Cardiac Cycle Lecture Notes

Introduction

• The cardiac cycle describes how heart muscles contract during one heartbeat.
• Heart is divided into left and right sides, separated by the septum.
• The left and right sides mirror each other, so focus is primarily on the left side.

General Structure of the Heart

• Left Atrium and Ventricle: Focus on these parts using generalized terms: Atrium, Ventricle, Vein, and Artery.

Blood Flow Process

Passive Filling

• Blood flows from the vein into the atrium.
• Atrioventricular Valve: Slightly open, allowing blood to trickle into the ventricle passively (muscles are relaxed).

Atrial Systole

• Definition: Wall of the atrium contracts to push blood into the ventricles.
• Pressure Dynamics: Pressure in the atrium becomes higher than in the ventricles, facilitating blood flow.

Ventricular Systole

• Definition: Wall of the ventricles contracts.
• Stage 1: Initial contraction increases pressure in the ventricles, closing the atrioventricular valves (first heart sound "Lub").
  • Prevents backflow into the atrium.
• Stage 2: Further contraction increases ventricular pressure beyond arterial pressure, opening the semilunar valves and allowing blood to flow into the arteries.

Ventricular Diastole

• Definition: Ventricular walls relax, reducing pressure below arterial levels.
• Action: Semilunar valves close to prevent arterial backflow (second heart sound "Dub").
• Further Relaxation: Pressure in ventricles drops below atrium, reopening atrioventricular valves, and blood trickles into ventricles.

Summary of Cardiac Cycle

• Cycle Order: Atrial systole -> Ventricular systole -> Ventricular diastole -> Repeat.
• Pressure Importance: Different pressures cause valves to open/close, controlling blood flow direction.
• Heart Sounds: "Lub-Dub" caused by valve actions, not muscle movements.

Key Points to Remember

• Pressure differences are crucial in valve operation.
• Heart sounds are due to valve closure, not muscle activity.