let's now step back and look at the heart from a different perspective and let's include understanding the presence and the function of valves this particular drawing indicates uh with initials the various chambers of the heart so right atrium right ventricle left atrium and left ventricle the valves that are in these areas we have the atrio ventricular valves which are between the right Atria and right ventricle and the left Atri and the left ventricle and then we have our semi lunar valves which on the right side of the heart would be our pulmonary valve and on the left side of the heart our aortic valve now as we look at Valves and how they function in the heart it's important to understand that valves will open or close primarily based on pressure differentials so if we have a valve and we have a pressure that's built up behind the valve that that pressure will cause that valve to swing open so when our ventricle contracts it's going to fi provide pressure that's going to in this case open up the pulmonary or the aortic valve and allow blood to pass on through now initially that blood flows through simply because of the pressure differences but realize that because of the force that's created there's quite a bit of inertia that develops with that flow of blood so the blood will actually stay open a little bit longer than we could explain simply from a pressure differential alone so our valve opens because of pressure in this direction But realize when the pressure changes that pressure then is going to cause that valve to close now if you look back at our first picture we show that on the atrial ventricular valves there appear to be some tendonous type attachments that attach to the leaflets of the valves these are called cord a tendon now if by just looking at the picture you might assume that the muscles that those tendonous attachments attached to would contract and cause those valves to open but that's not the case again the valves open and closed because of pressure differences these corded tendine they function so that when The ventricle does contract and there's quite a bit of pressure building up in the ventricular Chambers it keeps these atrial ventricular valves from ting and allowing the blood to flow backwards so the purpose of the valves is really to make sure that we have one-way flow of blood throughout the heart now with this information in mind we can now begin to talk about what's called the cardiac cycle so the cardiac cycle refers to the events that occur during the contraction and relaxation of the heart and we can divide the cardiac cycle into four different phases in this particular picture we are demonstrating the left side of the heart so this would be the left Atria the left ventricle this would be the aortic valve this would be our atrial ventricular valve or the mitro valve on the left side of the heart so realize that when we talk about the cardiac cycle what's happening on one side of the heart is also happening on the other as well so from Simplicity sake we'll simply refer to the left side of the heart at this point so the first phase of the cardiac cycle we refer to as ventricular filling and what you'll notice is that each of these names of the cycle are going to corespond with exactly what's happening during that time so during ventricular filling the pressure in the Atria is greater than the pressure in The ventricle because of that the atrial ventricular valve will be open and blood will be flowing through the V Atria Into The ventricle hence ventricular filling now initially that flow of blood is quite rapid we call that the rapid filling phase and then as more and more blood fills up The ventricle and the pressure begins to rise a bit that filling phase then slows down we call that diastasis now toward the end of ventricular filling if you think back to our electrical activity of the heart remember we had the p-wave which causes atrial contraction so toward the end of ventricular filling the Atria will contract and that will push just a little bit more blood into The ventricle the picture on the bottom here depicts ventricular volume and so this lighter colored purple indicates the phase of Rapid ventricular filling followed by diastasis followed by this small bump in volume caused by atrial contraction so that's the first phase of the cardiac cycle now what happens next is The ventricle is then depolarized IED and it's going to contract and once it begins to contract the pressure in The ventricle then increases and when it does the pressure in The ventricle becomes greater than the pressure in the Atria and the atrio ventricular valve closes and that accounts for the first heart sound when we're listening with a stethoscope so during this phase The ventricle continues to contract and pressure begins to build within the ventricle so the pressure in The ventricle remains greater than the pressure in the Atria but notice that this is isovolumetric ventricular contraction so that means the volume in The ventricle stays the same so from that name we can then deduce that all valves are closed so if the aortic valve is also closed that would tell us that the pressure in the aorta is greater than the pressure in The ventricle thereby causing that valve to remain closed now as we continue to contract The ventricle the ventricular pressure continues to increase and eventually that pressure will become greater than the pressure in the aorta when that becomes greater than the pressure in the aorta that aortic valve will then open and blood will then begin to be ejected that then marks the period the beginning of ventricular ejection so when ventricular ejection the pressure has become greater in The ventricle than the aorta so blood is leaving notice that the atrial ventricular valve remains closed so the pressure in The ventricle is greater than the pressure in the Atria and blood is now being rapidly ejected from The ventricle we can see that depicted on this graph this is showing the volume of blood in the ventricle decreasing quite rapidly now initially that flow is rapid because the pressure in The ventricle is so much greater than the aorta But realize toward the end it starts to kind of slow down the valve remains open though because of the inertia of the blood pushing forward keeps that valve open and blood continues to leave slowly now eventually the pressure in The ventricle becomes less than the pressure in the aorta once again and when that happens that's going to cause that aortic valve to close when the aortic valve closes that accounts for our second heart sound so this is the period of isov volumetric ventricular relax ation so again the name tells a lot isovolumetric means there's no change in volume which means all valve have to be closed so if all valves are closed we can look at this and say the pressure in the aorta must be greater than the pressure in The ventricle and the pressure in The ventricle must be greater than the pressure in the Atria that's what accounts for these valves remaining closed so as The ventricle continues to relax what's going to happen is the pressure in The ventricle continues to decrease and eventually that pressure in The ventricle will decrease to the point that it's below the atrial pressure and then that brings us back to the phase of ventricular filling now two other terms that we're going to introduce here are going to be two terms that can divide the cardiac cycle into two main categories the first term we call syy syy is when the ventricular muscle is Contracting so the ventricular muscle will be contracting during the phase of isovolumetric ventricular contraction as well as during ventricular ejection the other term is called diast and diast encompasses that period when The ventricle is relaxing so this will include isovolumetric ventricular relaxation and and ventricular filling now notice that we'll use these terms specifically in relation to the ventricle but rather than say ventricular syy or ventricular diast it's given that when we use those terms alone we're referring to the ventricle occasionally someone may want to be talking specifically about the function of the Atria and if that's the case they can sure describe the contraction of the Atria as atrial syy but typ typically we're finally focusing on the function of the ventricle so these terms will be used with that in mind