[Music] okay so now I'm going to deal with the histology of the cardiovascular system let's start with the basic structure of the circulatory system the circulatory system basically is a tube like blood vessels even the heart is a tube that is folded on itself during embryonic development so this tube has multiple layers and each one of them has a function so these three actually there are three layers we call them tunics like coats so there's the Tunica intima there's the Tunica Media and the Tunica adventitia the Tunica intima also called the Tunica interna because it is the inner layer here is the inner layer here and it is formed of a single layer of flattened epithelial cells these epithelial cells they they are called endothelial cells because they originate from the endoderm some other epithelial cells originate from the mesoderm so they are called mesothelial cells and other epithelium is derived from the ectoderm but these are derived from endoderm so they are called endothelial cells and they lie on a basement membrane as well as sub-endothelial connective tissue this is a connective tissue here lying just beneath the endothelial cells so all these constitute what we call the Tunica and Tema and then we have the Tunica Media which is as the name indicates the intermediate layer and this is predominantly made of muscle fibers smooth muscle fibers and some types of arteries there's a predominance of elastic fibers but in most of the other arteries and even in the veins there are smooth muscle fibers with collagen and and elastic fibers are present but there's a small amount of elastic fibers and in the heart the middle layer of the heart is The myocardium and so it's also formed of muscle fibers but they are not smooth muscle fibers they are cardiac muscle fibers then the trinica adventitia which is the outer layer Tonica externa and is formed of connective tissue and in certain arteries especially larger arteries there are small arteries as well supplying the wall so these are arteries of the arteries or vessels of the blood vessels and they have a special name we call them the vasavazorum and the reason is that the blood vessels are thick and so the the cells that are present in the wall cannot rely on oxygen that is present in the blood to reach them and that's why they are supplemented by blood vessels in the wall and these blood vessels this vazavazorum in the heart is represented as coronary circulation coronary arteries supplying the wall of the hearts the same idea now let's start with the first histology slide here this is a slide of a muscular or Distributing artery and you can see that this this is a special stain that stains elastic fibers so you don't see cells you don't see nuclei this will be the Tunica intimate which is lined by a simple squamous epithelium lying on a sub endothelial connective tissue and you can see that there is a layer of elastic fibers called internal elastic lamina this internal elastic lamina some textbook considered as part of the Tunica intimate some textbooks considered as part of the Tunica Media but it doesn't matter it is located at the border between Tunica antima and the Tunica Media and it contains and is formed of elastic fibers mainly so that's why it is prominent in the sustain then we have the Tunica Media which is mainly formed of smooth muscle fibers because this is a muscular artery but still we can see some evidence of elastic fibers and then there's the external elastic lamina another layer of elastic fibers that separate Tunica Media from the Tunica adventitia which is formed of connective tissue this is an another stain of an artery look at how the artery is uniformly shaped circular shape and you can see the three layers you can see the Tunica intimate the cells the epithelial cells are not clearly seen you can see some nuclei here but not clearly seen because of it looks crinkled because of the presence of the internal elastic laminar the internal elastic lamina is so obvious here and then you have multiple layers of smooth muscle fibers look at the nuclei only small amount of elastic tissue but there are collagen fibers and then you find another layer or or another collection of elastic fibers here which is the external elastic lamina separating the Tunica Media from the Tunica adventitia where connective tissue is formed you can see a nerve accompanying the blood vessel here's another nerve so these are Distributing arteries because they have a predominance of smooth muscle in their wall that's why they are called muscular arteries but why do we call them Distributing arteries because these smooth muscle fibers they will control the caliber of the artery and therefore they will help in distributing their blood to organs depending on the situation so for example let's say in situations of where there's sympathetic overstimulation like fight or flight these smooth muscle fibers and the blood vessels of the heart and the blood vessels of skeletal muscles are going to relax so that there will be more blood coming to the heart more blood coming to the skeletal muscles but at the same time the smooth muscle fibers in the wall of the blood vessels supplying the gut will contract and saw that the blood will be diverted from the gut to go to more needed organs the heart the skeletal muscles at this time that's why they are called Distributing arteries because they control the distribution of the blood and they are muscular because this can control over distribution is due to the fact that they have thick Tunica Media made of multiple layers of smooth muscle fiber this is another type of artery which is called elastic or conducting artery these are the large arteries that are connected to the heart like for example this section is from the aorta and again it is sustained with a special stain that shows the elastic fibers only because these are Big arteries you cannot see the entire artery in one section so this is a section in part of the artery and this is the luminal side and here is the where the endothelial cells are located and they lie on a sub endothelial connective tissue of the clinic and then you can see that the twinica media is all formed of multiple laminate of elastic fibers so you cannot distinguish an internal elastic lamina and external elastic lamina they are all elastic Calamity throughout the whole thickness of the Tunica Media and then there is the Tunica adventitious so that's why these are called elastic arteries because they have multiple layers multiple laminate of elastic fibers and at the same time they are conducting arteries because they conduct blood from the heart and in a moment I will give more details about the use of these multiple elastic fibers in the wall of these arteries then we have the other vessel here are the arterioles these are the small arteries they are less than 0.5 millimeter in diameter they have the same structure of the artery Tunica intimate and endothelial cells Tunica Media with smooth muscle fibers but the smooth muscle fibers that are present in the wall of the arterial um they are only limited to one to five layers of smooth muscle fibers because the article is very small so um up to five smooth muscle fibers layers of smooth muscle fibers are present in this wall in some arterios there is an internal elastic lamina but there is no way that there is an external elastic lamina and there is a layer of Advantage here you can see this arterial is accompanied by a vein a small vein which is called a venule and you can from here recognize that the venule has a very thin wall in comparison to the size of the Lumen and that's why it is easily compressed so it is not like uniformly rounded like in the artery because the arterial has a thicker wall than the venule and in both of them you can see that the Lumen have as red blood cells so they are definitely they are blood vessels and this is a vascular bundle which is also accompanied by a nerve and it is surrounded here by collagen fibers this is a dense irregular connective tissue so you can see collagen fibers in different orientation here's a longitudinal this is an oblique orientation and the hall is called a neurovascular bundle because it's a nerve and blood vessels artery arteriole and avenual and the nerve you can see that it has doesn't have a lumen that's why here I am saying that this is a nerve and the cells here the nuclei of the cells make sure that these are not of neurons because a nerve what's a nerve it's a bundle of axons so the neuron the cell body is not present in the nerve it's present in somewhere else either in a gangrian or it's present in the gray matter of the brain or in the spinal cord but why do we see cell nuclei here these are the cells the Schwann cells that provides the myelin sheath of the peripheral nervous system this is another slide showing you the muscular artery but at the same time it is showing a vein so so if you look at the vein you will see that the vein has a very thin wall in comparison to the size of this Lumen compressed because of it has a thin wall so it is easily compressed and not only this you will find that the vein also has the same features so the principle is the same there is a Tunica intima with endothelial cells they don't look like crinkles because there is no internal elastic laminar and there is a clinical media here but the silica media is very thin in comparison to the Tunica adventitia so that the wall of the vein in general is thin because the blood in the vein is under low pressure and the thickest tunic in the vein is not the attorney chem India like in the artery but it is the Tunica adventitia you can see here again thin Tunica Media in comparison to the technical advantage and again you can see a nerve another small artery here not an arteriole it's an artery and another vein in comparison to the artery you see that it is has a thin very thin wall in comparison to the size of its lumen so this is how to in summary to differentiate between an artery and vein the shape is less deformed than the artery as you compare it with the vein which is flattened because it has a thin wall it doesn't need a thick wall because the blood and the vein is under low pressure and the intimate is crinkled because of the presence of an internal elastic lamina here the intimate is smooth because there is no internal elastic lamina also sometimes in sections of the veins you you will find the presence of valves very thin a layer of valves like bicuspid valves if this is a section in the vein then the valves they open in the direction of the flow of blood and so they will allow unidirectional flow of blood towards the hearts and they will prevent the blood from returning back away from the hearts because if the blood tries to return these cusps will come together and will close this is an important mechanism for Venus return in the in the heart so when the vein because they have thin wow they are easily squeezed and when they are squeezed for example by muscle contraction like in the peripheral uh in the in the limbs the muscles are play an important role in the Venus Return of the blood they squeeze the art the veins and they allow unidirectional blood flow also the respiratory movements increasing intra-abdominal pressure will cause pressure on the veins that they have the blood has no way to pass but to the heart because of the presence of valves in the veins so you can see that morphology is related to function like in the large elastic or conducting arteries these multiple elastic laminee they will make sure that during diastere there is a continuous flow of blood in in the blood vessels because during history these elastic fibers they will stretch and so during diastole they will require back and they will create a pressure in in the wall of the of the blood vessel that will dampen the pulsatile flow of blood you can see here this is a section in the aorta don't confuse these These are these faults dark stains so these are like artifacts the not related to any structure and the endothelium as everywhere in the cardiovascular system the endothelium the lining epithelium will provide a smooth lining to facilitate the flow and prevent clotting of blood this is another section showing here an artery and again why this is an R3 because the profile the section is is not deformed like in the vein I can see an internal elastic lamina and I can see that there is an external elastic lamina and there is a thick layer of the thick multiple layers thick Tunica Media multiple layers of smooth muscle fibers and there is a clinical adventitia this means that the Lumen because of the thick Tunica Media with smooth muscle fibers the Lumen is controllable in size as I mentioned and these are called Distributing arteries because they Direct the blood flow as I mentioned in the example of directing blood flow from the GI to the car to the to the heart and to the skeletal muscle fibers at the times of fight or flight and the elastic tissue will limit the distention of the Lumen here this is the vein I can see that it has a distorted shape the thickness of the wall is a thin in comparison to the size of the Lumen the valves are not shown but as I mentioned the you might see sometimes you might see the valves which are functionally important to prevent backflow of blood um elastic fibers are not that much needed so you can see some elastic fibers but not in the form of an internal and external elastic lamina this is to show you the wall of the heart which is mainly a myocardium and these are cardiac muscle fibers cardiac muscle fibers although they are involuntary but they are striated so if you magnify more you can see the evidence of striation in addition to that the cardiac muscle fibers unlike the skeletal muscle fibers which are also striated but unlike them the cardiac muscle fibers they have centrally located nucleus sometimes two nuclei but usually a single nucleus per cardiac muscle fiber unlike the skeletal muscle fiber they will have the skeletal muscle fibers they have multiple nuclei and the nuclei are located at the periphery not centrally located like in the heart muscle and also the these muscle fibers are branched they are not purely cylindrical they have branches and so these are features of cardiac muscle fiber but the the other striking feature that you can see here is the presence of these vertical Junctions or dark stained areas these are called the intercalated discs because they are present in between cardiac muscle fibers and they look dark because the cell membrane here has junctional complexes so Junctions between cells mainly the junctional complexes are in the form of Gap Junctions that allow communication between the cardiac muscle fibers Communication in terms of like movement of electrolytes during contraction so that's why that's how these muscle fibers they act like a network or what you call sensitium because they can communicate with each other through the Gap Junctions but it's not only Gap Junctions that are present in these interculated disks but also there are desmosomes these desmosomes that will prevent separation between the cells because these cells are Contracting so they tend to separate and so the desmosomes will keep them together so the intercalated discs are a unique feature that is present in cardiac cardiac muscle fiber in the wall of the heart the wall of the heart is formed of three parts is formed of endocardium then myocardium and then we have the epicardium it's like in the Tunica antimatternica Media and Tunica adventitia but with slight changes here slight modifications so the my endocardium we have endothelial cells so clearly shown here simple squamous epithelial cells endothelial cells and there is a sub endothelial connective tissue you can see the connective tissue collagen fibers there are these nuclei are of fibroblasts nuclei and then The myocardium is obvious here and you can see that in the endocardium there are these cells in some places of the endocardium you can see these cells these are the purkinje fibers part of the conducting system of the heart they are modified cardiac muscle fibers and that's why they look like in one way or another they look like cardiac muscle they are cylindrical they have a centrally located nucleus but the cytoplasm although it looks striated but it is looks almost empty and especially around the especially around the nucleus because they have less contractile elements they are not structured they are not made for contraction they are made for conduction they also have intercalated discs so these are the purkinje fibers they are also they are present in the sub endothelial connective tissue of the endocardium not in The myocardium but in the endocardium purkinje fibers again same features like the myocardial cells but as I said that they like they have intercalated discs but they have they have large amount of glycogen and less amount of contractile elements so they look as if they are empty lighter stained cytoplasm so to wrap up there are three layers in the wall of a blood most of the blood vessels the arteries the veins the arterioles Tunica intimate Tunica Media and Tunica adventitia and why do I say most of the blood vessels because we have the capillaries they only have the Tunica enemy you can see this is an example of a capillary it should be also smaller than this maybe this one is also a capillary it's only formed of aligning of endothelial cells lying on a basement membrane and the reason for that the capillaries are for exchange between the blood and the interstitial interstitial fluids thank you very much [Music] thank you