Atrial Systole and Diastole – Cardiac Cycle

00:01 Now, let's talk a little bit about atrial systole.

00:04 We kind of ended on it being the last little kick of volume into the left ventricle.

00:11 It’s initiated by the P wave from the ECG.

00:16 It occurs, again, right at the end of ventricular diastole.

00:23 We talked about it also producing a little bit of pressure on its own.

00:28 The pressure that it induces involves squeezing in on the blood that's there right at the end and that again normally pushes about 10% into the left ventricle to cause the full filling.

00:42 So, someone really doesn't need to have a lot of atrial systole under most circumstances.

00:49 In fact, you could have a cardiac arrhythmia, known as atrial fibrillation, in which the atrium really don't contribute much to ventricular filling.

01:01 And you could be walking around and you might not even know that person has this arrhythmia.

01:08 In time, though, when atrial systole becomes very important are on things like during exercise where you have large volumes of blood coming back to the heart and the top part can help squeeze that blood into the left ventricle, so that you can get a better contraction out because that way you can utilize the Frank-Starling mechanism to push more fluid out per stroke without utilizing any more energy.

01:37 Atrial filling occurs throughout most of the cardiac cycle.

01:42 So, really, you're only contraction phase was during 1.

01:46 During 2 through 7, in which the ventricle is undergoing isovolumic contraction, rapid ejection, reduced ejection, isovolumic relaxation, rapid filling and reduced filling, all those are occurring at the same time that the atria are filling.

02:03 So it’s a longer point of time.

02:06 The interesting thing about the waveforms, I will highlight here this left atrial pressure.

02:15 It’s the dotted line on your screen.

02:18 You see that there is a small decrease in left atrial pressure immediately upon opening up of the aortic valve.

02:28 That's labeled C on your graph.

02:32 Then, pressure gradually builds up again until the mitral valve opens and you see a sharp decrease.

02:40 And that is denoted by this V.

02:44 So, in terms of the atrial filling pressures and the atrial contraction pressures, we can look A, C and V being those items that we want to make sure we have a good feeling for why they’re occurring. that we want to make sure we have a good feeling for why they’re occurring.