Video: Large intestine

Hello everyone! This is Megan from Kenhub, and welcome to another anatomy tutorial. In today's tutorial, we're going to be talking about the large intestine. The function of the large intestine is to absorb fluids and salts from the indigestible food matter and to collect and eventually relieve the body of waste matter. This tutorial will go over some anatomical features specific to the large intestine as well as discussing its different segments. To finish up the tutorial, we'll talk about a few pathological disorders which are related to the large intestine.

To begin this tutorial, we will describe the different sections of the large intestine. If we go through the length of the large intestine from left to right, we can see several sections. The large intestine is divided into the caecum (cecum) with its vermiform appendix, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum and anal canal.

The first part of the large intestine that we'll talk about is the cecum. This is an intraperitoneal pouch-like structure that is about 7.5 centimeters in diameters. It is usually in contact with the anterior abdominal wall and sometimes may cross the pelvic brim to lie in the true pelvis. In the superior border of the cecum, we can see this invagination known as the ileocaecal valve or the ileal papilla. This structure is located just in the opening between the small and large intestine. This valve prevents reflux from the cecum to the ileum and regulates the passage of contents from the ileum to the cecum.

In the next image, we can see the terminal portion of the ileum prior to the ileocaecal junction. In addition, in the lateral wall of the cecum, we can see this fold of peritoneum which is part of the mesentery. This large fan-shaped double-layered folded peritoneum connects the jejunum and the ileum to the posterior abdominal wall and its inferior end. As you can see in the top right image, it lies at the ileocaecal junction.

The second part of the large intestine we will talk about today is the vermiform appendix. The vermiform appendix is a narrow, hollow, and blind-ended four-inch tubular intraperitoneal structure. It arises from the posteromedial side of the cecum and it has large aggregations of lymphoid tissue in its walls. If you look carefully here near its base, it's suspended from the terminal ileum by its mesenteric fold which is known as the mesoappendix and which contains the appendicular blood vessels. Another important point to remember here is that although the point of attachment to the cecum is rather stable, the location of the rest of the appendix varies considerably.

Next, let's talk about the ascending colon which is a retroperitoneal section of the colon. This means that the anterior surface is covered by the peritoneum and, therefore, it's firmly attached to the posterior abdominal wall. You can try and remember the name of this section of the colon by thinking of the fact that waste material will ascend through it. It is found on the right side of the posterior abdominal wall and stretches between the cecum and the liver. The superior most part of the ascending colon turns right to form the next section. This curvature – as you can see in this image – is located before the liver and thus is called the hepatic flexure – or the right colic flexure.

At the hepatic flexure, the next section of the large intestine begins which is referred to as the transverse colon. This one is the largest of the sections of the large intestine and, as you can see in the image on the right, extends up to another flexure which is just below the spleen and, for that reason, it's called the splenic flexure – or the left colic flexure. An important point to remember here is that the transverse colon is an intraperitoneal section. This means that the peritoneum fully covers it and also creates the peritoneal fold that connects it to the posterior abdominal wall. This fold – which is highlighted the next image – is a part of the mesenteries and it's known as the transverse mesocolon. You can find out more about the mesenteries in our tutorial in Kenhub.

The next part of the large intestine is the descending colon. Like the ascending colon, the descending colon is retroperitoneal, meaning that it's firmly attached to the retroperitoneal space. Like the ascending colon, you can try and remember the name of this section of the colon by thinking of the fact that waste material will descend through it. It is found on the posterior left side of the abdominal wall extending from the splenic flexure to the sigmoid colon which is the next part of the intestine we will talk about.

The sigmoid colon is intraperitoneal which, as we mentioned before, means within the peritoneal sheaths in the abdominal cavity. It is suspended via the sigmoid mesocolon that you can see here in this image on the right. The sigmoid colon is the length between the descending colon and the rectum. Sigmoid means S-shaped, so this is an easy way of remembering this section of the colon as it's an S-shaped loop.

We will now move on to talk about the next section of the large intestine – the rectum. The rectum starts at the level of the third sacral vertebra and it's a 15-centimeter muscular tube which connects the colon to the anal canal where it's fixed and continuous. The very first part of the rectum is intraperitoneal but as it descends into the pelvic floor to the anal canal, it becomes retroperitoneal.

The final part of the large intestine is referred to as the anal canal. The anal canal arises from the lower end of the rectum and extends from the pelvic floor to the anus. So, now that we've looked all the parts of the large intestine, I'd like to make clear that it starts here in the cecum and terminates here at the rectum. Sometimes we used the term colon to describe this organ but if we want to be more accurate and anatomically correct, we must know that colon refers to the ascending, transverse, descending and sigmoid colon.

Also, please note that all parts of the large intestine except the transverse and sigmoid colon are retroperitoneal, meaning that their ventral surface is covered with a layer of peritoneum and, therefore, they are firmly attached to the abdominal wall. On the other hand, the transverse and sigmoid colon are intraperitoneal, meaning that they are covered fully by peritoneum and that they are actually suspended from the abdominal wall by mesenteries. That is the reason that these two parts of the large intestine are more mobile in relation to the other parts.

So until now, we have seen and described the different parts that make up the large intestine, now let's talk about some anatomical features which are specific to this structure. These features are helpful when trying to distinguish it from the rest of the alimentary canal.

The first feature that we will talk about is the taenia coli. The taenia coli are three thickened longitudinal bands of smooth muscle that span the length of the colon with the exception of the appendix and the rectum. Each of the three longitudinal bands has its own name and we will now look at these individually in more detail.

The taenia – which is highlighted in this image – is referred to as the taenia libera or the free taenia. These taeniae are not attached to the mesentery within the gastrointestinal tract, hence, its name. In the ascending and descending colon, these taeniae face anteriorly as you can see in this image of the descending colon. In contrast, they face downwards and slightly posterior in the transverse colon.

The taenia highlighted in this image is referred to as the taenia mesocolica – or the mesocolic taenia. This taenia face medially in the ascending and descending colon as you can see in this image on the right. The final taeniae coli which we will talk about are the taenia omentalis – or the omental taenia. This taenia face laterally in the ascending and descending colon and anteriorly in the transverse colon. Here we can see the taenia omentalis highlighted in green on the ascending colon and here on the transverse colon.

The second feature we will talk about are the haustra. These are the sacculations or bulging pouches you can see in this image which are formed by the contraction of the smooth muscular layer. These haustra give the colon its segmented appearance. This segmented appearance is preserved in the internal surface as well as you can see in these areas where we have removed part of the anterior wall of the colon. If we look carefully here in the mucosa of the colon, we can observe these folds or ridges between the sacculations. These are the semilunar plicae in plural – or plica for singular – of the colon and they are formed by the thickened underlying muscular fibers.

Another external feature of the large intestine that helps you distinguish from the small intestine are these structures here that you see now highlighted in green. These are referred to as epiploic appendages in plural or if we talk about one of them, the epiploic appendix. These elongated serosal fat saccules are distributed longitudinally in the ascending, transverse, descending and sigmoid colon.

The final feature we will mention is the greater omentum. The greater omentum is a double-layered fold of peritoneum which is attached to the posterior part of the stomach as you can see here in this image on the left. This omentum hangs from the stomach and inserts onto the anterosuperior aspect of the transverse colon as you can see in the image on the right.

Now that we've discussed each of the sections of the colon, we can now talk about the blood supply, lymphatic drainage and nerve supply of these different sections. Knowing the supply of these sections is important in clinical practice because if one of the arteries was, for example, obstructed, then you need to know which section of the large intestine is in risk of ischemia.

We will first talk about the blood supply of the cecum. The cecum is supplied by branches of the ileocolic artery which is a branch of the superior mesenteric artery and is seen here in this image. The cecum is drained by the ileocolic vein. The ileocolic artery and the ileocolic vein are also involved in the blood supply of the ascending colon. The cecum is drained by the ileocolic lymph nodes which are highlighted in green in this image. In the next image, we can see the superior mesenteric plexus highlighted in green. Branches from this plexus supplied the cecum.

So, to quickly summarize, the cecum is supplied by branches of the ileocolic artery and drained by the ileocolic vein. Lymph is drained by the ileocolic lymph nodes and the nerve supply of the cecum is derived from the superior mesenteric plexus.

Now let's move on to talk about the blood, lymphatic and nerve supply of the ascending colon. As I mentioned before, the ascending colon is similar to the cecum and that it's supplied by the ileocolic artery and drained by the ileocolic vein. However, it's also supplied by the right colic artery – another branch of the superior mesenteric artery – and it's highlighted in green in this image. The ascending colon is also drained by the right colic vein. The lymph from the ascending colon is filtered by the epicolic and the paracolic lymph nodes. Branches from the superior mesenteric plexus also supply the ascending colon as well as the cecum.

So, to quickly summarize, the ascending colon is supplied by the ileocolic artery and the right colic artery and drained by the ileocolic vein and the right colic vein. Lymph is drained from the epicolic and paracolic lymph nodes and the nerve supply of the ascending colon is derived from the superior mesenteric plexus.

So now let's move on to talk about the blood, lymphatic and nerve supply of the transverse colon. The transverse colon is supplied by the right colic artery which you can see in this image but also by two more arteries that we'll see in the next slides. A very important artery that supplies the transverse colon is the middle colic artery which is also a branch of the superior mesenteric artery and is highlighted in this image. Branches of the left colic artery also supply the transverse colon. Please note here that the left colic artery is a branch of the inferior mesenteric artery. So, these are the three main arteries involved in the blood supply of the transverse colon. The blood is mainly drained by its corresponding veins known as the right, middle and left colic veins. Nerve innervation is by the superior and inferior mesenteric plexuses and lymph is drained by the middle colic nodes.

So, to quickly summarize, the transverse colon is supplied by the right, middle and left colic arteries and drained by the right, middle and left colic veins. Lymph is drained from the middle colic lymph nodes and the nerve supply of the transverse colon is derived from the superior and inferior mesenteric plexus.

So now let's move on to talk about the blood, lymphatic and nerve supply of the descending colon. The descending colon is supplied by the left colic artery seen in this left image and the superior sigmoid artery as seen in the right image. Both these arteries are branches of the inferior mesenteric artery. The corresponding veins – the left colic vein and the superior sigmoid vein – drain the descending colon. Left colic nodes are involved in the lymphatic drainage of the descending colon. Now, moving on to the nerve supply of the descending colon. The descending colon is innervated by the superior hypogastric plexus sympathetically and is highlighted in green in the left image and, parasympathetically, it's innervated by the pelvic splanchnic nerves which are seen here on the right.

So, to briefly summarize, the descending colon is supplied by the left colic artery and the superior sigmoid artery and is drained by the left colic vein and the superior sigmoid vein. Left colic nodes are involved in the lymphatic drainage of the descending colon and the nerve supply is derived from the superior hypogastric plexus sympathetically and the pelvic splanchnic nerves parasympathetically.

The sigmoid colon has a very similar supply to that of the descending colon. It's supplied by the left colic artery and the superior sigmoid artery and drained by the left colic vein and the superior sigmoid vein. The paracolic and the epicolic nodes are involved in the lymphatic drainage of the sigmoid colon. The nerve supply is derived from the superior hypogastric plexus sympathetically and the pelvic splanchnic nerves parasympathetically.

Finally, we will talk about the blood supply, lymphatic drainage and nerve supply of the rectum and the anal canal. The rectum and anal canal are supplied by the rectal arteries which are highlighted here in these images. We can see the superior rectal artery, the middle rectal artery and the inferior rectal artery. These arteries are branches of either the inferior mesenteric artery or the internal iliac arteries.

The rectum and the anal canal are drained by the rectal veins which are highlighted in green in these images. Here, we see the inferior rectal veins, the superior rectal vein and the middle rectal vein. The epirectal and pararectal nodes are involved in the lymphatic drainage of the rectum and the anal canal. The nerve supply of the rectum and anal canal is derived from the superior hypogastric plexus seen here in the left image and the inferior hypogastric plexus highlighted in the right image.

So, to summarize, the rectum and anal canal are supplied by the rectal arteries and drained by the rectal veins. The pararectal and epirectal nodes are involved in the lymphatic drainage and the nerve supply is derived from the superior and inferior hypogastric plexuses.

Now that we've gone over the blood supply, lymphatic drainage and nerve supply of the large intestine, we can now move on to talk about a few clinical disorders which can affect the colon. One of these disorders is referred to as intestinal obstruction. This occurs when the bowel becomes twisted, blocked by food or stops working altogether. Symptoms include pain, vomiting, abdominal distension and constipation.

Colitis is referred to as inflammation of the walls of the colon. It can be caused by a bacterial infection or by inflammatory bowel disease commonly known as IBD. Diverticulosis occurs when certain areas of the colon's muscular wall become weak and the intestinal lining protrudes forming little pouches which can be subject to bleeding or inflammation.

The final clinical condition we'll mention is appendicitis. Appendicitis occurs when the appendix becomes inflamed due to a blockage of its lumen. The blockage is usually due to calcified feces known as fecalith. Those with appendicitis usually present with abdominal pain, nausea, vomiting and lack of appetite. Treatment for appendicitis is usually surgical removal of the appendix known as an appendectomy as this can prevent rupture of the appendix which can be a very serious complication of appendicitis.

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Now, good luck everyone, and I will see you next time.