Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on the retroperitoneum. The retroperitoneum is the collective term for the structures that are located posterior to the parietal peritoneum which we can see here highlighted in green. During this tutorial, we'll be looking at different organs and vessels found in the retroperitoneum and we'll do this by focusing mainly on the image in front of you which is an anterior view of the abdominal cavity. It's good to keep in mind that the structures we're talking about today are also referred to as retroperitoneal.
But before we start looking at the retroperitoneum, let's first have a look at some structures we can see within the parietal peritoneum. Here in green we see the median umbilical fold which is a ridge of the lower parietal peritoneum. If this part of the peritoneum were to be reflected back into its original anatomical position, the median fold would run from the umbilicus to the bladder. This fold contains the urachus, a remnant of the allantois which is a canal found in the fetus.
Another ridge of the parietal peritoneum is the medial umbilical fold. This is actually a paired structure and there is one found on either side of the body. As you may have noticed, these folds have a very similar name to the one we just discussed so it's very easy to get them mixed up. A good way to remember is that the median umbilical fold is closer to the median line of the body whereas the medial umbilical folds have an L in their names so are located more laterally. These folds are the remnants of the fetal umbilical arteries and can be used as a landmark during laparoscopic surgery for inguinal hernia repairs.
Moving superiorly, we can see another fold of peritoneum here which is known as the sigmoid mesocolon. The sigmoid mesocolon is a mesentery which is a double fold of peritoneum that attaches parts of the intestines to the posterior abdominal wall. A mesentery that specifically attaches to parts of the colon is called a mesocolon.
Now that I've explained these definitions, you'll understand that the sigmoid mesocolon functions to attach the sigmoid colon to the posterior abdominal wall. The mesentery that attaches to the small intestine, the small bowel mesentery, arises from this structure here, the root of the mesentery. The root of the mesentery runs diagonally from the end of the duodenum and is about fifteen centimeters long in adults.
Now let's move on to look at the retroperitoneal structures starting with the right kidney. The kidneys are important in maintaining electrolyte and water balance in the body as well as having an important role in endocrine function. The right kidney is located posterior to the parietal peritoneum and is slightly lower than the left kidney due to its location inferior to the liver. In this illustration on the right, we can see the diaphragmatic surface of the liver highlighted in green. We'll talk about the significance of this surface later on in the tutorial.
Superior and slightly medial to the right kidney is the right adrenal gland which is another retroperitoneal structure. The adrenal glands are also known as the suprarenal glands as they are located superior to the kidneys. They are endocrine organs that produce and secrete hormones and steroids such as adrenaline and cortisol.
The next structure we'll look at is the left kidney which we can see here highlighted in green. The left kidney is also located posterior to the parietal peritoneum and is usually larger and located slightly higher than the right kidney. Superomedial to the left kidney is the left adrenal gland which is also called the left suprarenal gland. The left adrenal gland is slightly larger than the right adrenal gland but they both weigh about 5 grams.
A significant portion of the diaphragmatic surface of the liver is covered by peritoneum, however, there's also a portion of the surface which is bare and comes in direct contact with the right hemidiaphragm. This bare area is an extension of the retroperitoneal space and, through it, pathological processes taking place in this space can spread to the thoracic cavity and vice versa.
We're now going to look at some veins of the retroperitoneum which are the hepatic veins. The hepatic veins arise inside the liver's tissues as smaller venules. They remain there for most of their course and exit to the retroperitoneal space only to merge with the inferior vena cava. In this illustration, we can actually see the three hepatic veins draining into the IVC.
Between the two kidneys is another retroperitoneal structure, the duodenum. The duodenum has four parts – the superior part, the descending part, the horizontal part and the ascending part. These are also commonly referred to as the first, second, third and fourth parts of the duodenum. Most of the duodenum is considered to be retroperitoneal. Only the superior part is intraperitoneal which means it's contained within the peritoneum. And in case you were wondering about the structure which can be seen above the first part of the duodenum, this is the hepatoduodenal ligament. It's not considered a structure of the retroperitoneum, however, it acts as a bridge for vessels and fatty tissue between regions of the retroperitoneal space.
Next, we're going to have a look at a glandular organ which is the pancreas. The pancreas also consists of four parts – the head, the neck, the body and the tail. The first three parts are considered retroperitoneal as they are covered by the parietal peritoneum anteriorly. However, like the superior part of the duodenum, the tail is intraperitoneal and is located within the splenorenal ligament.
The pancreas is considered to be an exocrine gland secreting enzymes into ducts which aid in the process of digestion. It also has an important endocrine function secreting the hormones insulin and glucagon which have an essential role in glucose homeostasis. Superior to the pancreas, we can see another retroperitoneal structure which is the abdominal esophagus. This part of the esophagus which we can see highlighted in green is the cardiac orifice which is the opening between the esophagus and the stomach.
Moving inferiorly, we can see more retroperitoneal structures. One of these is the ascending colon which is located on the right hand side of the abdomen. The ascending colon is the main portion of the large intestine that's involved with fluid and electrolyte absorption along with the transverse colon. In this image, we can see the part of the abdominal wall that lies posterior to the ascending colon. Medial to the ascending colon, we can see the right ureter – a thin tube that transports urine from the right kidney to the bladder. The ureters are about twenty five to thirty centimeters in length and have muscular walls which aid in the peristalsis of urine. Each ureter crosses over a common iliac artery at the point where they bifurcate into an internal and external iliac artery.
The other part of the colon that's retroperitoneal is the descending colon. The descending colon is about twenty five centimeters in length and is located between the transverse colon and the sigmoid colon. The anterior part and sides of the descending colon are covered by parietal peritoneum. As we saw earlier, in this image, we can see the part of the abdominal wall that lies posterior to the descending colon. Inferomedial to the descending colon, we can see this structure here which is the left ureter. Like in the right ureter, peristaltic contractions in the left ureter aid in the flow of urine from the left kidney to the bladder. If we take a closer look at the bottom part of this image, we can see the rectum which is located between the sigmoid colon and the anus. Only the middle third of the rectum is considered to be retroperitoneal.
There are also many blood vessels that are part of the retroperitoneum including the abdominal aorta which we can see in the middle of this image. The abdominal aorta begins at the level of T12 where it pierces through the diaphragm traveling from the mediastinum of the thorax into the abdomen. If we take a closer look at the abdominal aorta, we can see that it bifurcates into left common iliac artery and a right common iliac artery. This bifurcation occurs at the level of the fourth lumbar vertebra.
So branching from the abdominal aorta, we can see the right common iliac artery. The right common iliac artery is about five centimeters long and is located medial to the inferior vena cava. It's crossed by the right ureter where it bifurcates into the external and internal iliac arteries.
In the next illustration, we can see the right external iliac artery highlighted in green but the internal iliac artery is not visible. The external iliac arteries are considered to be the principal arteries of the lower limb whereas the internal iliac arteries are the main arteries of the pelvis. Corresponding to the right common iliac artery, we have the left common iliac artery which is about one centimeter shorter than its counterpart. At the level of the sacroiliac joint, the left common iliac artery bifurcates into external and internal iliac arteries. In the lower part of our image, we can see the left external iliac artery. Like the abdominal aorta and its branches that we've looked at so far, the left external iliac artery is covered by parietal peritoneum. This helps separate it from the sigmoid colon and parts of the small intestine anteriorly.
In the upper right hand part of our image, we can see the splenic artery highlighted in green. It's a branch of the celiac artery which is the first branch of the abdominal aorta. The splenic artery provides the main blood supply to the spleen which is an important organ involved in our immune function. Please note that in this illustration, we can't actually see the spleen but we can see where it would normally be located within the peritoneum. Despite the fact that the spleen is considered an intraperitoneal organ, most of the splenic artery is retroperitoneal.
Another vessel that arises from the abdominal aorta is the superior mesenteric artery which arises about one centimeter inferior to the celiac artery. We can see this artery within the root of the mesentery located next to the superior mesenteric vein. The superior mesenteric artery gives off many branches that supply both the small and large intestines. It branches off the abdominal aorta in the retroperitoneal space behind the pancreas. It then becomes intraperitoneal as it courses superficially to the duodenum to enter the area of the mesentery.
To the right of the superior mesenteric artery is the superior mesenteric vein which is located within the root of the mesentery. This vein drains blood from the small intestine, the cecum, the ascending colon and the transverse colon. It enters the retroperitoneal space after it courses through the root of the mesentery as it dives beneath the pancreas. Another branch of the abdominal aorta is the inferior mesenteric artery which we can see branching off to the left. It arises below the duodenum and about three to four centimeters above the bifurcation of the aorta.
Branching off from the left of the inferior mesenteric artery, we can see this vessel here, the left colic artery. Like the rest of the vessels we looked at, it's retroperitoneal or located posterior to the parietal peritoneum. It gives off many small branches that supply the transverse and descending colon. To the right of the abdominal aorta, we can also see this vessel here which is the inferior vena cava. The inferior vena cava is formed by the common iliac veins and is also covered by peritoneum. It travels superiorly to the heart coursing deep beneath the duodenum and the pancreas.
Structures that are part of the retroperitoneum are important to know in abdominal surgery and of curse may come up in exams. A tip to remember the main retroperitoneal structures is to remember the mnemonic Sad Pucker. S stands for the suprarenal glands, A stands for the abdominal aorta and the inferior vena cava as well as their branches, D stands for duodenum, P for pancreas, U for ureters and C for colon specifically the ascending and descending parts, K stands for kidneys, E stands for esophagus, and R stands for the middle third of the rectum.
To finish off this tutorial, we'll go over some clinical notes in relation to the retroperitoneum. Hematomas can occur within the retroperitoneum and these are often caused by major pelvic fractures, ruptures of arterial aneurysms or severe pancreatitis. Retroperitoneal hematomas also referred to as a retroperitoneal hemorrhage present with abdominal pain and hematuria and may lead to hypovolemic shock. Blood seeps into the retroperitoneal fat and can even form bruises visible on the skin of the flanks. This bruising is a clinical sign commonly known as the Grey Turner's sign. A CT scan is the investigation of choice and treatment involves nonsurgical and surgical approaches depending on the severity of the injury.
So that brings us to the end of our tutorial on the retroperitoneum. I hope you enjoyed it and thank you for listening.
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