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The Cardiac Cycle

The Cardiac Cycle

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The Cardiac Cycle

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  1. The Cardiac Cycle

  2. The Cardiac Cycle • The repeating pattern of contraction (systole) and relaxation (diastole) of the heart • Duration of cardiac cycle = 0.8 seconds • Diastole longer than systole • Ventricular contraction follows atrial contraction (0.1 to 0.2 second later)…why?

  3. The Cardiac Cycle • The end diastolic volume: the total volume of blood in the ventricles at the end of diastole (120 ml) • Stroke volume is the volume of blood pumped by each ventricle per beat (70 ml) • Residual volume: amount of blood left in each ventricle at the end of systole (50 ml)

  4. The Cardiac CycleIsovolumetric ventricular contraction • Ventricles contract • Ventricular pressure: increasing • Ventricular volume: no change • AV valves: closed.. prevent backflow of blood • Semilunar valves: closed (P in ventricles < P in vessels) • Heart sounds: 1st heart sound • ECG: QRS complex

  5. The Cardiac CycleEjection phase • Ventricular pressure: increasing > the pressure in the aortic and pulmonary vessels • Left ventricular pressure up to 120 mmHg • Right ventricular pressure up to 25 mmHg • Ventricular volume: decreasing • Semilunar valves: open • AV valves: closed.. prevent backflow of blood

  6. The Cardiac CycleIsovolumetric relaxation • Ventricles relax • Ventricular pressure: decreasing • Ventricular volume: no change • AV valves: closed • Semilunar valves: closed • Heart sounds: 2nd heart sound • ECG: T wave

  7. The Cardiac CycleRapid filling of the ventricles • Ventricular pressure: below atrial pressure ( slightly above zero) • Ventricular volume: increasing • AV valves: open when pressure in the atria> the pressure in the ventricles • Semilunar valves: closed • Passive ventricular filling via AV valves (80%)

  8. The Cardiac CycleAtrial systole • Active filling of the ventricles (20%) • Ventricular volume: slight rise • Ventricular pressure: slight rise • Semilunar valves: closed • AV valves: open • ECG: P wave

  9. The Cardiac Cycle • Isovolumetric contraction • Ejection phase • Isovolumetric relaxation • Rapid filling of the ventricles • Atrial systole

  10. Heart Sounds • The first heart sound: • Cause: closure of the AV valves • The second heart sound: • Cause: closure of the semilunar valves

  11. Cardiac Output

  12. Cardiac Output • Cardiac output is the volume of blood pumped by each ventricle per minute • CO = Stroke volume x Heart rate (L/min) (ml/beat) (beat/min) = 70 X 70 = 4900 ml/min = 5 L/min • Normal cardiac output (CO) = 5 L/min

  13. Cardiac Output

  14. Cardiac OutputRegulation of Heart Rate • Sympathetic stimulation •  HR (positive chronotropic effect) •  CO • Parasympathetic stimulation •  HR •  CO • Cardiac centers in the medulla oblangata

  15. Cardiac OutputRegulation of Stroke Volume End Diastolic Volume (EDV) • Frank- Starling Law of the Heart  venous return   EDV   length of cardiac muscle (stretch)  force of contraction  stroke volume   cardiac output

  16. Cardiac OutputRegulation of Stroke Volume • Positive ionotropic effect  strength of contraction • Sympathetic stimulation • Adrenaline • Negative ionotropic effect   strength of contraction • Parasympathetic stimulation • Acetylcholine • Vagal stimulation