Friday Physiology | Keep the Blood Flowing

Added: 2026-05-18

[00:00:00]the frank starling mechanism is one of the most famous and impressive findings in the history of cardiac physiology professor otto frank and some other colleagues already observed the basic principles underlying this concept in late 1800s in early 1900s professor starling consolidated some of these findings and a bit later both of their names were attached to this mechanism the frank starling law let this great example from more than 100 years ago be a friendly reminder for you whenever you do a literature search in pubmed please think twice before you exclude articles that are older than say a decade you might miss out on great findings that are still 100 valid today let's get started [Music] in this video i will explain the function and the regulation of the heart in the entire circulation although the heart is very small we can analyze many different physiological variables in this chart you can see how the pressure inside the atria inside the ventricles and inside the aorta change during one cardiac cycle can you find the key events that explain why valves open and close after replaying this amazing animation 100 times the heart serves both the pulmonary circulation and the systemic circulation with blood the overall physiology like pressure changes and functionality is roughly the same but one key difference is the overall pressure in each part of the system in the pulmonary circulation the overall blood pressure is significantly less stiff phonometry is the measurement of blood pressure and depending on your skills you could do that either manually with a stethoscope and a manual cuff or with an automatic device the concept is still the same you inflate your cuff around the left arm to completely block the flow of blood through the brachial artery from that height of pressure you will gradually lower the pressure while you listen for blood flow as the heart pumps discontinuously the blood pressure fluctuates between the systolic pressure and the diastolic pressure once you have lowered the cuff pressure to be just under the systolic pressure you will hear the blood flow only for small bursts exactly when the systolic pressure is reached under the cuff for just a split second and you have found your first value then you gradually lower the cuff pressure even further until you hear no more sound if that's the case you know you have brought the cuff pressure just under the diastolic pressure no more sound means that blood now flows smoothly without any turbulence so you can record your second value 120 over 80 is how we should report these two blood pressure values the pulse pressure is the difference between those two values and the mean arterial pressure is the average pressure but only after you've corrected for the time spent in diastole and systole as the diastolic phase is usually twice as long as the systolic phase the mean arterial pressure is one third of the pulse pressure above the diastolic pressure and therefore lower than the simple mathematical average would suggest hypertension is a prevalent disorder and is constantly putting more strain on the entire system many factors can play a role in the development of high blood pressure predisposition lifestyle exercise nutrition they can all have a direct or indirect effect in short blood pressure is increased by an increased blood flow or an increased resistance to the flow [Music] this second component is where many of the physiotherapeutic interventions are focused on or should be focused on let's start with exercise right next to professor frank and professor starling we can name professor thicke also in late 1800s he came up with a principle that was turned into the following equation the fig equation this equation can be used in different ways so depending on the variables that we can measure we can solve the remaining variable for example if we know that the avo2 difference is 5 stroke volume is 70 heart rate is 60 we can calculate the vo2 to be 2 liters per minute as a physio you would normally not have measurements of a vo2 difference or stroke volume but this equation still allows you to reason towards stroke volume just by measuring heart rate during steady state exercise for example if we assume that the avo2 difference is relatively constant at a certain workload any increase in vo2 can be attributed to an increase in cardiac output if we have measured heart rate solving stroke volume is then a piece of cake let's dissect this equation and separate each of these components and see how they can be measured and how they can be regulated credit output is the product of stroke volume and heart rate you simply multiply the number of beats per minute by the amount of blood pumped out per beat just make sure you use the correct units the heart rate frequency is measured in beats per minute and at rest values usually range between 50 and 80 while maximum heart rate for an average 20 year old student is around 200 beats per minute resting heart rate can change due to aerobic training maximum heart rate is not affected by training but it will roughly go down with one beat every year stroke volume is the amount of blood pumped out with every beat you could measure stroke volume by measuring the amount of blood in the heart after complete filling and diastolic volume and subtract the amount of blood left inside the heart after complete emptying and systolic volume stroke volume can vary between 70 milliliters and 100 milliliters at rest if you compare a sedentary person to an athlete maximum stroke volume values can go up to 200 milliliters depending on genetics and training status the control of stroke volume is done via three mechanisms preload contractility afterload the fringe starling mechanism is basically the length tension relationship of cardiac muscle cells the more the heart fills with blood the more cardiac fibers are stretched and via their position on the length tension curve you can see that that makes them capable of generating more force in plain english the more blood comes in the more blood flows out thanks to this mechanism extravenous return will therefore increase stroke volume as a physio you can think of many practical applications of this in your daily practice the normal ejection fraction of the heart at rest is around 50 this means that only half of what's in the heart at the end of the diastolic phase is actually pushed out the rest of the blood remains inside unlike skeletal muscle fibers your cardiac muscle fibers can increase the force of contraction if extra calcium is made available to these cells extra calcium means more crossbridges that can be formed more cross bridges means more force more force means more emptying and diastolic volume decreases stroke volume increases so you can see an higher ejection fraction the semilunar valve between the left ventricle and the aorta opens when the pressure in the left ventricle exceeds the pressure in the aorta if the pressure in aorta is greater than usual it will also require more pressure from the cardiac muscle cells in the wall of the left ventricle because the valve will only open if the pressure inside the left ventricle exceeds the pressure on the outside in the aorta again due to the length tension relationship that these cardiac muscle cells are in if they need to generate a higher amount of force they can only do that if they shorten less and therefore stroke volume will be less as a result so via these three factors your body can regulate stroke volume in this chart you can see how these components are connected in the regulation of cardiac output that was it for this video this was friday physiology i'll see you in the next video [Music]