The Cardiac Cycle, Animation

Summary

The cardiac cycle is the sequence of events in each heartbeat, divided into two main phases: systole (ventricular contraction) and diastole (ventricular relaxation). It begins with atrial depolarization, pushing blood into the ventricles. As ventricles contract, the AV valves close (producing the first heart sound, S1), followed by semilunar valve opening for blood ejection. After ejection, the semilunar valves close (producing the second heart sound, S2), and the ventricles relax as they refill with blood. The cycle repeats, ensuring efficient blood flow through the heart.

Takeaways

• 🔁 The cardiac cycle is a repeating sequence of events with every heartbeat, divided into systole and diastole.
• 📈 Systole is the ventricular contraction phase, while diastole is the relaxation phase.
• 💧 Blood flows from areas of higher pressure to lower pressure.
• 📉 Contraction increases pressure within a chamber, and relaxation decreases it.
• 🚪 The AV valves open when atrial pressure is higher than ventricular pressure and close when reversed.
• 🔄 Semilunar valves open when ventricular pressure exceeds aortic/pulmonary pressure and close when reversed.
• 🔊 The SA node's firing initiates the cycle, leading to atrial depolarization represented by the P-wave on the ECG.
• 🫀 Atrial contraction increases atrial pressure, pushing blood into the ventricles.
• 📉 Atrial contraction is a minor contributor to ventricular filling as the ventricles are nearly full from passive flow.
• 💥 The first heart sound, S1, marks the beginning of systole as the AV valves close.
• 🏋️‍♂️ Isovolumetric contraction is the phase where the ventricles contract without changing volume.
• 🚀 Ventricular ejection occurs when ventricular pressure exceeds that of the aorta and pulmonary artery.
• 📉 The T-wave indicates the beginning of ventricular repolarization, reducing the force of ejection.
• 💥 The second heart sound, S2, marks the end of systole and the beginning of diastole as the semilunar valves close.
• 🛑 Isovolumetric relaxation is the initial part of diastole where the ventricles relax with all valves closed.
• 🌀 Passive ventricular filling begins when ventricular pressures drop below atrial pressures, allowing blood to flow into the ventricles.
• 🫁 Atrial contraction completes the filling phase, preparing for the next cycle.

Q & A

What are the two major phases of the cardiac cycle?

The two major phases of the cardiac cycle are systole, which refers to ventricular contraction, and diastole, which refers to ventricular relaxation.

What triggers the cardiac cycle to begin?

The cardiac cycle begins with the firing of the SA node, which stimulates the atria to depolarize, represented by the P-wave on an ECG.

What is the significance of the P-wave in the cardiac cycle?

The P-wave represents atrial depolarization, which occurs just before atrial contraction, forcing blood into the ventricles.

What causes the first heart sound (S1) to occur?

The first heart sound (S1) occurs when the AV valves close as atrial pressure falls and the pressure gradient across the valves reverses, marking the start of systole.

What is isovolumetric contraction and when does it occur?

Isovolumetric contraction is a phase where the ventricles contract with all valves closed, so no blood is ejected and the ventricular volume remains unchanged. It occurs just after the AV valves close and before the semilunar valves open.

When does ventricular ejection begin?

Ventricular ejection begins when the pressure in the ventricles exceeds the pressure in the aorta and pulmonary artery, causing the semilunar valves to open and blood to be ejected from the ventricles.

What marks the end of systole and the beginning of diastole?

The end of systole and the beginning of diastole are marked by the closing of the semilunar valves when ventricular pressures drop below aortic and pulmonary pressures, producing the second heart sound (S2).

What happens during the isovolumetric relaxation phase?

During isovolumetric relaxation, the ventricles relax with all valves closed. Ventricular pressure drops rapidly, but the volume of the ventricles remains unchanged.

How does ventricular filling occur?

Ventricular filling occurs when ventricular pressures fall below atrial pressures, causing the AV valves to open. Blood flows passively into the ventricles, and atrial contraction completes the filling process.

What is the role of atrial contraction in ventricular filling?

Atrial contraction provides a final push of blood into the ventricles, but the majority of ventricular filling occurs passively through the open AV valves before the atria contract.