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Mediastinum

Contents of the mediastinum seen from the lateral views.

Show transcript

Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the mediastinum. Now, the mediastinum is an area found in the midline of the thorax between the left and right pleural sacs. Now, what we’re going to be doing here on this tutorial is using these two images that you see now on this screen which are just cuts of the thorax, as you can see here, sections of the thorax, where on this side here on this image on the left side, we’re looking at a lateral left side view here where you can see inside of the thorax. And, just for a bit of location, you can see here your heart covered by the pericardium. So, this is the anterior portion, so the chest, and this is the back. And here we have the same thing but this time we’re looking at a right lateral view where you can also see here the cut – so the thorax is cut – and you also see the heart covered by the pericardium. This is the chest and this is the back.

And there are a lot of structures here on these images that we’re going to be describing that you’ll find in the mediastinum. Now, anteriorly, the mediastinum is bordered by a bone, the sternum, and posteriorly by the vertebral column which you cannot see here but I’m just showing here where the mouse cursor where you can have an understanding where the posterior portion of the mediastinum will be. Inferiorly, the mediastinum is going to be bordered by this muscle here, the diaphragm.

And as I mentioned in this tutorial, we will take a look at the structures that are found in this space, namely, the different arteries, veins, nerves, organs, and also other structures that we’re going to be describing.

We’re going to start off with this one that you see here on this screen now, is a lateral left view here zoomed in. You see a highlight and the highlight is showing then the aortic arch. Now, the aortic arch also known as the transverse aorta can be seen here in this section and this part of the aorta that is located between the ascending and descending aorta. You see a little bit here of the ascending aorta, and then the descending aorta which is also known as the thoracic aorta – at least this portion here that is found on the thorax.

Now, the aortic arch will be giving off 3 branches which supply the head and upper limb including the brachiocephalic trunk, and then the brachiocephalic trunk will be dividing into the right common carotid artery and the right subclavian artery. But we’re going to be looking at these branches in different tutorials. Now the other branch will be then the left common carotid artery and then the left subclavian artery. Now, the aortic arch and its branches are found in the subdivision of the mediastinum known as the superior mediastinum.

The next structure that we’re going to be highlighting here, now still on the zoomed out left lateral view of the cut thorax, this is the descending aorta highlighted in green which is also known as the thoracic aorta – this portion here at least which you find on the thorax is then called the thoracic aorta. Now, as you probably know, this keeps going on all the way to the abdomen, the descending aorta, and at that point we will call it then the abdominal aorta.

Now, the descending aorta begins at the left side of the body of the 4th thoracic vertebra until the 12th thoracic vertebra. Now, it is also surrounded by the thoracic aortic plexus and forms an impression on the left lung, which on this image here as you can see we just removed the left lung, but you’d notice how this thoracic or descending aorta is causing or can cause an impression on the lung, left lung. Now, the descending aorta runs between the left subclavian artery and the first branch of the abdominal aorta, the celiac artery.

Moving on to the next slide, we’re going to highlight here the structure which is the left pulmonary artery, and the left pulmonary artery is a branch of the pulmonary trunk that carries deoxygenated blood from the right ventricle to the left lung. So, this is one of the rare arteries in your body that will be carrying deoxygenated blood instead of oxygenated blood.

Now, next on our list, we’re going to be highlighting this structure here that you see on the image coming from the aortic arch, this is known as the subclavian artery. Now, the subclavian arteries are a pair of large arteries in the thorax that supply the head and neck, shoulders, arms, and thorax. Now, the left subclavian which you see here highlighted on this image comes from this structure here which is the aortic arch. And then this artery, the left subclavian, will be supplying oxygenated blood to the left arm. Now, the right subclavian artery is a branch of the brachiocephalic trunk, so do not mix up these 2, and both arteries will be giving off several branches in the thoracic region, cervical region, and also to other muscles around this area.

Next, we’re going to be highlighting these arteries that you see here coming from the aorta. Now, this structure highlighted in green, they are known as the posterior intercostal arteries. There are 11 intercostal arteries that supply the intercostal spaces. The first 2 posterior intercostal arteries are branches of the subclavian artery whereas the rest of the intercostal arteries are branches of the descending aorta. And you can also see these structures from now the right lateral view of the cut thorax.

We’re moving on again to the lateral side as you can see here to this highlight, the structure here that you see between a vein and also here this nerve which is the phrenic nerve. Now, this highlight is showing you the pericardiacophrenic artery. This artery runs between the pleura and the pericardium to the diaphragm along with the phrenic nerve, as you can see here on this image, and it is a branch of the internal thoracic artery. Now, the pericardiacophrenic artery forms anastomosis with the musculophrenic artery and the inferior phrenic artery. So, this artery will be forming connections with the other arteries that I just mentioned. If you also switch to the image on the right side, you can see the highlight for the pericardiacophrenic artery.

We’re moving on to highlight now a main vein as you can see here still seen from the right lateral view, we’re looking at the superior vena cava which is a large vein of the heart that carries deoxygenated blood from the upper extremities to the right atrium. Now, the superior vena cava is formed by the union of the left and right brachiocephalic veins.

We’re moving onto the next structure that I’m highlighting here in green but now I’m looking, we’re looking at an anterior view of an open thorax where I just removed all the different organs and main structures that we’ve been looking at and now we’re left here with a bit of the superior vena cava, as you can see here where then the heart would be, and a bit here of the inferior vena cava, just a bit for location. And you can see then the different ribs and a tiny portion of the scalene muscles.

Now, with that, we’re highlighting then the azygos vein. Now, the azygos vein arises from the ascending lumbar vein. The azygos vein opens into the superior vena cava carrying deoxygenated blood to the right atrium. Now, the azygos vein is found on the right side of the thoracic vertebrae posteriorly. And now here you can see this image, again, from the images that we’ve been looking throughout this tutorial, but this time from a right lateral view. You can notice here clearly the highlighted structure which is then the azygos vein, which you see clearly here opening into the other structure that we just talked about, the superior vena cava.

The next structure that we’re going to be highlighting now, we’re zooming in from the left lateral view, this structure which is known as the hemiazygos vein. It is located on the left side of the vertebral column in the thoracic region. And on this image here you also see here a portion of the descending aorta, a little bit of the pericardium where the heart will be then found. If we go back to this image here of the anterior view of the open thorax, you see here the hemiazygos vein highlighted in green. And notice that this vein is draining into the other vein that we talked about, the azygos vein, which comes from the ascending lumbar vein receiving deoxygenated blood from the left posterior intercostal veins.

Next on our list, we’re going to be highlighting this vein which is known as the accessory hemiazygos vein. Now, the accessory hemiazygos vein drains the 4th to 8th posterior intercostal veins on the left side of the vertebral column. The accessory hemiazygos vein will be draining into the azygos vein. Now, one important point to make that the drainage of the accessory hemiazygos vein into the azygos vein may vary a bit.

Next, we’re going to be highlighting a set of structures that you see here from an anterior view of the open thorax and these are the posterior intercostal veins. Now, the posterior intercostal veins are 11 intercostal veins that drain the intercostal spaces posteriorly. Now, the 4th to 11th intercostal veins drain deoxygenated blood into the azygos or hemiazygos veins. And you can see here on this image of the right side, the different posterior intercostal veins draining into the azygos vein. The 1st and 3rd intercostal veins will be draining into the supreme and superior intercostal veins.

Next on our list, we’re going to be turning again to the left view to show you this structure which is the left superior intercostal vein which is highlighted here in green. The left superior intercostal vein will be draining deoxygenated blood into the brachiocephalic vein from the 2nd and 3rd left posterior veins.

Next on our list, we’re going to be zooming out this left lateral view to show you here this structure which is the subclavian veins, specifically, the left subclavian vein, which unites the internal jugular vein forming the brachiocephalic vein. So, any of the subclavian veins will be doing so. Now, you can also see here on this image on the anterior view how the subclavian veins are uniting with these veins here which are the internal jugular veins to then form these veins here which are known as the brachiocephalic veins. It is important to note that the thoracic duct, the largest lymphatic vessel, drains into the left subclavian vein.

We’re going to go back to the left lateral view here to highlight these 2 structures which are pulmonary veins. The pulmonary veins, unlike other veins, they carry oxygenated blood from the lungs to the heart. The pulmonary veins drain oxygenated blood from the lungs to the left atrium of the heart. There are 4 pulmonary veins with 2 veins carrying blood from each lung to the heart that will be pumped to the rest of the body. Now, notice here that we are looking at the left lateral view. So, these 2 pulmonary veins will be carrying blood from the left lung. Now, if we move to the image of the right side, you can also see these 2 highlights which are the 2 pulmonary veins that carry blood from the right lung. And if I show you here an image of the anterior view of the cut open thorax where you see here the 2 lungs, the heart and you find here the different pulmonary veins. So, the left one’s here, and the right one, and keep in mind that we’re looking at the subject’s perspective.

The next structure that we’re going to be talking about back to the left lateral view, now this is, this highlight is the vagus nerve, notice here. Now, the vagus nerve is the 10th cranial nerve. Its fibers consist of efferent motoric and parasympathetic fibers as well as afferent sensory fibers. And as you probably know, there is a left vagus nerve which you can see here on this image of the left view but if we turn here to the right side, you see here the right vagus nerve.

Moving on, we’re going to be highlighting here on the left side, the sympathetic trunk. Now, the sympathetic trunk refers to a chain of ganglia found on either side of the vertebral column running between the skull to the coccyx. Now, these ganglia are connected by nerve fibers and they communicate with the spinal nerves. If we also turn here to the right side, you see the right sympathetic trunk.

We’re going to move on and talk about now these nerves. From an anterior view, you see here highlighted in green the different intercostal nerves. Now, the intercostal nerves arise from the ventral rami of the thoracic spinal nerves. Now, these nerves run from the first thoracic vertebra T1 to then the 11th thoracic vertebra, also known as T11, in the costal groves along with the intercostal vessels as you can clearly see here on this image. Notice here highlighted in green, we have the intercostal nerves, but you see here a red line which is the arteries and also here the veins represented by this blue color. The intercostal nerves give off lateral cutaneous branches and the anterior cutaneous branches.

Next, we’re going to be talking about a nerve as well that you see here highlighted in green – we talked about before on a previous slide – this one is known as the phrenic nerve. The phrenic nerve is a paired nerve containing motor, sensory and sympathetic nerve fibers that supply the diaphragm, the pericardium, and the mediastinal pleura originating from level of the 4th cervical vertebra from the 4th cervical nerve. It also receives additional rami from C3 and C5. This nerve passes through an area subdivision of the mediastinum known as the middle mediastinum between the lungs and the heart to then the diaphragm as you can see here. And right now on this image we’re looking at the left phrenic nerve but we can move on here to the right side to show you then the right phrenic nerve.

We’re going to move on and talk about another nerve here on a zoomed image of the right view. This is known as the greater splanchnic nerve. The greater splanchnic nerve arises from the sympathetic trunk ganglia, as you can see here, this is the sympathetic trunk ganglia, at the level of T5 to T9, and it synapses with the celiac ganglion. The function of these nerves is to conduct pain from the upper abdominal organs, modulate activity of the enteric nervous system of the foregut and provides sympathetic innervation to the adrenal medulla. Now, the greater splanchnic nerves contain pre- and post-ganglionic fibers. Now, what we’re doing here also on this image is showing you an anterior view of the structures highlighted which are the greater splanchnic nerves arising from the sympathetic trunk ganglia, as you can see here.

The next structure that I’m going to be showing you here highlighted on these images of the cut of the thorax – notice here just for a bit of location – this structure highlighted is known as the brachial plexus which is formed by the ventral rami of the spinal nerves C5-T1. Now, as you probably remember, the brachial plexus provides cutaneous and motor innervation to the upper limbs.

One of the structures that I’m going to be highlighting here on the left lateral view is the diaphragm. And the diaphragm is made up of skeletal muscle and separates the thoracic and abdominal cavities. Very important muscle of respiration as you probably remember. Now, the diaphragm is innervated by the phrenic nerve. As I mentioned, the diaphragm is an important muscle of respiration, so breathing and contraction of this muscle enlarges the thoracic cavity which then reduces the intrathoracic pressure so that the lungs will be expanding and filling with air.

Now, there are several openings in the dome of the diaphragm that I’m going to be listing just for a reminder here that we’re going to be exploring the diaphragm in a lot more detail on a separate tutorial. But these openings where some structures will be passing through include the aortic hiatus where the aorta, azygos vein and the thoracic duct will be passing through. There’s also the esophageal hiatus where the esophagus, the esophageal arteries, anterior and posterior vagal trunks pass through there. There is also the vena caval foramen which encircles the vena cava and some branches of the right phrenic nerve. Also seeing the lesser aperture of the right crus, which transmits the lesser and greater right splanchnic nerves. Also on this list the less aperture of the left crus, which transmits to greater and lesser left splanchnic nerves and the hemiazygos vein. The sympathetic trunk runs behind the diaphragm as you can also see here from this image. And last on our list, we also find the Foramen Morgagni which contains the superior epigastric branch of the internal mammary artery and the lymphatics of the abdominal wall.

We’re going to move on and talk about now the right side where you can see the parietal pleura here highlighted in green and the parietal pleura is a serous membrane that lines the inside of the thoracic cavity covering then the diaphragm and the lungs. Now, this structure is innervated by the phrenic nerve and the intercostal nerves.

Another important structure worth highlighting here – I mentioned it before – this is the pericardium. Now, the pericardium is comprised of an external tough connective tissue layer known as the fibrous pericardium and an inner layer known as the serous pericardium, which is made of simple squamous epithelium. Now, this double-layered membrane covers the heart and also the roots of its great vessels as you can also see here, a bit of the pericardium covering the ascending aorta which then becomes the aortic arch here and the descending aorta as we mentioned before.

The next structure that we’re going to be highlighting here still on the image of the left side, we’re looking at the thymus, and the thymus is a specialized lymphatic organ inside of which T-cells of the immune system will be maturing. So it is located just behind the sternum in the anterior superior mediastinum, this organ begins to regress after puberty. And you can see here an image of the anterior view of the thymus.

The next structure you can see here on these images, now from the right view, you notice the trachea. The trachea extends from the pharynx and larynx to the lungs. Sometimes, it is also referred to as the windpipe. This cartilage-lined tube bifurcates at the level of T5 into 2 primary bronchi. Now, this tube is a passageway for inhaled and exhaled air from the oral and also nasal cavities to then the lungs and vice versa. Now, the inner surface of your trachea is actually lined up with a type of tissue known as the pseudostratified columnar epithelium – quick note on histology here on this tutorial. You also see here on this image on the left side, you see highlight one of the bronchi. This one is the left main bronchus. Now, the left and right main bronchi arise from that bifurcation of the trachea at the level of T5. Both main bronchi enter the lungs. The left main bronchus will be branching into the 2 secondary bronchi corresponding to the 2 lobes of the left lung while the right main bronchus that you see here on this image now highlighted in green divides into 3 secondary bronchi also corresponding to the 3 lobes of the right lung. So remember that the left lung has 2 lobes and the right lung has 3 lobes.

Another structure we’ve been seeing here on these images now on the left view, you see a little bit of the esophagus. Now, the esophagus connects the pharynx with your stomach and this long fibromuscular tube is part of the alimentary canal. It is lined with the type of tissue with a long name known as the nonkeratinized stratified squamous epithelium. You can see a bit better here on this image on the right view, you notice clearly the esophagus highlighted in green.

And we’re ready to move onto another structure, other structures that you see here highlighted in green which are known as the intercostal muscles. And the intercostal muscles are a group of intrinsic muscles occupying then the spaces known as the intercostal spaces. And as a reminder here, there are 3 types of intercostal muscles. Now, the external intercostals, there are the internal intercostal muscles, and the innermost intercostal muscles. The intercostal muscles are innervated by the intercostal nerves that arise from the thoracic nerves.

And as we continue exploring this image here, we’re going to be highlighting this structure here which shows you the first rib. Just for a bit of location here of what we’re looking at this image. So this is a cut of the first rib, which is found on the thoracic ribcage and is the most curved rib. If we move onto this image here of the anterior view, where you see the 2 first ribs highlighted in green, you can see that the first rib articulates with the first thoracic vertebra, as you can see on this image. Specifically, it is articulating on the posterior part of the rib. Then anteriorly, it is articulating with the first rib cartilage, which then articulates with the manubrium of the sternum.

If we go back to this image here, you see another bone highlighted. If you could guess – if this is your chest – this cut here will be then a cut of the clavicle. This is also known as the collarbone and is a long flat bone of the upper limb that articulates with the manubrium of the sternum proximally and the acromion of the scapula at its distal end. The articulation of the clavicle with the manubrium of the sternum and the acromion of the scapula form then 2 joints. One is known as the sternoclavicular joint. As the name indicates, this with the manubrium of the sternum and also the clavicle. And then the acromioclavicular joint, with the acromion of the scapula and then the clavicle. And if I show you here this image of the anterior view of the clavicle highlighted in green, you see here one of these 2 joints namely the acromioclavicular joint formed by the acromion, this projection of this triangular-shaped bone, the scapula. So the acromion of the scapula and the acromial end of the clavicle which articulates with the acromion.

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