26. The cardiac cycle Flashcards
Basic about cardiac cycle
The coordinated electrical activity and mechanical properties of the heart make it possible that the heart functions all trough a lifetime. The basic unit of this functioning that is repeated uninterruptedly is the cardiac cycle.
Composed of two elements: systole (contraction) and diastole (relaxation).
Total length of cardiac cycle in dog: 800 msec
Ventricular systole (270 msec):
- Tension in the ventricles closes the cuspidal valves (to the atria).
- The ventricle continues to increase its tension while all valves are closed.
- In this phase the contractile components (CC) shorten and stretch the serial elastic elements (SEC).
- There is no volume change, while the tension increases: therefore the name of this contraction is “isovolumetric contraction”.
- The increasing tension deforms the heart (ovoid spherical)
- Including in the ventricular systole:
- Isovolumetric contraction: When the internal pressure of the ventricles goes just above the pressure found in the aorta and the pulmonary artery, the semilunar valves open. 50 msec.
- Auxotonic contraction: Blood enters the large arteries while the tension further increases. 220 msec.
- Fast ejection: 80% of the stroke volume. Lasts for 90 msec.
- Slow ejection phase: Tension drops, but ejection continues. Lasts for 130 msec.
Ventricular diastole (530 msec)
- Including in it:
- Isovolumetric relaxation: The pressure in the ventricles falls below the pressure of the atria. A passive opening of the cuspidal valves occurs and the filling phase commences. Lasts for 120 msec.
-
Isotonic relaxation: Isotonic filling. Lasts for 410 msec. Can be divided into:
- Fast filling: 60% of filling. Lasts for 110 msec.
- Reduced filling: A smaller fraction of blood enters the ventricles. At the end of this phase the SA node generates a new AP: the depolarization and the subsequent contraction of the atria commences. Lasts for 190 msec.
- Atrial systole: Lasts for 110 msec.
Parameters ofthe cardiac cycle
- Pressure
- Volume
- Valves
- Heart sounds
- Jugular pressure
Pressure
Pressure: The minute to minute changes of pressure values in the pulmonary veins, in the left atrium, in the left ventricle, and in the aorta determine the position of the valves and accordingly the flow of blood in these compartments.
Volume
Volume: The volume is constant in the isovolumetric stages of cardiac cycle. In the early ejection phase of systole the volume of the ventricle decreases rapidly and then the rate of emptying slows down. In a medium size dog its value is about 60ml (ESV, end systolic volume). During diastole a rapid filling phase is followed by reduced filling (EDV, end diastolic volume), and its about 140 ml. The difference of EDV and ESV is the stroke volume (SV), and its about 80 ml.
Valves:
Valves: Semilunar valves (aorta and pulmonary artery) are closed during diastole and open during systole, while the cuspid valves (from atria) are open during diastole and closed during systole.
Heart sounds:
-
1st sound: During and just after the closure of the cuspidal valves (systolic heart sound). Three components:
- Vibration of muscle contraction (weak)
- Turbulence of blood due to closure of the valves (most pronounces) o Turbulence of blood caused by fast ejection (weak)
-
2nd sound: Generating during and just after the closure of the semilunar valves (diastolic heart sound). Very much pronounced in large animals.
- First the aortic valve closed, followed by the semilunar valves of the pulmonary artery.
- 3rd sound: Originates from the rapid filling of the ventricle
- 4th sound: Result of the turbulent flow caused by the atrial contraction (may lack).
Jugular pressure:
Jugular pressure: With the sudden onset of cardiac relaxation at the beginning of the diastole the basis of the heart moves cranially and this increases the pressure in the jugular vein.
