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Heart in situ

Heart in situ seen from the anterior view.

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This patient is in critical condition. We need to perform an emergency thoracotomy to repair her wounds. We’ll start with incisions up the mid-axillary lines bilaterally, across the clavicles, over the manubrium, and across the costal margin. Now, cut the ribs and clavicles. Now, let’s remove the chest wall and save this patient. Wow, that sounds intense. The surgeon has performed an advanced surgical procedure called the thoracotomy. Let’s step aside and let the healthcare team treat this patient. We’ll check in with them later.

Meanwhile, we can explore the anatomy we see in this view. Here we are looking at the thorax with the front part of the chest wall removed, and since none of the structures have been removed or disturbed, we can say that everything is in situ, which translates to “in its original place.” In this video, we’ll focus particularly on the heart as it sits in its normal anatomical position a.k.a. in situ.

Before we delve into the nitty-gritty of our tutorial, let’s briefly go over what we’ll be looking at today. So, first, we’ll go over the anatomy of the heart that can be seen in the normal anatomical position, and this will include looking at some of the heart’s chambers, atrial appendages, and external features. Then, we’ll look at some important neighboring structures including different tissues and organs. Next, we’ll look at important neurovasculature including arteries, veins and nerves. And we’ll wrap things up by looking at a clinical scenario related to the heart in situ. But first let’s start by looking at the belle of this video’s ball – the heart.

In order to see this muscular organ in all its glory, we have to first remove this structure which is the pericardium. This layer of tissue surrounds and protects the heart. Although it’s very important, don’t worry too much about it now, we’ll get to it later. With the pericardium out of the way, we can identify several key anatomical structures of the heart. We’ll be going through the chambers, the appendages, as well as some of the heart’s external features.

And we’re going to start by identifying its chambers. And from this perspective, we can clearly see three of the four chambers – these being the right atrium, the smaller right ventricle, and the larger left ventricle. The fourth chamber – the left atrium – is actually best seen from the back or the posterior view. We’d had to lift the heart out of the pericardial sac in order to see it, but then the heart wouldn’t be in situ anymore. But as a quick refresher, here’s what the left atrium looks like from a posterior view.

Okay, let’s take a second to look at these chambers in a little bit more detail, starting with the right atrium.

So, blood enters this chamber through two large veins called the superior and inferior vena cavae although we can only see the superior with the heart in situ. The inferior cava is hiding behind the heart. And we’ll look at the superior vena cava in more detail a little bit later. The blood within these veins has just travelled all throughout the body delivering its oxygen to various muscles and organs, and now it’s returning to the heart to be rejuvenated.

Next, the blood enters the right ventricle by traveling through the tricuspid valve, also known as the right atrioventricular valve – and we can’t see that valve with the heart in situ as we’d have to cut open the heart and have a look inside. From the right ventricle, the blood travels to the lungs to pick up oxygen via the pulmonary trunk which we’ll get to in just a bit.

The largest of the four chambers seen here is the left ventricle, and this sits just inferior and anterior to the left atrium which, remember, we can’t see since it’s on the posterior side of the heart, and the blood within this chamber was just revitalized or oxygenated at the lungs, has traveled through the left atrium, and is ready to be pumped out to the rest of the body to deliver its delicious oxygen.

Next, we’re going to be looking at two mini-chambers of the heart, and these include the left atrial appendage and the right atrial appendage. And this little eel-like projections act as reservoirs for when the larger four chambers become overfilled, and you may hear these structures also being called auricles which actually means little ear, referring to their shape.

If we look at the left atrial appendage a little more closely, we can see that it is perched just on top of the left ventricle. Meanwhile, the right atrial appendage sits just on top of the right ventricle and you can probably see how prominent it is. With the heart in situ, it actually covers a lot of the right atrium.

Okay, so now that we’ve covered the chambers and appendages seen with the heart in situ, we’ll check out three of the heart’s external features as seen from this view. So, first stop is the conus arteriosus and then we have the anterior interventricular sulcus, and finally, we have the apex of the heart.

The conus arteriosus is this cone-shaped pouch that we can see highlighted now and notice that it is at the upper most part of the right ventricle and it actually gives rise to the pulmonary trunk that we saw earlier. It might help to think of it as a small piece of plumbing that connects the right ventricle and the pulmonary trunk.

Just to the left of the conus arteriosus, we have a shallow groove called the anterior interventricular sulcus. Now, don’t let this big name scare you. Anterior just means that’s on the front of the heart, and thinking about the chambers that we looked at earlier will help you figure out the next word. So, interventricular just refers to the fact that it is located between the right and the left ventricles. The last word, sulcus, is another term for groove or depression. And it’s long and narrow and that’s where you’d find the interventricular branch of the left coronary artery – one of the arteries that we’ll look at later.

So, normally, when you hear apex you may think of top as the top of a mountain. However, this word really just means the pointy end. In the case of the heart, the apex isn’t’ pointing up, it’s actually pointing downwards and to the left, and we can see here how the apex of the heart is part of the left ventricle which we saw earlier.

So now that we covered the heart, we can start investigating some important things that are close by. We’ll start by looking at relevant lung tissue layers located near the heart that we can see with the heart in situ including the pericardium and the mediastinal part of the parietal pleura and there are also some key organs that we can see from this view, too, like the thymus, the lungs, and the diaphragm. And lastly, we’re going to be looking at the neurovascular structures we can see with the heart in situ.

So we’re going to start with a structure that we saw at the beginning of the video, and that’s the pericardium. And this layer of tissue forms a sac around the heart and actually has two layers – the fibrous pericardium and the serous pericardium – and what we see highlighted now is the fibrous pericardium which is the most superficial of the two layers.

Next, we have the serous pericardium, which can actually be divided into two parts. On the inner surface of the fibrous pericardium, we have the parietal layer of the serous pericardium and adhered directly onto the heart is the visceral layer of the serous pericardium or more simply known as the epicardium. Between the parietal and visceral layers of the pericardium is a fluid, and this is simply called the pericardial fluid. The pericardium helps to protect the heart from infection while fixing it to the mediastinum and also provides some lubrication from the heart as it vigorously pumps blood throughout the body.

The mediastinal part of the parietal pleura is just one section of a continuous layer of tissue that lines the inside of the thorax which collectively is called the parietal pleura. The mediastinal part of this tissue is the part that is contact with the mediastinum and if it helps, you can think of the thorax as being like a room with the pleura being the wallpaper that’s stuck to the inside walls of the room. The mediastinal part of the parietal pleura is just one part of the wallpaper that faces the space found between the two lungs.

Okay, looking at the upper part of the mediastinum, we can see the lymphoid organ known as the thymus, and if we zoom in, we can see how close the thymus lies to the great vessels of the heart, which we’ll go over in more detail when we discuss the neurovascular structures.

When we look at the sagittal view, we can see how the thymus sits right at the front of the mediastinum directly behind the sternum, and this organ would be one of the first thing the doctor at the beginning of the video would have seen when performing the thoracotomy. However, normally, as we age, this organ slowly undergoes involution or regresses into fatty tissue.

On either side of the heart, we can see the right and left lungs and if we take a closer look, we can actually identify some of the lobes and fissures associated with each lung.

So let’s start with the right lung, which we can see highlighted now and this lung has three lobes – the superior, middle, and inferior. And these lobes are separated from each other by fissures. And since we have three lobes, we know that there must be two separate fissures, so let’s have a look. So, first separating the superior and middle lobes is the horizontal fissure and separating the middle and inferior lobes is the oblique fissure.

Let’s quickly jump over to the left lung now, and on this side, we only have two lobes as three lobes would just make it a bit too crowded with the heart in the way. So over here we can see the superior lobe, and just inferior to that, we can see the inferior lobe. So, just like we saw on the right side, these lobes are going to be separated by a fissure and in this instance, we only have one fissure that is separating our two lobes, and this fissure is called the oblique fissure since it is in an oblique or diagonal orientation.

If we keep our focus near the left lung, we can see one important structure traveling towards it and that’s the left main bronchus. And here, you can see that it’s kind of hidden behind some of the great vessels, so if we move these out of the way, we can see the whole left main bronchus and its counterpart on the other side, the right main bronchus, and you can see that the left main bronchus comes off the trachea at a little bit more of an angle compared to the right main bronchus which appears more continuous with the trachea. And this is why if you were to accidentally inhale a piece of candy down the wrong pipe, it would most likely fall into the right main bronchus.

So if we go back to our image with the heart in situ, we can see the last neighboring structure before we get to the neurovasculature and that’s the thoracic diaphragm. Sometimes, it’s also referred to as the respiratory diaphragm or even more simply just the diaphragm. And this thin sheet of skeletal muscle extends across the bottom of the ribcage and serves as a boundary between the upper thoracic cavity and the lower abdominal cavity. And this muscle is very important as it plays a crucial role in breathing. As it contracts and pulls down on the thorax, the thoracic cavity gets bigger and air is drawn into the lungs, and when it relaxes, the thoracic cavity shrinks and air is forced out of the lungs.

Okay, so now that we’ve looked at the heart and some of the surrounding organs and structures, we’re ready to move on to the arteries, veins and nerves that can be seen with the heart in situ. And get your notepad ready, there’s quite a few to get through.

So, we’re going to start by looking at the arteries, and we know that normally arteries are responsible for carrying oxygenated blood, however, we’re going to be looking at one very unique artery that actually carries blood that is deoxygenated. Then we’ll move on to the veins and some of the veins that’re close by and have similar names to the arteries so hopefully that part will be a breeze. And finally, we’ll wrap up this section by looking at some major nerves, and this will be a nice way to end since the structures we’ve looked at prior will be either good landmarks of finding these nerves or actually innervated by these nerves.

Okay, so we’ve got a lot to get through so let’s just jump right into it.

So the first artery that we’re going to be looking at is this big one here, and that’s the aortic arch. The blood that’s being pumped through the aortic arch is highly oxygenated and just came out of the left ventricle – one of the four chambers that we looked at previously via the ascending aorta. And it’s easy to remember the name of this one, since this vessel is actually in the shape of an arch, which can be seen a little bit better if we remove some of the surrounding structures. And you can also see there are three big branches that come off the arch and ascend upwards, and these three branches are what we’ll look at next.

With the heart in situ, you can only see a sliver of the first branch of the aortic arch known as the brachiocephalic trunk and it’s kind of hidden by some of the veins that we’ll get to in a little bit and usually hides behind the thymus, but that’s already been removed. So, let’s remove a few more structures so that we can see it a little bit more clearly, and here we go, that’s a lot better.

So now we can see that this artery is actually pretty short before it splits into the right common carotid artery and the right subclavian artery, and we’ll look at those next. So coming off the brachiocephalic trunk and going up the neck is the right common carotid artery, and we can see it pretty well here, but it’s important to see it while the heart is in situ since that’s the whole point of the video.

Alright, so let’s add all our structures back in. It’s going to hide this artery a little bit, but it’s important to see where it’s at in relation to everything else. And there we go. So now we can see that it sits behind the vein and nerve that we’ll talk about in just a minute.

The second branch of the brachiocephalic trunk is the subclavian artery, and this branch heads a little bit more lateral towards the right arm where it will supply blood. It also gives off some branches which supply the neck and parts of the brain.

Up next is the left common carotid artery which is the second branch of the aortic arch and this is different than what we saw on the right side. Rather than coming off a common trunk, the left common carotid and the left subclavian arteries come directly off the aortic arch, and like its right-sided counterpart, the left common carotid goes up to supply the head and the neck.

Just nearby, we can see the third and final branch of the aortic arch which is the left subclavian artery, and like on the right side, this vessel is responsible for supplying the left upper limb, the neck, and parts of brain with oxygenated blood.

Coming off the left and right subclavian arteries are the internal thoracic arteries and here we’re only seeing a small stub of this vessel but actually it’s quite long. So if we add the chest plate back on, we can see that the internal thoracic artery actually travels down the whole length of the front of the thorax on either side of the breast bone or the sternum.

So if we head back to our heart in situ image, we can see the next artery on our list, and this is the pericardiacophrenic arteries. It’s a little bit of a mouthful, but don’t let that intimidate you as these guys are just named based of what they supply. So the first part of the word ‘pericardiaco’ means that it supplies the pericardium – the layer of tissue that we looked at previously – and the last part of the word ‘phrenic’ refers to the fact that it supplies the diaphragm, which is the thin muscle at the bottom of the thorax that we already discussed. And not only are these little guys sort of hard to pronounce, they’re also a little bit hard to identify, and zooming in definitely helps us see them a little bit better.

The next artery we’re going to be looking at is a little bit tricky since it’s not a true artery, but it used to be. So, in the developing embryo, there is a small arterial shunt between the aortic arch and the pulmonary trunk, and this shunt is called the ductus arteriosus, and it allows the blood to bypass the lungs since our lungs aren’t functional in utero. However, after we’re born and our lungs start working, this shunt closes off, and what’s left behind is this little guy, the ligamentum arteriosum.

Okay, so we’ve got three more arteries to go, and the next ones are a little bit more closely associated with the actual heart, so they might be a little bit easy to remember.

So, first, we’re going to look at one vessel that supplies the actual heart muscle tissue and it’s called the right coronary artery, and you can see that it’s located just below the right atrium and just above the right ventricle, and it travels around the right part of the heart and sits sort of like a crown.

The next artery that we’ll look at also supplies the heart muscle tissue and it’s called the interventricular branch of the left coronary artery. It’s a branch of the left coronary artery which we can’t see from this view but it would be located around about here. And we mentioned the interventricular branch of the left coronary artery earlier when we talked about the anterior interventricular sulcus which is where you would find this artery snugly fitting right between the two ventricles.

And, finally, here we have the pulmonary trunk. We briefly saw this structure earlier when we talked about the conus arteriosus which sits just inferior to the pulmonary trunk, and this artery is unique because it actually carries blood that is deoxygenated. Remember that the blood leaving the right ventricle is heading for the lungs to pick up oxygen and it gets to the lungs by traveling through the conus arteriosus and then the pulmonary trunk. And it might be easy to remember that arteries are traveling away from the heart and aren’t necessary always carrying oxygenated blood.

Okay, so the next set of neurovascular structures we’re going to be looking at are the veins, and we’re going to start off more distally and follow the drainage pattern towards the heart.

So the first veins that we’re going to be looking at are these guys which are the left and right internal jugular veins and they’re responsible for draining the head and the neck of deoxygenated blood. And you’ll find them just anterior to the common carotid arteries that we looked at previously.

So now we know that the blood from the head is traveling downwards in the internal jugular veins. The blood from the upper limbs meanwhile is traveling back to the heart in the veins that you see highlighted now called the left and the right subclavian veins. And as the name suggests, you would find these just beneath the clavicle or the collarbone. The blood from the internal jugular veins and the subclavian veins then merge together to form the left and the right brachiocephalic veins.

And I known this is kind of a lot to take in right now, so it might help if we break down the word parts of brachiocephalic. So, ‘brachio’ refers to arm which is where the subclavian vein is receiving blood from and ‘cephalic’ refers to head which is where the internal jugular vein is receiving blood from. And if we put these two together, we get brachiocephalic meaning arm and head which is where all the blood in this vein is coming from.

The left and the right brachiocephalic veins then drain into this vein which is the superior vena cava and from there, the blood finally reaches the heart by entering into a chamber we looked at previously which is the right atrium.

Just one vein to go and it’s a mouthful, but hopefully it sounds familiar – the pericardiacophrenic veins travel right along with the aforementioned pericardiacophrenic arteries. And similar to their arterial counterparts, the pericardiacophrenic veins are associated with the pericardium and the diaphragm, however, unlike the arteries that supply these structures with blood, these veins are responsible for draining these structures of deoxygenated blood.

Okay, so now that we’ve looked at the arteries and veins seen with the heart in situ, we’re ready to move on to the nerves, and the first nerves that we’re going to be looking at are the vagus nerve. We might also hear them referred to as cranial nerve ten. Seen over here is the left vagus nerve and it can be kind of tricky to find during dissection, so let’s have a look at some key landmarks for this nerve.

In the neck, you can see that it is situated between the common carotid artery and the internal jugular vein and you’ll notice that it then travels just in front of the aortic arch before it dives behind the left main bronchus. Finally, it travels through the diaphragm before reappearing in the abdomen. What a journey! And it might be interesting to know that vagus in Latin actually means ‘wanderer’ which makes perfect sense when you consider its crazy course through the body.

On the right side of the body, we’ll find the other vagus nerve and with the heart in situ, you can’t quite see as much of the right vagus nerve, but you can see how it is also situated between the common carotid artery and the internal jugular vein on the right side. It will also dive behind the right primary bronchus and will pierce the diaphragm to enter the abdomen. We just can’t see this relationships on the right side because the heart is in the way.

Next, we’re going to talk about the left recurrent laryngeal nerve which is actually a branch of the left vagus nerve. And you can kind of see it here but let’s remove some structures and zoom in a little to make things a little bit clearer. There we go. Hopefully that clears things up a little bit, and you can see that this nerve actually loops underneath the aortic arch before climbing up the neck next to the trachea, and this nerve is very important as it helps control small muscles in your larynx or voice box. After all, without it, I wouldn’t be able to narrate this video.

The last nerves that we’re going to be looking at are the phrenic nerves which are responsible for innervating the large flat muscle that we looked at earlier, the diaphragm. So these guys travel in the same direction as the vagus nerve, so it can be sometimes confusing to correctly identify them in the lab. There are a few key differences that we’ll look at to help you differentiate between the two.

So you can see here that the phrenic nerve is smaller in diameter than the larger vagus nerve, and also as it travels down the thorax, it lays anterior or in front of the two primary bronchi, whereas the left and right vagus nerves traveled behind these structures. Also, since the phrenic nerve actually innervates the diaphragm, it will stop at this muscle whereas the vagus nerves just kept on going right into the abdomen. Keeping these things in mind will hopefully help avoid any confusion.

Okay, so now that we’ve looked at all the relevant anatomy in this image, let’s check back in with the patient from the beginning of the video.

So, earlier, we saw the trauma surgeon successfully removing the chest plate in a procedure called a thoracotomy, and with the chest plate removed, the healthcare team then performed what’s called a cardiac massage. This doesn’t quite mean the same thing as getting a good old fashion foot rub at the end of a long day, although it is kind of a similar idea. Essentially, it’s a resuscitative procedure which involves the application of rhythmic pressure to the patient’s heart in order to restore and maintain a sufficient blood flow after cardiac arrest or ventricular fibrillation, and this keeps the patient alive while the doctor could repair a serious internal injury that the patient suffered. But, fortunately, the team knew their anatomy, avoided damaging important structures, and save the patient.

And there you have it, the heart in situ.

Before I let you go, let’s quickly recap what we went through today.

So, first we went through the heart and the anatomy that can be seen in its normal anatomical position and this included three of the four chambers, being the right atrium, the right ventricle, and the left ventricle. We also looked at the left atrial appendage and the right atrial appendage. Finally, we looked at some external features of the heart and these included the conus arteriosus, the anterior interventricular sulcus, and the apex of the heart.

We then moved on to neighboring structures which included the lungs, their lobes and their fissures that separate the lobes. We also looked at parts of the airway with particular focus on the left main bronchus since this can be seen with the heart in situ. We checked out the pericardium and talked about its various layers, and then we looked at the lymphoid organ known as the thymus and the flat muscular diaphragm.

We then delved into a lot of neurovasculature and we started by looking at some major arteries, and these included the aortic arch, the brachiocephalic trunk, the right and left common carotid, the right and left subclavian, the internal thoracic, the pericardiacophrenic, the ligamentum arteriosum, the right coronary, the interventricular branch of the left coronary, and the pulmonary trunk. And then we moved on to the veins which included the left and right internal jugular, the left and right subclavian, the left and right brachiocephalic, the superior vena cava, and the pericardiacophrenic.

We finished our neurovasculature section by identifying some major nerves – the left and the right vagus nerve, the left and the right phrenic, and the left recurrent laryngeal. And we then wrapped things up by investigating some clinical notes on the thoracotomy and cardiac massage procedures.

And that now brings us to the end of our tutorial on the heart in situ. I hope you enjoyed it. Thanks for watching!

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