2.4 Phases of Heartbeat

The heartbeat is a continuous, rhythmic cycle of contraction and relaxation of the heart chambers. This cycle is called the cardiac cycle. It consists of alternating phases of systole (contraction) and diastole (relaxation) that ensure blood is pumped efficiently through the body.

Definition

Cardiac Cycle: The cardiac cycle is a sequence of one complete heartbeat. It includes the alternating relaxation and contraction of the atria and ventricles, along with the opening and closing of heart valves. The normal cardiac cycle is about 0.7 to 0.8 seconds.

Systole: Systole means "to contract." It refers to the contraction phase of the heart chambers.

Diastole: Diastole means "to dilate." It refers to the relaxation phase of the heart chambers.

Main Idea

• Atrial diastole — atria relax and fill with blood
• Atrial systole — atria contract and push blood into ventricles
• Ventricular systole — ventricles contract and pump blood out
• Ventricular diastole — ventricles relax and fill with blood

These phases ensure that blood flows in one direction and that the heart pumps efficiently.

Core Concept

• Cardiac cycle = one complete heartbeat
• **Systole** = contraction
• **Diastole** = relaxation
• Atrial systole = atria contract
• Ventricular systole = ventricles contract
• Atrial diastole = atria relax
• Ventricular diastole = ventricles relax
• Valves open and close in response to pressure changes
• Heart sounds = "lub" (AV valves close) and "dub" (semilunar valves close)
• Normal cardiac cycle = 0.7 to 0.8 seconds
• Heart muscles rest = 0.1 to 0.3 seconds between beats

Detailed Explanation

Phases of the Cardiac Cycle

1. Atrial Diastole

• Blood enters the right atrium from the body through the vena cavae.
• At first, the bicuspid and tricuspid valves are closed.
• As the atria fill with blood, pressure in them rises.
• Eventually, pressure becomes greater than that in the relaxed ventricles.
• The AV valves are pushed open.
• Blood flows from atria to ventricles.

2. Atrial Systole

• The two atria contract simultaneously.
• Blood is pushed through the atrioventricular valves into the still relaxed ventricles.
• In this phase:
• Semilunar valves are closed.
• Tricuspid and bicuspid valves are open.

3. Ventricular Systole

• Almost immediately, the ventricles contract.
• Pressure in the ventricles rises.
• This pressure closes the atrioventricular valves, preventing blood from returning to the atria.
• Pressure forces open the semilunar valves of the aorta and pulmonary artery.
• Blood enters these vessels.
• In this phase:
• Tricuspid and bicuspid valves are closed.
• Semilunar valves are open.

4. Ventricular Diastole

• The high pressure developed in the aorta and pulmonary artery tends to force some blood back towards the ventricles.
• This backflow closes the semilunar valves of the aorta and pulmonary artery.
• Backflow in the heart is prevented.
• In this phase:
• Bicuspid and tricuspid valves are open.
• Aortic semilunar valve and pulmonary semilunar valve are closed.

Summary Table of Cardiac Cycle Phases

Duration of Cardiac Cycle

• The normal cardiac cycle is 0.7 to 0.8 seconds.
• This depends on the capability of cardiac muscle to contract.
• The heart muscles rest 0.1 to 0.3 seconds between beats.

Timing of Cardiac Cycle (at 75 beats/min):

Heart Sounds

When a stethoscope is used to listen to the heart sounds, distinct sounds are normally heard.

Note: "Lub" is also written as "lubb" and "dub" as "dupp."

Intermediate Level

Detailed Phases of the Cardiac Cycle

Pressure Changes During Cardiac Cycle

Valve Actions During Cardiac Cycle

Cardiac Cycle and Heart Rate

Formula / Mathematical Form

Cardiac Output:

CO = HR × SV

• CO = cardiac output (mL/min)
• HR = heart rate (beats/min)
• SV = stroke volume (mL/beat)

Cardiac Cycle Duration:

Duration = 60Heart Rate

Example: If heart rate = 75 beats/min: Duration = 6075 = 0.8 sec

Solved Example

Given: Heart rate = 75 beats/min

Required: Duration of cardiac cycle

Formula: Duration = 60Heart Rate

Substitution: Duration = 6075

Calculation: Duration = 0.8 sec

Final Answer: The duration of the cardiac cycle is 0.8 seconds.

Important Points

• 1. Cardiac cycle is one complete heartbeat.
• 2. Normal cardiac cycle is 0.7 to 0.8 seconds.
• 3. Systole means contraction.
• 4. Diastole means relaxation.
• 5. Atrial diastole: atria relax and fill with blood.
• 6. Atrial systole: atria contract; blood pushed to ventricles.
• 7. Ventricular systole: ventricles contract; blood pumped out.
• 8. Ventricular diastole: ventricles relax; blood fills.
• 9. AV valves open during diastole; close during systole.
• 10. Semilunar valves open during systole; close during diastole.
• 11. First heart sound ("lub") = AV valves closing.
• 12. Second heart sound ("dub") = semilunar valves closing.
• 13. Heart muscles rest 0.1 to 0.3 seconds between beats.
• 14. Cardiac cycle duration depends on heart rate.
• 15. At 75 beats/min, cardiac cycle = 0.8 seconds.

Remember

• - Cardiac cycle = one heartbeat
• - **Systole** = contraction
• - **Diastole** = relaxation
• - Atrial diastole = atria relax; fill with blood
• - Atrial systole = atria contract; push blood to ventricles
• - Ventricular systole = ventricles contract; pump blood out
• - Ventricular diastole = ventricles relax; fill with blood
• - AV valves = open during diastole; close during systole
• - Semilunar valves = open during systole; close during diastole
• - **"Lub"** = AV valves closing
• - "Dub" = semilunar valves closing
• - Normal cardiac cycle = 0.7–0.8 sec
• - Heart rest = 0.1–0.3 sec between beats

Common Mistakes

• 1. Confusing systole and diastole — Systole = contraction; diastole = relaxation.
• 2. Mixing up the phases — Atrial diastole → atrial systole → ventricular systole → ventricular diastole.
• 3. Thinking AV valves open during systole — AV valves close during systole.
• 4. Thinking semilunar valves open during diastole — Semilunar valves close during diastole.
• 5. Confusing "lub" and "dub" — "Lub" = AV valves closing; "dub" = semilunar valves closing.
• 6. Forgetting the duration of the cardiac cycle — 0.7 to 0.8 seconds.
• 7. Believing the heart rests between beats — Heart muscles rest 0.1–0.3 seconds.
• 8. Mixing up the pressure changes — Ventricular pressure rises during systole.
• 9. Thinking the atria and ventricles contract at the same time — Atria contract first, then ventricles.
• 10. Confusing cardiac cycle with cardiac output — Cardiac cycle is one heartbeat; cardiac output is volume per minute.

Quick Revision

The cardiac cycle is one complete heartbeat, lasting 0.7 to 0.8 seconds. It consists of four phases: atrial diastole (atria relax and fill with blood), atrial systole (atria contract and push blood into ventricles), ventricular systole (ventricles contract and pump blood out), and ventricular diastole (ventricles relax and fill with blood). AV valves open during diastole and close during systole. Semilunar valves open during systole and close during diastole. The first heart sound ("lub") is caused by AV valves closing. The second heart sound ("dub") is caused by semilunar valves closing. The heart muscles rest 0.1 to 0.3 seconds between beats.

60-Second Revision

• Cardiac cycle = one heartbeat
• Systole = contraction; diastole = relaxation
• Atrial diastole = atria relax; fill
• Atrial systole = atria contract; push blood
• Ventricular systole = ventricles contract; pump out
• Ventricular diastole = ventricles relax; fill
• AV valves = open during diastole; close during systole
• Semilunar valves = open during systole; close during diastole
• "Lub" = AV valves closing
• "Dub" = semilunar valves closing
• Normal cardiac cycle = 0.7–0.8 sec
• Warning: AV valves close during systole; semilunar valves close during diastole

NEXT TOPIC

• Chapter:
• 2. Blood Circulatory System of Man

• Topic:
• 2.5 CONDUCTING SYSTEM OF THE HEART

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