Physiology: Cardiac Cycle

Question 1

What is responsible for the heat sounds S1 (Lub) and S2 (Dub)?

Answer 1

The closing of the heart valves

S1 = atrioventricular valves
S2 = Semilunar valves

Question 2

Define one cardiac cycle

Answer 2

All events from the beginning of one heartbeat to the beginning of the next

Question 3

Define diastole

Answer 3

• Ventricles relax and fill with blood

- Starts with S2 sound (dub)

Question 4

Define systole

Answer 4

• Atria contract
• Ventricles contract
• Starts with S1 sounds (Lub)

Question 5

Give the average lengths of systole and diastole

Answer 5

• Systole ~0.3secs

- Diastole ~0.5secs

Question 6

List the events of the cardiac cycle

Answer 6

1) Isovolumetric ventricular relaxation
2) Passive filling of ventricles

3) Atrial Contraction
4) Atrioventricular valves close
5) Isovolumetric ventricular contraction
6) Ventricular ejection
7) Semilunar valves close

Question 7

Describe the events of passive filling

Answer 7

• Very low pressure in ventricles and atria
• AV valves open, allowing venous return to flow into the ventricles
• Ventricles become 80% full through passive filling
• Aortic pressure drops to ~80mmHg

Question 8

Describe the events of atrial contraction

Answer 8

• Ventricular filling is completed
• Brings end diastolic volume up to ~130ml (resting adult)
• Starts at the P-wave on an ECG
• Ends at the start of the QRS complex on an ECG

Question 9

Describe the events of isovolumetric ventricular contraction

Answer 9

• Starts AFTER the QRS complex
• Ventricular contraction starts
• When ventricular pressure exceeds atrial pressure the AV valves shut (Lub)
• Aortic valve is still closed already, so blood is isolated
• Ventricular pressure rises steeply

Question 10

What causes the atrioventricular valves to close?

Answer 10

Rising ventricular pressure

When ventricular pressure exceeds atrial pressure the AV valves shut

Question 11

Describe the events of ventricular ejection?

Answer 11

• Ventricular pressure exceeds aortic/pulmonary artery pressure, so semilunar valves open
• Stroke volume is ejected, and the end systolic volume remains in the ventricle
• Aortic pressure rises
• The T-wave on an ECG signals ventricular repolarization
• Ventricles start to relax, ventricular pressure starts to drop
• When ventricular < aortic/pulmonary artery pressure, the semilunar valves shuts

Question 12

What causes the semilunar valves to open?

Answer 12

Ventricular pressure exceeding aortic/pulmonary artery pressure

During ventricular ejection

Question 13

What causes the semilunar valves to shut?

Answer 13

Ventricular pressure falling lower than aortic/pulmonary

During ventricular ejection

Question 14

Describe the events of isovolumetric ventricular relaxation

Answer 14

• Starts as semilunar valves close
• Blood is sealed in ventricles, as the AV valves are still closed
• Pressure drops as muscle relaxes
• When the ventricular pressure falls below atrial pressure the AV valves open

Question 15

When do the AV valves open?

Answer 15

When the ventricular pressure is less than the atrial pressure

During isovolumetric ventricular relaxation

Question 16

How is the decrease in arterial pressure during diastole minimised?

Answer 16

The walls of the arteries close to the heart are elastic

Thus they recoil as the pressure drops, decreasing the volume. Thus the pressure doesn’t drop as far