we are now going to start talking about arteries and their functions okay um before we start discussing that we do need to talk a little bit about vascular distensibility so all blood vessels are distensible they have this elastic nature and arteries actually play a very important role that allows them to buffer fluctuations in pressure so if you think about it your cardiac cycle is composed of a systole and diastole it's cystitis a period of ejection and diastole there is no ejection the heart is just refilling with blood and getting ready for the next subsequent contraction so in systole you will have blood flow because the heart is contracting and providing that pressure for flow but what about in diastole is there blood flow during diastole or not well the answer is yes our blood flows normally it's not pulsatile by the time blood reaches the um the tissues and the organs blood flow is very much constant and even within the large arteries you still have blood flow during diastole so where does that blood flow comes from well because your arteries are not rigid pipes now if you think about pumping blood through rigid pipes and if your motor or your pump is only you know pumping and then relaxing or stopping and then pumping again at these intervals you will have pulsatile flow but your arteries are not rigid pipes so what happens is when your heart contracts during systole your arteries not just propel blood forward but they also expand outwards and when they expand outwards a little bit they actually store some energy the moment your heart stops and relaxes your blood vessels recoil and that recoil of the large elastic arteries helps provide blood flow during diastole and so nice compliant distensible arteries are actually very important for proper circulatory function if these arteries lose that property and they become pretty stiff and they're not expanding we will start experiencing pulsatile flow now pulsatile flow is actually pretty dangerous especially when you're thinking about high flow organs or organs that require constant blood flow like the brain the kidneys and the eyes and this may cause a problem called microvascular hypoperfusion and ischemia leading to a lot of other issues that can be very significant arteries are distensible but veins are even more distensible than arteries and that's why they can hold a lot more blood and they serve as a reservoir for holding blood now if we understand extensibility we need to also talk about compliance and compliance is a relationship between volume and pressure delta volume over delta p you can also think of compliance as distancibility times volume when we're talking about compliance there's always a volume component it's the total quantity of blood that can be stored in a given portion of the circulation and is more important from a physiological perspective so we talked about arteries and veins and veins can store more blood within you know a similar area therefore veins are actually a lot more compliant than arteries but arteries are still compliant and so when we say vascular compliance always think about this relationship delta v over delta p so in order to understand this um a little bit better we have three different scenarios here so in the first scenario assume here that we have this rigid pipe right very rigid and it's not going to expand whatsoever and if we try to inject this rigid pipe with some fluid volume is not going to change volume is always going to be zero however this imaginary tube can accept infinite amount of pressure so if this is the case then compliance would be zero because remember again compliance would be delta v over delta p in this case zero divided by infinity we still get zero so compliance is zero um in another scenario imagine here with me that you have a tube that can actually expand infinitely um if this was the case you try to inject fluid in there and then volume is just always expanding as a result you have absolutely no change in pressure pressure is always going to be zero and so again infinity over zero undefined or you're still infinity now in the third scenario this is what resembles what happens physiologically you have finite compliance you tend to increase the amount of volume so volume will change but also pressure will change both of them will end up changing and that is compliance it is simply how much volume changing per pressure change that is what compliance is all about here the these volume pressure curves express the relationship of delta p to delta v now this is a little bit different now compliance would be volume and you would have volume here and you would have pressure here that would be compliance this is the inverse of compliance this is elastance okay elastance is pressure over volume so compliance was volume over pressure elastance is pressure over volume and note here that you know previously we said that the veins would have a higher compliance in the arteries but now because we flipped it around arteries will have a higher elastance than veins this if we read these bullet points that we have here that the venous system um can uh accept a large change or in order in order to shift in order to shift pressures in the venous system right you need a very large change in the amount of volume and so if we're here and all of a sudden or actually let's start here all of a sudden we add a little bit of volume we move up here what's the amount of increase in pressure that we have it's kind of minimal not much but look at the arterial system if we are normally here and if we increase volume by just a little bit compare the amount this is actually a lot uh compare the amount of pressure that you see increasing and so this explains why one very important reason why we never almost never receive any intra-arterial injections whenever we're hooking a patient up on some fluids it's always through the venous system because the venous system because of its high compliance and low elastance it can accept a lot of the fluid without any significant change in pressure but the same is not said about the arteries any slight increase in in volume in the arterial system will lead to a significant increase in pressure so one more thing here um to to bring up just to show you the difference between elastance and compliance and the role of the veins in the arteries so we mentioned right now elastance is going to be pressure over volume while compliance is volume over pressure and we also mentioned that veins have a higher compliance and arteries have a higher elastance so what this means in in terms of let's say compliance first given a constant pressure veins will increa given a constant pressure veins will increase their volume much much much more than arteries but also given a constant volume arteries will increase their pressure much more than veins so this is just one more way for you to kind of wrap your minds around elastance and compliance and what it means for both arteries and veins vessels also express this relation or this property called delayed compliance or stress relaxation which is a property of biological tissues and basically what happens is when you have a increase in volume right like what happens here for example you will have a significant increase in pressure but with some time because these blood vessels have have this property of stress relaxation and delayed compliance they will kind of adjust leading to a drop in pressure it's not going to go all the way back to where it was but it's going to drop from here down to here and it also works the other way right which is also called reverse stress relaxation and it's one important way of how the vasculature responds to hemorrhage arterial pressure pulsations um so when we look at our the pressure wave we talked about how it looks like we talked about that rise the sharpness tejura and then another fall back down we mentioned that systolic pressure will be up here diastolic pressure would be down here if we if the arteries were not distensible at all and meaning our arteries were very very stiff blood would only flow during systole and we would have no blood flow during diastole like we explained before the factors that affect the pulse pressure and increase make it increase in size or decrease in size are mainly stroke volume and compliance so the higher the stroke volume the higher the pulse pressure but the higher the compliance the lower the pulse pressure so there's an inverse relationship between arterial compliance and pulse pressure and so we can kind of summarize this relationship like we see here pulse pressure is proportional directly proportional to stroke volume but inversely proportional to compliance of the arteries and this is what we talked about before normally your large arteries work as a buffer system um the left ventricle contracts pumping blood out into the aorta the your not not just pushes blood forward but it also expands outwards kind of storing some energy so the moment the heart goes into diastole like you see here these large vessels recoil and this recoil helps push blood further down the arterial tree therefore you have a constant flow of blood and you don't have pulsatile blood flow however in the case of stiff arteries right the ventricle contracts but these are very stiff they don't expand forward and also because they don't expand forward the velocity of blood flow is actually higher so velocity is higher in stiff arteries right because you're not wasting any energy to expand the vessel because it's just very stiff and it's not expanding but as a result you get no flow during diastole and this leads to pulsatile flow which can be a problem to some organs arterial stiffness changes with age or arteries as they age they become stiffer so this is what it would howard a pulse pressure would look like in a compliant artery during systole you see that nice expansion and then therefore you get the recoil in diastole and this is what the pulse pressure curve would look like but in a non-compliant artery or in a stiff artery right your ar you have the flow during systole but you have no no flow during diastole like we mentioned and so the pulse pressure looks different high flow during systole but then a significant drop during diastole with age both men and women our arteries our arterial system in general just becomes a lot more stiff and this is one important reason why our blood pressures increase with age but i want you to note here that you see this linear increase in systolic blood pressure that's not the case for diastolic blood pressure we will have this increase at first followed by a decrease and this decrease and big difference between your systolic blood pressure and diastolic blood pressure is because you lose the flow of blood during diastole so you have very low pressures during diastole this is we would call our the pulse pressure and you see here it widens it increases as we age and so when you're aging you would expect systolic to go up and diastolic to go down therefore leading to a widened pulse pressure meaning your systolic and diastole just go further apart and the difference between the two is greater here's a very interesting study looking at arterial compliance and stiffness across different age groups and different recreational um or different exercise levels so here the researchers broke them down into sedentary recreationally active and endurance trained individuals and in every of these three categories they broke them down according to age into young middle or old and what you see um consistently the young individuals have lower stiffness and the older we get the more the stiffness is but see how high the stiffness is in sedentary individuals it's kind of similar and recreationally active but you see in endurance trained individuals the stiffness is lower this is actually more pronounced when you start looking only at the older age group and compare endurance trained older individuals with sedentary older individuals right and look at the difference in stiffness which is also going to lead to differences in arterial health and so endurance training can actually help with arterial stiffness help with him help maintain the viability of our arteries and therefore also blood pressure and a lot of other different factors and the same is said here about arterial compliance and you can see that with endurance trained athletes the changes in arterial compliance are not as great well looking here um they compared what happens before training to after training and this is the beta stiffness index which is of stiffness in the arteries before training and you see with training it goes down and compliance with training goes up so your arteries get healthier with exercise but this is looking specifically at aerobic exercise because things are a little bit different with resistance training so here they're looking at what's called pulse wave velocity right this is between cf between the carotid and the femoral or fdp between the femoral and the dorsalis pedis and so the stiffer your arteries are right if you have high stiffness this will cause an increase in pulse wave velocity blood will flow down your arteries faster but the more compliant your arteries are the lower the pulse wave velocity so decreases in pulse wave velocity is a good thing this is what we want to see we want to see decreases in pulse wave velocity so if you look here the solid line is aerobic exercise this is pre-training and then you see that after training you have decreases in pulse weight velocity that is a good thing but if you look at aerobic exercise it's kind of different with aerobic sorry with resistance exercise you start off here and resistance training actually causes an increase in arterial stiffness now there's a lot of conflicting reports and how we interpret this data is you know can be is open to discussion because there is a lot of other factors that come to play i'm not here trying to tell you that resistance training is harmful um but it there is a component that we need to consider when we're looking at arterial stiffness there are other reports that show otherwise that show that resistance training does not increase arterial stiffness but even if that was the case resistance training does induce other physiological adaptations that are also very important and these include vascular adaptations as well but when it comes strictly looking at arterial stiffness it is aerobic exercise that provides the most benefit looking at the pulse pressure contours this is what it would normally look like different pathologies and different factors are going to change how our pulse pressure looks like so in arterial sclerosis in which our arteries become stiff we talked about this you will have high flow during systole and high pressures because of the non-compliant arteries but because diastole you won't have much flow there you have those significant drops with aortic stenosis in which the aortic valve is very narrow this is going to lead to a reduction in stroke volume and therefore this is going to bring the pulse pressure down with a patent ductus arteriosus where you have an opening in the aorta that causes blood to flow back into the pulmonary artery because of that you will have the rise and a very quick drop in your diastolic blood pressure and finally aortic regurgitation which is kind of the opposite of the aortic stenosis now the aortic valves they can open fine but the problem is they don't close well so that they're leaky and because of that it's like you have an open door between the earth and the left ventricle so whatever you pump out can fall back in and your pulse pressure will look something like that and so this is what aortic valve stenosis looks like look at the opening very very narrow very narrow this is a patent ductus arteriosus right blood comes normally will go out from here through the aorta but you have this abnormal opening so some blood is just going to move back right back into the pulmonary artery and this is what it would look like um damping of the pulse pressure and so when we look at different anatomical sites throughout the vasculature the aorta the femoral artery radial artery arterials and capillaries the pulse pressure starts to get dampened as we go further and further away and this damping is caused by the increase in resistance which we said is mostly going to be present at the level of the arterials as well as the change in the compliance of the vessels so damping is going to be proportional to the product of resistance in compliance we know that most resistance is found at the level of the arterials but also the least compliance is found at these levels and that's why it's at the level of the arterials where you see the most amount of pulse pressure damping happening right here when we measure blood pressure what are we doing we are applying a cuff and we're increasing the pressures in that cuff until we occlude blood flow and then slowly reopen the blood pressure cuff and the when you hear that first sound right that's when blood is starting to flow again and that's what we would call your systolic blood pressure because it's at that pressure point that you restore blood flow and then you will start hearing some noise until things become silent again at the point where it becomes silent that is going to be your diastolic blood pressure so the systolic pressure corresponds to the first tapping sound while the diastolic pressure corresponds to the muffling of sound and the final disappearance or silence that we hear at the end um this slide summarizes some of the determinants of arterial pressure some of them are physical and some of them are physiological so articular pressure is determined by cardiac output and peripheral resistance is actually a very important equation for you guys to remember so we mentioned that blood flow is pressure over resistance so therefore you can say that pressure equals blood flow multiplied by resistance we can also rearrange this and say that blood pressure is going to equal cardiac output times total peripheral resistance and that's what we see right here so it's important that you know this equation because it's going to be very very helpful to you but also the physical properties that affect pulse pressure are mainly going to be the arterial compliance and changes in arterial volume those are going to have a significant effect on pulse pressure thank you