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:
- 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.