D.4 Internal and external factors influence heart function Developments in scientific research followed improvements in apparatus or instrumentation—the invention of the stethoscope led to improved knowledge of the workings of the heart.

The sound of the heartbeat Normal heart sounds are caused by the atrioventricular valves and semilunar valves closing causing changes in blood flow. Q: Which valve closing is the “lub” and which is the “dub”?

Application: Causes and consequences of hypertension and thrombosis. Skill: Analysis of epidemiological data relating to the incidence of coronary heart disease. Skill: Interpretation of systolic and diastolic blood pressure measurements.

Skill: Measurement and interpretation of the heart rate under different conditions.

Structure of cardiac muscle cells allows propagation of stimuli through the heart wall. Define propagation. Q: How do branching and intercalated discs in the structure of cardiac muscle assist the propagation of stimuli in the heart?

Signals from the sinoatrial node that cause contraction cannot pass directly from atria to ventricles. Q: What are the features of the cells in the heart that keep the contraction from passing from the atria to the ventricles? There is a delay between the arrival and passing on of a stimulus at the atrioventricular node. Q: List two features of the AV node that lead the delayed initiation of contraction of the ventricles. This delay allows time for atrial systole before the atrioventricular valves close. Q: What does the phrase “atrial systole” mean in terms of blood volume? Conducting fibres ensure coordinated contraction of the entire ventricle wall. Q: List two characteristics of Purkinje fibres that allow them to conduct signals at high speeds.

Application: Use of artificial pacemakers to regulate the heart rate.

Application: Use of defibrillation to treat life-threatening cardiac conditions.

Skill: Mapping of the cardiac cycle to a normal ECG trace.