hi learners it's m from sono nerds and this video is on unit 18 hemodynamics unit 18 hemodynamics hemodynamics is a study of blood as it moves through the circulatory system and understanding hemodynamics is really going to help us understand doppler so there are two important ideas that we want to keep in mind as we learn about hemodynamics first volume flow rate now sometimes we'll just refer to this as flow rate or even just flow but we are interested in knowing how much blood is moving through any given point of the circulatory system typically we're going to express this as a volume over time kind of like gallons per second or milliliters per second and we also want to understand the idea of velocity now velocity is going to apply a little bit more in the next unit when we learn about doppler where volume flow rate asks the question how much velocity is going to ask the question how fast blood is flowing through any given point in the circulatory system now velocity is going to recognize a speed and a direction so the speed part of this is expressed as a distance over time like centimeters per second the direction at least an ultrasound is usually going to refer to towards the transducer or away another example speed could be expressed as 50 miles per hour where velocity would be 50 miles per hour heading east velocity needs a direction so in this unit we are going to really be focusing on hemodynamics of blood and in section 18.1 we're going to focus on the flow of fluid in general so studying hemodynamics really takes a lot of cues from fluid dynamics and that's because the vessels in the body are basically just a big system of pipes with a slightly thicker fluid blood flowing through them now the heart is going to act as a pump that propels the fluid through the body the blood is susceptible to extrinsic physical forces such as pressure resistance and size of the tube or vessel in our case and this is going to change how the blood flows so we need to know how these physical properties affect blood flowing through the body and how the body can affect the flow both in how much is flowing and how it is flowing now there are a few key terms that you should be familiar with as we talk about how fluid flows the first one is viscosity now viscosity is the resistance of a fluid to flow so viscosity is going to describe the thickness of the fluid so if you think about pouring water out of a pitcher and pouring honey out of a pitcher which one would allow more to flow over one minute giving us that volume flow rate while the water which has low viscosity flows very easily compared to the honey which has a high viscosity would flow very slowly so again viscosity is the resistance of fluid to flow you can think of how thick is the fluid the thicker it is the more resistance it's going to have to moving or to flowing and it'll be important to remember that viscosity is expressed in units of poise and because we're learning about hemodynamics let's bring it back to the discussion around blood so average blood is about five times thicker than water however there are medical conditions that can change the thickness of the blood by changing the amount of red blood cells anemia which means few red blood cells and is shown on the image to the right is going to cause the blood to be thinner or less viscous because it has fewer solid components in it opposite of that then is polycythemia which means too many red blood cells and this is going to cause the blood to be thicker or more viscous again that's because it has more solid components it's going to resist flow more a blood test called hematocrit is going to tell us the percentage of blood that is made up of red blood cells so a high hematocrit lean itself more to polycythemia where a low hematocrit is going to be more anemia the next concept that i need you to understand is pressure and pressure in the circulatory system in fluid dynamics is the driving force behind fluid flow so for fluid to move for fluid to flow there must be a pressure difference when we're measuring pressure we're usually going to use a units of force per unit of area so we've already learned about pascals but another example would be like the air pressure in our tires that's pounds that's your force per square inch that's your area so force per unit area pounds per square inch so if we imagine a tube that represents our circulatory system and we have a certain amount of pressure on one side and a certain amount of pressure on the other side if those pressures are equal we are not going to see any flow there must be a difference of pressure for fluid to flow so equal pressures result in no flow however when we're talking about our circulatory system we have two main sources that are going to exert pressure on the system being the heart and gravity so now when we increase the pressure on one side of the tube and that could be either by raising it or pushing on it like a pump would we would now create a pressure gradient so that pressure gradient is the difference between pressure on one side of the tube to pressure on the other side the gradient is the difference between the two fluid is going to flow from high pressure to low pressure and it's also going to flow from high resistance to low resistance so if we look back at our tube if we raise one end of it we're putting more gravitational energy into the system and we are going to see a pressure difference between pressure one and pressure two now if pressure one is greater than pressure two we're going to see the flow move towards pressure too high pressure to low pressure same idea with the pump if we've got force on one side of the tube that is going to push the fluid to the lesser pressure on the other end of the tube so fluid will only flow when there is a pressure gradient which means a change or a difference in pressures on either end of the tube the last trim that i want to make sure that we cover is volumetric flow rate and yes we did already touch on this but we are working towards one of our first formulas related to flow so i just want to make sure we cover it again remember that volumetric flow rate is sometimes just referred to as flow and it's the amount of blood that passes a point in the system over a certain amount of time another example that we can use is when we think about our faucets we consider an output of about one gallon per minute to be adequate flow in our homes so volumetric flow rate uses units of milliliters per minute or maybe milliliters per second some sort of volume over time the adult cardiac output is actually about five thousand milliliters per minute and with five liters or so in the body which is equal to five thousand milliliters it takes about one minute for all of your blood to circulate and return to the heart so if we were to look at any one point in the circulatory system we would see on average a flow rate of about 5 000 milliliters per minute and that is going to bring us to our first formula in this unit this is the formula for flow rate also known as just flow or volume flow rate now this formula does assume a long straight tube our circulatory system is not a long straight tube but it helps us to predict what is going to occur in the tubes in our bodies the other thing i want you to note is that the triangle next to the p is the greek letter delta and that means change so if you remember when we were talking about our pressure gradient that was the change in pressure from one end to the other that's what's being expressed in this formula it's the change in pressure from one end of the tube to the other end so when we look at this formula we see that q is our volume flow rate that is what q stands for and it's measured in milliliters per second p again stands for pressure and this is going to be measured in some sort of force over area for example a dyn per centimeter squared lastly then we have r which stands for resistance and again this is measured in poise so volume flow rate is equal to the change in pressure divided by the resistance knowing our formula relationships we can confidently say that when the change in pressure increases we are going to see that the flow rate will also increase and that is because the numerator and the quotient are directly related to one another and this is true if you think about it if you are slowly pouring water out of a pitcher and you just kind of have it lifted a little bit you can get a small little trickle of water coming out of that pitcher however you raise the pitcher almost upside down you can get a very large amount to come out you're increasing your flow when you change from that horizontal position to more of the vertical position you're increasing the change in the pressure gradient so when there is a larger difference between pressure one and pressure two that's that change in pressure we're going to see more flow now opposite of that if we were to increase our resistance we are going to see a decrease in the flow rate and this kind of goes back to our honey versus water example water has very low resistance so that would be a decrease in resistance therefore we'd see an increase in flow rate however honey is very viscous a lot more resistance so that resistance is going to increase and then we see a decrease in the rate of flow so we can say that resistance and flow rate are inversely related and we know that again because the denominator and the quotient are always inversely related now there is a little bit more to the resistance the r that we can discuss and learn a little bit more about there is a formula that just relates back to the resistance or the r from that previous formula so if we break that down we can say that resistance is now equal to 8 multiplied by the length of the tube multiplied by viscosity which is divided by pi multiplied by the radius of the tube to the fourth power now i know this is a lot to take in we're going to break this down you do not need to memorize this but we do need to understand the concepts so let's just look at this formula again look at what all the symbols mean again r the capital r stands for resistance it is equal to eight which is a constant so that's just going to stay the same multiplied by the length of the tube so if we have a longer tube or a short tube that's going to affect the resistance now we're going to multiply that by the viscosity of the fluid in the tube so increase viscosity decrease viscosity we need to know what happens with that and that viscosity is divided by pi which is again another constant multiplied by the radius of the tube to the fourth power and again we need to know what happens when we change the radius of the tube so length viscosity and radius are all going to affect the resistance that we see in this tube so again you do not need to know the resistance formula but we do want to know how length viscosity and radius are going to affect flow rate so let's take a look through it if we increase the viscosity that is going to increase our resistance and when we increase our resistance we decrease the flow rate so thicker fluid makes more resistance which makes less flow of the fluid if we increase the length of the tube that is also going to increase resistance which decreases the flow rate so a longer tube is going to cause more resistance which decreases the amount of flow an example of the length affecting our flow rate would be your garden hose pull it out for the first time connect it to the house and you turn it on and if you have a really really short hose that water is going to come out almost immediately you're going to have a large volume over a short amount of time coming out of the hose compare that to to a very long hose maybe 50 100 foot hose you're going to see a very low trickle low volume coming out of the hose initially so the length of the tube increases resistance which is going to decrease the rate of flow lastly then when we decrease the radius of the tube that is going to increase our resistance so smaller tubes bigger resistance when we increase our resistance we see a decrease in flow so again small tubes more resistance low flow again we can compare this to hoses your garden hose compared to a fire hose your garden hose is not going to put out a four alarm fire that's why we've got giant fire hoses so it can move a lot of water in a short amount of time so small hoses more resistance less flow so now if we combine the volumetric flow rate which was q being equal to the change in pressure divided by resistance and our resistance equation which we learned was resistance multiplied by eight by the length multiplied by viscosity divided by pi multiplied by the radius to the fourth we get what we call the poise equation and by using the positi equation we can start to make very educated guesses about how blood is going to behave in the body there are a couple formats that you might see the positive equation in the first one on top here we are using radius just like we did for the resistance equation but it can also be expressed using diameter and that just changes some of our constant numbers 8 to 128 either is correct and in the end the same concepts are true for both so let's take a look at those and we're going to see the same pattern that we see in all of our other formulas so here's our big take away from the pose equation if we increase length then we are going to decrease our flow rate if we increase viscosity then we decrease our flow rate if we increase our pressure gradient then we increase the flow rate and lastly if diameter or radius increase then we also see an increase in the flow rate so of the equations to memorize or to spend a little bit more time with understand that the volumetric flow rate q equals change in pressure over resistance combined with what resistance equation is gives us poised equation and here's where we're really going to see those extra factors from the body how the effect flow rate within the tubes of our vessels now out of all of these takeaways from looking at the poise equation the biggest one really is this last one here that if we increase the radius or increase the diameter we're going to see an increase in flow rate and that is something that physiologically happens in our bodies we have arterials which are very very very tiny arteries kind of last thing before connecting to capillaries and transitioning into veins now the arterials as all arteries do the arterials do as well have muscular walls and these can either relax or they contract and when they relax they make the diameter bigger when they contract they make the diameter smaller and this is a fantastic tool that the body has to control the rate of flow of blood into certain organs by changing the diameter the body can either increase or decrease the flow of blood to an organ now for example when we eat we want to use our intestines our intestines need to help digest all that food get our nutrients and circulate that around the body so the blood vessels that supply the intestines are going to dilate increase their diameter or increase their radius which is going to increase the amount of flow that travels to them once we're done digesting we don't need to have as much blood flow going to the intestines we just need enough to keep the tissue alive so the vessels are going to contract on themselves thus decreasing the flow rate one last little side note then about jose's equation then is that jose's equation assumes laminar flow and we're going to learn what laminar flow is next so section 18.2 types of flow now there are really two distinct ways in which we want to look at how blood flows through the body first we want to know how do streamlines of blood flow within a vessel and then we also want to know how does the heart or respiration affect that blood flow the streamlines of the blood is going to relate back to some basic principles of fluid dynamics but we're going to see it applied to blood flow as well when we take a look at how the heart or respiration affects blood flow that's going to be directly related to the physiology of the body what vessel are we looking at and how do these extrinsic factors affect the blood flow so let's first take a look at how those streamlines affect flow within the vessels so there are two types of flow that you need to know laminar and turbulent flow now laminar flow is broken down into three more categories we have plug parabolic and disturbed now we're going to take a look at each of these individually but first we need to define what streamlines are when fluid flows it has streamlines these streamlines you can think of as kind of layers of fluid that are flowing through the area by observing these streamlines we can now categorize the type of flow that we are seeing so if we see all streamlines are all layers traveling parallel straight and at the same speed we would say that this is laminar plug flow quite often we see laminar plug flow at the beginning of vessels as that fluid travels into the vessel those outer layers are going to start to slow down due to friction then that next innermost layer has some friction with the outermost layer and the next layer and the next layer next layer until what we end up seeing is the very central portion of the vessel ends up being the fastest moving part of the blood because it has the least amount of resistance so when we see parallel straight and varying speed streamlines we are going to call this laminar parabolic flow the parabolic part comes from looking at the profile of those streamlines how they look in the blood and you'll notice that they make a slight curve along the front here that is a parabola so we call this parabolic flow as blood flows through a vessel it may encounter a branch or a slight narrowing and this can cause parallel but not quite straight streamlines we might see same speeds and we might see more of the parabolic speeds but the biggest point on this one is that they are not straight and when we see those parallel not straight stream lines we are looking at laminar disturbed flow and we most often see this again near little disturbances or branching of the vessels so the big thing we've seen with laminar flow up until this point is that it is parallel stream lines now when we look at streamlines and they become chaotic we call that turbulent flow the streamlines are not well organized or even really well seen there's going to be swirling of the blood which we call eddies or vortices we typically still see forward flow but it can be at different speeds different directions and we're most commonly going to see turbulent flow following flow that is extremely fast and very fast flow tends to occur in a severe stenosis so when we see a severe narrowing of the vessel as the blood comes out of that narrowing that is most likely where we will see that turbulent flow so again blood is coming in to the stenosis or to the narrowing it's going to start to change directions change speeds through that narrowing and right as it comes out of it and enters back into a wide open vessel we are going to see very chaotic turbulent swirling blood and that's the turbulent flow that's the chaotic eddies and vortices the blood is still moving forward but it can have some different speeds and swirls and backwards motion very chaotic eventually as that blood continues down the vessel it will return to some sort of probably parabolic flow but it does take a little bit of time for that turbulent flow to normalize back into our parabolic flow now the interesting thing between laminar flow and turbulent flow is that they sound different so laminar flow is actually relatively silent when blood is flowing smoothly through a vessel you really can't hear it and think about sitting next to a calm peaceful river you don't really hear much of the river flowing through but you can see that it's moving compare that though then to turbulent flow which is actually pretty loud when you are sitting next to maybe a rapids or if there's a waterfall that's loud you can hear the water flowing through that area and the same is true when we're listening to blood flowing through the body now if we take a stethoscope and listen to a patient's carotid artery in their neck or sometimes we listen to abdominal vessels or even the heart we can hear that turbulent flow and when we can hear the turbulent flow it's called a brewee so b-r-u-i-t brewey and it has a very kind of wind-blowing sound to it it's more like it's going to go along with the heartbeat as the pulse is going by you can hear this kind of turbulent flow within it now what's also interesting is that you can feel certain types of turbulent flow for example when patients have dialysis graphs those are a connection between an artery and a vein typically in the arm where those two connect causes very turbulent flow and if you were to put your hand in that area you can feel the vibrations of that turbulent flow so when you can feel turbulent flow we call it a thrill so again if you can hear it it's a brewee if you can feel it it's a thrill now in the next unit we're going to learn more about spectral tracings which is the image that you can see on your screen now and spectral tracings are the result of performing a pulse wave doppler and the white line that you can see there represents individual blood cells that the machine is detecting the reflection of when we see laminar flow observed in a vessel with spectral tracing it's going to show a very thin line again most of those red blood cells are traveling at or near the same speed we're getting that parabolic flow so they're going to be very similar it's going to be a nice organized line of blood going through there and laminar is the normal physiological state of blood if a vessel is wide open we expect to see laminar flow through it however when that turbulent flow is observed with spectral tracing we're going to show a line that starts to fill in as there's red blood cells traveling at all different speeds all different directions causing turbulent flow to be visible in our spectral tracing so as you can see in this image we no longer have that nice smooth clear open window through here we've now filled this in with spectral tracing information and this is a visual representation of turbulent flow and again we typically see turbulent flow like these after a critical stenosis in a vessel or in a valve of the heart another concept that we look at when we are looking at laminar versus turbulent flow is something called reynolds number now reynolds number is a unitless number and it can predict the type of flow that will be present it's going to take into consideration the velocity the density and the viscosity of the blood moving through a radius of the tube so yes there is a formula for it but i don't even want to show it to you because it is absolutely unnecessary what you need to know is what the numbers mean so again remember reynolds number unitless and it's going to give us a value that predicts laminar or turbulent flow so if we see a value of reynolds number less than 1500 we know we have laminar flow if we see a value between 1500 and 2000 it's kind of indeterminate it could be shifting towards turbulent flow it could just be kind of a disturbed flow but we're not real sure at this point once reynold's number goes over 2000 then we are predicting turbulent flow through the area so as a review laminar versus turbulent flow parallel streamlines they can be going the same speed that's plug parabolic has the center going a little bit faster or disturbed which changes the direction making them not quite straight anymore but the big part about laminar is that they are parallel streamlines turbulent then is very chaotic flow blood flowing in all different directions streamlines basically gone we're going to see vortices and eddies with the turbulent flow laminar is very smooth it is normal for blood flow to be laminar we expect laminar flow in most of our vessels turbulent flow typically is due to some sort of pathology so this is at a stenosis either in the vessel or of a valve laminar flow is typically silent where turbulent flow is very loud laminar is predicted by a less than 1500 reynolds number value or turbulent is going to be over 2000 so now that we know the type of flow that we are going to see really in any vessels we can see turbulent flow and veins we can see turbulent flow in arteries probably a little bit more common in arterial high pressure flow but now that we know the laminar and turbulent flow now we can discuss how our anatomy then begins to affect the flow in the blood as well so when we consider the vessels in how blood is flowing we have three common types of flow that we see so we can have pulsatile flow which you are seeing here that pulsatile flow refers to blood that's going to move at different velocities it's typically caused by cardiac contractions and because of that we are going to most likely see pulsatile flow in the arteries pulsar tile flow is most likely going to have a very high flow rate and that's very much in relationship to the heart and the high pressure that comes along with it so if we look at this diagram here we have a spectral tracing and then a diagram of the spectral tracing showing the variable velocities this baseline here is no flow so we are seeing a change or an increase in flow in one direction then we see the flow come back a different direction and then we see the flow go forward again in a different direction coming back and so you'll see kind of these forward movements and backward movements forward movements backward movements as the heart contracts this is during systole you get that strong heart contraction lots of pressure forces the blood forward as the heart relaxes we're going to switch into diastole this is usually going to cause the blood to flow backwards a little bit we've lost all that pressure that was moving it forward so we see a little bit bit of backwards flow and then we're going to see either like a rebound of the vessel or a closing of the valve in the heart which might cause a little bit more forward flow again so anytime you're seeing an increase decreasing increase decrease increase decrease we call that pulsatile flow and this is going to be giant increases that match up with the cardiac contractions so that's the big part pulsatile flow cardiac contractions usually seen in arteries now we also have something called phasic flow phasic flow is also going to have blood that's moving at variable velocities but this is going to be much more related to respiration when we breathe in and breathe out and because it's related to respiration we typically see this type of flow in veins usually a very low flow rate and low pressure with basic flow so again in this diagram we have a spectral tracing of phasic flow and then we also have just a schematic drawing of it as well and again what we're seeing is the baseline here this is the baseline here and because this specific example is in the leg as the patient exhales we're going to see an increase in flow so we're still seeing a variability in the velocities of flow that are occurring here and as they inhale we see the cessation or even backwards flow of blood and then as they exhale the blood will move forward again when it's closer to the baseline it's moving slower as the graph makes it further away from the baseline those are going to be increased velocities so we see with the breathing a change in the velocities of the blood flowing again this is typically caused by respiration because respiration changes the pressure variance in our chest and in our abdominal cavity and it's going to most likely affect the veins as they are bringing blood back to the heart because they are less affected by the cardiac contractions so this is all occurring at the same time that the arteries are more affected by the pulsatile cardiac contractions so again phasic flow is most likely going to be in veins and we're going to see variable velocities that match up more with respiration now there's a third type of blood flow that we see in vessels and that's called steady flow and this is going to be when blood is moving at a constant speed again we have our pulse wave tracing or our spectral tracing and then a diagram of it the baseline would represent no flow but we can see that we have reflections of blood cells that are moving at a pretty constant 20 centimeters per second so steady flow is when we see very little variation it's relatively constant and this is typically going to be in seen in veins while holding your breath so if a person were to take a breath in and hold it that blood still needs to get back to the heart so the blood will continue flowing forward at a relatively constant speed another really common vessel that shows steady flow with a breath in is the portal vein that's a big vein that heads to the liver so steady flow is not as commonly seen but it is a type of flow that we can see within vessels all of the types of flow that we talked about laminar turbulent disturbed parabolic those can happen in pulsatile or it can happen in steady or it can happen in phasic so you can have phasic laminar parabolic flow or you can have phasic turbulent flow it all depends on how the blood is flowing through the tube and typically what tube it's in in relationship to our body section 18.3 energy now we talked about needing a pressure gradient or difference in pressure for fluid to flow that pressure is actually related back to energy that is within the circulatory system the heart when it's relaxed is going to fill the left ventricle and the muscles are kind of static holding what we call potential energy once the left ventricle contracts it takes the potential energy in the muscles squeezes transferring that potential energy into pressure and kinetic energy that potential energy is then transferred into pressure and kinetic energy into the circulatory system now as the blood moves through the body some of that pressure energy and kinetic energy are going to be converted into other types of energy so it's going to exit the circulatory system and be converted into other types of energy so we need to spend a little bit of time understanding how energy is transferred in the circulatory system now one of the really big pieces of energy is the law of conservation and that tells us that energy cannot be created or destroyed but it can be transformed so again if we think about the heart and the energy that it provides to the circulatory system it's providing the forward momentum for the circulatory system and that energy that the heart has just didn't come out of nowhere it was transformed from the food that we ate and the bodily functions that we have in the ions moving in between things and electrical energy that comes from that so there's all this energy always in the world none of it has been created out of nothing and none of it's been destroyed we just keep transforming it so the energy in the heart in the muscles transforms from this like muscular energy pushing on the blood making it kinetic energy that kinetic energy is going to go through the circulatory system causing the blood to move and so we see energy constantly being transformed and then eventually we'll put more food in our body and more processes will happen and energy will be restored to the circulatory system through the heart and all this stuff again so again energy cannot be created or destroyed it can only be transformed and a really common way that we like to display this is something called a newton's cradle which you can see on your screen now so as the ball is pulled back and released it goes from potential energy to kinetic energy the ball is moving anytime something's moving it has kinetic energy as that ball is moving it has momentum and that energy is transferred to the next ball which is transferred to the next ball to the next ball and then we see it being transferred to the last ball which kicks it out providing that kinetic energy almost at the same rate as the energy that the first ball had and so we see that the energy was conserved through all those elements eventually giving kinetic energy to the last ball well as that ball travels up is affected by gravity that kinetic energy is transferred to potential energy for just like a brief fraction of a second as it falls back to earth it gathers kinetic energy again and then the whole process starts over and it transfers through all the balls and makes the other side kick out now eventually we know that friction and momentum will be lost from this system because it's been transferred to some sort of energy typically heat outside the system so eventually this will stop because we don't have perpetual motion machines so all this is to introduce us to the idea that even though our heart and gravity are providing energy into the circulatory system it is going to lose some energy to transformation so there are three types of transformation or energy loss that we see from the circulatory system the three most common ways that we lose that energy due to transformation are going to be viscous loss frictional loss and inertial loss now viscous loss should sound familiar we talked about it being the viscosity of fluid the thicker it is the more energy it's going to take to move that fluid so viscous loss is usually due to a fluid having to overcome its own stickiness to move so in this example here you can see all of these cups have different viscosities within them they're all held up at the same angle at the same height so it has the same potential energy working on the viscous fluid found within them the ones that are barely flowing or not flowing really at all have a lot more viscosity to them so it's going to take more energy to move that fluid and because there's not enough energy in the system it doesn't flow so when blood is really thick or we have polycythemia it's going to take more energy for the blood to flow versus blood that's very thin and another idea that you can kind of think of with that is if you've ever known someone to be on blood thinners their blood is very thin it has low viscosity if they were to cut themselves they potentially could bleed out because their blood is so thin it's just going to flow very easily out of a wound so viscous loss blood has to overcome its own stickiness now frictional loss we see this all the time especially in the colder environments if you're a little chilly and you rub your hands together you start to feel heat so friction is probably the most common transformation from the circulatory system and it's transformed into heat so our circulatory system loses a lot of its energy to heat into the surrounding vessels remember when we were talking about parabolic flow we talked about how the outer stream lines are going to drag along the walls of the vessel those are going to cause friction on the next streamline and so on and so on and so we see frictional energy being a very large cause of transferred energy and again that is going to be in the form of heat the third type of loss on inertial loss is going to refer more to how blood has to change directions while it's flowing and in doing so it's going to lose some of its kinetic energy now a really good example of this is merging on a highway everybody's driving along very fast all of a sudden a lane is closed well now if you live anywhere like i live everybody is trying to get into the same lane traffic slows down we've lost our kinetic energy because we've got to change directions and figure out how to fit through this area the lane closure would be very similar to a stenosis in a vessel or in a valve as the blood reaches it it's got to kind of slow down figure out how to rearrange itself and then figure out how to move through that area after the stenosis then just like in changing lanes once the road opens up everybody separates out and depending on how aggressive drivers there are some will start going really fast some are still trying to figure out how to get their car up back up to speed it gets a little chaotic possibly after that opening and that's what we're going to really see in the blood as well we're going to lose some energy as we reach areas that we need to start to change direction we also see inertial loss just in the positivity of vessels that forward and backwards movement causes kinetic energy to be lost during that motion so inertia loss is the third type of way that we lose energy via transformation from the circulatory system now we've heard the word stenosis a few times we've talked about it in relationship to turbulent flow and now we've talked about it in relationship to an inertial loss so it turns out that a stenosis has actually a really big effect on flow through vessels so stenosis is a narrowing of the lumen of a vessel or a valve and again that stenosis is going to cause a significant change in the way that blood flows looking at these graphics here a really common space that we see a sonatic blood vessel is in the carotid arteries the crowded arteries bring blood to our brain so naturally we would want a lot of blood always flowing to our brain but when there's a narrowing as you can see here in the magnified area we are going to see blood having a problem flowing through that area it's going to lose inertia it's going to start to flow chaotically back here there's a lot of ways that this stenosis is going to affect the normal flow of the blood pathology within the vessels and valves causes blood flow to change and can affect what happens downstream over on this side we have a couple examples of aortic valves we have a normal one up here opens and shuts all the way in aortic reguration which is not quite what we're talking about but this is a valve that's not closing all the way so it allows blood to flow backwards it doesn't close the system quite all the way and so we start to see blood flowing back into the left ventricle with an aortic stenosis that's actually really going to cause the blood in the left ventricle to have a difficult time getting out of the left ventricle just like blood is going to have a difficult time continuing towards the brain and the aortic valve is responsible for blood getting to the rest of our body so this can really cause a lot of issues downstream anytime we are narrowing the vessels we can expect to see changes in how the blood is flowing and that is significant to ultrasound because we can monitor those changes with doppler tracings so what do we see when a stenosis is present in the pathway of blood flowing well there are five things there are five changes that we need to be aware of the first one is that blood flow is going to change directions as it flows into the narrowing and then back out of it so again we have our blood flowing through the vessel encounters the stenosis it has to narrow change its pathway through here and when it comes out of it it turns into that chaotic turbulent flow so stenosis causes the blood to change directions as it flows into the narrowing and back out secondly we're going to see that velocities increase again we have blood flowing into the sonatic area and then back out of it the issue is that blood can't slow down here just because it met the sonatic area we have to keep up the flow rate remember we talked about the flow rate for the circulatory system being about 5 000 milliliters per minute we got to keep that up otherwise if blood just slowed down here and wasn't making it back to the heart we're going to have some bigger issues that we're not keeping blood flowing we're going to have blood backing up not a good thing so we got to increase the velocity through a stenosis because we got to make sure all this blood can still continue through trying to maintain that flow rate so again increase in velocity in the narrowing we're then going to see turbulent flow distal to a narrowing so remember we've gone through the stenosis and when we come out of it we see that turbulent flow that again is going to be the vortices in the eddies the turbulent chaotic flow behind the stenosis we're also going to see a pressure gradient within the stenosis and we're actually going to see a decrease in pressure so blood is flowing through this area this is a higher pressure over here versus down here and we know that because blood is flowing this direction but when we get to the stenosis we need a pretty severe pressure gradient or high pressure to low pressure because we got to get that blood through there very fast and so when the pressure decreases velocity is going to increase to keep that blood flowing at the same rate through that area so pressure gradient decreased pressure through the stenosis and then the last thing that we see with the stenosis as the blood is flowing up to the stenosis we are going to see a loss of pulsatility remember we were able to see how the blood reacted to the cardiac contractions but as that blood gets up to the stenosis it's almost like a lot of it is kind of hitting a wall so instead of seeing that pulsatility in our waveforms what we see is almost blood just kind of hitting a wall and stopping and so loss of pulsatility we stop seeing the cardiac contractions having as great an effect on the blood flow now what's really neat about ultrasound and stenosis is that we are actually able to detect a lot of these changes using our machine and we're going to learn a little bit more about that in the doppler section but just to kind of recap when we use doppler technology with ultrasound if we sample in the stenosis we will be able to see increased velocities on our recordings if we were to sample beyond the stenosis we'll see turbulent flow in our doppler tracings and then we can also see that loss of pulsatility if we are coming up to a stenosis and place a sample doppler before it we're going to see the pulsatility decrease and when you see a decrease in pulsatility you expect a distal stenosis so these are markers that we see that kind of help us let us know that a stenosis is going to come up or is occurring where we can find the greatest amount of stenosis and then by using like calculations and numbers from years and years of studying ultrasound information we can then predict actually how stenos something is for example this leans a little bit more to the vascular side of things but if we are sampling in the middle of the stenosis we expect that we're going to have really high velocities so for frame of reference a lot of the time blood is flowing um maybe about we'll just say 90 centimeters per second when you're in the middle of that stenosis you might see velocities 500 centimeters 700 centimeters per second and the higher that velocity is the more cenos the vessel most likely is and so we can kind of start to predict you know 300 centimeters might be 70 to 90 percent the nose so by using our doppler information and our knowledge of hemodynamics we can start to put the pieces of the puzzle together to really help us understand how the patient's blood flow is related back to a pathology i think this discussion really lends itself to us taking a moment here and just realizing again our knowledge of physics is super important to creating diagnostic images we just learned about how a stenosis is going to affect blood flow it's got five things that it's going to do to the blood flow three of those things we can detect with our ultrasound machine by using doppler information we're going to see high velocities we know that's in a stenosis we're going to see turbulent flow we know that's past the stenosis we know that there's going to be a loss of pulsatility before the stenosis so when we're interrogating a patient's blood flow be it in the heart or in the vessels and we see that loss of pulsatility we can think oh there might be stenosis coming up and as we're going through getting our doppler information we see extremely elevated velocities now you know you're in a stenosis you're proving there's a stenosis because we learned about the formulas and the hemodynamics of blood flow so now we're showing yes there is a stenosis and we're going to take it a step further by understanding what's happening in that stenosis and that is big reason why we're learning ultrasound physics it's good to know that velocities increase in a stenosis it's better to know why it's occurring and what you can do to show and to prove to hovers of looking at your images that you have identified an area of concern that needs to be followed up so really knowing your ultrasound physics does lend itself to really good patient care so let's move to bernoulli's principle this is the y a velocity is increasing in a stenosis and pressure decreasing in a stenosis bernoulli's principle tells us that pressure has to be low in a stenosis and the velocity has to be high in the stenosis because we need to make sure that the law of conservation is being followed so as we have blood flowing through a vessel towards the stenosis it's going to have different types of energy in it it's got pressure energy and it's got kinetic energy remember the kinetic energy is the actual movement of the blood the pressure energy is what is kind of the propelling force behind it so as we're getting up to that stenosis pressure energy is going to be a little bit higher than kinetic energy we have blood moving relatively free through the vessel now when we get up to the stenosis we have to maintain energy through the stenosis we know that velocities are going to increase which means we have to have more kinetic energy the only way we can get more kinetic energy is by converting some of that pressure energy so pressure energy has to be converted to kinetic so now we've reduced pressure increased kinetic so we can increase velocities we have to maintain the flow rate so blood can still get back to the heart at the rate that we need it to so pressure decreases in the stenosis velocities increase which means kinetic energy increased now on the back side of the stenosis this is distal that's where we're going to start to see that turbulent flow but we start to see pressure energy increase again and kinetic energy reduce again because we are conserving the energy in the system so again this is mostly just a representational view of bernoulli's principle but we have 10 units of energy here six of them are pressure four of them are kinetic as the blood flows through the stenosis we have to convert some of that pressure energy into kinetic so we still have 10 units of energy in here but now we've reduced to three pressure and seven kinetic that's going to cause velocities to increase pressure decreases when we get to the outside of it again conserving energy we still have 10 units of energy in here but now we have more pressure energy so we've returned to six pressure energies compared to four kinetic energies so again bernoulli's principle tells us velocities increase in stenosis pressure decreases that's bernoulli's principle section 18.4 hydrostatic pressure moving away from blood flow in a stenotic area i want to bring us back to the idea that we had a couple sources that caused pressure changes in the circulatory system now we talked about the heart being a pump and having the forward moving energy that was one of the ways the other one that i mentioned was gravity and gravity is going to be translated into hydrostatic pressure and we can actually see that in the hydrostatic pressure formula so hydrostatic pressure is going to describe the relationship between the weight of the blood the gravity on the blood and the height of the blood so in our formula we have the capital p representing pressure is going to be equal to the height of fluid lowercase h multiplied by gravity the lowercase g multiplied by the density of fluid which is represented by the greek letter rho which kind of looks like a lowercase p because we know our relationship values anytime we increase height gravity or density we should see an increase in hydrostatic pressure if we take a step back and just look at hydrostatic pressure from the idea of fluid dynamics then it's worth mentioning that hydrostatic pressure is in relation to a column of fluid and in that column of fluid we know that's going to be acted upon by gravity the height of the column and the density of the fluid within that column so if we are looking at these two columns of fluid this fluid is taller there is more levels of density within it therefore there's more weight pushing on this bottom part of the fluid compared to this bottom part of the fluid the ball at the top of either of them has the least hydrostatic pressure there's really only gravity working on it at this point the hydrostatic pressure is going to be cumulative as it goes through the column the pressure of this ball is going to be added to this ball to this ball to this ball and so the fluid is the same idea that the fluid on top is going to add to the pressure of the fluid underneath it and so on and so forth until we have relatively high pressure at the bottom of the column while humans technically aren't columns of fluid we do still technically have a height or acted on upon gravity and our blood has density to it so we do have hydrostatic pressure within our bodies now the interesting thing about hydrostatic pressure especially when related to humans is that we want to have kind of a baseline where we consider there to be no hydrostatic pressure and so for humans we use the heart kind of as the baseline so if we're looking at a patient that is standing up and we think of the heart as our baseline then we are going to say that the heart has zero millimeters of mercury of hydrostatic pressure this is going to be our baseline there is no extra pressure in that area we then know that hydrostatic pressure above the heart is going to be negative hydrostatic pressure below the heart is going to increase and be positive and the further away we get from the heart we are going to increase even more just like we did in our column of blood the bottom bowel the bottom fluid has the most pressure on it has the most hydrostatic pressure in fact so again in humans we consider heart to be baseline pressure above the heart is negative pressure below the heart is positive and increases in pressure as we move to the feet our column of blood is getting heavier at the feet and you may have noticed as well we are going to use units of millimeters of mercury that's that mm millimeters mercury is abbreviated hg on the periodic table as our unit for hydrostatic pressure and this concept becomes important when we think about taking our blood pressure we use the arm cuff at the level of the heart because we want to know what the true pressure is within the vessels so we take the blood pressure where we expect there to be zero hydrostatic pressure we don't have any outside forces on the value that we're getting from that blood pressure cuff so when we want an accurate blood pressure reading we try to take it on a sitting patient at the level of the heart and we do that because the measured pressure that we get with the blood pressure cuff should be equal to the true blood pressure plus the hydrostatic pressure so again if heart is zero millimeters of mercury of hydrostatic pressure then we should be getting a true reading of our blood pressure so so far we've been talking about a patient that has been standing or at least sitting in a position in which hydrostatic pressure would be apparent but what do you think happens when we lie the patient down and the whole body is now at the level of the heart now we have created a situation in which there is no hydrostatic pressure added to the system so now in theory our supine patient are lying down patient has zero hydrostatic pressure added to their circulatory system so let's take a look at a couple examples let's say that we have a patient with a blood pressure of 110. now this is the very true blood pressure we somehow were able to like sneak into a vessel measure the pressure in there we know it's 110. now we are outside of the body we are the nurse the caregiver practitioner who wants to take a blood pressure on this patient with your arm cuff so if that patient is lying down remember that their body is all at the same level of the heart so that means that there is no added hydrostatic pressure in the circulatory system and if we know that blood pressure plus the hydrostatic pressure should equal our measured pressure that we get with the blood pressure cuff then we'll see that if we could put a blood pressure cuff around the head we should get 110 millimeters of mercury at the arm we should get 110 millimeters of mercury at the thigh 110 at the ankle 110 we should see a consistent blood pressure throughout the body because we are not adding any hydrostatic pressure we're not adding height we're not adding gravity we're not adding density to the column that's going to change though when we turn the patient into a standing or even sitting position specifically in that standing position we are increasing the hydrostatic pressure so again we're going to say that our patient has 110 millimeters of mercury blood pressure we just know that's what their pressure is in their circulatory system we now stand them up and we get out our magical blood pressure cuff and we put it around their head and because we are taking a pressure above the heart we are going to have a negative hydrostatic pressure so we're going to have a negative millimeters of mercury so we're still going to take our blood pressure add that negative hydrostatic pressure and then we'll see that the measured pressure in the head is going to be less than the true blood pressure again that's because we have negative hydrostatic pressure above the heart now if we take our blood pressure cuff and move it to the arm again we have 110 just true blood pressure going on we're adding zero millimeters of mercury because we're at the level of the heart on the arm therefore we're going to get a measured 110 millimeters of mercury for their blood pressure we then move their cuff down to their thigh we are below the heart so we are going to have a positive hydrostatic pressure so we're going to take that 110 add the 50 millimeters of hydrostatic pressure and now we're going to see a measured pressure of 160 millimeters per mercury and if we move finally our blood pressure cuff down to the ankle we again are adding even more hydrostatic pressure to the system and so we are going to see even a higher measured pressure at 210 millimeters of mercury the big takeaway from this is that when we are looking at measured pressure that is the reading that you would get from like a blood pressure cuff outside the body when we are looking at that measured pressure is affected by the hydrostatic pressure on the body if we are at the level of the heart we are not adding any hydrostatic pressure hydrostatic pressure is only zero if the patient is supine and that is going to be throughout their whole body or if they're standing it'll only be at the level of the heart when we have that standing patient and we're looking at measured pressures we need to know that the hydrostatic pressure is going to affect the measured pressure we are subtracting pressure from the system when we are above the heart and we are adding hydrostatic pressure to the system when we are below the heart and the further we get from the heart the more the hydrostatic pressure is going to increase now you don't need to know these numbers these are going to be variable from person to person because remember it's gravity height and density is how we figure out this number so again the big thing that you need to know negative hydrostatic pressure above the heart zero at the heart positive and increasing pressure as we move below the heart the last section of hemodynamics then is section 18.5 vessel considerations the vessels and their anatomy do affect some of the blood flow so we need to understand how the vessel anatomy can change blood flow now the circulatory system starts at the heart and the left ventricle is going to create that pressure and then propels blood into the aorta and the aorta becomes arteries then arterioles and then capillaries and those capillaries are the very tiniest vessel they're going to allow for the nutrients and waste exchange to occur and they're really only big enough to allow one red blood cell through from side to side now those capillary sun are going to converge into what we call venules those are going to converge into veins the veins become the vena cavas and the vena cava is going to return to the heart taking a brief look at the vessel anatomy then we do need to know that both veins and arteries are made up of the same layers on the very very inside of the arteries and veins we have the tunica intima the next layer out is the tunica media and you'll notice that the size of the tunica media varies quite a bit for the arteries and veins the arteries have to be able to handle that increased pressure from the aorta and the heart and so the tunica media on the arteries is a very elastic muscular layer and that's why we can feel our pulse because we can feel the muscle being stretched out and bouncing around in the arteries so the tunica media is much thicker on the arteries and not only does it allow arteries to accept high pressure but it also is going to allow for some control of the blood flow in the arterial side of things the outermost layer then is the tunica adventitia and that is mostly made up of connective tissue and on our really large vessels it has something called the vasovasarum these are going to be tiny little blood vessels that actually supply the outside layer of the large vessels in the body one final anatomy note is that the veins actually have valves within them and they are going to keep blood from flowing backwards i had mentioned earlier that the arterials are the tiniest vessel right before turning into capillaries and that the arteries do have that muscular wall the arterials have the capability of what we call vasoconstriction and vasodilation in vasoconstriction the arterials are going to increase the pressure because they are creating a smaller diameter or smaller radius of the vessel opposite of vasoconstriction then is vasodilation in this instance the muscular wall of the arterial relaxes and that's going to allow for increased flow because now we have increased the diameter or increased the radius of the vessel so this vasoconstriction and vasodilation is directly related to the flow rate and passes equation that we learned earlier as the diameter of the vessel changes it's going to change the flow rate now the veins of the system don't have the large muscular layer but they are very flexible tubes and so the venous system at any point is holding about two-thirds of the total volume of the blood in the body the venous system has something called high capacitance and that means the ability to hold on to a volume of blood because the walls of the veins are very flexible in a low pressure setting they're almost going to be completely collapsed upon one another but as the body needs more volume to return to the heart they can dilate therefore lowering the resistance and increasing the diameter causing the flow to increase so again this is really showing us posse's law in action when we dilate the vessel make it bigger we're making the radius diameter increase which will increase the flow rate heading into our last concept then on hemodynamics is respiration and venous flow now we've been talking about forces that propel the blood forward we've got the heart contractions and we have gravity and when we think about arterial flow we know it has a lot of pressure it's pushing the blood into our organs through those capillaries and now it's got to head back to the heart and we know that the veins take the blood back to the heart but what's interesting about this is that the blood in the veins is no longer really affected by the cardiac contractions so how does the blood get back well the biggest concept is that there still has to be a pressure difference to make the blood flow back to the heart and there is but it's actually a really really tiny difference the venous system has about 15 millimeters of mercury of pressure in it and the right atrium of the heart where the blood empties into has about eight so there is a very tiny pressure difference that pressure difference is going to keep blood flowing in the correct direction towards the lower pressure but there are also some other mechanisms within the body that help encourage blood flow back to the heart as well the first one that we can see on the screen now are that veins contain valves as blood is flowing back through the veins the valves will periodically close keeping blood flowing in the correct direction and just like the heart acts as a pump for the arterial side of things we have something called the calf muscle pump as the calf muscles squeeze the sole sinuses which are kind of like these little cups that hold blood in the veins of the leg they are squeezed of their blood that extra pressure that extra volume into the system is going to help propel blood towards the heart and then remember those veins shut keeping that blood moving in the correct direction the third mechanism that encourages blood flow are changes in respiration because as we breathe in and breathe out we are changing the pressure in the thoracic and abdominal cavities and the pressure change in the thoracic and abdominal cavities is going to affect the amount of blood flow that is coming in from the veins but it does this in a very particular way there's a large muscle that sits in between the thoracic and abdominal cavity and it's called the diaphragm and when we inhale the diaphragm is going to be pushed down into the abdominal cavity when the diaphragm is down towards the abdominal cavity that causes a decrease in pressure in the thoracic cavity and an increased amount of pressure in the abdominal cavity because there is a decreased amount of pressure in the thoracic cavity venous blood flow from the arms and the heads is going to increase it is easier for that blood to get back to the heart because there is more pressure in the abdominal cavity it makes it more difficult for blood from the legs to return when the patient exhales the diaphragm bounces back up into the thoracic cavity when it goes up into the thoracic cavity the thoracic cavity pressure increases and then the abdominal pressure decreases when the abdominal pressure is lower that's going to increase the flow from the legs and decrease the flow from the arms and the head so again to recap when we inhale diaphragm moves down thoracic pressure decreases abdominal pressure increases venous flow from the arms and the head increase because it's easier for it to go into the thoracic cavity where the heart is venous flow from the legs stops or decreases because it's harder for it to come into the abdominal cavity before getting into the chest when we exhale the diaphragm moves back up into the thoracic cavity this causes the thoracic cavity pressure to increase the abdominal pressure to decrease the decrease in abdominal pressure encourages blood flow to increase from the legs and the high pressure in the chest causes blood flow from the arms to decrease because now it's harder for it to get into the chest and that brings us to the end of our conversation surrounding hemodynamics so remember we started out this unit talking about fluid dynamics and passes law basically telling us that flow rate is related to the length of the tubes the viscosity of the fluid the radius or diameter of the tubes and the change in pressure or the pressure gradient within the tube knowing passes formula will be very helpful but in the end you really need to understand the concepts of what's going to increase or decrease the flow rate we then learned about the different types of blood flow laminar versus turbulent you should be able to describe both of them and what it looks like what they sound like what causes them you'll also want to be able to discuss the types of flow that we see in arteries and veins pulsatile versus phasic versus steady then you should also be able to describe the ways in which we lose energy out of the circulatory system and specifically how a stenosis is going to change the way blood flows remember bernoulli's principle is very highly linked to blood flow through a stenosis so you should be able to in your own words describe bernoulli's principle next we talked about hydrostatic pressure make sure you understand the difference between a standing and a supine or lying down patient why would take patients blood pressure with them either lying down or at the level of the heart if they are sitting knowing that hydrostatic pressure is negative above the heart and positive below the heart and then lastly you should be able to describe what happens when we inhale and exhale how that affects the venous flow and how vasoconstriction and vasodilation can also affect the flow rate in the arteries remember you do have activities in the workbook and some open-ended questions at the end of the workbook to double check your knowledge of the material presented