Lecture 4 - Cardiac Physiology

Main function of the heart?

To pump blood through the pulmonary and systemic circuits.

What initiates the heartbeat?

Electrical impulse from the sinoatrial (SA) node.

Where is the SA node located?

In the right atrium near the superior vena cava.

What is the intrinsic rate of the SA node?

About 60-100 beats per minute.

What is the function of the atrioventricular (AV) node?

Delays the impulse before it passes to the ventricles.

Where does the impulse travel after the AV node?

Bundle of His → bundle branches → Purkinje fibres.

Define depolarisation.

Electrical activation causing cardiac muscle contraction.

Define repolarisation.

Restoration of the resting electrical state after contraction.

What does the P wave on an ECG represent?

Atrial depolarisation.

What does the QRS complex represent?

Ventricular depolarisation.

What does the T wave represent?

Ventricular repolarisation.

Define systole.

Contraction phase of the cardiac cycle.

Define diastole.

Relaxation and filling phase of the cardiac cycle.

What causes the "lub" heart sound?

Closure of the atrioventricular valves (mitral and tricuspid).

What causes the "dub" heart sound?

Closure of the semilunar valves (aortic and pulmonary).

Define stroke volume.

Volume of blood ejected by one ventricle per beat.

Define cardiac output.

Volume of blood pumped by each ventricle per minute.

Formula for cardiac output?

CO = stroke volume × heart rate.

Normal adult cardiac output?

Approximately 5 litres per minute at rest.

Define ejection fraction.

Percentage of ventricular blood volume ejected during systole.

Typical normal ejection fraction?

Around 60 %.

What factors influence heart rate?

Autonomic nervous system, hormones, temperature, and emotions.

What effect does sympathetic stimulation have on heart rate?

Increases heart rate and contractility.

What effect does parasympathetic stimulation have?

Decreases heart rate.

What is preload?

Degree of ventricular stretch at the end of diastole.

What is afterload?

Resistance the ventricle must overcome to eject blood.

What is contractility?

Force of ventricular contraction independent of preload and afterload.

How does exercise affect cardiac output?

Increases heart rate and stroke volume.

Define blood pressure.

Force of blood against arterial walls.

Equation for mean arterial pressure (MAP)?

MAP ≈ diastolic pressure + ⅓ (pulse pressure).

Normal resting MAP?

Approximately 93 mmHg.

What are baroreceptors?

Pressure sensors in carotid sinus and aortic arch that regulate blood pressure.

What is an electrocardiogram (ECG)?

A recording of the heart's electrical activity.