the large intestine is a part of the digestive tract specialized in absorbing water from the residual digested food coming from the small intestines while forming and storing feces until defecation occurs the large intestine or colon begins at the ilocal junction where it is continuous with the ilum which is the final part of the small intestine there are three main features that distinguish the large intestine from the small intestine besides the fact that the large intestine has a well larger caliber first the large intestine has momental appendices which present as fatty outgrowths covered by visceral perenium second there are the Tena coli which are three strips of smooth muscle that run lengthwise from the base of the appendix through the colon and merge at the recto sigmoid Junction to form a longitudinal layer around the rectum the third and final differentiating feature is that the large in has HRA which are pouch-like bulges of the intestinal wall that form between the tener when they contract the large intestine has several major components the seeum appendix ascending transverse and descending colon sigmoid colon rectum and anal Canal the seeum is the first part of the large intestine and it receives the terminal ilium which invaginates into the medial side of the seeum the seeum lies in the right ilc fossa in the right lower quadrant of the abdomen and it looks a bit like an intestinal pouch it is intrarenal mobile and doesn't have its own mesentary on its posteromedial wall inferior to the ilocal junction the seeum has a blind-ended organ called the appendix which can vary in length up to and over 10 cm the appendix is usually retral meaning behind the seeum but its position can vary a bit as well it's full of lymphoid tissue and its proximal part has attachment to the seeum by a small mesentary called the meso appendix now if we open up the large intestine and look inside we can see that the terminal ilum protrudes a bit into the seeum forming the ilal Pilla with the ilal orifice at its Center the orifice is usually closed due to Tonic contraction and the lips of the ilal Pilla serve as passive flaps to prevent regurgitation of feal matter from the seeum into the ilium just inferior to the ilal orifice there's the opening to the appendix finally starting at the seeum and throughout the whole large intestine there are multiple semi lunar folds that are found in between and separate the HRA from one another then the seeum continues as the second part of the large intestine or the ascending colon the descending colon extends superiorly from the seeum along the right side of the abdominal cavity towards the right lobe of the liver inferior to the liver and below the ninth and 10th ribs the colon turns to the left forming the right colic flexure also called the hepatic flexure the ascending colon is positioned retrop peronally therefore just behind the peritoneum laterally between the ascending colon and the lateral abdominal wall is a shallow trench covered by parietal perenium called the right paraca gutter medial to the ascending colon is the small intestine while anteriorly is the greater momentum now after the ascending colon comes the transverse colon which extends from the right colic flexure or hepatic flexure and travels horizontally to the left once At the Spleen it turns downwards in front of the lower portion of the left kidney to become the descending colon and this gives us the left colic flexion or splenic flexure which is more Superior than the hepatic flexure and has a more acute angle the transverse colon is intraperitonial and therefore mobile and can hang down to the level of the umbilicus it has a mesentary called the transverse mesocolon that is rooted alongside the inferior border of the pancreas Loops down and can extend past the iliac crests the fourth part of the colon is the descending colon that stretches from the left colic Flex fure lateral to the left kidney down to the left ilc faasa where it connects to the sigmoid colon just like the ascending colon the descending colon is retrop peraonal and covered by perenium on its anterolateral side also similar to the right side of the body there's a left paracolic gutter between the descending colon and the left abdominal wall in the left ilc fossa the descending colon ends and the fifth part of the large intestine known as the sigmoid colon follows as it descends towards the third sacral vertebra the sigmoid colon makes an S shape as it connects and continues as the rectum the sigmoid colon is intrarenal very mobile and has a mesentary called the sigmoid mesocolon the rectum is the terminal part of the large intestine is fixed retroperitoneally and is continuous inferiorly as the anal Canal also note that the Tina coli mentioned previously end at the rectum where they form a continuous muscle layer and this transition Point indicates the recto sigmoid Junction let's take a quick break and see if you can identify all parts of the large intestine now let's talk about the arterial supply of the large intestine the seeum is supplied by the superior mesenteric artery through its terminal branch called the iloc colic artery and the appendix is supplied by the appendicular artery which in turn is a branch of the iloc colic artery the ascending colon receives arterial blood from the branch of the SMA called the right cholic artery and also from the iloc colic artery and these two arteries also Anastos with each other the transverse colon is supplied by a branch of the SMA called the middle kolic artery the middle kolic artery Anastos is with the right kolic artery and they together Supply the right kolic flexure the descending and sigmoid colon are supplied by the inferior mesenteric artery or IMA for short that gives two branches the left kolic artery and the sigmoid arteries which Anastos with each other the left kolic artery also anastomoses with the middle kolic artery at the left colic flexure which IT Supplies the IMA gives one last terminal branch called The Superior rectal artery that supplies the Superior part of the rectum now looking at the blood supply of the whole large intestine from the seeum to the rectum we can see that all adjacent major arteries including the iloc colic right middle left kolic and sigmoid arteries share anastomoses with each other this gives rise to the marginal artery which runs the entire length of the colon close to the mesenteric border now Venus blood from the seeum and appendix drain into the iloc colic vein the ascending colon drains into the iloc colic and right colic vein and from the transverse colon into the middle colic vein which all drain into the superior mesenteric vein blood from the descending and sigmoid colon is collected into the left cholic veins and sigmoid veins which drain into the inferior mesenteric vein which further drains into the splenic vein and finally into the hepatic portal vein lymphatic vessels of the large intestine drain into the local lymph nodes around the intestines and then into lymph nodes following the AR that Supply those regions therefore the lymph from the seeum appendix ascending and transverse colon flows to the iloc colic nodes right cholic nodes and middle kolic nodes respectively which all drain into the superior mesenteric lymph nodes lymphatics from the descending and sigmoid colon drain into the left kolic lymph nodes along the left kolic artery and eventually pass into the inferior mesenteric lymph nodes and finally let's look at the innervation of the large intestine before we start though recall that sympathetic fibers to the large intestine derive from the abdomino pelvic splenic nerves composed of thoracic and Lumbar splenic nerves and parasympathetic fibers are from the vagus nerve and pelvic spanic nerves so for the seeum appendix ascending and transverse colon sympathetic fibers derive mainly from the Lesser thoracic splank niic nerves coming from T10 and t11 while the parasympathetic fibers are derived from the vagus nerves together they go to the superior and inferior mesenteric plexuses where only the sympathetic Fiers synapse in different prevertebral ganglia to their post synaptic neurons then both of these nerve Pathways pass through to the periarterial plexuses where the parasympathetic fibers will eventually synapse to their post synaptic fibers through intrinsic ganglia located within the intestinal walls after synapsing in their respective ganglia both nerve Pathways eventually go on to inate the seeum appendix ascending and transverse colon for the descending and sigmoid colon the sympathetic innervation is mainly from the least thoracic Spann nerves coming from T12 as well as the lumbar splenic nerves coming from L1 to L3 which all go on to travel through the superior and inferior mesenteric plexuses and Superior hypogastric plexus where they will synapse with fibers event traveling through the periarterial plexuses before reaching their end destination in regard to the parasympathetic inovation the vus nerve stops its parasympathetic innervation at the level of the left cholic flexure so below this point the parasympathetic intervation now comes from the pelvic splenic nerves which derive from the anterior Rami of spinal nerves S2 to S4 these fibers then travel through the inferior hypogastric plexus and eventually synapse in intrinsic ganglia throughout the abdominal viscera near the structures they inate after synapsing in their respective ganglia both nerve Pathways eventually go on to inate the descending and sigmoid colon remember sympathetic innervation reduces peristaltic and secretory activity to stop digestion where parasympathetic increases peristaltic and secretory activity facilitating digestion and last there are the autonomic visceral afren fibers which conduct reflex and pain Sensations for the large intestine reflex sensation always follows parasympathetic fibers however the visceral afference for pain nerve fibers can differ depending on if they are sensing above or below the pelvic pain Line This pelvic pain line represents the inferior limit of the perenium that extends into the pelvic cavity and abdominal pelvic organs can be either Superior or inferior to this line now recall that for structures above the pelvic pain line or in contact with the abdominal peritoneum visceral afron sensing pain travel with sympathetic fibers whereas for structures below the pelvic pain line typically sub peronal pelvic viscera visceral afron sensing pain travel with the parasympathetic fibers however an exception to this is that the path of the visceral afrine sensing pain for the large intestine does not follow the pelvic pain line rule we just described and instead the dividing line on whether pain travels with sympathetic versus parasympathetic is determined by a line going through the middle of the sigmoid colon instead of the pelvic pain line before we hit the recap let's take a break and see if you can identify the main arteries of the large intestine all right as a quick recap the large intestin consists of the seeum appendix ascending transverse descending and sigmoid colon and the rectum and anal Canal the ascending and descending colon and the rectum are retrop peronal while the seeum transverse and sigmoid colon are intrarenal on the outside the large intestine has momental appendices tenia coli and hastra that differentiate it from the small intestine as well as semi lunar folds on the inside the seeum has an ilal orifice and ilal Pilla at the iloc colic Junction and an opening for the base of the appendix the large intestine is supplied from SMA and IMA via the ilic left cholic artery middle kolic artery and right kolic artery sigmoid and Superior rectal arteries veins follow arteries and have the same names as they do and eventually drain into into the hepatic portal vein lymphatics drain into the nodes of the main arteries supplying the colon and eventually drain into the superior and inferior mesenteric nodes sympathetic innervation derives from the abdomino pelvic splank niic nerves specifically the Lesser splank niic nerve least splank niic nerve and lumber spanic nerves while the parasympathetic fibers derive from the vagus nerves or pelvic splank nerves visceral afren fibers transmitting pain from the large intestine follow sympathetic fibers above the middle of the sigmoid colon and parasympathetic fibers below the middle of the sigmoid colon and remember reflex sensation always follows parasympathetic fibers helping current and future clinicians Focus learn retain and Thrive learn more