8.3.38 · Transport in Mammals
The Cardiac Cycle — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
8.3.3 The Cardiac Cycle — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the pressure, volume and valve changes through one heartbeat.Definition — the sequence of events in one heartbeat, including muscle contraction and relaxation.Terms — systole is contrac…
- Continuity
- Blood movement
- Volume and pressure
- Atrial systole
- Ventricular systole
- Semilunar valve
Section 1 of 10What the cardiac cycle is
1Definition
- Definition the sequence of events in one heartbeat, including muscle contraction and relaxation.
2Terms
- Terms systole is contraction and diastole is relaxation.
3Stages
- Stages atrial systole, ventricular systole and diastole.
4Duration
- Duration at about 70 beats per minute one cycle lasts roughly 0.8 s.
5Continuity
- Continuity one cycle follows another with no gap where blood stops flowing.
6Two sides
- Two sides both sides of the heart follow the same sequence at the same time.
Section 2 of 10Pressure principle
Blood movement
- Blood movement blood always moves from higher to lower pressure.
Volume and pressure
- Volume and pressure when a chamber's volume decreases pressure rises, and when its volume increases pressure falls.
Valves
- Valves they open when pressure behind them exceeds pressure in front, and close when pressure in front is greater.
Section 3 of 10Stages of the cycle
1
Diastole
- Diastole the muscle is relaxed.
2
Blood returns through the vena cava and pulmonary veins into the atria, and much flows straight into the ventricles through open AV valves.
3
Arterial pressure exceeds ventricular pressure, so the semilunar valves are closed.
4
Atrial systole
- Atrial systole atrial walls contract, raising atrial pressure above ventricular pressure.
5
The remaining blood is pushed into the ventricles with the AV valves open; this adds little because the ventricles are mostly full.
6
Ventricular systole
- Ventricular systole ventricular pressure exceeds atrial pressure, so the AV valves shut.
- (first heart sound)
7
For a short time all valves are shut and pressure builds at constant volume (isovolumetric contraction).
8
When ventricular pressure exceeds aortic and pulmonary artery pressure, the semilunar valves open and blood is ejected.
9
As the ventricles empty and relax, pressure falls below arterial pressure and the semilunar valves close (second heart sound).
10
The closure causes a small rise in aortic pressure, the dicrotic notch.
11
When ventricular pressure drops below atrial pressure, the AV valves reopen and the cycle restarts.
Section 4 of 10Valve states
| Stage | Atria | Ventricles | AV valves | Semilunar valves |
|---|---|---|---|---|
| Atrial systole | Contracting | Relaxed, filling | Open | Closed |
| Ventricular systole | Relaxed, filling | Contracting | Closed | Open once pressure exceeds arterial pressure |
| Diastole | Relaxed | Relaxed | Open | Closed |
Section 5 of 10Reading pressure and volume graphs
AV valve
- AV valve it closes where ventricular pressure first crosses above atrial pressure, and opens where it falls below.
Semilunar valve
- Semilunar valve it opens where ventricular pressure crosses above aortic pressure, and closes where it falls below.
Peak heights
- Peak heights the ventricular peak is higher than the atrial peak because the ventricle wall is much thicker.
Ventricular volume
- Ventricular volume it is highest at the end of diastole and lowest at the end of systole;
- the difference is the stroke volume.
Left and right sides
- Left and right sides both show the same pattern, but the right side reaches much lower pressures.
Section 6 of 10Calculations and links
1Cycle length
- Cycle length measure the time between two identical points, such as two ventricular pressure peaks.
2Heart rate (beats per min)
60 ÷ cycle length in seconds.
3Cardiac output
Cardiac output stroke volume × heart rate.
4Blood pressure readings
- Blood pressure readings systolic pressure is the peak in ventricular systole and diastolic pressure is the lowest value during relaxation.
5Exercise
- Exercise stroke volume and heart rate both rise, increasing cardiac output to meet oxygen demand.
6Coordination
- Coordination a wave of excitation makes the atria contract before the ventricles (see 8.3.4).
Section 7 of 10More from the specification
1Atrial and Ventricular Systole
- Contraction of the thin muscular walls of the left and right atrium increases chamber pressure, forcing blood through open atrioventricular valves into the relaxed ventricles prior to vigorous ventricular contraction.
2Ventricular Pressure Dynamics on Cardiac Graphs
- A classic pressure-volume graph illustrates that when the thick muscular walls of the left ventricle contract, ventricular pressure exceeds aortic pressure, forcing open the aortic semilunar valve to eject blood into the aorta.
3Atrioventricular Valve Action
- The atrioventricular valves, anchored to papillary muscles by fibrous cord tissue, close abruptly when ventricular pressure exceeds atrial pressure, preventing backward regurgitation of blood.
- (bicuspid and tricuspid)
4Anatomical Separation of Heart Halves
- The left and right chambers of the heart are completely separated by a central muscular septum, maintaining total separation between oxygen-rich systemic blood and oxygen-poor pulmonary blood.
5Atrium
- Upper chamber of the heart that receives blood.
6Tissue
- A group of cells of similar structure that perform the same function.
7Coronary arteries
- Blood vessels that supply the heart muscle with oxygen and glucose.
8Atrioventricular valve
- Tricuspid or bicuspid valve between an atrium and ventricle.
Section 8 of 10More on this topic
Reading the graph
- Reading the graph take the time between two identical points, such as two successive peaks.
AVN
- AVN the wave reaches the atrioventricular node, where a short delay lets the atria empty.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Systole | Contraction of the heart muscle. |
| Diastole | Relaxation of the heart muscle. |
| Cardiac cycle | The sequence of events in one complete heartbeat. |
| Isovolumetric contraction | Ventricular contraction with all valves shut, so pressure rises at constant volume. |
| Dicrotic notch | A small rise in aortic pressure when the semilunar valve closes. |
| Stroke volume | Volume of blood pumped by a ventricle in one beat. |
Section 10 of 10Exam technique
1
Saying the ventricles contract before the atria — atrial systole comes first.
2
Saying valves are pushed open by muscle — they respond to pressure differences.
3
Saying the heart stops between beats — cycles run continuously and blood keeps flowing.
4
Saying the AV valves are open during ventricular systole — they are shut, producing the first heart sound.
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