🫀 [#1] CICLO CARDÍACO: FASES DO CICLO CARDÍACO | MK Fisiologia

Summary

In this video, Mirian Kurauti from MK Fisiologia explains the cardiac cycle, detailing both the electrical and mechanical events that occur during a heartbeat. She reviews how action potentials originate in the sinoatrial node, propagate through the atria, atrioventricular node, His-Purkinje system, and ventricles, generating the phases of systole and diastole. The video describes the mechanical consequences, including atrial contraction, ventricular isovolumetric contraction, ejection, and relaxation, highlighting the role of heart valves. Mirian also introduces the four main phases of the cardiac cycle and their subdivisions, preparing viewers to understand the Wiggers diagram in upcoming content.

Takeaways

• 😀 The cardiac cycle involves all electrical and mechanical events that occur during a heartbeat.
• 😀 Electrical events in the heart can be observed through an electrocardiogram (ECG), where the P wave, QRS complex, and T wave represent atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively.
• 😀 Atrial repolarization is not visible in the ECG because it occurs during the ventricular depolarization (QRS complex).
• 😀 The contraction phase of the cardiac cycle is called systole, and the relaxation phase is called diastole.
• 😀 The electrical events (action potentials) determine the contraction (systole) and relaxation (diastole) of both the atria and ventricles.
• 😀 The mechanical events in the cardiac cycle begin with atrial contraction (systole), which pumps blood into the ventricles, increasing ventricular pressure and closing the atrioventricular valves.
• 😀 After atrial depolarization and contraction, the action potential is transmitted through the AV node, His bundle, right and left bundle branches, and Purkinje fibers to the ventricles, causing ventricular contraction (systole).
• 😀 During ventricular contraction (systole), all heart valves are closed, and this is called isovolumetric contraction.
• 😀 As ventricular pressure rises, the semilunar valves open, allowing blood to be ejected into the pulmonary artery and aorta (ejection phase).
• 😀 Once ventricular repolarization begins, the ventricles relax (diastole), and the pressure decreases. When ventricular pressure falls below arterial pressure, the semilunar valves close, marking the beginning of isovolumetric relaxation.
• 😀 The heart cycle is divided into four main phases: ventricular filling, isovolumetric contraction, ventricular ejection, and isovolumetric relaxation, all defined by the opening and closing of the heart valves.
• 😀 Some textbooks further divide the ventricular filling and ejection phases into 'rapid' and 'slow' phases, depending on how quickly the blood enters or exits the ventricles.
• 😀 The Wiggers diagram can be used to visually represent the phases of the cardiac cycle, though it may seem complex at first.
• 😀 A review of the key points: the cardiac cycle involves both electrical and mechanical events, with the heart valves’ movements marking the start and end of each phase.

Q & A

What is the cardiac cycle?

The cardiac cycle refers to all the electrical and mechanical events that occur in the heart during a single heartbeat.

What are the electrical events in the cardiac cycle?

The electrical events involve the generation of action potentials at the sinoatrial (SA) node, conduction through atrial fibers, the atrioventricular (AV) node, His bundle, right and left bundle branches, Purkinje fibers, and finally, ventricular fibers.

What do the different waves in the electrocardiogram (ECG) represent?

The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization.

Why is atrial repolarization not seen on an ECG?

Atrial repolarization occurs during ventricular depolarization, which is represented by the QRS complex, masking the atrial repolarization wave.

What is the difference between systole and diastole?

Systole is the phase of contraction of the heart muscles (atria or ventricles), while diastole is the phase of relaxation.

How do electrical events influence the mechanical events of the heart?

The electrical events determine the contraction (systole) and relaxation (diastole) of the atrial and ventricular muscles, thus controlling the mechanical actions of the heart.

What happens during atrial systole?

During atrial systole, the atria contract, pushing blood into the ventricles, completing their filling.

What is isovolumetric contraction?

Isovolumetric contraction occurs when the ventricles contract but there is no blood ejection because the heart valves are closed, and the volume of blood in the ventricles does not change.

What causes the semilunar valves to open?

The semilunar valves open when the pressure inside the ventricles becomes greater than the pressure in the arteries (pulmonary artery and aorta), allowing blood to be ejected.

What happens during isovolumetric relaxation?

Isovolumetric relaxation occurs when the ventricles are relaxing but there is no change in blood volume inside the ventricles because all valves are closed.

What are the four phases of the cardiac cycle?

The four phases are ventricular filling (when AV valves open), isovolumetric contraction (when AV and semilunar valves are closed), ventricular ejection (when semilunar valves open), and isovolumetric relaxation (when all valves are closed).

What is the difference between rapid and slow filling phases?

Rapid filling occurs when blood flows quickly from the atria to the ventricles immediately after the AV valves open, while slow filling happens when blood continues to flow into the ventricles more slowly due to ongoing venous return.

What is the purpose of the Wiggers diagram in the cardiac cycle?

The Wiggers diagram visually represents the electrical and mechanical events of the cardiac cycle, helping to correlate the timing of heart actions, such as pressure changes and valve movements, with the ECG.