The colon, or large intestine, is the final part of the gastrointestinal tract. It runs from the cecum to the anal canal, receiving the partially digested remains of food from the small intestine, from which it absorbs water and electrolytes to form faeces. Anatomically, the large intestine can be divided into four parts – the ascending colon, transverse colon, descending colon, and sigmoid colon – which together form an arch, encircling the small intestine.
In this article, we shall look at the anatomy of the colon, examining the structures and relations of the four parts, the neurovascular supply, and clinical correlations.
The human colon is an average of 150cm in length, and is broken down into four sections (in order from proximal to distal) – the ascending colon, transverse colon, descending colon, and sigmoid colon.
The ascending colon is the first section of the large intestine, and begins at the cecum. It is a retroperitoneal structure, meaning it runs superiorly, or upwards, from the cecum. When it meets the right lobe of the liver, it makes a 90 degree turn and begins to move horizontally – this is known as the right colic flexure, or hepatic flexure, and marks the beginning of the transverse colon.
The transverse colon is the second part of the large intestine, extending from the right colic flexure to the spleen, where it makes another 90 degree turn and points inferiorly. This turn is known as the left colic flexure, or splenic flexure. The transverse colon is held in place by the phrenicocolic ligament, which attaches it to the diaphragm. It is intraperitoneal, meaning that it is enclosed by the transverse mesocolon, and is notable for being the least fixed part of the colon – in tall, thin individuals, it may dip down towards the pelvis.
The third section is the descending colon, which begins after the left colic flexure and moves inferiorly towards the pelvis. It is retroperitoneal in the majority of individuals, and passes over the lateral border of the left kidney. When it begins to turn medially, it transitions into the sigmoid colon.
The fourth and final part of the colon is the sigmoid, a 40cm-long structure which lies in the left lower quadrant of the abdomen – extending from the left iliac fossa to the level of the S3 vertebra. The sigmoid's characteristic “S” shape is created by its journey along the posterior pelvic wall, which is held in place by a mesentery – the the sigmoid mesocolon. This mesentery gives the sigmoid colon a greater degree of mobility than the other parts of the large intestine.
The large intestine can be easily distinguished from the small intestine due to its larger diameter and the presence of certain features, which are absent in the small intestine. On the surface of the large intestine, there are omental appendices – small pouches of peritoneum, filled with fat. Longitudinally along the surface of the large bowel are three strips of muscle, known as the teniae coli, which contract to shorten the wall of the bowel, producing the sacculations known as haustra. The presence of teniae coli and haustra ceases at the rectosigmoid junction, where the smooth muscle of the teniae coli broaden to form a complete layer within the rectum.
Also found in the abdomen are two spaces between the ascending/descending colon and the posterolateral abdominal wall, known as the paracolic gutters. These structures are clinically significant, as they allow material released from a diseased or infected organ to accumulate in a different part of the abdomen.
The colon has numerous important anatomical relations with other organs in the abdomen, as can be seen in Table 1 below:
The neurovascular supply to the colon is closely linked to its embryological origin – the ascending colon and proximal two thirds of the transverse colon originate from the midgut, whereas the distal third of the transverse colon, descending colon and sigmoid colon derive from the hindgut.
The anatomy of the colon is of great importance clinically, as there are many conditions which can affect any of its four sections. Therefore, it is important for clinicians to be familiar with the anatomy of the colon in order to diagnose and treat any abnormal conditions that may arise.
The large intestine is also important for understanding the pathogenesis of abdominal diseases and conditions, as it is known, for example, that the paracolic gutters can act as a space into which material released from affected organs can accumulate. Knowing the anatomy is also important for understanding the embryology of congenital anomalies, such as malrotation of the bowel.
The anatomy of the colon is also important for the performance of many medical procedures. Knowing the relative positions of different organs to the colon can help guide the performance of surgical and endoscopic procedures, allowing them to be done safely and accurately. The mobility of the transverse and sigmoid colon can also have implications for certain surgical procedures, in cases where the positions of these sections are abnormal.
The anatomy of the colon is critical for both the clinical evaluation and treatment of gastrointestinal disorders. Its four sections have distinct relationships with other abdominal organs, and each section has its own neurovascular supply. Additionally, knowing the anatomy of the colon can be useful for understanding the pathogenesis of abdominal diseases, performing medical procedures safely and accurately, and diagnosing congenital malformations. For these reasons, an understanding of the colon's anatomy is essential for any clinician.
The colon (large intestine) is the 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, descending and sigmoid. These sections form an arch, which encircles the small intestine.
The colon averages 150cm in length, and can be divided into four parts (proximal to distal)- ascending, transverse, descending and sigmoid. The ascending colon begins as 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.
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. The transverse colon is the least fixed part of the colon, and is variable in position (it can dip into the pelvis in tall, thin individuals). Unlike the ascending and descending colon, the transverse colon is intraperitoneal and is enclosed by the transverse mesocolon.
After the left colic flexure, the colon moves inferiorly towards the pelvis - and is called the descending colon. It is retroperitoneal in the majority of individuals, but is located anteriorly to the left kidney, passing over its lateral border. When the colon begins to turn medially, it becomes the sigmoid 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. The long length of the mesentery permits this part of the colon to be particularly mobile.
The paracolic gutters are two spaces between the ascending/descending colon and the posterolateral abdominal wall.
As a general rule, midgut-derived structures are supplied by the superior mesenteric artery, and hindgut-derived structures by the inferior mesenteric artery. The ascending colon receives arterial supply from two branches of the superior mesenteric artery; the ileocolic and right colic arteries. 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). The venous drainage of the colon is similar to the arterial supply-
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.
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.
The large intestine, or colon, is a key part of the human anatomy, essential for the proper functioning of the abdominal organs. This structure is also clinically important, as it helps to collect material released from inflamed or infected abdominal organs. In order to better understand the anatomy and importance of the large intestine, it is important to understand its characteristic features, as well as its anatomical relations in the abdomen.
The large intestine has several distinct features, which help to distinguish it from the small intestine. Attached to the surface of the large intestine are omental appendices, which are small pouches of peritoneum filled with fat. In addition, there are three strips of muscle, known as the teniae coli, which run along the length of the large bowel. These strips are known as the mesocolic, free and omental coli. The teniae coli are able to contract, which helps to shorten the wall of the bowel and produces sacculations known as haustra. The large intestine is also much wider in diameter than the small intestine, with these features ceasing at the rectosigmoid junction. Here, the smooth muscle of the teniae coli enlarges to form a complete layer within the rectum.
The colon has numerous anatomical relations in the abdomen that it is important to understand. Generally, structures derived from the midgut are supplied by the superior mesenteric artery, while those derived from the hindgut are supplied by the inferior mesenteric artery. For example, the ascending colon is supplied by two branches of the superior mesenteric artery, the ileocolic and right colic arteries. The transverse colon receives arterial supply from both the midgut and hindgut, and thus is supplied by branches of both the superior mesenteric artery and 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 similarly receives its arterial supply from branches of the inferior mesenteric artery, known as sigmoid arteries. The marginal artery (of Drummond) is another important vessel that provides collateral supply to the colon and helps to maintain the arterial supply in the case of occlusion or stenosis of one of the major vessels.
The venous drainage of the colon is similar to its arterial supply. The midgut structures (ascending colon and proximal 2/3 of the transverse colon) receive their sympathetic, parasympathetic and sensory supply from the superior mesenteric plexus, while the hindgut structures (distal 1/3 of the transverse colon, descending colon and sigmoid colon) receive their sympathetic, parasympathetic and sensory supply from the inferior mesenteric plexus. The lymphatic drainage of the ascending and transverse colon is into the superior mesenteric nodes, while the descending colon and sigmoid colon 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 then on to the cisterna chyli, before finally emptying into the thoracic duct. It is clear, therefore, that the large intestine plays an important role in the body's anatomy and functioning, and that its anatomy is complex and fascinating.
