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[lowkey🧃 @_fwhez]

🧵 THE CARDIAC CYCLE EXPLAINED FROM SCRATCH ❤️

If you've ever wondered how the heart fills with blood, pumps it, and prepares for the next beat, this thread is for you.

Let's break down the cardiac cycle step by step.👇

1/ The cardiac cycle is the sequence of events that occurs during one complete heartbeat.

It consists of two major phases:
• Diastole (relaxation and filling)
• Systole (contraction and ejection)

One heartbeat lasts approximately 0.8 seconds in a healthy adult with a heart rate of 75 beats per minute.

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2/ Before understanding the cardiac cycle, you must understand blood flow.

The pathway is:

Body → Superior & Inferior Vena Cava → Right Atrium → Tricuspid Valve → Right Ventricle → Pulmonary Valve → Pulmonary Artery → Lungs → Pulmonary Veins → Left Atrium → Mitral Valve → Left Ventricle → Aortic Valve → Aorta → Body.

Every event in the cardiac cycle revolves around moving blood efficiently through this pathway.

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3/ The cycle begins with atrial diastole and ventricular diastole.

At this point:
• All four chambers are relaxed.
• Venous blood flows into the atria.
• Blood passively enters the ventricles.
• The AV valves are open.
• The semilunar valves remain closed.

About 70–80% of ventricular filling occurs passively, without the atria contracting.

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4/ Next comes atrial systole.

The SA node generates an electrical impulse, causing both atria to contract.

This contraction pushes the remaining 20–30% of blood into the ventricles.

This final portion is called the atrial kick.

Patients with atrial fibrillation lose this atrial kick, reducing ventricular filling.

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5/ Once the ventricles are full, ventricular systole begins.

The electrical impulse reaches the ventricles through:
• AV node
• Bundle of His
• Right and Left Bundle Branches
• Purkinje Fibres

This coordinated conduction ensures both ventricles contract almost simultaneously.

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6/ The first event of ventricular contraction is isovolumetric contraction.

Here:
• Ventricular pressure rises rapidly.
• AV valves close.
• Semilunar valves remain closed.
• No blood leaves the ventricles yet.

The closure of the AV valves produces the first heart sound (S₁), commonly heard as "lub."

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7/ As ventricular pressure continues to increase, it eventually exceeds the pressure in the aorta and pulmonary artery.

The semilunar valves open.

This marks the beginning of the ventricular ejection phase.

Blood is pumped:
• Right ventricle → Pulmonary artery.
• Left ventricle → Aorta.

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8/ The left ventricle generates significantly higher pressure than the right ventricle.

Why?

Because it must pump blood throughout the entire body.

The right ventricle only pumps blood to the lungs, which require much lower pressure.

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9/ After ejection, the ventricles begin to relax.

This is known as isovolumetric relaxation.

During this phase:
• Semilunar valves close.
• AV valves remain closed.
• Ventricular pressure falls rapidly.
• Ventricular volume remains unchanged.

Closure of the semilunar valves produces the second heart sound (S₂), heard as "dub."

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10/ Once ventricular pressure becomes lower than atrial pressure, the AV valves reopen.

Blood once again flows from the atria into the ventricles.

A new cardiac cycle begins.

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11/ The cardiac cycle is closely linked to the ECG.

• P wave → Atrial depolarization → Atrial systole.
• QRS complex → Ventricular depolarization → Ventricular systole.
• T wave → Ventricular repolarization → Ventricular relaxation.

Understanding this relationship makes ECG interpretation much easier.

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12/ Clinically, disturbances in the cardiac cycle lead to disease.

Examples include:
• Valve stenosis or regurgitation.
• Heart failure.
• Arrhythmias.
• Myocardial infarction.
• Cardiomyopathies.

A nurse who understands the cardiac cycle can better recognize abnormal heart sounds, ECG changes, and signs of poor cardiac output.

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[lowkey🧃 @_fwhez]

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