergent and the trajectory of the obstruction less rapid.Caecum (large intestine) • The cecum is the most proximal part of the large intestine. • Located between the ileum (distal small bowel) and the ascending colon. • Caecum acts as a reservoir for chyme which it receives from the ileum. We shall look at the anatomy of the cecum – ➢anatomical structure ➢neurovascular supply ➢lymphatic drainage.Caecum (Anatomy and relations) • The cecum is the most proximal part of the large intestine • Found in the right iliac fossa (RIF) of the abdomen. • Lies inferiorly to the ileocecal junction • Can be palpated if enlarged due to faeces, inflammation, or malignancy. • Superiorly, the cecum is continuous with the ascending colon. • Unlike the ascending colon, the cecum is intraperitoneal. Neurovascular Supply (Caecum) • The cecum is derived from the embryologic midgut. • Therefore, the vascular supply is via branches of the superior mesenteric vessels. • Arterial supply is from the ileocolic artery, a branch of the superior mesenteric artery. • It subsequently divides into anterior and posterior cecal arteries, which directly supply the cecum. • Venous drainage is provided by the corresponding ileocolic vein and empties into the superior mesenteric vein.Lymphatic Drainage of Caecum • Lymph from the cecum drains into the ileocolic lymph nodes (which surround the ileocolic artery).Appendix • The appendix is a narrow blind-ended tube that is attached to the posteromedial end of the cecum (large intestine). • It contains a large amount of lymphoid tissue but is not thought to have any vital functions in the human body (vestigial organ). we shall look at the anatomy of the appendix – ➢its anatomical structure and relations ➢neurovascular supply ➢lymphatic drainage.Appendix (anatomy and relations) • The appendix originates from the posteromedial aspect of the cecum. It is supported by the mesoappendix. • The position of the free-end of the appendix is highly variable and can be categorized into seven main locations depending on its relationship to the ileum, caecum or pelvis. • The most common position is retrocecal. • They may also be remembered by their relationship to a clock face:Appendix (anatomical structure and relations) They may also be remembered by their relationship to a clock face: • Pre-ileal – anterior to the terminal ileum – 1 or 2 o’clock. • Post-ileal – posterior to the terminal ileum – 1 or 2 o’clock. • Sub-ileal – parallel with the terminal ileum – 3 o’clock. • Pelvic – descending over the pelvic brim – 5 o’clock. • Subcecal – below the cecum – 6 o’clock. • Paracecal – alongside the lateral border of the cecum – 10 o’clock. • Retrocecal – behind the cecum – 11 o’clock.Neurovascular Supply (Appendix) • The appendix is derived from the embryologic midgut. • Therefore, the vascular supply is via branches of the superior mesenteric vessels. • Arterial supply is from the appendicular artery (derived from the ileocolic artery, a branch of the superior mesenteric artery). • Venous drainage is via the corresponding appendicular vein. • Both are contained within the mesoappendix.Neurovascular Supply (Appendix)Lymphatic Drainage (Appendix) • Lymphatic fluid from the appendix drains into lymph nodes within the mesoappendix and into the ileocolic lymph nodes (which surround the ileocolic artery).The colon (large intestine) • distal part of the gastrointestinal tract • extending from the cecum to the anal canal. • It receives digested food from the small intestine, from which it absorbs water and electrolytes to form faeces. • Anatomically, the colon can be divided into four parts – ➢Ascending ➢Transverse we shall look at the anatomy of the colon – • its anatomical structure and relations ➢Descending • clinical correlations. ➢Sigmoid. These sections form an arch, which encircles the small intestine.Ascending colon • The colon begins as the ascending colon, • a retroperitoneal structure • which ascends superiorly from the cecum. • When it meets the right lobe of the liver, it turns 90 degrees to move horizontally. • This turn is known as the right colic flexure (or hepatic flexure), and marks the start of the transverse colon.T ransverse Colon • The transverse colon extends from the right colic flexure to the spleen. • where it turns another 90 degrees to point inferiorly. • This turn is known as the left colic flexure (or splenic flexure). • Here, the colon is attached to the diaphragm by the phrenicocolic ligament. • Unlike the ascending and descending colon, the transverse colon is intraperitoneal and is enclosed by the transverse mesocolon.Descending Colon • After the left colic flexure(splenic flexure), the colon moves inferiorly towards the pelvis – and is called the descending colon. • It is retroperitoneal in the majority of individuals. • Located anteriorly to the left kidney, passing over its lateral border. • When the colon begins to turn medially, it becomes the sigmoid colonSigmoid colon • The 40cm long sigmoid colon is located in the left lower quadrant of the abdomen. • Extending from the left iliac fossa to the level of the S3 vertebra. • This journey gives the sigmoid colon its characteristic “S” shape. • The sigmoid colon is attached to the posterior pelvic wall by a mesentery – the sigmoid mesocolon.Characteristic features of Large intestine • Attached to the surface of the large intestine are omental appendices – small pouches of peritoneum, filled with fat. • Running longitudinally along the surface of the large bowel are three strips of muscle, known as the teniae coli. • The teniae coli contract to shorten the wall of the bowel, producing sacculations known as haustra. • The large intestine has a much wider diameter compared to the small intestine.Anatomical RelationsAnatomical RelationsNeurovascular Supply (colon) • The neurovascular supply to the colon is closely linked to its embryological origin: • Ascending colon and proximal 2/3 of the transverse colon – derived from the midgut. • Distal 1/3 of the transverse colon, descending colon and sigmoid colon – derived from the hindgut.Arterial supply (colon) • The ascending colon receives arterial supply from two branches of the superior mesenteric artery; ➢ileocolic ➢right colic arteries • The ileocolic artery gives rise to ➢Colic ➢anterior cecal ➢posterior cecal branches – all of which supply the ascending colon.Arterial supply (colon) • The transverse colon is derived from both the midgut and hindgut, and so it is supplied by branches of the superior mesenteric artery and inferior mesenteric artery: • Right colic artery (from the superior mesenteric artery) • Middle colic artery (from the superior mesenteric artery) • Left colic artery (from the inferior mesenteric artery) • The descending colon is supplied by a single branch of the inferior mesenteric artery; the left colic artery. • The sigmoid colon receives arterial supply via the sigmoid arteries (branches of the inferior mesenteric artery).Marginal artery of DrummondVenous drainage (colon) The venous drainage of the colon is similar to the arterial supply: • Ascending colon – ileocolic and right colic veins, which empty into the superior mesenteric vein. • Transverse colon – middle colic vein, which empties into the superior mesenteric vein. • Descending colon – left colic vein, which drains into the inferior mesenteric vein. • Sigmoid colon – drained by the sigmoid veins into the inferior mesenteric vein.Innervation (colon) • The innervation to the colon is dependent on embryological origin: • Midgut-derived structures (ascending colon and proximal 2/3 of the transverse colon) receive their sympathetic, parasympathetic and sensory supply via nerves from the superior mesenteric plexus. • Hindgut-derived structures (distal 1/3 of the transverse colon, descending colon and sigmoid colon) receive their sympathetic, parasympathetic and sensory supply via nerves from the inferior mesenteric plexus:Lymphatic drainage (colon) • The lymphatic drainage of the ascending and transverse colon is into the superior mesenteric nodes. • The descending colon and sigmoid drain into the inferior mesenteric nodes. • Most of the lymph from the superior mesenteric and inferior mesenteric nodes passes into the intestinal lymph trunks • and on to the cisterna chyli – where it ultimately empties into the thoracic duct.Rectum (large intestine) • Most distal segment of the large intestine • Important role: - Temporary store of faeces • Continues proximally → with the sigmoid colon • Terminates → anal canal we will discuss the anatomy of the rectum – ➢structure ➢anatomical relationshipsRectum (anatomical structure) • The rectum begins at the level of the S3 (as a continuation of the sigmoid colon). • It is macroscopically distinct from the colon, with an absence of taenia coli, haustra, and omental appendices. The course of the rectum is marked by two major flexures: ➢Sacral flexure ➢Anorectal flexureRectum (anatomical structure) • There are additionally three lateral flexures (superior, intermediate and inferior) • formed by transverse folds of the internal rectum wall. • The final segment of the rectum, the ampulla, relaxes to accumulate and temporarily store faeces until defecation occurs. • It is continuous with the anal canal; which passes through the pelvic floor to end as the anus.Peritoneal coverings • In the superior third of the rectum, the anterior surface and lateral sides are covered by peritoneum. • The middle third only has an anterior peritoneal covering • Lower 1/3 has no peritoneum associated with it. • In males, the reflection of peritoneum from the rectum to the posterior bladder wall forms the rectovesical pouch. • In females, the peritoneum reflects to the posterior vagina and cervix, forming the rectouterine pouch (pouch of Douglas).Rectum (anatomical relations)Rectum (neurovascular supply) • The rectum receives arterial supply through three main arteries: • Superior rectal artery – terminal continuation of the inferior mesenteric artery. • Middle rectal artery – branch of the internal iliac artery. • Inferior rectal artery – branch of the internal pudendal artery.Rectum (neurovascular supply) Venous drainage is via the corresponding: - • Superior rectal vein • Middle rectal vein • Inferior rectal vein Superior rectal vein empties into → portal venous system Middle rectal vein and Inferior rectal vein → systemic venous system Anastomoses between the portal and systemic veins are located in the wall of anal canal, making this a site of portocaval anastomosis.Rectum (innervation) • The rectum receives sensory and autonomic innervation. • Sympathetic nervous supply to the rectum is from the lumbar splanchnic nerves and superior and inferior hypogastric plexuses. • Parasympathetic supply is from S2-4 via the pelvic splanchnic nerves and inferior hypogastric plexuses. • Visceral afferent (sensory) fibres follow the parasympathetic supply.Rectum (Lymphatic Drainage) • Lymphatic drainage of the rectum is via the pararectal lymph nodes, which drain into the inferior mesenteric nodes. • Additionally, the lymph from the lower aspect of the rectum drains directly into the internal iliac lymph nodes.Anal canal • Final segment of the gastrointestinal tract • It has an important role in defecation and maintaining faecal continence. We shall look at the anatomy of the anal canal – ➢Position ➢Structure ➢Relations ➢Neurovascular supplyAnal canal (anatomical position) • The anal canal is located within the anal triangle of the perineum • between the right and left Ischioanal fossae. • It is the final segment of the gastrointestinal tract, • around 4cm in length. • The canal begins as a continuation of the rectum • passes inferoposteriorly to terminate at the anus.Anal Canal (anatomical structure) • Anal Sphincters • The anal canal is surrounded by internal and external anal sphincters, which play a crucial role in the maintenance of faecal continence: • Internal anal sphincter – surrounds the upper 2/3 of the anal canal. It is formed from a thickening of the involuntary circular smooth muscle in the bowel wall. • External anal sphincter – voluntary muscle that surrounds the lower 2/3 of the anal canal (and so overlaps with the internal sphincter). It blends superiorly with the puborectalis muscle of the pelvic floor.Internal structures • The superior aspect of the anal canal has the same epithelial lining as the rectum (columnar epithelium). • However, in the anal canal, the mucosa is organized into longitudinal folds, known as anal columns. • These are joined at their inferior ends by anal valves. • Above the anal valves are small pouches which are referred to as anal sinuses – these contain glands that secrete mucus.Internal structures • The anal valves collectively form an irregular circle – known as the pectinate line (or dentate line). • This line divides the anal canal into upper and lower parts, which differ in both structure and neurovascular supply. • This is a result of their different embryological origins: • Above the pectinate line – derived from the embryonic hindgut. • Below the pectinate line – derived from the ectoderm of the proctodeum.Anal canal (anatomical relations)Anal canal (arterial supply) (venous drainage)Anal Canal (nerve supply)(lymphatics)Calot’s triangle • Calot’s triangle (cystohepatic triangle) is a small anatomical space in the abdomen. • It is located at the porta hepatis of the liver – where the hepatic ducts and neurovascular structures enter/exit the liver. We shall look at the: - ➢borders ➢contentsClot’s triangle (borders) Calot’s triangle is orientated so that its apex is directed at the liver. The borders are as follows: • Medial – common hepatic duct. • Inferior – cystic duct. • Superior – inferior surface of the liver.Calot’s triangle (contents) The contents of Calot’s triangle include: • Right hepatic artery – formed by the bifurcation of the proper hepatic artery into right and left branches. • Cystic artery – typically arises from the right hepatic artery and traverses the triangle to supply the gall bladder. • Lymph node of Lund – the first lymph node of the gallbladder. • LymphaticsThe inguinal (Hesselbach’s triangle) • The inguinal triangle (Hesselbach’s triangle) is a region in the anterior abdominal wall. • It is alternatively known as the medial inguinal fossa. • First described by Frank Hesselbach, a German surgeon and anatomist, in 1806. we shall look at the anatomy of the inguinal triangle – ➢its borders ➢contentsThe inguinal (Hesselbach’s triangle) (borders) • The inguinal triangle is located within the inferomedial aspect of the abdominal wall. It has the following boundaries: • Medial – lateral border of the rectus abdominis muscle. • Lateral – inferior epigastric vessels. • Inferior – inguinal ligament. • Direct hernia lie in the triangle • Indirect hernia lie laterallyThanks to our partners!@supta_uk @SUPTAUK www.supta.uk