2.4 Phases of Heartbeat

The phases of a heartbeat, also known as the cardiac cycle, involve the rhythmic contraction (systole) and relaxation (diastole) of the heart chambers to pump blood throughout the body.

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Cardiac Cycle: The complete sequence of events in the heart during a single heartbeat. It involves alternating periods of contraction (systole) and relaxation (diastole) of the atria and ventricles. A normal cardiac cycle lasts approximately 0.7 to 0.8 seconds.

Systole vs. Diastole:

• Systole: The phase of contraction of the heart muscle (myocardium).

• Diastole: The phase of relaxation of the heart muscle.

• Atrial systole/diastole refers to the contraction/relaxation of the atria.

• Ventricular systole/diastole refers to the contraction/relaxation of the ventricles.

• When used alone, "systole" and "diastole" typically refer to the ventricular phases.

For more on how blood pressure is measured during these phases, see Blood Pressure→.

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Phases of the Cardiac Cycle

1. Atrial Diastole (Atrial Filling)

• The atria are relaxed.
• Blood from the body enters the right atrium via the vena cavae, and oxygenated blood from the lungs enters the left atrium via the pulmonary veins.
• Initially, the atrioventricular (AV) valves (the tricuspid on the right and bicuspid/mitral on the left) are closed.
• As the atria fill with blood, the pressure inside them increases, eventually exceeding the pressure in the relaxed ventricles. This pressure difference pushes the AV valves open, allowing blood to begin passively flowing into the ventricles.

2. Atrial Systole (Ventricular Filling)

• The two atria contract simultaneously.
• This contraction actively pushes the remaining blood from the atria through the open AV valves into the still-relaxed ventricles, "topping them off".
• During this phase, the semilunar valves (the aortic and pulmonary valves) are closed.

3. Ventricular Systole (Ventricular Ejection)

• Almost immediately after atrial systole, the ventricles contract forcefully.

• The pressure inside the ventricles rises sharply, causing two key events:

  • Closing of AV Valves: The high ventricular pressure forces the tricuspid and bicuspid valves to snap shut, preventing blood from flowing back into the atria. The vibration from this closure creates the first heart sound, "lub".
  • Isovolumetric Contraction: A brief period where both AV and semilunar valves are closed, and pressure builds without volume change.
  • Opening of Semilunar Valves: As ventricular pressure continues to rise and exceeds the pressure in the aorta and pulmonary artery, it forces the semilunar valves open.

• Blood is ejected from the right ventricle into the pulmonary artery (to the lungs) and from the left ventricle into the aorta (to the rest of the body).

4. Ventricular Diastole (Ventricular Relaxation)

• The ventricles relax, and the pressure within them drops.
• The high pressure of the blood in the aorta and pulmonary artery creates a slight backflow towards the ventricles.
• This backflow causes the aortic and pulmonary semilunar valves to snap shut. The vibration from this closure creates the second heart sound, "dub". This prevents blood from returning to the ventricles.
• Isovolumetric Relaxation: The period where all valves are closed as the ventricles relax before the AV valves open again.
• As the ventricles relax, the AV valves remain closed until the pressure in the filling atria once again exceeds the ventricular pressure, and the cycle repeats.

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Summary of Valve States and Blood Flow

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Heart Sounds

• "Lub" (First Heart Sound): A low-pitched sound caused by the closure of the atrioventricular (tricuspid and bicuspid) valves at the beginning of ventricular systole.
• "Dub" (Second Heart Sound): A higher-pitched sound caused by the closure of the semilunar (aortic and pulmonary) valves at the end of ventricular systole.

Abnormalities in these sounds can indicate Heart Problems→.

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Diagram of the Cardiac Cycle

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Possible Questions and Answers

Q: What is the cardiac cycle? A: The cardiac cycle is the sequence of events during one complete heartbeat, involving the contraction (systole) and relaxation (diastole) of the heart's chambers.

Q: What is the difference between systole and diastole? A: Systole is the phase of muscular contraction of a heart chamber, where blood is pumped out. Diastole is the phase of muscular relaxation, where the chamber fills with blood.

Q: What causes the "lub" and "dub" heart sounds? A: The "lub" sound is caused by the closure of the atrioventricular (bicuspid and tricuspid) valves. The "dub" sound is caused by the closure of the semilunar (aortic and pulmonary) valves.

Q: What prevents the backflow of blood from the ventricles into the atria? A: The atrioventricular (AV) valves (tricuspid and bicuspid) close firmly during ventricular systole, preventing backflow into the atria.

Q: What prevents the backflow of blood from the major arteries back into the ventricles? A: The semilunar valves (aortic and pulmonary) close during ventricular diastole, preventing blood from flowing back into the ventricles from the aorta and pulmonary artery.

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• The cardiac cycle is the complete process of one heartbeat (approximately 0.8 seconds).
• Systole is the contraction phase; diastole is the relaxation phase.
• Atrial systole fills the ventricles with blood.
• Ventricular systole ejects blood into the aorta and pulmonary artery.
• Heart valves (AV and semilunar) open and close in response to pressure changes, ensuring one-way blood flow.
• The closing of these valves produces the characteristic "lub-dub" heart sounds.

Biological Significance: The precisely timed and coordinated cardiac cycle ensures the continuous and efficient circulation of blood, which is vital for delivering oxygen and nutrients to all body tissues and removing metabolic waste products. This process is regulated by Baroreceptors→ to maintain homeostasis.