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Peritoneal relations

Peritoneal cavity as seen in a parasagittal section.

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You’ve probably heard that the gastrointestinal tract within your abdomen is several feet long. But have you ever wondered how these loops of intestine don’t get tangled or bounce out of control when you run, jump or dance? Fortunately, many of your abdominal organs are held in place by a serous membrane called the peritoneum. Serous membranes line several body cavities where they secrete lubricating serous fluid to reduce friction from movement of muscles. And in this tutorial, we’re going to talk about the peritoneum and its relation to organs in the abdomen and the pelvis.

So to begin, I’m just going to give you a quick overview, and we’re going to start by distinguishing the two types of peritoneum which are the visceral peritoneum and the parietal peritoneum and describe their interactions with organs, and we will discuss the borders and structures of the abdomen and the pelvis as they relate to the peritoneum.

In the abdomen, we’ll look at the peritoneal reflections, the intraperitoneal structures and the retroperitoneal structures, and in the pelvis, we’re going to be looking at the organs, the muscles, the pouches and some other structures. And finally, we’ll discuss the clinical significance of the peritoneum.

First, though, let’s talk about the structure of the peritoneum.

The peritoneum is a double layer of simple squamous serous tissue called the mesothelium. And there are two layers of peritoneum as we mentioned before, and these two layers are known as the parietal peritoneum and the visceral peritoneum. And these two layers of peritoneum are continuous with one another and have a potential space between them filled with serous fluid. And this space allows the layers to effortlessly slide over each other, as you can see in our image. Together, both layers anchor organs and provide support for their movements.

So we’re going to look at both of these layers and the first of these two layers that we’ll look at in a bit more detail is the parietal peritoneum. This is a serous tissue layer adjacent to the abdominal cavity. And in this image, we can see it along the boundaries of the abdomen.

The parietal peritoneum is sensitive to pressure and pain found here is well-localized, unlike pain in the second layer of peritoneum, which is, of course, the visceral peritoneum. This is also a serous tissue layer that is in direct contact with the abdominal organs. We can see it here on the liver, and we can also see a part of the gastrointestinal tract. The movement of food through the stomach and the intestines causes those organs to stretch as the visceral peritoneum is sensitive to the stretching, however, painful stretching of visceral peritoneum is not very well-localized. So pain signals here that are felt elsewhere in the body are called referred pain.

So now that we’ve gone over the structure of the peritoneum itself, let’s go over how it is organized in the abdomen. And peritoneum may or may not completely surround an organ, so let’s take a look at what they might look like.

Organs that are completely surrounded by a peritoneum are called intraperitoneal organs, and most intraperitoneal structures are associated with the gastrointestinal tract as this organization allows for both support and movement. For example, the transverse colon could look something like this. The lumen that food passes through would be here, and this is the wall of the colon over here, and over here, we can see the peritoneum passing completely around the colon.

Retroperitoneal organs, on the other hand, are not enclosed within the peritoneal cavity. Instead, the peritoneum covers retroperitoneal organs only on their anterior surface as you can see here. The organs then have a free surface on the posterior abdominal wall. And something important to note is that retroperitoneal organs such as kidneys typically exhibit less mobility than intraperitoneal organs, because their functions do not require expansion or the mixing of contents. And we’re going to cover both intraperitoneal and retroperitoneal organs by their region.

Let’s start with the abdominal organs.

The abdomen is bounded by the diaphragm, the pelvic rim, and the anterior and posterior abdominal walls. So let’s start by looking at the anterior abdominal wall. This over here is the Camper’s fascia, and it is one of two layers that make up the superficial fascia of the anterior abdominal wall. And the Camper’s fascia lies directly beneath the skin of the anterior abdomen and it’s a layer of a subcutaneous fatty tissue that varies in thickness from person to person. Camper’s fascia is the outermost or most superficial layer of superficial fascia of the anterior abdomen and is continuous with the superficial fascia of the thigh and the dartos fascia of the penis.

The deeper layer of superficial fascia of the anterior abdominal wall is known as Scarpa’s fascia, and this is made up of membranous fascia that lies deep to the Camper’s fascia we just saw. And Scarpa’s fascia is a thin layer of connective tissue that resembles wispy, spider web-like fibers in humans. Scarpa’s fascia is continuous with the fascia of the urogenital region, so fluid accumulated in the scrotum or labia majora could ascend the abdominal wall deep to this layer. And Scarpa’s fascia is sandwiched between Camper’s fascia and the muscles of the abdominal wall. And we can see one of these muscles over here – the rectus abdominis muscle.

So, the rectus abdominis muscle is the most midline of the anterior abdominal wall muscles, and it’s made up of two columns of muscle on either side of a thick band of dense connective tissue called the linea alba. The muscle fibers of the rectus abdominis are arranged along the superior-inferior axis so that when this muscle contracts, the spine is flexed. The rectus abdominis also compresses the abdominal cavity and in doing so, it causes abdominal organs to push against the diaphragm and aids in exhalation.

Let’s just take a few minutes to look at the diaphragm right now.

So, the diaphragm marks the most superior extent of the abdomen and it’s comprised of skeletal muscle that inserts on a central tendon near the spine and decreases intrathoracic pressure when it contracts. The decreased pressure causes the lungs to expand resulting in inhalation. Although it is a muscle situated between the thorax and the abdomen, the diaphragm receives its innervation from a nerve emerging in the neck – the phrenic nerve.

The subphrenic recess is the space directly inferior to the diaphragm and it overlies the liver and spleen and can be displaced by several inches during inspiration. And some of the ligaments that stabilize the liver can be found in this area. One ligament – the falciform ligament – extends inferiorly down the front of the liver and attaches it to the anterior abdominal wall. The most distal part of it is called the round ligament of the liver.

Next, let’s move on to talk about the reflections of the peritoneum.

So during fetal development, the migration of abdominal organs causes the peritoneum to fold over itself in some areas and creates peritoneal reflections, which divide the abdomen into two spaces – the greater sac and the lesser sac. The lesser sac, also known as the omental bursa, is bounded anteriorly by the lesser omentum, superiorly by the liver and the diaphragm, posteriorly by the peritoneum overlying the posterior abdominal wall and its organs, and inferiorly by the transverse mesocolon greater omentum.

The greater omentum is actually the largest peritoneal reflection and it hangs off the greater curvature of the stomach and the transverse colon as a layer of fatty peritoneum, and part of the lesser sac rests between its two layers as we just saw.

The lesser omentum up here is a smaller peritoneal reflection that attaches to the lesser curvature of the stomach and the inferior surface of the liver. As its name suggests, it is usually less substantial than the greater omentum and lies anterior to the lesser sac here.

The most superior extent of the lesser sac is called the superior recess of the omental bursa. And as you can see, it lies posterior to the liver just below the diaphragm.

The lesser sac communicates with the greater sac through the omental foramen and it’s a small opening posterior to the hepatic portal vein, the hepatic arteries and the cystic duct. The greater sac is the larger part of the peritoneal cavity that houses the intestines and it’s bounded anteriorly by the anterior abdominal wall, the liver, the posterior abdominal wall, and the pelvis.

Now, let’s have a look at some of the structures within the peritoneum.

Remember that the intraperitoneal structures are those that are completely encased by peritoneum. So, this is the stomach. It’s a hollow J-shaped organ that receives food from the esophagus as a bolus. In this illustration, we can see the stomach from an anterior view, and the smaller part of the J is called the lesser curvature and is attached to the lesser omentum. The larger part of the J is called the greater curvature, and the stomach is made up of three layers of smooth muscle in differing orientations that churn its contents while glands in its mucosa break down proteins and other macromolecules.

After being mixed and partially digested by the stomach, the food you eat makes moves into the small intestines, and the first part of your small intestines is called the duodenum. The duodenum is shaped like a C with four parts, and these four parts are the first or the superior part, the descending part, the horizontal part, and the ascending part. In this illustration, the part of the duodenum that you see highlighted in green is the horizontal part of the duodenum or the bottom part of the C.

Now all of these highlighted structures here are part of the lumen of the small intestines and it’s very convoluted, so in this image, we can see it in multiple places, and as food passes through the small intestines, small microvilli absorb proteins, fats, and nutrients that were broken down in the stomach, and as you can see the small intestine is long and folds upon itself so it is suspended by a mesentery for support.

So this is the mesentery here and besides providing support for the intestine, the mesentery also transmits the neurovasculature supplying the small intestines and also some of the large intestine. And most of the arteries travelling through the mesentery arise from the artery that we’ll look at next which is the superior mesenteric artery.

The superior mesenteric artery is the second unpaired branch of the abdominal aorta and it passes inferiorly over the duodenum and fans out into a wheel of arteries to reach all of the small intestines and the first half of the large intestine. And from this perspective, we can see some of its smaller branches within the fat of the mesentery.

One branch of the superior mesenteric artery, the middle colic artery, travels to the transverse colon and we can see it here because the middle colic artery travels in the midsagittal plane. The transverse colon seen here is the second part of the large intestine and it extends across the abdomen from the ascending colon at the right colic flexure to the descending colon at the left colic flexure. Because of its proximity to the stomach, the transverse colon needs to be somewhat mobile, and it is suspended by the next structure we’re going to be looking at which is the transverse mesocolon.

And although most of the large intestine is retroperitoneal, a layer of peritoneum extending from the transverse mesocolon completely surrounds the transverse colon so it is considered an intraperitoneal organ.

So now let’s move on to some accessory digestive structures that we can see in our image here.

So this large solid organ is the liver, and the liver sits in the superior part of the abdomen and detoxifies the blood from your intestines. It attaches to the anterior abdominal wall by the falciform ligament and is usually restricted to the right side of the body but in some cases can extend to the left as well. Blood from the spleen and the small and large intestines reaches the liver through the hepatic portal vein, and we can only see a small part of it here but this vein is fairly large. It’s formed by the merging of the superior mesenteric vein and the splenic vein. And about seventy five percent of the blood supply to the liver passes through the portal vein.

The rest of the blood supply to the liver comes from the proper hepatic artery which is over here, and when it reaches the liver, the proper hepatic artery splits into two smaller arteries called the left and right hepatic arteries. The proper hepatic artery is a continuation of another artery coming from the celiac trunk. The celiac trunk arise directly off the abdominal aorta as the first unpaired branch of the abdominal aorta. It’s a short artery with three main branches that provide the blood supply to the foregut and to the accessory digestive organs, and there are several branches of the celiac trunk and these are the left gastric artery, the common hepatic artery, and the splenic artery.

From this perspective, we can only see the common hepatic artery and the splenic artery, so we’ll just take a quick look at these.

The splenic artery is a large convoluted artery that courses towards the spleen on the left side of the body, and it’s partially imbedded in the pancreas and provides some of the blood supply to this organ as well.

The pancreas is here and it’s an endocrine organ that secretes insulin from the islets of Langerhans and glucagon from alpha cells and together these hormones maintain a healthy level of glucose within the blood. And over here, we have the splenic vein, and this vein receives blood from the spleen, the pancreas and the inferior mesenteric vein. Rather than running through the pancreas like the splenic artery, the splenic vein passes posterior to the pancreas and is not as convoluted.

Now let’s move on to talk about some of the retroperitoneal structures in the abdomen.

So remember that the retroperitoneal organs are those with one side covered by peritoneum and one side against the posterior abdominal wall. And the first structure we’re going to be talking about that has these characteristics is the esophagus – and you can see this highlighted in green on our image. It’s a hollow structure and is an organ of the foregut that transmits food from the pharynx to the stomach and it also empties into the stomach at the esophagogastric junction.

This large blood vessel over here is the abdominal aorta, and the aorta is the largest artery in the body. It arises immediately from the heart and features many paired and unpaired branches like the celiac trunk that we saw earlier.

The renal artery is another branch of the aorta, and this paired artery supplies the kidneys and adrenal glands with oxygenated blood. This is the accompanying renal vein. The renal veins drain deoxygenated blood from the kidneys and adrenal veins into the inferior vena cava. The left renal vein also receives blood from the left gonadal vein which drains blood from either the ovaries or the testes.

Alright, so now that we’ve covered the intraperitoneal and retroperitoneal organs of the abdomen, let’s have a look at the structures in the pelvis.

So, pelvic structures are subperitoneal, which means that they are inferior to the peritoneum of the abdomen, and we’ve been looking at the male pelvis for a little while so we’re going to cover the shared organs of the pelvis and the structures of the male reproductive system.

So let’s start with the urinary bladder, and this organ is, of course, found in both the male and female pelvis but it varies in its location slightly to compensate for the differing reproductive organs. So, the bladder is lined internally by a special kind of epithelial cell called urothelium, and the multiple layers of smooth muscle making up the wall of the bladder are collectively called the detrusor muscle. The bladder receives urine production by the kidneys and holds it until urination, and when the bladder is full, the detrusor muscle contracts and urine is expelled through the urethra. And this action is, of course, the same for both males and females.

Moving upwards, there is a thin cord of connective tissue stabilizing the bladder on the anterior abdominal wall known as the urachus, and in humans, it is covered by a layer of parietal peritoneum called the median umbilical fold. The urachus is a remnant of the allantois which is a structure that drains the fetal urinary bladder.

Our next structure is the prostate, and the prostate is a male reproductive organ so it’s obviously not found in the female pelvis. The prostate produces a milky secretion that protects sperm from the acidity of the vaginal wall and it also contains the first part of the male urethra which is known as the prostatic urethra.

Posterior to the urinary bladder and prostate in males lies the rectum, which is the final part of the large intestines. And on this image, we’re looking at a cross-section so we can see the transverse rectal folds inside it. The rectum receives contents from the descending colon and will hold them until defecation occurs, and in the female pelvis, the rectum lies posterior to the uterus rather than the bladder.

So the position of the structures within the pelvic cavity creates pouches between organs, and these pouches are named for the organs that border them. So, let’s take a look at the pouches found in a male pelvis.

So, this is the retropubic space, and it’s found just deep to the pubic bones and the pubic symphysis. The retropubic space is usually filled with connective tissue and is bordered posteriorly by the urinary bladder. In males, there’s another space between the urinary bladder and the rectum called the rectovesical pouch, and the word “recto” refers to the rectum while the word “vesicle” means bladder, and the male pelvis only contains one pouch.

In the female pelvis, however, the presence of a uterus creates two spaces. So over here, we have a female pelvis without peritoneum covering the muscles or the organs, and this is the bladder and over here, is the rectum. Firstly, we have a vesicouterine pouch between the urinary bladder and the uterus, and over here, we have a rectouterine pouch between the uterus and the rectum. All three pouches contain loops of small intestines and any fluids that may accumulate in the peritoneal cavity.

Alright, now let’s have a look at some structures of the male reproductive system. So, let’s begin by looking at some structures associated with the penis. The penis is a male reproductive organ and the spongious body in green here is also known as the corpus spongiosum and it’s the more ventral body of the penis and it contains the spongy urethra. The cavernous bodies, also known as the corpus cavernosa, are the more dorsal bodies, and although we can only see one here, there are two cavernous bodies on either side of a dorsal vein, and they contain a network of blood vessels that make up the erectile tissue of the penis.

This small piece of connective tissue is the suspensory ligament of the penis, and it attaches to the pubic symphysis as well as the dorsal surface of the penis to hold the penis close to the pubic bone. This very small structure is the bulbourethral gland, also called Cowper’s gland, and it is a paired gland just posterior to the bulb of the penis. The bulbourethral glands secrete mucoproteins that lubricate the urethra during ejaculation. A homologous gland does exist in the females although it has a different name, and this is the greater vestibular gland and it lubricates the vagina rather than the urethra.

Alright now that we’ve seen the organs in the pelvis, let’s have a look at some muscles. The external anal sphincter surrounds the – you guessed it – the anus. It is a band of circular skeletal muscle that contracts to prevent leaking of contents from the rectum and it is tonically contracted but relaxes during defecation.

Over here is another circular band of skeletal muscle known as the external urethral sphincter and it surrounds the urethra and contracts to prevent the leaking of urine. In males, the urethral sphincter muscles are fairly simple and only surround the urethra, however, the presence of the vagina in females makes the urethral sphincter muscle a bit more complicated. And in women, it is made up of three parts – a sphincter urethrae muscle around the urethra, a urethrovaginal muscle that surrounds the urethra and the vagina, and a compressor urethra muscle that surrounds the anterior urethra and pushes it against the vagina.

This muscle over here is the bulbospongiosus, and this skeletal muscle originates from a midline raphe and fans out like a quill of an arrow. It covers the bulb of the penis and is thought to contribute to erection.

And finally, let’s have a look at a few more connective tissue structures in the pelvis and then discuss the significance of the peritoneum. So, the pubic symphysis is a fibrocartilaginous joint between the two pubic bones in the front of the pelvis, and here you can see the pubic symphysis from an anterior view, and this joint allows for some mobility of the pelvis which is particularly important for childbirth.

The rectoprostatic fascia is a thin layer of connective tissue between the rectum and the prostate, and it is found just inferior to the rectovesical pouch.

And finally this over here is the urogenital diaphragm, and it’s a collection of muscles and connective tissue that prevents the pelvic floor from collapsing. The external urethral sphincter that we saw earlier is a part of the urogenital diaphragm along with the levator ani muscles, the peroneal fascias, and several other urogenital muscles.

Alright, thanks for sticking with me throughout the tutorial so far. Before we get to our summary, let’s just have a quick look at some clinical notes that are relevant to the peritoneum.

So, we’ve included this image of the greater omentum just to give you another perspective on its location, and although it is a fold of fatty connective tissue, the greater omentum plays an active role in immune defense. The omentum contains milky spots full of lymphoid aggregates that provide protection by gathering antigens, pathogens and particulates from the peritoneal cavity, and they can even promote inflammation and fibrosis.

Now we’ve reached the end. Let’s quickly go over what we’ve learned.

So in this tutorial, we discussed the peritoneum and its relation to organs in the abdomen and in the pelvis. First, we distinguished the two types of peritoneum and these were the visceral peritoneum covering the organs directly on their surface and the parietal peritoneum adjacent to the abdominal cavity. Then we described the organization of peritoneum in relation to the other structures – intraperitoneal structures that are completely surrounded by peritoneum and retroperitoneal structures that have a free surface on the posterior abdominal wall.

Then we looked at muscles, organs and some other structures in the abdomen and pelvis, and we started with some components of the anterior abdominal wall which included the Camper’s fascia, Scarpa’s fascia, and the rectus abdominis. And some components of the superior boundary of the abdomen, the diaphragm is the major respiratory muscle and the subphrenic recess is a space just inferior to it.

After going over the boundaries of the abdomen, we named some areas where the peritoneum reflects upon itself and creates spaces and these included the greater omentum which is a large peritoneal reflection hanging off the greater curvature of the stomach, and the lesser omentum which is a smaller peritoneal reflection attached to the lesser curvature of the stomach. These folds create a peritoneal cavity separated into two sacs – the greater sac encompassing a larger part of the peritoneal cavity and the lesser sac near the accessory digestive organs – and these communicates through the omental foramen near the accessory digestive organs that communicate through the omental foramen posterior to the hepatic portal vein. Within the peritoneal cavity, the superior recess of the omental bursa represents the most superior part of the lesser sac.

Once we went over the reflections and spaces in the abdomen, we then began looking at the structures within them starting with the intraperitoneal structures. The stomach is a hollow digestive organ that receives food from the esophagus and it passes its contents to the duodenum which is the first part of the small intestine. The contents of the duodenum continue into the next part of the small intestine that is suspended from the posterior abdominal wall by a mesentery, and the mesentery transmits neurovasculature to the small intestine and some of the large intestine.

The transverse colon is the second part of the large intestine and it’s suspended by the transverse mesocolon to provide it with some mobility. The pancreas is an endocrine organ lying posterior and slightly inferior to the transverse colon, and the liver is a large organ out lateral to the right of both of these.

Before moving on to the pelvis, we also looked at some retroperitoneal structures which include the esophagus which is a foregut organ that transmits food from the pharynx to the stomach, the aorta which is the largest artery in the body and it features many branches including the renal arteries that supply the kidneys and the adrenal glands. The renal arteries are accompanied by renal veins draining the same structures.

After going over the structures of the abdomen, we next moved into the pelvis beginning with some organs and spaces that they create. The urinary bladder which contains urine until it is ready to be expelled and it’s held in place by the urachus, a remnant of the allantois. The prostate is a male reproductive organ just inferior to the urinary bladder and anterior to the rectum which is the final part of the large intestine.

Within the pelvis, these organs create the retropubic space between the pubic bone and the urinary bladder, and the rectovesical space between the urinary bladder and the rectum. Just inferior to the rectovesical space is a thin line of fascia called the rectoprostatic fascia. Then we went over some of the parts of the male reproductive system which included the cavernous body of the penis which contains erectile tissue, the spongious body of the penis which contains the spongy urethra, the suspensory ligament holds the penis close to the pubic bones, and the bulbourethral glands secrete a fluid to neutralize acid in the urethra. And finally, the bulbospongiosus muscle covers the bulb of the penis.

Next, we looked at some other muscles and connective tissue structures of the pelvis. The external anal sphincter prevents contents from the rectum from leaking and the external urethral sphincter prevents contents from the urinary bladder from leaking. The pubic symphysis is a fibrocartilaginous joint between the two pubic bones and the urogenital diaphragm is a collection of muscles and connective tissue that prevents the pelvic floor from collapsing.

And there we go! Now we’ve finished the end of our tutorial. Thanks for watching. Happy studying, see you next time!

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