welcome to section six of Cardiology in this section I will be discussing the Starling curve and cardiac and Vascular function curves let's get started this is figure 2.8 from you text which shows a Starling curve the y-axis is cardiac output and the x-axis is preload let's look at the normal line first as you can see as preload increases the cardiac output increases but this only occurs up to a point and then we can see that the curve begins to taper off in order to understand this let's take a step back and pull up a blank screen a call from musculoskeletal histology that a sarcomere is the fundamental unit of striated muscle and cardiac muscle is a type of striated muscle so I'll draw the sarcomere here so in the sarcomere we can see that the Z line is right here and coming off we have actin right here in between the strands of actin there is the myosin he's represent the myosin heads so as the volume of blood in the heart increases or as preload increases the sarcomere gets stretched and the tension between the myosin heads and the actin increases the increased tension allows the heart to contract with greater force and pump a greater volume of blood so cardiac output increases so again we have increased preload which causes increased tension which results in increased cardiac output however if the volume increases too much then the tension between the myosin heads and actin is such that it's harder for the heart to pump out the same volume of blood I think this is easier to understand if you do the following exercise so flex your wrist and then try to make a fist you'll notice it's a lot more difficult than just trying to make a fist without flexing your wrist this is similar to the idea I'm describing with the heart the sarcomeres have an optimal length tension relationship which is the basis for understanding the Stirling curve so now that you understand the normal curve let's explain the other two curves recall from section 3 that I know tropism or inotropy refers to changes in contractility so we can see pause the divinest ropey and negative inotropic here remember contractility is directly related to the concentration of intracellular calcium so if there's increased contractility this means there's increased cytosol 'ok calcium if more calcium is available then more troponin c is able to move tropomyosin from the actin sites which means more myosin combined actin resulting in a more forceful contraction so positive inotropic epinephrine and norepinephrine increase contractility we can see that the curve shifted here if we look at the graph we can see that under these conditions the heart pumps more blood with the same preload compared to normal so if we just draw a line here we can see that cardiac output it's gone from here to here with the same preload unlike the normal curve the length tension relationship of the sarcomere is not a major contributor to increase cardiac output the major contributor is increased intracellular calcium this makes sense the volume in the heart hasn't increased so the length tension relationship of the sarcomere couldn't have changed either conversely negative inotropes like beta-blockers will decrease the concentration of intracellular calcium causing the cardiac output to decrease with the same preload so again if we draw the dashed line we can see that the preload is the same the cardiac output decreased from right here to right here it's important to know that changes in afterload also have a similar effect to changes in inotropy in other words a decrease in afterload means the heart has to pump against loss resistance so the cardiac output will increase for a given preload in this case decreasing afterload would cause the Starling curve to shift left an increase in afterload would cause the Starling curve to shift right increase afterload in summary two things can cause shifts in the Starling curve inotropy and afterload okay with this in mind let's see what your text says the Starling curve is a way to graphically measure cardiac output as a function of preload cardiac output increases as preload increases up to a point changes in inotropy cause shifts in the Sterling curve and changes in afterload also cause shifts in the Sterling curve okay moving on to cardiac and Vascular function curves this is figure 2.9 from your text which shows a normal cardiac and Vascular function curve this figure can be very confusing for a lot of students because it's an image of two graphs overlapped on top of each other in order to make things more confusing there are multiple variables on the axes the x-axis has right atrial pressure or end diastolic volume and the y-axis has cardiac output or venous return let's talk about the x-axis for a moment and draw this out on a blank screen okay so here's a sketch of the heart this is the right atrium the right ventricle pulmonary artery the left atrium the left ventricle and the aorta imagine for a moment that 100 milliliters of blood enters the right atrium and this blood exerts a force of 5 millimeters of mercury when the x axis is labeled right atrial pressure we're focusing on the 5 millimeters of mercury of pressure in the atrium rather than the hundred milliliters of blood itself eventually the same 100 milliliters of blood gets pumped into the right ventricle the pulmonary artery to the left atrium and the left ventricle typically the volume of blood in the left ventricle is used to determine end diastolic volume or edv so you can see that if the hundred milliliters of blood entered the right atrium then that same hundred milliliters of blood must enter the left ventricle a few moments later so when the x axis is labeled and diastolic volume we're focusing on the hundred milliliters of blood not the pressure exerted on the walls of the ventricles by the hundred milliliters of blood in other words right atrial pressure and end diastolic volume are essentially telling us the same thing it's just being expressed in different units with that in mind let's return to the figure because the x axis has two variables that are essentially telling us the same thing I like to just focus on the variable of the x axis that makes more sense conceptually whenever I analyze these figures I'll explain this more in a second on the other hand the y axis has two variables that are telling us two different things the first variable is cardiac output and the second is venous return remember cardiac output is how much blood leaves the heart in venous return is how much blood returns to the heart it's important to know that the y-axis variables coincide with the green and blue curves seen in this figure so the cardiac output variable of the y-axis is illustrated with the green curve the venous return variable of the y-axis is illustrated with the blue curve let's take a step back and separate the two curves for a moment and then return to this image [Music]