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Structures seen on the abdominal surface of the diaphragm.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on the abdominal surface of the diaphragm. During today's tutorial, we'll be looking at the diaphragm which is a musculotendinous structure that separates the thorax from the abdomen. More specifically, we're going to mention the structures that can be seen on the abdominal surface of the diaphragm that you can see here in this image. It's important to note that this is the anterior aspect of the diaphragm. We can use structures such as the sternum and the costal cartilages to orientate ourselves whereas here we have the posterior aspect of the diaphragm where we can see structures such as the ribs and the lumbar vertebra.
Viewing the diaphragm in this way means that the right side of the image actually represents the left side of the body and vice versa, therefore, it's always good to know the orientation of an image before we start looking at it. So before we go over the different structures, it'd be good to know a bit about the diaphragm itself.
The diaphragm, which we can see here highlighted in green, is in fact a skeletal muscle that separates the thoracic cavity above from the abdominal cavity below. So you can think of the diaphragm as the floor of the thoracic cavity as well as the roof of the abdominal cavity. The major function of the diaphragm is to assist with breathing. During inspiration, the diaphragm contracts and moves downwards which increases the volume of the thoracic cavity and helps draw air into the lungs. Before we look at the structures on the abdominal surface of the diaphragm, let's have a closer look at this structure above the diaphragm which is called the thoracic cage.
The thoracic cage is also known as the rib cage and is the bony framework of the thoracic cavity. It's made up of the sternum, the costal cartilages, twelve pairs of ribs, and the twelve thoracic vertebrae. On the abdominal surface of the diaphragm, we can see the sternum as well as the lower costal cartilages and some of the ribs. The costal cartilage of ribs seven to twelve attach to the diaphragm as well as the xiphoid process of the sternum. We can also see some vertebrae here but they're not part of the thoracic cage as they're lumbar vertebrae. Now, let's move on to the structure we can see highlighted in this illustration which is the central tendon of the diaphragm.
This structure is a thin, strong aponeurosis situated in about the center of the diaphragm. An aponeurosis consists of layers of flat broad tendons. As I've mentioned previously, the diaphragm is a musculotendinous structure. So here we have the tendinous component or the central tendon which is highlighted in green and surrounding it we have the muscular component. The central tendon of the diaphragm plays a role in respiration. When the diaphragm contracts during inspiration, this draws the central tendon down thus flattening out the diaphragm. This increases thoracic volume and decreases intrathoracic pressure drawing air into the lungs due to a pressure gradient. A good way to remember that an increase in thoracic volume causes a decrease in intrathoracic pressure is remembering that volume and pressure are inversely proportion. So if the volume increases, the pressure must then decrease.
Now, let's continue by looking at the openings in the surface of the diaphragm. There are three openings in total and they're known as the esophageal hiatus, the hiatus of the inferior vena cava, and the aortic hiatus. We will start with the hiatus of the vena cava which is also known as the caval opening. This opening allows the inferior vena cava and sometimes the phrenic nerve to pass from the thorax into the abdomen. This opening is located at the level of the eighth thoracic vertebra or T8. Let's now take a short look at the esophageal hiatus. This opening in the diaphragm allows the esophagus and the vagus nerve to pass from the thorax into the abdomen. This hiatus is elliptical in shape and is located at the level of the tenth thoracic vertebra or T10.
Finally, let's have a look at the aortic hiatus. This opening is located here at the level of the twelfth thoracic vertebra or T12 and allows the abdominal aorta to pass from the thoracic cavity to the abdominal cavity. There are few other structures that pass through the aortic hiatus. These include the thoracic duct and the azygos vein.
Let's now have a look at some of the branches of the abdominal aorta which we can see on the abdominal surface of the diaphragm. These arteries you can see branching from the aorta in green are known as the inferior phrenic arteries. In this image, they arise from the aorta just superior to the celiac trunk but there's a lot of anatomical variation in their origin. The inferior phrenic arteries are the main vessels that supply the diaphragm with blood. Each of the inferior phrenic arteries gives off a branch named the superior suprarenal artery which we can see highlighted in green here and here. The superior suprarenal arteries supply the anteromedial part of the suprarenal glands otherwise known as the adrenal glands.
Okay, now let's have a look at another very important branch of the abdominal aorta – the celiac trunk. This artery branches from the aorta at T12 and as you can see, it has three main branches. The first branch is the common hepatic artery, the second branch is the left gastric artery, and the third and final branch is the splenic artery. We will now look at each of these branches in turn starting off with the common hepatic artery.
The common hepatic artery branches off to the right hand side of the body to supply the liver, the pylorus of the stomach, the pancreas and the duodenum. The common hepatic artery usually arises from the celiac trunk but there are some cases where it can arise directly from the aorta. The second branch that stems from the celiac trunk is the left gastric artery which we can see here. This artery supplies the greater curvature of the stomach as well as the esophagus. And finally the third branch of the celiac trunk which is shown here is the splenic artery. This artery branches off to the left part of the body and it runs along the superior part of the pancreas. It supplies the stomach, the pancreas and the spleen.
Now after talking about the vessels related to the diaphragm, let's continue and have a look at some of the nerves located on the abdominal surface of the diaphragm starting with the phrenic nerve. You should notice from the illustration that there are two phrenic nerves – a left phrenic nerve which we can see here and a right phrenic nerve which we can see here. Both of these nerves originate in the neck from the third to fifth cervical nerves then travel through the thorax and finally enter the abdomen by piercing the muscular part of the diaphragm. It's important to see here that the right phrenic nerve enters the abdomen through the hiatus of vena cava. In contrast, the left phrenic nerve passes through the diaphragm via its own small hiatus which we can see here.
The phrenic nerve is a very important nerve having both motor and sensory functions. Each of the two phrenic nerves innervate the ipsilateral side of the diaphragm and at the same time it receives sensory information from the diaphragm and transmits this to the brain. So it's very important at this point to remember that the phrenic nerve is very closely associated to the diaphragm and therefore it's a very important nerve for the breathing process.
An important clinical note to mention is that irritation to the phrenic nerve leads to the activation of the hiccup reflex. A hiccup is a spasmodic contraction of the diaphragm which pulls air upwards in the esophagus that is eventually stopped by a closed larynx. Another important clinical note is that since the phrenic nerve supplies the diaphragm, lesion to one or both of the phrenic nerves can cause a condition called diaphragmatic paralysis. Lesions can be caused by trauma, neuropathies or compression from other structures such as a tumor.
Another interesting clinical note is that pain arising from regions supplied by the phrenic nerve can cause referred pain to other areas supplied by C3 to 5 such as the shoulder. For example, a ruptured spleen can cause irritation of the diaphragm and phrenic nerve resulting in pain in the tip of the shoulder.
Let's take a quick look at other nerves located on this diaphragmatic surface. Here we can see the splanchnic nerves which are highlighted in green on either side of the aorta. There are many types of splanchnic nerves in the body which are usually classified by their location. These nerves here are the thoracic splanchnic nerves. They arise from the sympathetic trunk in the thorax before traveling inferiorly and piercing the diaphragm to enter the abdomen. The main function of these nerves is to provide sympathetic innervation to the abdominal organs.
The final structures we'll look at are the muscles located near the abdominal surface of the diaphragm. The muscle we can see here highlighted in green is the psoas major muscle. This muscle doesn't actually attach to the diaphragm but has its origin at the transverse processes of the T12 to L5. Along with the iliacus muscle, it forms iliopsoas. The psoas major muscle is a hip flexor meaning it contributes to flexion at the hip joint. The other muscle that we can see here is the quadratus lumborum muscle. This muscle originates from the iliac crest and runs anteriorly to insert into the last rib as well as the transverse processes of L1 to 4. This muscle is involved in lateral flexion and extension of the lumbar vertebrae.
So that concludes this tutorial on the abdominal surface of the diaphragm. I hope you enjoyed it and thank you for listening.
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Now, good luck everyone, and I will see you next time.