Cardiac Cycle Overview

Overview

This lecture explains the cardiac cycle, detailing the sequence of electrical and mechanical events that occur during one heartbeat, and connects them to changes in pressure, volume, and heart sounds.

Cardiac Cycle Basics

• The cardiac cycle refers to all events from the start of one heartbeat to the start of the next.
• Blood returns to the heart via the atria: left atrium from lungs, right atrium from body.
• Atria contract to push blood into ventricles; ventricles contract to push blood out of the heart.

Electrical Events (ECG)

• The P wave represents atrial depolarization, initiating atrial contraction (atrial systole).
• The QRS complex represents ventricular depolarization, triggering ventricular contraction (ventricular systole).
• The T wave shows ventricular repolarization, signaling ventricular relaxation.

Mechanical Events and Pressures

• Atrial contraction increases atrial pressure and causes blood to flow into ventricles, increasing ventricular volume.
• Ventricular contraction causes a sharp rise in ventricular pressure.
• Isovolumetric contraction is when ventricles contract with all valves closed, so volume remains constant while pressure rises.
• Once ventricular pressure exceeds aortic pressure (~80 mmHg), the semilunar valve opens and blood is ejected into the aorta, reducing ventricular volume.
• After ejection, ventricles relax, pressure falls, and the semilunar valves close, leading to isovolumetric relaxation.
• When ventricular pressure drops below atrial pressure, atrioventricular valves open for ventricular filling, and the cycle repeats.

Heart Sounds (Phonocardiogram)

• The first heart sound ("lub") occurs when atrioventricular valves close during ventricular contraction.
• The second heart sound ("dub") happens when semilunar valves close after ventricular relaxation.

Key Terms & Definitions

• Cardiac Cycle — Complete sequence of heart events from one heartbeat to the next.
• Atria — Upper chambers of the heart that receive blood returning to the heart.
• Ventricles — Lower chambers of the heart that pump blood out.
• Systole — Contraction phase of the heart chamber.
• Diastole — Relaxation phase of the heart chamber (implied, though not explicitly named).
• Depolarization — Electrical activation causing muscle contraction.
• Repolarization — Return to resting electrical state, muscle relaxation.
• Isovolumetric Contraction/Relaxation — Phases where ventricular volume does not change due to all valves being closed.

Action Items / Next Steps

• Review the flow of blood through the heart if unclear on the pathway.
• Pause and rewatch sections to reinforce understanding of each phase.