L10 - Cardiac Cycle

Question 1

What are the 4 main stages of the cardiac cycle?

Answer 1

Diastole 
1. Inflow of blood 
4. Isovolumetric relaxation 
Systole 
2. Isovolumetric contraction 
3. Outflow of blood

Question 2

Cardiac cycle method overview

Answer 2

1. Initiated with the P wave
2. Both atria contract simultaneously due to faster conduction velocity in Bachmanns branch
3. Atria contract so blood pressure in each atrium increases
4. Blood is squeezed into the ventricles – 0.1 sec
5. Atrial pressure begins to fall causing a pressure gradient reversal across the AV valves
6. This causes the valves to float upward before closure

Question 3

How does blood flow into the ventricles?

Answer 3

80% of blood flows into ventricles passively
Left atria only contributes to 10% of flow into ventricles
Additional atrial contraction – atrial kick (only important in exercise)

Question 4

Phase 1 of the cardiac cycle overview

Answer 4

Ventricular volumes are maximal - end-diastolic volume (EDV)
Left ventricular EDV is 120 ml and represents the ventricular preload
- During exercise the preload increases

Question 5

At the end of phase 1 what are the diastolic pressures?

Answer 5

LV = 8-12 mmHg

RV =3-6 mmHg

Question 6

Phase 2 of the cardiac cycle overview

Answer 6

All valves closed
QRS complex representing ventricular depolarisation
Triggers excitation-contraction coupling, myocyte contraction and increased intraventricular pressure
Rate of pressure development becomes maximal
AV valves close when intraventricular pressure exceeds atrial pressure - first heart sound (S1)
- This sound is split - mitral valve closes earlier than tricuspid valve
Ventricular contraction triggers contraction of the papillary muscles

Question 7

How does ventricular contraction trigger contraction of papillary muscles?

Answer 7

Their chordae tendineae are attached to the AV valve leaflets
This tension on the valve leaflets prevents them bulging back into atria, becoming incompetent

Question 8

Phase 3 of the cardiac cycle overview

Answer 8

Between AV valve closure and AP valve opening, ventricular pressure rises rapidly with no change in volume
Rate of ventricular pressure increase is determined by rate of contraction of muscle fibers
- Determined by speed of excitation-contraction coupling
C-wave in the LAP due to bulging of mitral valve leaflets back into left atrium

Question 9

Phase 4 (ejection) of the cardiac cycle overview

Answer 9

AP valves open
AV valves closed
Ejection begins when intraventricular pressure exceeds aorta and pulmonary artery pressures
- A/P valves open
Maximal outflow velocity is early in the ejection phase
- Maximal aortic and pulmonary artery pressures are achieved
No heart sounds
Left atrial pressure initially decreases as the atrial base in pulled downward
Blood continues to flow into the atria from venous inflow tracts
Atrial pressures begin to rise

Question 10

Phase 5 (isovolumetric relaxation) of the cardiac cycle overview

Answer 10

AP valves close
- When intraventricular pressures fall
- Causes S2
- Causes a small backflow of blood into the ventricles
- Notch in the aortic and pulmonary artery pressure tracings
After valve closure, aortic and pulmonary artery pressures rise - dicrotic wave
Followed by a slow decline in pressure
Rate of pressure decline in ventricles is determined by rate of relaxation of muscle fibers - lusitropy
- Regulated by sarcoplasmic reticulum re-sequestering calcium
Ventricular volumes do not change because valves are closed
- Volume of blood that remains in a ventricle -end-systolic volume - 50 ml in left ventricle
- Stroke volume - difference between EDV and ESV = 70 ml
Left atrial pressure continues to rise because of venous return from the lungs
- Peak LAP - v-wave.

Question 11

Phase 6 (rapid fillinf) of the cardiac cycle overview

Answer 11

Ventricles continue to relax - intraventricular pressures fall below atrial pressures
- AV valves open
- Passive ventricular filling begins
- Intraventricular pressure continues to briefly fall as ventricles still undergoing relaxation
- Then begin to rise as they become less compliant
– Reduces the pressure gradient across AV valves so rate of filling falls
AV valve opening causes a rapid fall in LAP
- Followed by the y-descent of the LAP
Ventricular filling is silent
- S3 - tensing of chordae tendineae and AV ring
In resting heart, ventricle is 90% filled by the end of this phase
- Occurs before atrial contraction - passive

Question 12

When is S1 heard?

Answer 12

At the end of diastole and beginning of ventricular systole

Question 13

What causes S1?

Answer 13

Produced by vibrations generated by closure of the mitral and tricuspid valves

Question 14

When is S2 heard?

Answer 14

At the end of systole

Question 15

What causes S2?

Answer 15

Produced by the closure of the aortic and the pulmonary valves

Question 16

When is S3 heard?

Answer 16

At the beginning of diastole

Question 17

What causes S3?

Answer 17

Produced during the rapid entry of blood from the atrium to the ventricle (low pitched)