hi everyone welcome to learn a level biology for free with Miss Esther ik this video is on the cardiac cycle looking at the volume and pressure changes that result in this cycle if you haven't already watched the video on the heart structure make sure you click the link at the top here watch that first and then come back to this one because it will make a lot more sense if you've learned all the structures in detail before this if you are new here make sure you click to subscribe and click the under notification button so you know when all the latest videos are uploaded and give it a thumbs up today if you do find it helpful if you want to make notes grab a paper and pen before you begin so in the last video we went through the structures of the hearts looking at the structure and the function of the four chambers the four major blood vessels and the valves and in a particular importance for this video will be the valves and the thickness of the muscular walls so the cardiac cycle is looking at the pressure and the volume changes within the heart how that controls the opening and closing of the valves and how the valves make sure that the blood flows in one direction I'm going to split into these three key stages diastole atrial systole and ventricular systole now I do want to point out at this stage that these two terms are pronounced differently by so many people so I say diastole some people say distally I say systole some people say systole so diastole distally systole systole whichever your teacher says whichever you say I'm going to pronounce it that way pronounce it how you like as long as you spell it correctly then both are accepted so diastole at this stage we're going to begin here on the diagram and the inner circle is showing you what's happening to the atria and the outer circle is showing what's happening to the ventricles diastole means relaxing so if the atria and ventricles are in diastole that means both of those muscles are relaxing systole means contracting so we can see at this stage in the diagram the atria are contracting and at this stage in the diagram the ventricles are contracting so we're going to go through each of those stages in more detail so we're starting with diastole and both the atria and the ventricles are relaxing or in diastole at this stage so because the muscular wall and the muscles are relaxing we've got a larger volume and blood will flow into the atria during diastole so we've got here atrium ventricular muscles are relaxed the blood will enter because the blood is entering the pressure will start to rise as you are pouring in more and more liquid into that small space so that is our diastole atrial systole so we're now onto this stage in the diagram so the ventricles at this point are still in diastole so the ventricles are still relaxed but now you can see that the atria contract or they're in systole and that's what we've shown in this diagram here with a darker purple is so the atria start to contract as they contract that decreases the volume of the atria and if the volume decreases the pressure will increase and that increase in pressure behind the atrioventricular valves forces open the atrioventricular valves and so blood starts to pour into the ventricles in atrial systole the next stage is our ventricular systole so this darker green section and at this point that means the ventricles are contracting so it's a short delay before that happens and then the ventricles start to contract from the bottom upwards and as those ventricles contract they've got a really thick muscular wall so you get a big contraction that causes a decrease in the volume of the ventricles and because the volume decreases we get a big big increase in the pressure so we have a really high pressure in the ventricles that forces the atrioventricular valves shut and the semilunar valves open so as a result the blood moves from the ventricles up through the pulmonary artery and the aorta so that is what causes the semolina valves to open and the blood to leave the heart and then we get back to the beginning of the cycle then all of the muscle relaxants so the blood will then re-enter through the atria so that's the cardiac cycle the cardiac output is a math skill which is linked to this so you can work out the volume of the blood which is going to leave one of the ventricles in one minute and that's what the cardiac output is and the way you calculate it is heart rate times stroke volume and the heart rate is how many times your heart beats within one minute the stroke volume is the volume of blood which leaves the heart every beat so you could be asked to work that out now the valves play a key role in the cardiac cycle they are controlling that the blood flows in one direction or unidirectional flow the blood doesn't flow back into the atria so just to recap on the valves there's two sets we have the semilunar and the hv8 ventricular valves so maluna are the ones that are found between the ventricles and the arteries so you have them in the aorta and the pulmonary artery atrioventricular valves are as the name suggests that found between the atria and ventricles sometimes I call bicuspid and tricuspid and that's referring to how many flaps they're made of so bicuspid we can see two flaps tricuspid there are three flaps and the valves themselves they open when the pressure is higher behind the valve they'll close when the pressure is higher in front and this then prevents the backflow of blood and we're gonna have a look at this in detail before looking at pressure change graphs so if we think about the atrioventricular valves first they will open we said when there's a higher pressure behind compared to in front so behind the valve is the atria so if you have a higher pressure in atria compared to the ventricles that is what causes the AV valves to open if you have a higher pressure in the ventricles compared to the atrium that will cause the atrioventricular valves to close next then the semilunar valves and what causes those valves to open so they're always open when there's a high pressure behind than in front and in this case what's behind them as the ventricles so if you have a really high pressure in the ventricles so during ventricular systole that will cause the semolina valves to open the semilunar valves will close which we can see in this diagram they're closed when the pressure is higher in the arteries compared to the ventricles and whatever in arteries here that's specifically referring to that a otter and the pulmonary artery so bear those facts in mind before when we now go on to interpreting the graph looking at these pressure changes so what's a really common question linked to the cardiac cycle it's for you to be given a graph similar to this without these four labels and you would then have to draw on when each the valves opens and closes and it's all due to this knowledge of valves open when the pressures higher behind they close when the pressure is in front and then you need to be able to visualize the position of the valves so you know what is behind and what is in front now I've added on to mine whether the muscles are in systole or diastole so we've got the left atria and starting in systole then in diastole left ventricle at that point would start in diastole then it contracts and systole and then it's relaxed as well so this is just showing us what would be happening to the pressure in the aorta the ventricles and the atrium at each of these stages in the cardiac cycle so the stars' of this graph were beginning with atrial systole so the atria contracting that is why the pressure slightly increases the pressure then decreases they because the blood is being pumped out of atria then we go into ventricular systole and this is when the ventricles contract and this causes a much much bigger increase in pressure change and that's because the ventricles have a really really thick muscular wall compared to the muscular wall in the atria so they can contract with a greater force and generate a much higher pressure when the blood is then pumped out of the ventricles to the rest of the body we have a slight decrease as the volume of blood decreases but when the ventricles relaxed and therefore the volume of the chamber increases that's what causes the big decrease in pressure and when both the atria and ventricles relaxed the pressure in both chambers is very very low and that's why the blood will then move in to the heart through the atria so if we have a look at the four valves when they open and close thinking about the semilunar valves first we've said so many valves are positioned here which is between the ventricles and the arteries now we've only got data on this graph for the aorta and ventricles and atria so we must be thinking about simile in advance then which we can actually see in this diagram but it's between the left ventricle and the aorta behind the semilunar valve is the ventricle in front is the aorta so a semilunar valve will open when the pressure is higher behind it which is in the ventricle that is why it's at this point here where the semilunar valve first opens when the pressure is higher in the ventricle compared to the aorta that is when it forces the valve open and it remains open until this point when the pressure is now higher in the aorta compared to in the vent cool and that is what causes assembly in valves to shut thinking about the atrioventricular valves these the ones that are found between the atria and the ventricles so we have the atria in behind and the ventricles in front so the valves will open when the pressure is higher in the atria and the first time we see that crossing over of the atrial pressure exceeding the ventricular pressure is actually this point here so that's the first time they cross over so when the pressure in the atria now exceeds that in the ventricle that is what forces opened the atrioventricular valves the whole cycle is going on and then looping rounds it's still open and they'll close at this point when we see the first crossover when the ventricular pressure exceeds that in the atria it forces the atrioventricular valves shut so that's how you'd interpret the data on this type of graph so in summary then we've got this table looking at atrial systole ventricular systole and diastole and you need to come up with a description what is happening to the volume the pressure in the atria volume and pressure in the ventricles so if you want to have a go pause now draw the table and practice if not just carry on straight through with me now so the description what atrial systole means is for a trio contracting when tricular systole the ventricles are contracting diastole both sets of chambers are relaxed what happens to the volume then if the atria is contracting that means the volume will decrease if you've got a smaller volume that increases the pressure now at this stage that does actually result in an increased volume in the ventricles and a slight decrease in pressure because of that ventricular systole the ventricles are contracting now that actually has no impact on the atria at all but it will have a big impact on the ventricles so that large contraction causes a large decrease in the volume of the ventricles and therefore a large ink in the pressure of the ventricles so lastly when both sets of chambers are relaxing because they're relaxed that will result in an increase in the volume now the reason the pressure actually starts to increase in the atria at this point is because the volume is larger blood will start to flow into the atria and that's what causes the increase in the pressure the volume of the ventricles there's a slow increase in both the ventricle volume and pressure and that's because as the Blood starts to flow into the atria it will actually then start to trickle down into the ventricles as well and that's why there's this slow increase in volume and the pressure so in summary the cardiac cycle is made up of three stages diastole atrial systole and ventricular systole the pressure and volume changes within each chamber of the heart is what causes the valves to open and close and that ensures blood flows in one direction or its unique directional and those pressure and volume changes can be represented on the graph so that you can identify it when the valves are open or closed so that's it for the cardiac cycle head over to miss Esther ich komm and go to the practice questions topic 3 for questions to test your knowledge on this if you have found this helpful then please give a like a thumbs up below [Music] [Music] [Music]