Can anyone help me with this practice exam question?

I'm doing a question that was on an exam paper a few years ago. It says 'the graph shows changes in the volume of blood in the left ventricle as the heart beats' Then there is the graph, which I don't think I need to explain,...the volume goes up then down. 'The horizontal line labelled A (where the volume goes down) on the graph shows when the blood is leaving the ventricle. Explain, in terms of blood pressure, why blood does not flow back into the atrium during this period'. This is a two mark question. I can easily explain is using valves, but blood pressure? Any ideas?

Reply 1

Dimez

Where the volume goes down, that's basically telling you that the ventricle is contracting, hence increasing the pressure within. There is increased blood pressure within the ventricle, thus stopping blood from flowing back into the ventricle from the atrium.

Reply 2

Dan3274

The blood in the ventricles is under increasing pressure so to prevent the blood from entering the atria, the atrioventricular valves close.
These valves have small tendons (if i remember rightly) which prevent them from opening both ways, so blood can leave the atria, but cannot re-enter them.