[Music] cardiac conducting system is how we generate and spread the electrical impulses to then signal those cardiac contractile cells to contract if we look at instead of individual cells if we look at the heart as a whole um let's talk about how the order of Contracting Atria and ventricles and relaxing them they have to contract and relax in a nice coordinated way so that we can properly push the blood through the heart and through the body so the cardiac cycle essentially is the period between the start of one heartbeat and the beginning of the next so one full heartbeat essentially is the cardiac cycle the two ideas that are involved with a cardiac cycle would be contraction of the heart chambers and relaxation of the heart chambers when a heart chamber contracts it's called syy when a heart chamber relaxes that's called diast so the heart is alternating Cy diast Cy diast contract relax contract relax the cardiac cycle is divided up into phases so at the beginning of the cardiac cycle we have what we call atrial Cy atrial syy syy means contraction so atrial syy is contraction of the Atri during atrial syy okay so I would like to point out that when this begins there is already blood in the Atria and in the ventricles both all right so we already have blood in all four heart chambers there's already a lot of blood in the Atria and the ventricles step one atrial syy the Atria begin Contracting and they force some not all they force some of their blood down through their atrio ventricular valves into the ventricles now again the ventricles already had blood in there so atrio syy puts more a little more blood some extra blood down into the ventricles and the ventricles now become fully engorged with blood once atrial Cy ends then the Atria enter diast so atrial diast in atrial diast the Atria again are relaxing and expanding and so that creates a little bit of a vacuum and we want to so those heart valves the atrial ventricular valves here and here they have to slam shut so that blood does not go backwards into the Atria from The ventricle into the Atria now if it was just from the Atria relaxing alone that's not a strong enough Force to fully close those atrial ventricular valves so atrial Cy they contract push blood down into the ventricles atrial diasti they relax and when they begin their relaxing relaxing diasti then we begin the ventricular syy so that the ventricles then contract after the Atria do their Contracting then the ventricles begin Contracting and when the ventricles first start Contracting right the pressure is pretty low so they first start Contracting and the the blood pressure inside starts to go up a little bit and that pressure is enough to then make sure that those atrial ventricular valves slam shut and the blood cannot go back up into the Atria now when The ventricle when when the ventricles first enter syy and start building up pressure right yes that's enough to cause these valves to slam shut so that doesn't regurgitate or go backwards but the pressure still is not yet high enough to open the semi lunar valves to push the blood out okay so you see here that all four heart valves are shut at the beginning of ventricular cyti and if they're all shut but the ventricles are Contracting that means the pressure really starts building up inside the ventricles at this point in time this the ventricles will be holding the maximum amount of blood that they can fill with during this particular cardiac cycle this is known as the end diastolic volume or edv end diastolic volume at the end of diast right at the end here this is the maximum amount of blood the ventricles can hold so the ventricles continue to contract and the pressure GS keeps going up and up and up and up but no matter what if everything works properly that pressure is never going to be strong enough to push open the atrio ventricular valves so we're not going to regurgitate blood back up into the Atria however the ventricles will be able to squeeze enough to generate enough pressure to push open these semi lunar valves so the pressure builds up to a certain amount and it becomes finally great enough to force open those semi lunar valves while this is happening while that pressure is building up but before the valves push open this type of contraction is known as an isovolumetric contraction ISO means the same volume right is the volume or the amount of blood in the ventricles so during an isov volumetric contraction those ventricles are Contracting but because all the valves are still shut right we're not changing the volume of blood in the ventricles blood's not going in blood's not going out the ventricles are still Contracting but the volume of blood doesn't change it's an isov volumetric contraction again they continue to contract eventually the pressure gets so high that it pushes open those semi lunar Valves and blood gets ejected from the ventricles out into either the pulmonary trunk or the aorta depending on which ventricle it is uh the amount of blood that the ventricles eject during one cardiac cycle is known as the stroke volume right we call it the stroke volume the amount of blood that comes out of a ventricle during ventricular syy this is not all the blood in a ventricle the ventricles do not force out all the blood there is always blood that remains in the ventricles when it's done so the amount of blood that's left in the ventricles when it's fully contracted after it's after it's achieved its stroke volume once it's forced out as much blood as it can the remaining blood is called the ESV end systolic volume so up here the end diastolic volume it's how much blood the ventricles can hold maximum the NC systolic volume is how much blood is left in the ventricles after assistly after they contract so once they finish their their syy their contraction then the ventricles enter diast they relax now the Atria are already in diasti they're already relaxing and now the ventricles are relaxed too so because they're relaxed the pressure starts dropping inside and look look at the valves so right now at the very beginning of ventricular diasti they're starting to relax we have the semi lunar valves closed which is good because the ventricles relaxed and expanding we don't want the blood to regurgitate flow backwards into the ventricles so that's why the semi lunar valves are closed but the pressure is not strong enough in the Atria to open up the atrio ventricular valves and so they're also closed at the beginning of ventricular diast okay so because no the ventricles already have blood in there right the enolic volume that amount of blood they're relaxing but at the beginning no Bloods going into the ventricles yet so we call this an isov volumetric relaxation right so they're relaxing the ventricles are relaxing but the volume of blood is not changing so isos same volumetric volume the volume of blood in the ventricles is not changing because those AV valves are still closed no new blood is entering but the ventricles are still relaxing is volumetric relaxation now eventually as The ventricle continue to relax the pressure gets really low in the ventricles but blood still keeps going in to the Atria so the pressure builds up in the Atria so look what happens at a certain point the blood going into the Atria builds up gets high the ventricles keep expanding and relaxing and the pressure keeps dropping really low at some point the pressure in the Atria becomes so much bigger than in the ventricles that the pressure is able to push the AV valves up open and blood can now start flowing from the Atria into the ventricles okay so this is that that second half of ventricular diast the Atria are still in diast they're still relaxed so they're not Contracting but blood is still passively flowing into the Atria and down into the ventricles they're all passively filling even though they're all relaxed in diast so to briefly review we're going to start with all four chambers in diast and being relaxed then we have atrial syy the Atria contract and force more blood into the already full ventricles and so they expand more then the Atria relax so atrial diast then the ventricles take their turn to contract ventricular cyly so they contract the AV valve slam shut so blood can't go backwards they continue to contract until the pressure becomes big enough to force open the semi lunar valves and the blood flows from the ventricles into their respective arteries the ventricles then relax and now all four chambers are in diast they're all relaxed blood go blood goes into the Atria and then once the pressure builds up enough the AV valves pop open and blood then starts filling the ventricles and that is the end of the cardiac cycle when they're all relaxed but they're all filling with blood still and then we started again atrial syy atrial diast ventricular syy ventricular diast