L10: The Cardiac Cycle Flashcards
Function of ventricles
- pumping action of heart
- thick muscular walls —> power contraction
- high pressure high velocity
Function of valves
- Prevent backflow of blood
- electrical insulation
AV valve: chordae tendinae to prevent valve leaflets from inverting
Semilunar valve: back flow of blood fills pockets of valve
Function of atria
- atrial contraction is weak (20% of ventricular filling)
- passive conducting chamber
- receives and store venous return during ventricular contraction
- build up atrial pressure
- rushes down pressure gradient when AV valve open
Normal values for systemic pressure and pulmonary pressure
Systemic BP: 120/80 mmHg
Pulmonary BP: 25/10 mmHg
7 Phases of cardiac cycle
- Atrial contraction
- depolarisation at SA node
- spread throughout L/R atria —> contraction —> AP ↑ 5mm Hg (a wave)
- force a little more blood into 70% filled ventricle —> VP and Vvol ↑ further - Isovolumetric contraction
- ventricular contraction —> VP>AP —> AV valve close
- semilunar valve still closed since VPVP>AP
- ventricular volume is not changed
- AP little ↑ due to bulging of AV valve (c wave) - Rapid ventricular ejection
- VP>aP —> semilunar valve open —> blood rapidly ejected from ventricle
- AP ↓ (x descent) then ↑ (v ascent) gradually —> AV valve pulled downward by ventricle contraction then filled by venous return - Reduced ventricular ejection
- VP ↓: ventricle stop contracting and relax
- aP ↓: due to blood leaving aorta to periphery
- VP↓ > aP↓ —> VP ejection however continues due to high velocity of blood from ventricles —> negative P gradient gradually decelerate flow to zero
- Semilunar valve closed at the end - Isovolumetric relaxation
- ventricular relaxation and repolarisation
- aP>VP>AP —> AV valve and semilunar valve closed —> ventricular volume unchanged
- VP ↓ quickly
- aP ↑ slightly: backflow of blood from aorta - Rapid filling
- atrium is already filled during ventricular systole
- when VP ↓ below AP —> AV valve open —> blood rush from atrium to ventricle
- AP ↓ (y descent)
- first quarter of diastole - Slow filling
- blood flows passively from venous return to atrium to ventricle
—> Right VP: maximum 25 mmHg, pulmonary arterial pressure correspondingly lower
Changes in atrial pressure during cardiac cycle
- Increase during atrial contraction (5-10mmHg) (a wave)
2. increase during atrial filling / accumulation of venous return (ventricular systole) (v ascent)
Changes in aortic pressure during cardiac cycle
- Maximum value at end of rapid ejection
- Decrease steadily until next ejection phase
- Dicrotic notch when aortic valve close: back flow of blood fills pocket to close valve
ECG
P wave: atrial depolarisation (just before atrial contraction)
QRS complex: ventricular depolarisation (just before isovolumetric contraction)
T wave: ventricular repolarisation (during reduced ejection)
Q: interventricular septum, R: main body of ventricular muscle (predominantly left ventricle), S: upper septum and high posterior wall (predominantly right ventricle)
Heart sound
S1: closure of AV valve + tensing of valves and muscle (lower frequency, longer)
- intensity depends on HR and velocity of blood (contractility)
S2: closure of semilunar valve (during inspiration, closure of pulmonary valve slightly after aortic valve —> splitting of S2) (shorter, sharper)
S3: vibration of ventricles / valves during rapid filling due to impact of blood flow (normal in young people / children, hard to hear in adult due to thickened wall) (relaxed ventricles —> dampen the sound)
S4: during atrial contraction, atrial pressure is abnormally high or ventricle abnormally stiff (pathological)
4 Heart sound areas:
- 2nd intercostal space: pulmonic area (右), aortic area (左)
- 5th intercostal space: mitral area (apex beat) (右), tricuspid area (左)
