Hi, in the heart, blood flows through the AV valves and enters the ventricles once the atria have filled up with blood. What opens the AV valves? Fluid pressure from the blood or the SA node pulses?
The opening and closing of the atrioventricular valves is controlled by the differences in pressure across them: When blood passes from an atrium to a ventricle, the atrioventricular valve is pushed open (and the papillary muscles relax and the chordae tendineae lose their tension, becoming slack). At the next stage in the sequence the ventricle contracts. This forces blood out from the ventricle into the artery via which it leaves the heart. This contraction of the ventricle and the corresponding increase in blood pressure within that ventricle also presses the cusps of the atrioventricular valve together until their edges meet, closing the valve. At the same time, the papillary muscles contract and the chordae tendineae tighten - both of which also help to prevent the cusps from enabling the atrioventricular valves to open. When in the next stage of the sequence the ventricle relaxes and the atrium contracts, the blood pressure situation is reversed, the relaxation/tension of the papillary muscles and the chordae tendineae also reverses, and the atrioventricular valve allows blood to pass from the atrium to the ventricle again.
