Anatomy and Physiology / Unit 8: The Cardiovascular and Lymphatic Systems
8.5 The Cardiac Cycle and Conduction
Notes
The cardiac cycle is one heartbeat: systole (contraction) and diastole (relaxation). At 75 beats per minute it lasts about 0.8 seconds.
- Atria contract first, filling ventricles; then ventricles contract, closing AV valves and opening semilunar valves to eject blood.
- Conduction system: sinoatrial (SA) node (the pacemaker, in the right atrium, about 60-100 beats per minute) then the atrioventricular (AV) node (delays the signal so atria finish emptying), then the AV bundle (bundle of His), bundle branches and Purkinje fibers, which spread the impulse through the ventricles.
- Electrocardiogram (ECG): P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).
- Cardiac output (CO) = heart rate (HR) times stroke volume (SV). Example: HR 70 per minute and SV 70 mL gives 4,900 mL per minute, about 4.9 L/min.
- Sympathetic nerves raise heart rate; parasympathetic (vagus nerve) slows it.
Practice
Which structure is the normal pacemaker of the heart?
The SA node fires fastest and sets the heart rate.
A person has a heart rate of 80 beats per minute and a stroke volume of 70 mL. What is cardiac output?
CO = 80 times 70 mL = 5,600 mL per minute.
Why does the AV node delay the impulse?
The delay ensures efficient filling of ventricles.
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