Small and Large Intestines

Small Intestine For digestion & absorption Extends from pyloric part of stomach to the ileocecal valve 6m long & 2.5cm wide Consists of three parts: Duodenum, Jejunum, Ileum Jejunum and Ileum Jejunum and Ileum are intraperitoneal structures They attach to the posterior abdominal wall by the root of mesentery The mesentery has two parts: a free part and an attached part. The free part covers several parts of Jejunum and Ileum, and the attached part holds them up and is about 15 cm long The Jejunum and Ileum are retroperitoneal They attach at the anterior to the greater omentum, and attached to the post. abdominal wall by root of mesentery Root of mesentery (duodenojejunal flexure) and terminates at ileocecal junction It passes anterior to 3rd part of duodenum, IVC, abdominal aorta, anterior surface of right major muscles, and right ureter Small intestine contains four layers: Mucosa: contains villi and folds for absorption of nutrients Submucosa: contains nerves and vessels Muscular Layer: Outer longitudinal smooth muscles, inner circular smooth muscles Serosa: Serous layer since small intestine is intraperitoneal Differences Between Jejunum and Ileum Feature Jejunum Ileum Location Up & Left Down & Right Diameter High Low Position Horizontal Vertical Mesentery Clear, low fat Dark, high fat Vasculature Long & low Short & many Lymphatic Plaques None Peyer's patches Length 2/5th 3/5th Nearly all chemical digestion and nutrient absorption occurs in the small intestine. The inner layer of the Jejunum has many prominent folds called Plicae circulares, while the inner layer of the Ileum has very discrete and small amounts of them. The mesentery covering the Jejunum has low fat and few arterial arcades, while the mesentery covering the Ileum has high amounts of fat and arterial arcades. Efferent from the arterial arcades towards the Jejunum and Ileum are short arteries called Vasa recta arteries. They are shorter in the Ileum and longer in the Jejunum. The inner surface of the Ileum, there are accumulations of lymphoid tissue called Peyer's patches. These Peyer's patch lymphoids do not exist in the Jejunum. Meckel's Diverticulum It is an embryological abnormality It is a remnant of vitelline duct (or proximal part & yolk sac) which runs from umbilical cord to ileum It is an uncommon finding and occurs in only 2% of people It is important to consider diagnosing for the possibility of Meckel's diverticulum because it does reduce symptoms in a small number of patients. Typical symptoms/findings include bleeding, hemorrhage, diverticulitis, ulceration, and obstruction Superior Mesenteric Artery It is an anterior branch of the abdominal aorta It arises from the abdominal aorta inferior to the celiac trunk, at the level of L1 vertebra It gives out several branches including: Inferior pancreaticoduodenal arteries Jejunal & Ileal arteries Middle colic artery Right colic artery Ileocolic artery Inferior pancreaticoduodenal artery is the first branch of superior mesenteric artery, and after it exits, it divides into anterior and posterior branches, and anastomoses with superior pancreaticoduodenal artery which is a branch of gastroduodenal artery. The Jejunal & Ileal arteries are 12-15 branches arising from the main trunk of mesenteric artery to the left side. They pass between the layers of the mesentery and form anastomoses with each other, which give out vasa recta striate arteries supplying the walls of the small intestine. The number of arterial arcades increases distally along the gut. The Striate arteries (vasa recta) of the Jejunum are long and close together "While the ones from the Ileum are generally short and flat apart from each other." "The Middle colic is another branch of Sup. mesenteric artery arising from the right side of the artery. It goes reaches to the transverse colon and divides into left and right branches. The right branch anastomosis with right colic artery and the left branch anastomosis with left colic artery." "The Right colic artery arises from the right side of the Sup. mesenteric artery and moves in a retroperitoneal position to supply the ascending colon. Near the colon, the right colic artery divides into ascending and descending branches. The ascending branch anastomosis with middle colic artery (right branch) and the descending branch anastomosis with the ileocolic artery." "The ileocolic artery is the last branch of Sup. mesenteric artery. It posses obliquely downward and to the right going to the right iliac fossa. It divides into superior (cecal) branch and descending branch. The superior branch supplies the first part of the ascending colon, and the inferior branch divides into anterior cecal and posterior cecal branches supplying the cecum, appendix and terminal part of ileum. Finally, the Ileal branch of Ileocolic artery anastomosis with the main trunk of the Superior mesenteric artery." Large Intestine The large intestine extends from the termination of the distal end of the ileum (ileocecal valve) to the anus. It's about 1.5m long. It's different from small intestine by that the position is more fixed. It consists of seven parts: Cecum Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anus It begins from the right iliac fossa as the cecum with an associated appendix vermiform. Then it continues upwards as the ascending colon through the right flank. Just below the liver, it terminates and bends to the left, forming the right colic flexure (hepatic flexure). Then it crosses the abdomen as the transverse colon to reach the left hypochondrium. At this point, just below the spleen, the large intestine bends downwards, forming the left colic flexure (splenic flexure) and continues as the descending colon through the left flank and into the left groin. It enters the upper part of the pelvic cavity as the sigmoid colon and terminates as the anal canal.   The large intestinal wall is of four layers (inside to outside): Mucosa - contains folds Submucosa - contains nerves and vessels Muscular layer - Longitudinal (outer) & circular (inner) segregate together to form 3 layers called taenia coli Serosa - Serous layer General features of Large Intestine: It has a larger diameter compared to small intestine. "It has a fat covering of peritoneum called Appendices epiploicae" "It has longitudinal muscles that are more concentrated in specific areas called Taeniae coli (teniae liberae). There are not three tenia coli in the ascending, transverse and descending colons but only two in sigmoid colon. Because of these Tenia coli longitudinal muscles, the walls of the large intestine sacculate and form Haustra coli, which makes the large intestine appear sacculated." "On the external wall's surface of large intestine, there are flat projections covered with peritoneal folds and are called Epiploic appendices" "The area of the cecum at ileocecal junction has some opening called ileocecal orifice through which the small intestine enters the large intestine. It is formed by two folds of superior and inferior folds which join together from the lateral and medial side, forming frenulums (medial and lateral frenulum), forming the ileocecal valve." "Cecum contains an appendicular orifice for the appendix" "Cecum is located at the right iliac fossa and starts inferior to the ileocecal junction and continues downward." "The Appendix exits through the postero-medial wall of the cecum. The opening is called Gerlach's crevice." "Vermiform appendix" It is a blind intestinal diverticulum of 10cm length It is an intraperitoneal organ connected by the mesoappendix to the ileum and cecum (mesoappendix holds the appendicular artery, lymph vessels) The base of appendix is stable, while the extended part is varying in position. It can be anterior or posterior to the ileum, and is named pre-ileal or post-ileal position. It can also be in a position descending or continuous to the pelvic cavity and is called pelvic position. It could also be in a position posterior to the cecum but pointing to the right, called sub-cecal position or retrocecal position. Surface Anatomy of Appendix The surface anatomy of the base of the appendix is the point between the lateral and middle third of a line drawn from the umbilicus to the ASIS. This point is called McBurney's point. People suffering from appendicular problems often have problems at this point, and it is where they're diagnosed. Cecum The cecum, in the posterior, is related to iliacus and psoas major muscles, femoral and lateral cutaneous nerve, and thigh. Ascending Colon It is 20cm long. Located in the right lumbar region, or right flanks. Starts at the ileocecal junction and terminates at the right colic flexure. Ascending colon is a Secondary Retroperitoneal structure (because it was secondarily retroperitoneal at different developmental stages). Ascending colon is related posteriorly to: Psoas major Quadratus lumborum Transversus abdominis Four nerves: Femoral nerve Lateral cutaneous nerve of thigh Ilioinguinal nerve Iliohypogastric nerve Transverse Colon It is 45-50cm long. Located in the umbilical and epigastric regions. It is intraperitoneal and hangs downwards, suspended by the transverse mesocolon (mesentery of transverse colon). Transverse Colon Transverse mesocolon's root starts from the left kidney and passes the pancreas and second part of duodenum and terminates at the right kidney. Transverse colon starts from hepatic flexure and terminates at the splenic flexure. Hepatic flexure (HCF) Turns down and forward Liver anterosuperior lateral Gallbladder anteromedial Quadratus lumborum Diaphragm Descending Colon It is 25cm long. Starts from splenic flexure and terminates at the left pelvic brim or inlet, where it becomes continuous with the sigmoid colon. It moves across the left hypochondrium, left flank/lumbar region, and left groin iliac region. It is a secondary peritoneal structure. It is related posteriorly to the same structures as the ones in ascending colon: the four muscles and four nerves (in addition to two other structures) Iliacus Psoas major Quadratus lumborum Transversus abdominis Diaphragm Subcostal nerve Iliohypogastric nerve Sigmoid Colon It is about 40cm long Begins at the descending colon at the pelvic brim or inlet Hangs down into the pelvic cavity in the form of a loop and it is continuous with the rectum at the level of S3 vertebra where it becomes attached to the posterior abdominal wall by the fan-shaped sigmoid mesocolon It is an intraperitoneal structure The beginning part and end part of sigmoid colon are fixed, but the middle region is sort of movable, so the sigmoid colon is variant in position and this sometimes causes Sigmoid Volvulus Large & Small Intestine Large & Small intestine are supplied by superior mesenteric artery. It supplies the jejunum and ileum of small intestine, and the ascending and transverse colon. The other parts of large intestine (the distal end of transverse colon, descending colon, sigmoid colon, rectum) are supplied by Inferior mesenteric artery. Inferior Mesenteric Artery Inferior mesenteric artery is an anterior branch of abdominal aorta, and it has three branches: Left colic artery Sigmoidal artery Superior rectal artery Left Colic artery is the first branch of inferior mesenteric artery. It ascends in a retroperitoneal position and divides into ascending and descending branches. The ascending branch passes anterior to the left kidney and enters the transverse mesocolon and anastomosis with the left branch of middle colic artery. Descending Colon The descending branch passes inferiorly, supplying the lower part of the descending colon and anastomoses with the first sigmoid artery. Sigmoid Artery   The sigmoid artery consists of two or three branches which descend on the left through the sigmoid mesocolon, supplying the lower parts of the descending colon and the sigmoid colon. It anastomoses superiorly with the descending branch of left colic artery and inferiorly with the superior rectal artery. Superior Rectal Artery Superior rectal artery is the terminal branch of inferior mesenteric artery. It descends to the rectum, supplying the rectum's superior part with blood. Arterial Arcades These branches form the Drummond or marginal arch at the wall of large intestine. Another arch exists, called Ridan arch, located at the transverse colon, and is formed by right colic, middle colic, ileocolic arteries, anastomosing together. Venous Drainage Venous drainage of large intestine is to the superior and inferior mesenteric veins. The inferior mesenteric vein, posterior to the body of pancreas, joins with the splenic vein. Then, posterior to the neck & pancreas, they join with the superior mesenteric vein to form the portal vein. Then the portal vein enters the liver to take in venous drainage, then drains to the IVC as the hepatic vein.