hello everyone this is the first video about vessels and circulation and these are our learning objectives in terms of blood vessels we already defined that arteries carry blood away from the heart and veins carry blood towards the heart throughout our body we find arteries veins and capillaries what happens is that arteries carry blood away from the heart they start branching and they bring so much and become very thin and this very thin blood vessel is called capillary capillaries are the most abundant of our blood vessels but they are also the smallest and thinnest out of our blood vessels because the capillaries are so thin nutrients gases and wastes get exchanged at the capillary level with the surrounding tissues and I like to say that the function of arteries and veins is to transport blood but all magic happens at the capillary level the walls of our arteries and veins are organized in three tunicates or layers and they're named adventitia media in intima layer now think about the name intima it sounds like the most intimate one right and that's correct the Tunica intima is the layer that lines the lumen or the inside of our blood vessels this layer is made of simple squamous epithelium so a single layer of epithelial cells that provides a very smooth lining for the blood to flow through in the capillaries the blood vessels that allow exchange of nutrients with the surrounding tissues are basically just made of intima layer this very thin layer of endothelial cells now our arteries and veins which are much larger than capillaries they have on top of the Tunica intima the tunica media and the tunica media is the smooth muscle layer of these vessels both veins and arteries have tunica media and they're primarily made of smooth muscle which as we already learned is the type of muscle that we cannot control consciously since we cannot control it it's controlled by our autonomic nervous system the part of our nervous system that works automatically so our autonomic nervous system can control how big or dilated our blood vessels get or how constricted they get and to cause vasoconstriction the smooth muscle in the tunica media contracts leading to a decrease in size of the vessel lumen when vasoconstriction happens an increase in resistance happens leading to decreased blood flow in that vessel now the opposite to that is the vasodilation and vasodilation happens when these muth muscle relaxes and with that we have a wider vessel lumen and we feel wider vessel lumen we get a decreased resistance and an increase in blood flow through that vessel now on top of the tunica media we have the tunica adventitia also called Tunica externa and this is the layer that we see on the outside of the blood vessels this layer is basically dense fibrous connective tissue in a search it helps to hold the vessels to the surrounding tissues now it turns out that some of our larger and thicker vessels have their own blood vessels we think the vessel itself and that's called visa VA's arm veins of Ozora is Latin for vessel of the vessel so the same way as our heart has its own blood supply the walls of bigger arteries and veins have smaller blood vessels to provide them with oxygen in nutrients because remember this is all made up with cells every part of our body is made up of cells and I like to say that because sometimes it's hard for us to think that every single part of our body is made up of cells so in the Tunica externa of some large blood vessels like the aorta and vena cava we find a small blood vessels because these large blood vessels they're so big that even though they are transporting blood they are so thick that they actually need their own blood supply now looking at the differences between an artery in a vein we can see first of all that arteries have much thicker walls than veins and we can also notice that the looming of a vein appears to be much wider than the lumen of an artery focusing on the three main layers we just talked about we can see that the Tunica intima which lines the lumen of our blood vessels looks very similar in both arteries and veins then on top of the Tunica intima we have the tunica media which is the smooth muscle layer in the middle we can see that the tunica media the layer that we find a smooth muscle is much thicker in arteries then it is in veins which means that we have a lot more smooth muscle in an artery than in a vein now why our arteries would have thicker tunica media than our veins and the reason is because the arteries carry blood with higher pressure than veins as you know arteries carry blood away from the heart and that means that the arteries are the ones closest to where the pressure is generated hence they are the vessels that are going to experience the highest amount of blood pressure on the other hand veins the blood vessels that are carrying blood towards the heart they are the farthest away from where the pressure is generated so that the pressure is so low inside the veins that in fact some of our larger veins have the blood pressure very close to zero now besides the much thicker tunica media we see in arteries arteries have elastic fibers within the tunica media in bands of elastic tissue in between the three layers of the blood vessel itself so you can see that between the intima and the media layer we have the internal elastic membrane in between the media in the adventitia layer we have the external elastic membrane both of these elastic membranes are network of elastic fibers and all these is to give more elasticity to the artery so it can sustain the pressure of the blood flowing through it in other words if arteries were not that thick and not that elastic the pressure of the blood traveling through them would possibly rip them apart now you might be wondering why do we have a smooth muscle in the vein if the vein does not have to sustain high blood pressure and the reason for that is because when veins constrict it helps the squeezed blood back towards our heart for example if it is necessary to increase the venous return to the heart to increase cardiac output we have the constriction of the smooth muscle on the tunica media and that helps to squeeze some blood back towards our heart now if we compare the lumen of an artery in a vein we can easily see that veins have a larger diameter than arteries hence veins hold much more blood than arteries do and in fact veins have between 65 to 70 percent of our blood volume at test what means that only 30 to 35 percent of our blood is actually inside arteries hence most of our blood is found at the venous end and this is why veins are called blood reservoirs arteries are the blood vessels transporting blood away from the heart and veins are the blood vessels transporting blood towards the heart one thing that I would like to point out is that although pulmonary arteries carry deoxygenated blood this deoxygenated blood is high in pressure because it is coming from the right ventricle so the pulmonary artery needs to have thicker walls like all other arteries have it just happens that the pulmonary artery carries deoxygenated blood but that has nothing to do with the pressure that the blood is flowing through the vessel now the pulmonary veins which are carrying blood towards the heart they have low blood pressure because they are just bringing blood from the lungs back to the heart so the walls of the pulmonary veins are thinner even though they are carrying oxygenated blood now in this slide we can see that we have different types of arteries and different types of veins in regards to arteries we have elastic arteries muscular arteries and arterioles the elastic arteries are the largest of our arteries so this includes the pulmonary trunk as well as the aorta in its associated branches for example the break of cephalic trunk the left common carotid in the left subclavian artery elastic arteries as the name implies are the more elastic ones they are more stretchable and that's a good thing because these are the arteries that need to sustain the higher amount of pressure what makes them more elastic is just the fact that they have more elastic fibers so the elastic arteries are the ones that have the external and the internal elastic membrane also called external and internal elastic layers as you can see here basically the elastic arteries have more elastic fibers now as the elastic arteries we have the muscular arteries another name implies the muscular arteries have lots of his mouth muscle so since these muscular arteries have lots of smooth muscle these vessels can change their diameter and they can cause vasoconstriction and vasodilation and with that these muscular arteries can regulate blood pressure and also blood flow examples of muscular arteries include the femoral artery in the radio and ulnar arteries now muscular arteries eventually branch into arterioles and the arterioles are the thinnest out of all arteries we have our Terios have very thin walls with some is muth muscle cells but nonetheless the presence of this is most muscle cells give them some capability of regulating blood pressure in blood flow at the arterial level now the arterioles branch into capillaries which form the capillary bed and the capillaries are the smallest blood vessels we have they are made of simple squamous epithelium so basically a layer of endothelial cells held together by lots of tight junctions and desmosomes and they have a basement lamina surrounding it the diameter of a capillary is almost as wide as one red blood cell so that the red blood cells need to get squeezed when passing through the capillary bed just at the level of capillaries we have exchange between the blood and the surrounding tissues because capillaries are the only blood vessels thin enough to actually allow the exchange of nutrients and wastes to happen our organs are full of capillary beds which are basically wide areas of capillaries forming big blankets big networks of little vessels that spread out and within the tissues allowing for nutrient and waste exchange to happen if we zoom in at the structure of a capillary bed we see an arteriole and in a venue end and blood flows from there to yo end across the capillary towards the venue end as you can see here we have some sort of main street which is called thoroughfare channel from which all these capillaries branches emerge from if we look here at the sides of this Main Street we see pre capillary sphincters and these filters are like wings of smooth muscle that help regulating blood flow through the capillary bed now since we have a smooth muscle on the sides of this main street we cannot call this main street a capillary because capillaries do not have smooth muscle on their walls and we call it a mater tario so mater tario is like a blood vessel in between an arteriole and a capillary and interestingly materials are exclusively found in the mesenteric circulation now imagine that most or all of these pre capillary sphincters at the entrance of the capillary bed contract simultaneously what will happen is that blood will have no other option than to go directly from their TV oh and to the venue end through the material and in this situation the matter Theriault act as a thoroughfare channel and the entire capillary network is bypassed so for instance when we are running we need more blood flowing through our skeletal muscles then through our digestive system so in order for that to happen we have the constriction of the pre capillary sphincters that were the entrance of the capillary bed of our digestive organs and once they constrict less blood will flow through the capillary bed of the digestive organs allowing more blood to be available for our skeletal muscle cells and we keep running so mater Terios are capable of regulating blood flow into capillaries and with that blood delivery to a particular tissue can be quickly increased to respond to the need of our body at that specific moment now we have organs such as our heart and our brain that require a rich blood flow all the time and to guarantee this rich blood flow we have more than one artery supplying blood to one specific area in this arteries that supply blood to the same area are called collateral arteries what happens is that these collateral arteries kind of fuse and form together a capillary bed in this capillary bed formed by two arteries is called arterial anastomosis the cool thing about this is that if one artery gets blocked the other one will supply blood to the capillary bed which ensures a blood supply for that specific area of the tissue and this is a very smart way of our body to ensure constant blood supply throughout specific organs now the word anastomosis means interconnection in in our body we have anastomosis interconnections between different types of blood vessels not just between arteries and collectively we refer to them as vascular anastomosis so the interconnection between two arteries is called arterial anastomosis can you guess what the interconnection between veins is called yes it is called venous anastomosis an evening interconnection is between an artery and a vein like the case we have here when the pre capillary sphincters at the entrance of capillaries contract and forces the blood going straight from there tear you end to the venue end what type of interconnection of anastomosis would it be called it would be called on arterial venous anastomosis now we have three main types of capillaries we have the continuous capillaries the fenestrated capillaries and the synthoid capillaries the most common type is the continuous capillaries in this type we have a simple squamous epithelium which is formed by a single layer of endothelial cells held together by tight junctions if you recall tight junctions hold cells together so strongly that it prevents water from passing between them and in the case of the continuous capillaries that's very important because we do not want to lick a lot of fluid from our capillaries into the nearby tissues this is the main type of capillary found in our brain this will be the capillary making the blood-brain barrier which is the barrier that prevents substances present in our blood stream from getting into our brain tissue now the fenestrated capillaries is a type of capillary that contains a small pores which are called fenestrations these little pores allow very rapid movement of fluid and proteins that need to be exchanged but cells cannot cross the fenestration this type of capillary is found on the choroid plexus of our brain and also in our kidneys so places where we need to have filtration of blood in fact our kidneys filter about seven liters of blood every hour so that means that our entire blood volume is filtered every hour in less than one hour actually and that can just happen because these fenestrations in the capillaries let water waste and is more ions to pass through and then they get removed from our blood stream now the last type of capillaries we have is called sinusoid capillary and these are the least abundant type of capillaries these capillaries have large gaps in the capillary wall called sinusoids these gaps are so big that blood cells can actually enter and leave the blood stream through them so where do you think it would be helpful to have this type of capillaries it would be useful to have this type of capillaries in places where new blood cells are produced so our red bone marrow which is the place where new red blood cells are produced has this type of capillaries new red blood cells are made and then they go into our blood stream through these big gaps we also have the sinusoid capillaries in our liver and spleen because these are organs that help breaking down damaged red blood cells but in order for these organs to their job the red blood cells need to leave the blood stream and then they get engulfed by new cells right outside the sinusoid capillary vessels so these sinusoid capillary vessels they have a discontinuous basement lamina which allow for the large gaps to exist on the other hand there are the two types they continues in the fenestrated capillaries they both have continuous basement lamina now going back to this diagram we see that after the blood passes through the capillary bed the blood flows to the venues and venues are the smallest veins we have then we have the venues getting together into medium sized veins and the medium-sized veins converge into large veins examples of medium-sized veins includes the radial vein and the tibia vein and also the popliteal vein an examples of large veins are the superior and inferior vena cava one interesting thing is that most arteries and veins run parallel to each other and when that happens we have medium sized veins traveling with muscular arteries and we have large veins traveling with elastic arteries if we take a closer look at a medium sized vein we see that these veins have valves in these valves are folds of the Tunica intima that prevents the blood back flow so this vein valves guaranteed the one-way flow of blood they guaranteed that the blood actually keep moving towards our heart instead of pulling in our legs now there are two other factors that influence the direction that the venous blood flow in they're called muscular skeletal pump and the respiratory pump one thing that we know about fluids is that they always flow from high pressure to low pressure so if we have a pressure difference in high blood pressure in a low blood pressure the blood will flow towards the low pressure point now if we pay attention to this diagram we can see that skeletal muscle activity can you squeeze the vein nearby and it's the squeezing on the vein that increases the pressure in that particular area of the vein now if the blood pressure is increased in that particular spot the blood will flow towards the low pressure point nearby which could be above below the squeezing site now luckily we have valves that prevent backflow that way we don't just squeeze blood in both directions when the skeletal muscle contracts we have blood being only pushed up in the direction these valves permit because these valves are the ones blocking the backflow of blood these valves are establishing the unit direction of venous flow so our skeletal muscles itself can work as a pump to pump blood towards the heart and in fact this is called the skeletal muscle pump the other pump we have is called the respiratory pump and this one relies on changes in pressure inside our thoughts so during inhalation our thoracic wall expands and if we hear you very deeply we can really see it expanding with that we increase the volume of our thorax and when we increase the volume of our thorax it decreases the pressure in the thorax area if we have lower pressure in the large veins of our thorax then blood will flow from the higher pressure point towards the low pressure point which is inside our thorax so just the action of inhaling helps bring in blood towards the heart we know that arteries have high pressure and veins have very low pressure and we can also say that the blood pressure decreases from the aorta to the vena cava because the aorta is the closest artery to the heart and the heart is what generates the blood pressure and the vena cava is the vein located the farthest away from where the pressure was generated so the blood pressure decreases from the aorta to the vena cava when our heart contracts and relaxes we have the systole and diastole and this leads to the generation of the systolic pressure and the diastolic pressure the blood pressure in our arteries has a systolic and a diastolic pressure and this is what we call post systolic pressure is the high peak of the post and it is generated while the ventricles are in systole and diastole ik pressure is the low peak of the post and it is the pressure that remains during diastole now the blood post's continues through the arteries until it reaches the arterioles but it disappears by the time the blood gets to capillaries and the reason for that is that capillaries are just made of simple squamous epithelial there are no elastic fibers or a smooth muscles in the capillary walls and if there are no elastic fibers or is moved muscles the capillaries cannot expand and contract hence capillaries cannot pause they post these appears at the capillary level and then there is like a continuous blood pressure from capillaries all the way through the veins so what's important to notice is that under normal conditions veins also do not post and if we feel a vest posing it's probably not a vein veins would just pose under abnormal conditions and an example is when the right side of the heart is failing and the right ventricle is not pumping blood to the lungs in a proper manner and this leads to the backup of blood in the veins and then you can start seeing the jugular vein posing but again veins would just pose in abnormal conditions here we have a pie chart just giving us a visual representation of where our blood is located so as I mentioned before our veins act as blood reservoirs and the reason we say that is because most of our blood is found inside our veins basically 64% of our blood is inside our veins and this percentage includes large veins medium-sized veins and venues the heart itself just holds 7% of the blood and this is the blood found in the heart chambers itself this is not taking into consideration the amount of blood inside the vessels of the heart and as you could suspect if we compare the amount of blood found in the systemic circuit with the amount of blood found in the pulmonary circuit we have much more blood in the systemic than in the pulmonary circuit and with these we finish this part please let me know if you have any questions bye