Cardiac Cycle – Complete Notes
Introduction
The cardiac cycle refers to the sequence of mechanical and electrical events that repeat with every
heartbeat. It ensures the continuous pumping of blood throughout the body. One complete cardiac
cycle lasts about 0.8 seconds in a person with a normal heart rate (75 beats per minute).
Phases of Cardiac Cycle
1. Diastole (Relaxation Phase)
During this phase, the heart muscles relax and the chambers fill with blood.
1 Atrial Diastole: Atria relax and blood flows from veins into atria.
2 Ventricular Filling: AV valves (mitral & tricuspid) open and blood flows into ventricles.
3 Atrial Systole: Atria contract and push remaining blood into ventricles.
2. Systole (Contraction Phase)
During this phase, the ventricles contract and pump blood out of the heart.
1 Isovolumetric Contraction: Ventricles contract, all valves closed, pressure increases.
2 Ventricular Ejection: Semilunar valves open and blood is ejected into aorta and pulmonary
artery.
Heart Sounds
1 S1 (Lub): Closure of AV valves.
2 S2 (Dub): Closure of semilunar valves.
Pressure Changes
1 Atrial pressure is lowest.
2 Ventricular pressure rises during systole.
3 Aortic pressure increases during ejection.
Clinical Importance
1 Abnormal heart sounds indicate valve disorders.
2 Irregular cardiac cycle can lead to arrhythmias.
3 Important in understanding ECG and blood pressure.
Quick Summary
Introduction
The cardiac cycle refers to the sequence of mechanical and electrical events that repeat with every
heartbeat. It ensures the continuous pumping of blood throughout the body. One complete cardiac
cycle lasts about 0.8 seconds in a person with a normal heart rate (75 beats per minute).
Phases of Cardiac Cycle
1. Diastole (Relaxation Phase)
During this phase, the heart muscles relax and the chambers fill with blood.
1 Atrial Diastole: Atria relax and blood flows from veins into atria.
2 Ventricular Filling: AV valves (mitral & tricuspid) open and blood flows into ventricles.
3 Atrial Systole: Atria contract and push remaining blood into ventricles.
2. Systole (Contraction Phase)
During this phase, the ventricles contract and pump blood out of the heart.
1 Isovolumetric Contraction: Ventricles contract, all valves closed, pressure increases.
2 Ventricular Ejection: Semilunar valves open and blood is ejected into aorta and pulmonary
artery.
Heart Sounds
1 S1 (Lub): Closure of AV valves.
2 S2 (Dub): Closure of semilunar valves.
Pressure Changes
1 Atrial pressure is lowest.
2 Ventricular pressure rises during systole.
3 Aortic pressure increases during ejection.
Clinical Importance
1 Abnormal heart sounds indicate valve disorders.
2 Irregular cardiac cycle can lead to arrhythmias.
3 Important in understanding ECG and blood pressure.
Quick Summary
