In a healthy mammalian heart, contraction of the four chambers is coordinated by the action of the sinoatrial node (SAN) and atrioventricular node (AVN). Fig. 2.1 shows blood pressure changes that occur in the left ventricle, left atrium and aorta during one cardiac cycle. E, F, G and H are the points at which a valve opens or closes as a result of blood pressure changes.

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [2] sinoatrial node / SAN, sends out, wave of excitation / impulses / electrical impulses ; R nerve impulses wave of excitation / AW, spreads across atrial wall ;
2. [2] any two from non-conducting fibres between atrial and ventricle walls ; impulse must pass down septum ; AVN (in interatrial septum) delays impulse ; (gives) time for atria to, complete contraction / empty;
3. [3] one mark if F and G are semilunar and E and H are bicuspid; one mark for E and G opens ; one mark for F and H closes ; E bicuspid/ (left) atrioventricular, valve, closes F semilunar/aortic, valve opens G semilunar/aortic, valve closes Η bicuspid / (left) atrioventricular, valve opens
4. [2] (during contraction) left ventricle, generates / AW, higher pressure (than left atrium); A do not produce the same pressure when contracting any one from (because) wall of left ventricle thicker / more (cardiac) muscle, so reaches higher ; data from Fig. 2.1 to show difference in pressure;