19.3 Cardiac Cycle - Anatomy and Physiology 2e | OpenStax

Learning Objectives

• Understand the relationship between blood pressure and blood flow.
• Summarize the events of the cardiac cycle.
• Compare atrial and ventricular systole and diastole.
• Relate heart sounds detected by auscultation to the action of heart valves.

Definition of Cardiac Cycle

• The cardiac cycle begins with atrial contraction and ends with ventricular relaxation.
• Systole: Contraction phase where the heart pumps blood into circulation.
• Diastole: Relaxation phase where chambers fill with blood.
• Both atria and ventricles undergo systole and diastole.

Phases of the Cardiac Cycle

Atrial Systole and Diastole

• Atrial contraction follows depolarization (P wave in ECG).
• Pressure rises within atria, pumping blood into ventricles through open atrioventricular valves.
• Contributes 20-30% of ventricular filling (atrial kick).
• Lasts about 100 ms and ends prior to ventricular systole.

Ventricular Systole

• Follows depolarization of ventricles (QRS complex in ECG).
• Two phases lasting a combined 270 ms.
• Initial phase: Isovolumic contraction where ventricular pressure rises but no blood is ejected; closes tricuspid and mitral valves.
• Second phase: Ventricular ejection where pressure exceeds pulmonary trunk and aorta, opening semilunar valves.
• Stroke volume: 70-80 mL with an end diastolic volume of 130 mL and an end systolic volume of 50-60 mL.

Ventricular Diastole

• Follows repolarization (T wave in ECG).
• Two phases lasting 430 ms.
• Early phase: Isovolumic ventricular relaxation; pressure drops below pulmonary trunk and aorta, closing semilunar valves.
• Late phase: Blood flows from atria to ventricles as atrioventricular valves open.

Heart Sounds

• S1 (Lub): Closing of atrioventricular valves during ventricular contraction.
• S2 (Dub): Closing of semilunar valves during ventricular diastole.
• Additional sounds (S3, S4) can indicate abnormal conditions.
• S3: May indicate congestive heart failure if heard in older adults.
• S4: Indicates stiff or hypertrophic ventricle.

Murmurs

• Unusual heart sounds from turbulent blood flow.
• Graded 1-6 based on severity.
• Auscultation involves listening through a stethoscope to assess.

Stethoscope Placement

• Proper placement is essential for hearing valve sounds effectively.

Conclusion

Understanding the cardiac cycle is crucial for assessing heart function and diagnosing potential cardiac issues. Auscultation and ECG are key tools in evaluating the heart's health.