Cardiac Cycle Overview

Cardiac Cycle Timing

• Average duration: 0.8 seconds per cycle.
• Two heart sounds per cycle:
  • Lub (S1): Closure of the atrioventricular (AV) valves (mitral and tricuspid).
  • Dub (S2): Closure of the semilunar valves (aortic and pulmonary).

Focus on Left Side of the Heart

• Exam questions often focus on the left side.
• Pressure changes are typically represented in a graph.

Graph Components

• Y-axis: Pressure (kilopascals).
• X-axis: Time (up to 0.8 seconds).
• Lines on the graph:
  • Red line: Represents the aorta (generally high pressure).
  • Purple line: Represents the left ventricle (high pressure during systole).
  • Green line: Represents the left atrium (lower pressure).

Recognizing Lines in the Graph

• Aorta: Red line remains high due to higher pressure in arteries.
• Left Ventricle: Purple line shows high pressure during contraction, thicker wall supports higher pressure.
• Left Atrium: Green line shows lower pressure, thinner wall.

Cardiac Cycle Phases

1. Atrial Systole

• Pressure in the left atrium > pressure in the left ventricle.
• Blood flows from the left atrium to the left ventricle.
• Semilunar valves are closed to prevent backflow.

2. Isovolumetric Contraction

• Left ventricle contracts, raising pressure.
• Pressure in the left ventricle > pressure in the left atrium.
• AV valves close (first heart sound, "Lub").
• Pressure in the aorta > pressure in the left ventricle, semilunar valves closed.

3. Ventricular Systole

• Left ventricle pressure > aorta pressure.
• Semilunar valves open, blood flows into the aorta.

4. Isovolumetric Relaxation

• Pressure in the left ventricle decreases.
• Aortic pressure > left ventricle pressure.
• Semilunar valves close (second heart sound, "Dub").

5. Ventricular Diastole

• Left atrium pressure > left ventricle pressure.
• AV valves open, blood flows into the ventricles.

Summary

• Line Identification: Aorta (red), Left Ventricle (purple), Left Atrium (green).
• Intersection Points:
  • Determine valve action (open/close).
• Understanding Graphs: Use logic to determine phases and valve actions.

Exam Tips

• Avoid abbreviations like LV or AV in exams.
• Use full terms: "left ventricle," "atrioventricular valve."