The sinoatrial node (SAN) and the atrioventricular node (AVN) have an important role in the control of the cardiac cycle. The timing of atrial and ventricular systole and diastole must be controlled so that blood passes through the heart efficiently.

1. (a)Fig. 1.1 is a summary of blood flow through the right side of the heart during one cardiac cycle. Three boxes in Fig. 1.1 are not complete. Complete boxes 3, 5 and 7 in Fig. 1.1 using only the terms systole and diastole. 1 blood arrives at the heart in the superior vena cava and inferior vena cava 2 blood passes into the right atrium 3 atrial ventricular 4 blood is forced into the right ventricle 5 atrial ventricular 6 blood leaves the right ventricle to enter the pulmonary arteries 7 atrial ventricular [Figure 1.1][2]
2. (b)Impulses sent out by the SAN pass to the AVN, where there is a short delay. With reference to Fig. 1.1, explain why it is important for the control of the cardiac cycle that there is a short delay at the AVN after impulses have been sent out by the SAN.[2]
3. (c)Changes in blood pressure occur in the heart during the cardiac cycle. These changes cause the opening and closing of the bicuspid and tricuspid (atrioventricular) valves and the aortic and pulmonary (semilunar) valves. Explain how blood pressure changes: • cause the opening of the tricuspid valve • cause the opening of the pulmonary valve • help the flow of blood through the heart.[3]

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