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 ileocecal junction, where it is continuous with the ileum, 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 omental appendices, which present as fatty outgrowths covered by visceral peritoneum. Second, there are the tinea coli, which are three strips of smooth muscle that run lengthwise from the base of the appendix through the colon and merge at the rectosigmoid junction to form a longitudinal layer around the rectum. The third and final differentiating feature is that the large intestine has hostra, which are pouch-like bulges of the intestinal wall that form between the tinea when they contract. The large intestine has several major components.
The cecum, appendix, ascending, transverse, and descending colon, sigmoid colon, rectum, and anal canal. The cecum is the first part of the large intestine, and it receives the terminal ileum, which invaginates into the medial side of the cecum. The cecum lies in the right iliac fossa in the right lower quadrant of the abdomen, and it looks a bit like an intestinal pouch. It is intraperitoneal, mobile, and doesn't have its own mesentery. On its posteromedial wall, inferior to the ileocecal junction, the cecum has a blind-ended organ called the appendix, which can vary in length up to the middle of the iliac.
up to and over 10 cm. The appendix is usually retrocecal, meaning behind the cecum, but its position can vary a bit as well. It's full of lymphoid tissue and its proximal part has attachment to the cecum by a small mesentery called the mesoappendix.
Now if we open up the large intestine and look inside, we can see that the terminal ileum protrudes a bit into the cecum, forming the ileal papilla, with the ileal orifice at its center. The orifice is usually closed due to tonic contraction, and the lips of the ileal papilla serve as passive flaps to prevent regurgitation of fecal matter from the cecum into the ileum. Just inferior to the ileal orifice, there's the opening to the appendix.
Finally, starting at the cecum and throughout the whole large intestine, there are multiple semilunar folds that are found in between and separate the hostra from one another. Then, the cecum continues as the second part of the large intestine, or the ascending colon. The ascending colon extends superiorly from the cecum along the right side of the abdominal cavity towards the right lobe of the liver.
Inferior to the liver and below the 9th and 10th ribs, the colon turns to the left, forming the right colic flexure, also called the hepatic flexure. The ascending colon is positioned retroperitoneally, therefore just behind the peritoneum. Laterally, between the ascending colon and the lateral abdominal wall is a shallow trench covered by parietal peritoneum called the right paracolic gutter.
Medial to the ascending colon is the small intestine, while anteriorly is the greater omentum. 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 flexure, or splenic flexure, which is more superior than the hepatic flexure and has a more acute angle. The transverse colon is intraperitoneal and therefore mobile and can hang down to the level of the umbilicus. It has a mesentery called the transverse mesocolin 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 flexure, lateral to the left kidney, down to the left iliac fossa, where it connects to the sigmoid colon. Just like the ascending colon, the descending colon is retroperitoneal and covered by peritoneum on its anterolateral side. Also similar to the right side of the body, there's a left pericolic gutter between the descending colon and the left abdominal wall.
In the left iliac 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 intraperitoneal, very mobile, and has a mesentery 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 tinea coli mentioned previously end at the rectum, where they form a continuous muscle layer and this transition point indicates the rectosigmoid 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 cecum is supplied by the superior mesenteric artery through its terminal branch called the ileocolic artery, and the appendix is supplied by the appendicular artery, which in turn is a branch of the ileocolic artery. The ascending colon receives arterial blood from the branch of the SMA called the right colic artery, and also from the ileocolic artery, and these two arteries also anastomose with each other.
The transverse colon is supplied by a branch of the SMA called the middle colic artery. The middle colic artery anastomoses with the right colic artery, and they together supply the right colic flexure. The descending and sigmoid colon are supplied by the inferior mesenteric artery, or IMA for short, that gives two branches, the left colic artery and the sigmoid arteries, which anastomose with each other.
The left colic artery also anastomoses with the middle colic 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 cecum to the rectum, we can see that all adjacent major arteries, including the iliocolic, right, middle, left colic, 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, venous blood from the cecum and appendix drain into the ileocolic vein, the ascending colon drains into the ileocolic 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 colic 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 arteries that supply those regions. Therefore, the lymph from the cecum, appendix, ascending and transverse colon flows to the iliocolic nodes, right colic nodes, and middle colic nodes, respectively, which all drain into the superior mesenteric lymph nodes. Lymphatics from the descending and sigmoid colon drain into the left colic lymph nodes along the left colic 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 abdominopelvic splanchnic nerves composed of thoracic and lumbar splanchnic nerves, and parasympathetic fibers are from the vagus nerve and pelvic splanchnic nerves. So, for the cecum, appendix, ascending, and transverse colon, sympathetic fibers derive mainly from the lesser thoracic splanchnic 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 fibers synapse in different prevertebral ganglia to their postsynaptic neurons. Then, both of these nerve pathways pass through to the periarterial plexuses. where the parasympathetic fibers will eventually synapse to their postsynaptic fibers through intrinsic ganglia located within the intestinal walls.
After synapsing in their respective ganglia, both nerve pathways eventually go on to innervate the cecum, appendix, ascending, and transverse colon. For the descending and sigmoid colon, the sympathetic innervation is mainly from the least thoracic splanchnic nerves coming from T12. as well as the lumbar splanchnic 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 eventually traveling through the periarterial plexuses before reaching their end destination.
In regard to the parasympathetic innervation, the vagus nerve stops its parasympathetic innervation at the level of the left colic flexure, so below this point the parasympathetic innervation now comes from the pelvic splanchnic 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 innervate. After synapsing in their respective ganglia, both nerve pathways eventually go on to innervate 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 afferent fibers, which conduct reflex and pain sensations for the large intestine. Reflex sensation always follows parasympathetic fibers. However, the visceral afferents 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 peritoneum 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 afferent sensing pain travel with sympathetic fibers, whereas for structures below the pelvic pain line, typically subperitoneal pelvic viscera, visceral afferent sensing pain travel with the parasympathetic fibers.
However, an exception to this is that the path of the visceral afferent 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 pain. 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.
Alright, as a quick recap... The large intestine consists of the cecum, appendix, ascending, transverse, descending, and sigmoid colon, and the rectum and anal canal. The ascending and descending colon and the rectum are retroperitoneal, while the cecum, transverse, and sigmoid colon are intraperitoneal. On the outside, the large intestine has omental appendices, tinea coli, and hostra that differentiate it from the small intestine, as well as semilunar folds on the inside. The cecum has an ileal orifice and ileal papilla at the ileocolic junction and an opening for the base of the appendix.
The large intestine is supplied from SMA and IMA via the ileocolic, left colic artery, middle colic artery, and right colic artery, sigmoid, and superior rectal arteries. Veins follow arteries and have the same names as they do and eventually drain 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 abdominal pelvic splanchnic nerves, specifically the lesser splanchnic nerve, least splanchnic nerve, and lumbar splanchnic nerves, while the parasympathetic fibers derive from the vagus nerves or pelvic splanchnic nerves. Visceral afferent 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.
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