# Cardiac cycle and atrial contraction

During atrial contraction ("a" in the figure), why does the ventricular pressure match the atrial pressure? The ventricular pressure generally stays the same throughout passive filling until it reaches the point where atrial contraction occurs. Why is there a sudden change in the ventricle pressure during atrial contraction? I can understand the atrial pressure would increase when the atria contract, but the ventricles have not contracted yet so the ventricle pressure shouldn't increase. The ventricle pressure can increase greatly when the volume exceeds a certain value and the elastic tissue of the heart cannot stretch anymore. However, the volume here is only about 110ml, so it has not reached that point. I can only think of one explanation:

1. The pressure is equilibrated between two sides of an open valve.

However, this doesn't explain why the atrial pressure is slightly greater than ventricle pressure during filling; it also doesn't explain why the aortic pressure is slightly greater than the ventricle pressure near the end of ejection.

• Two things to think about: 1) Does the pressure in a balloon increase only when it is full and cannot stretch anymore, or does it increase while it is stretching? 2) If A and B are connected, under what conditions will there be flow from A to B? Pressure_A == Pressure_B? Pressure_A>Pressure_B? Pressure_A<Pressure_B? May 8, 2019 at 22:18
• Well I've mentioned that ventricular pressure generally stays the same throughout passive filling. So stretching, based on your question, still doesn't explain why there is a sudden change during atrial contraction. Are you saying stretching of the ventricles only occurs during atrial contraction? However, your final point does rule out my hypothesis or attempt at explaining it.
– Rome
May 8, 2019 at 22:30
• Maybe one more thing to think about: 3) While there is flow from A to B, is there any circumstance where B could have greater pressure than A? May 8, 2019 at 22:37
• Well if we are talking about systole, the ventricles relax (A) near the end, as blood flows into the output blood vessels (B). As they relax, the pressure of A decreases and becomes less than B. Also, if we are talking about the atria (A) and ventricles (B), the same situation would occur. I'm not sure how your question explains my main query though. If you would like to clarify?
– Rome
May 8, 2019 at 23:22
• Mostly just wanted to make sure you were putting some thought into answering the question yourself because I think that's the best way to learn about these things. I think you've done sufficient thought that I can provide an answer explaining things - let me know if anything remains unclear. May 9, 2019 at 17:08

While blood is flowing into the ventricle, it can never be at a higher pressure than where blood is flowing from: if it was, the flow would be going in the other direction. Flow is always from higher to lower pressure, if there is no pressure difference there is no flow.

Before atrial contraction, the ventricle can have no more pressure than the uncontracted atrium, which in turn can have no more pressure than the veins (vena cava or pulmonary depending on which side of the heart we are talking about). When the atrium contracts, it increases the pressure in the atrium, which causes a flow of blood into the ventricle. Whenever a fluid flows, there is a pressure drop that depends on the resistance (very much like voltage in an electrical circuit). However, the AV valve is fairly big and open, so there is little pressure drop from the atrium to ventricle.

The pressure is equilibrated between two sides of an open valve.

...is mostly true if the valve is large enough. There is still some pressure drop, though: if there wasn't, there wouldn't be flow.

There is no need for the ventricle to be at maximum capacity for pressure to increase. Imagine if you squeeze one side of a balloon: the pressure in the balloon increases, which you can tell because the balloon stretches and expands in the areas you are not squeezing; it need not be at the maximum capacity of the balloon for this to happen. Same for the heart.

As far as the higher pressure in the aorta, I this diagram is slightly exaggerating where the pressure difference starts, but as the ventricle relaxes there is a brief time where you get a small backward flow because the relaxing ventricle ends up having less pressure than the proximal aorta. This pressure drop is what closes the aortic valve (or the pulmonary valve, same process).

• Thanks for you answer. I agree with most of what you have said but there are a few points that I am not entirely convinced with. Firstly, the figure was from the Guyton and Hall textbook and corroborated by others (e.g. aafp.org/afp/2004/0601/p2609.html). I do agree that flow is from higher to lower pressure but I don't agree that "if there is no pressure difference there is no flow". For one, the figure shows that ventricle volume continues to fall even when aortic P is higher than ventricular P, likely do to kinetic energy propelling it forward.
– Rome
May 9, 2019 at 21:02
• "if there is no pressure difference there is no flow" is physics (or at least no *net*/bulk flow...you will still have diffusion equally in both directions) and there is no controversy. The figure is a cartoon for demonstration. There could be some inertial effect but you wouldn't expect it to be a major contributor for the overall flow. May 9, 2019 at 21:06
• Secondly, I didn't mean that the ventricle had to be at a maximum capacity. What I wanted to understand is why there is a "sudden" increase in pressure when there was only a slight increase during passive filling. The main idea I was putting forward was that once the heart reaches a certain volume there is "sudden" rise in pressure for given volume b/c elastic tissue can't stretch as well after this point. I now realise that I may have underestimated what that pivot point is for the "sudden", also I didn't account that the ventricles are still contracting for some time.
– Rome
May 9, 2019 at 21:07
• physics.stackexchange.com/questions/157038/… and Bernoulli's equation
– Rome
May 9, 2019 at 21:10
• There isn't enough velocity or constriction for it to matter on the filling side. On the outflow side you might get a little effect of velocity but not much. I'd suggest starting with understanding how fluid moves without momentum before thinking about the exceptions, that's the level I based my answer on. May 9, 2019 at 21:18