Cardiac Cycle Overview

Overview

This lecture explains the mechanical phases of the cardiac cycle, including key terminology, phase sequence, pressure gradients, valve mechanics, and heart sounds.

The Cardiac Cycle Basics

• The cardiac cycle includes all physical events in one heartbeat (about 0.8 seconds).
• Systole refers to contraction (atria or ventricles), while diastole refers to relaxation.
• The quiescent period is when both atria and ventricles are relaxed (about 0.4 seconds per cycle).
• Heart beats roughly 70–75 times per minute.

Phases and Valve Function

• Blood flows from high-pressure to low-pressure areas, driven by heart contraction/relaxation.
• Atrial systole is short; ventricular systole is longer.
• When both atria and ventricles are relaxed, blood trickles into atria and then ventricles.
• As atria contract, blood moves into the ventricles, increasing ventricular pressure.

Isovolumetric Phases

• Isovolumetric contraction: all valves closed, ventricles begin to contract, no volume change.
• Isovolumetric relaxation: all valves closed, ventricles begin to relax, no volume change.
• Both isovolumetric phases are brief and crucial for proper valve and pressure function.

Ventricular Ejection and Pressure Gradients

• Ventricular pressure must exceed aortic and pulmonary trunk pressure to open the semilunar valves.
• Blood is ejected into arteries during ventricular systole.
• Drop in ventricular pressure causes brief backflow, closing semilunar valves (dicrotic notch).

Relationship to Electrical Activity (EKG)

• P wave: atria depolarize and contract (atrial systole).
• QRS complex: ventricles depolarize and contract (ventricular systole).
• T wave: ventricles repolarize and begin to relax (ventricular diastole).
• Mechanical events follow electrical events shown on EKG.

Heart Sounds

• "Lub" sound: closing of atrioventricular (AV) valves.
• "Dub" sound: closing of semilunar valves.
• Heart murmurs are often due to turbulent blood flow, not just valve problems.

Key Terms & Definitions

• Systole — contraction phase of a heart chamber.
• Diastole — relaxation phase of a heart chamber.
• Quiescent period — time when whole heart is relaxed.
• Isovolumetric contraction/relaxation — phases when all heart valves are closed, and no blood volume changes in ventricles.
• Pressure gradient — difference in pressure causing blood to move from high to low pressure.
• Dicrotic notch — small dip in aortic pressure due to brief backflow when semilunar valves close.

Action Items / Next Steps

• Review the cardiac cycle diagram in your textbook.
• Look up the term "dicrotic notch" for further clarification.
• Practice associating EKG waves with corresponding mechanical heart events.