19.3 Cardiac Cycle - Anatomy and Physiology 2e

Learning Objectives

• Describe 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.

Overview of the Cardiac Cycle

• The cardiac cycle starts with the contraction of the atria and ends with ventricular relaxation.
• Systole: Period of contraction pumping blood into circulation.
• Diastole: Period of relaxation where chambers fill with blood.
• Both atria and ventricles undergo systole and diastole to ensure efficient blood pumping.

Phases of the Cardiac Cycle

Pressures and Flow

• Blood flows according to pressure gradients, from regions of higher pressure to lower pressure.
• During diastole, blood flows into the atria from veins, with atria filling the ventricles passively.
• Atrial systole increases pressure, pushing remaining blood into ventricles.
• Ventricular systole pumps blood into the pulmonary trunk and aorta.

Atrial Systole and Diastole

• Occurs after atrial depolarization (P wave on ECG).
• Atrial contraction ("atrial kick") adds 20-30% to ventricular filling.
• Ends before ventricular systole begins.

Ventricular Systole

• Follows ventricular depolarization (QRS complex on ECG).
• Isovolumic contraction: Initial phase where pressure rises but blood volume remains the same.
• Ventricular ejection phase: Blood pumped as ventricular pressure exceeds that in the pulmonary trunk and aorta.
• Stroke volume: The amount of blood ejected (~70-80 mL).
• End Systolic Volume (ESV): Blood remaining in ventricles (~50-60 mL).

Ventricular Diastole

• Follows ventricular repolarization (T wave on ECG).
• Isovolumic relaxation phase: Pressure falls, semilunar valves close, atrioventricular valves remain closed.
• Late ventricular diastole: Pressure decreases further, atrioventricular valves open, blood flows into ventricles.

Heart Sounds

• S1 ("lub"): Closure of atrioventricular valves during ventricular contraction.
• S2 ("dub"): Closure of semilunar valves during ventricular diastole.
• S3 and S4: Can indicate certain health conditions if heard in adults.
• Murmurs: Unusual sounds due to turbulent blood flow, graded on a scale of 1 to 6.

ECG Correlation

• P wave: Atrial depolarization.
• QRS complex: Ventricular depolarization.
• T wave: Ventricular repolarization.

Auscultation

• Using a stethoscope to listen to heart sounds.
• Proper placement over the chest improves the detection of heart sounds and murmurs.

Summary

The cardiac cycle is a coordinated sequence of electrical and mechanical events ensuring efficient blood flow. Understanding these phases highlights the balance and functionality within the heart's operation, crucial for maintaining circulatory health.