A-level Biology Notes

Describe the process of the cardiac cycle?

-filling stage= atria relaxed, ventricles relaxed, Atrioventricular valve open, semilunar valve closed

-Atria contracts= the SAN located in the Right atrium initiates the heart beat and sends the impulse across both atria making them contract, this pushes all the remaining blood into the ventricles so it becomes full.

-Ventricles contract= the AVN picks up the impulse, delays it (stops the atria and ventricles contracting at the same time, so the atria empties and the ventricles fill), sends the impulse down the septum in the bundles of his, then at the apex the impulse goes up both walls of the ventricles in the purkine fibres, so the ventricles contract from the base upwards, pushing the blood up through the arteries, when the ventricles start to contract the Atrioventricular valve closes then the semilunar valve opens and blood leaves the heart 

-Ventricles relax- the semilunar valve closes then the Atrioventricular valve opens and filling starts again.