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.

00:15 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.

00:27 The pressure that it induces involves squeezing in on the blood that's there right at the end.

00:33 And that again, normally pushes about 10%, into the left ventricle to cause the full filling.

00:41 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 you the atrium really don't contribute much to ventricular filling.

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

01:07 On time though when atrial systole becomes very important around things like during exercise.

01:14 We 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.

01:24 Because that way you can utilize the Frank-Starling mechanism to push more fluid out per stroke without utilizing any more energy.

01:35 Atrial diastole occurs after atrial systole.

01:39 Remember that this atrial kick is what creates the A wave.

01:43 However, during the diastole portion of the atria, these are phases two through seven.

01:49 There are two other waves we need to be concerned about.

01:54 The first of these is known as the C wave.

01:57 The C wave occurs during the contraction of the ventricles.

02:01 This is actually a pressure reverberation that occurs.

02:05 It either occurs during phase two as shown in the graph, or sometimes even a little bit into phase three.

02:14 The other way we need to be concerned about is the V wave.

02:17 The V wave occurs after you start to get blood filling the atria.

02:23 It continues to fill the atria until suddenly the AV valves open and there is a sudden pressure fall.

02:31 That's known as the V wave.

02:35 So in terms of the atrial filling pressures, and the atrial contraction pressures, we can look at A, C, and V being those items that we want to make sure we have good feeling for why they are occurring.