Hey everyone! This is Nicole from Kenhub, and in this tutorial, we will be looking at the veins of the thorax, looking at the veins that drain the intrathoracic structures as well as the veins that drain both the anterior and the posterior thoracic walls and the mediastinum.
So in our first image here, we're looking at a right lateral view of the heart with the wall pulled back to expose the right atrium. And in the right atrium of course, we have the pectinate muscles along the wall here and the tricuspid valve going into the right ventricle just here. And in terms of our vessels, we have our superior vena cava in green here as well as our ascending aorta and the pulmonary artery popping out just under here and the right pulmonary veins are hanging out around here. And the superior vena cava which we're here to talk about on this slide is a large valveless venous vessel that drains the head, the neck, the upper limb and the thorax.
The superior vena cava is formed by the union of the brachiocephalic veins and opens into the right atrium of the heart which we can see it doing just here. And before we move on, I quickly just want to talk about the tracheal veins which are small veins that drain the trachea. And these veins which we can't see on this slide obviously are interesting to note because they open into the different venous vessels at different levels of the trachea. For example at the level of the neck, the tracheal veins drain into the superior vena cava. However, further down the trachea, these veins open into the left brachiocephalic vein or the inferior thyroid vein or the internal thoracic vein depending on which level of the trachea they arise from.
Now on this slide, we're having a look at our azygos venous system and the azygos venous system is directly related to the superior vena cava. And the azygos venous system is a system of veins that drains the viscera within the mediastinum, the back and the thoraco-abdominal walls. And this system consists of the azygos vein, the hemiazygos vein, the accessory azygos vein, and the left superior intercostal vein. And I know we've listed the azygos vein on this list first but we're going to start with the accessory hemiazygos vein as the azygos system can be separated into two sides – the left and the right side – and I'll explain that a little bit more later but for the moment we're going to start with the left side, and that includes the accessory hemiazygos vein which we have here highlighted in green. And if you can't see it, I'm just pointing it out here with my arrow.
And the accessory hemiazygos vein drains the superior left hemithorax. It also receives tributaries from the fourth to eighth lumbar veins as the fourth to eighth intercostal veins on the left side which we can see here and the left bronchial veins which we can't see. And the accessory hemiazygos vein goes on to open into the azygos vein which is running down the length of the thorax on the right just here. And the hemiazygos vein like the accessory hemiazygos vein also drains into the azygos vein and it's the asymmetric counterpart to the azygos vein on the left side and it crosses to the right side at the level of the seventh thoracic vertebra T7 to join with the azygos vein. So as we can see counting down our ribs 1, 2, 3, 4, 5, 6, 7 – here we have the hemiazygos crossing over at the level of T7 as we said earlier.
And the hemiazygos vein receives tributaries from the ninth to eleventh posterior intercostal veins and the left superior phrenic vein. Occasionally, it also receives the left renal vein and inferior vena cava. And as we mentioned before, the accessory hemiazygos vein and the hemiazygos veins receive venous drainage of the intrathoracic structures on the left side while the structures on the right drain into the azygos vein which we're now going to talk about.
And the azygos vein itself receives both the accessory hemiazygos, the hemiazygos veins, and the right bronchial veins. The azygos vein arises from the posterior aspect of the inferior vena cava at the level of the renal veins and descends in the posterior mediastinum to the right of the vertebral column eventually draining into the superior vena cava just before it pierces the pericardium. And we can see it running along the passageway just described here so rising inferiorly to run along lateral to the vertebral column. And in the last two slides I've mentioned that the bronchial veins open into the accessory hemiazygos and azygos veins on the left and right side respectively but it also should be noted that the bronchial veins also communicate with the pulmonary veins at the hilum resulting in a small amount of deoxygenated bronchial blood being mixed in with the much larger outflow of oxygenated pulmonary blood as it makes its way to the left atrium.
The next veins we're going to look at are the pulmonary veins which we just mentioned. And our pulmonary veins are a little bit special as they do not carry deoxygenated blood like other veins being the only veins along with the fetal umbilical veins that carry oxygenated blood. And there are four pulmonary veins – two on the left which we can see here just below the aorta and the left bronchus and two on the right below the right bronchus and the right pulmonary artery. And these four veins carry oxygen-rich blood from the left and right lungs to the left atrium. And this blood will eventually be circulated to the rest of the body.
So the pulmonary veins carry oxygenated blood from the lungs to the heart and the pulmonary arteries carry deoxygenated blood from the right ventricle of the heart to the lungs for gaseous exchange and do keep that in mind as it's very important not to mix the two up.
So just going back to our veins that do carry venous blood, the next visceral branches of the superior vena cava we will look at are the esophageal veins. And as the name suggests, these veins drain the esophagus and, similarly to the tracheal veins, they drain into different venous vessels depending on which level they arise from. For example, the esophageal veins that arise from the cervical part of the esophagus drain into the inferior thyroid vein or the left brachiocephalic vein whereas the esophageal veins arising from the thoracic part of the esophagus drain into the accessory hemiazygos vein at the upper left side into the hemiazygos vein at the lower left side and into the azygos vein on the right side. And we can see those on our image here. A branch is going into the accessory hemiazygos vein here into the hemiazygos vein here and into the azygos vein just here. And in the abdomen, the esophageal veins drain into the left gastric vein.
And while we're here, I'm just going to mention a brief clinical note. So the esophageal veins are important because in the case of portal venous flow obstruction to the liver for example as a result of portal hypertension, blood may be diverted through the esophageal veins into the superior vena cava, however, this may result in the formation of esophageal varices due to the added amount of pressure put on the small veins.
And the next veins of the thorax we'll look at here are the internal thoracic veins. And the internal thoracic veins are quite unique as they arise from the venae comitantes surrounding the internal thoracic arteries which you can see running along here. And just to explain quickly what the venae comitantes are, the venae comitantes are paired accompanying veins which run alongside an artery usually to receive the pulsations of the artery which in turn aid venous return. And in most cadaver cases of the venae comitantes accompanying the internal thoracic arteries, the two vessels merge to form a single vein which then travels along the medial aspect of its corresponding internal thoracic artery and continues until it opens into its respective brachiocephalic vein. And in this image, you can see the venae comitantes merging to form the corresponding internal thoracic veins at the level of the third rib.
And the internal thoracic veins receive tributaries from the thymic veins which drain the thymus, the anterior intercostal veins which drain the anterior intercostal spaces and note that there are two anterior intercostal veins in each intercostal space, the pericardiacophrenic veins which accompany the pericardiacophrenic arteries from the base of the diaphragm and the pericardium, the mediastinal tributaries which are small veins from the mediastinum, and the musculophrenic veins which accompany the musculophrenic arteries.
And now let's have a look at our cardiac veins which open into the coronary sinus which subsequently drains into the right atrium. And there are three cardiac veins – the great cardiac vein, the middle cardiac vein and the small cardiac vein. But first off, we'll just have a chat about the great cardiac vein which we can see highlighted in green here running in the anterior ventricular groove. And the great cardiac vein is the largest of the three cardiac veins and drains the anterior aspect of the heart receiving the left marginal vein, the anterior interventricular vein and the posterior vein of the left ventricle.
The middle cardiac vein highlighted in green here also drains into the coronary sinus. It commences at the apex of the heart and descends in the posterior longitudinal sulcus ending in the coronary sinus, and we can see the coronary sinus running along the left atrioventricular groove just here. And, finally, the small cardiac vein – another tributary of the coronary sinus – drains the inferior and lateral wall of the right ventricle occasionally emptying into the middle cardiac vein and it travels in the right coronary groove and receives the right marginal vein.
And finally for this tutorial, we have the superior intercostal veins which are also paired veins, and the superior intercostal veins drain the second, third, and fourth intercostal spaces on the left and right side of the body. And although they are not seen in our illustration, it should be noted that just above the superior intercostal veins are the paired supreme intercostal veins that drain the first intercostal spaces.
Now going back to our superior intercostal veins, the left superior intercostal vein also receives tributaries from the second, third and fourth posterior intercostal veins which are these veins seen just here on the posterior side of the body and opens into the left brachiocephalic vein. Although it does not open directly into the azygos vein, the left superior intercostal vein is still considered a part of the azygos venous system as we saw at the beginning of this tutorial. The right intercostal vein also drains the second, third and fourth posterior intercostal spaces on the right side, however, unlike its counterpart on the left side, the right superior intercostal vein opens directly into the azygos vein.
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Now, good luck everyone, and I will see you next time.