HTI5201 Chap.4 Cardiac Conduction, Cycle and Output
Cardiac Conduction, Cycle and Output
Synchronise electrical, pressure and flow events
The cardiovascular sequence starts with ordered electrical activation, but an electrical event is not itself ejection. Muscle activation changes chamber pressure, pressure differences determine valve state, and valve state determines whether changing volume fills or empties a chamber.
Stroke volume connects that cycle to cardiac output, while vascular resistance and vessel properties shape distribution. This chapter aligns those event streams and uses explicit arithmetic to show how heart rate and stroke volume jointly determine flow per minute.
What this chapter covers
- 01
Pacemaker and conduction sequence
- 02
Electrical and mechanical delay
- 03
Chamber pressure changes
- 04
Valve opening and closure
- 05
Stroke volume determinants
- 06
Cardiac output calculation
Worked example · free
Separate stroke volume from cardiac output
- 3Subtract end-systolic from end-diastolic volume.
- 3Multiply stroke volume by heart rate.
- 2Convert to litres per minute.
Key terms
- End-Diastolic Volume
- Ventricular volume at the end of filling before contraction.
- End-Systolic Volume
- Ventricular volume remaining after ejection.
- Cardiac Cycle
- Ordered electrical, pressure, valve and volume events of one heartbeat.
Cardiac Conduction, Cycle and Output FAQ
Why does electrical activation precede contraction?
The action potential initiates processes that raise intracellular calcium and activate the contractile machinery. Those steps take time, so the mechanical response follows electrical depolarisation. An answer that equates the electrical trace directly with blood ejection skips the pressure and valve events in between.
What opens and closes a cardiac valve?
A valve responds passively to the pressure difference across it. It opens when pressure behind it exceeds pressure ahead in the permitted direction and closes when that gradient reverses. The valve is not commanded to move by the conduction system, even though electrical activation helps create the pressures.
Can cardiac output rise when stroke volume falls?
Yes. Cardiac output is the product of heart rate and stroke volume. If heart rate rises proportionally more than stroke volume falls, the product can increase. The direction cannot be inferred from one factor alone when both change, so calculate or compare proportional changes.
How should I join an ECG event to a blood-flow consequence?
Use the intermediate mechanical links. Name the electrical activation, connect it to coordinated myocardial contraction, state the resulting chamber-pressure change, determine valve state from the pressure difference, and only then describe filling or ejection. An ECG does not directly measure valve motion or stroke volume.
The reasoning becomes defensible when each transition names the physical variable that carries the effect forward.
Exam move
Prepare a cardiac-cycle table with five aligned rows: electrical event, atrial and ventricular pressure, valve state, chamber volume and blood movement. Fill the table from a blank template until you can explain why each valve changes state from the pressure difference across it. Do not use a phase name as the reason a valve opens. Add heart sounds only after the pressure and closure events are in place.
In a separate conduction sketch, trace sinoatrial initiation, atrial activation, atrioventricular delay and rapid ventricular distribution. For every ECG label, write what electrical event it represents and one mechanical quantity it does not measure directly. This protects against treating a waveform as pressure, force or ejected volume. Quantitative practice should always proceed in two stages.
Compute stroke volume as end-diastolic volume minus end-systolic volume, then compute cardiac output as heart rate multiplied by stroke volume. Carry mL per beat and beats per minute through the multiplication so the cancellation produces mL per minute. Convert to litres only at the end.
Before calculating, predict whether each factor rises or falls; after calculating, check whether the product matches that qualitative expectation. Create changed cases in which heart rate and stroke volume move in the same direction and in opposite directions. The opposing cases are most valuable because guessing from one factor fails.
For vessels, build a structure-function ladder from elastic artery to vein and add the pressure, resistance or exchange job performed at each level. Practise explaining how total output can rise while one tissue receives less flow because arteriolar resistance redistributes perfusion. Once a week, write an integrated answer starting with an ECG event and ending with capillary exchange.
Every transition must pass through muscle activation, chamber pressure, valve state, ejected volume, arterial flow and regional resistance. If a link is unsupported, stop at that link rather than covering the gap with a general statement about circulation.
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