Check All That Occur During Ventricular Systole

Ventricular systole, a crucial phase in the cardiac cycle, involves a complex interplay of events that ensure efficient blood ejection from the heart. Understanding what happens during this period is essential for comprehending overall cardiovascular function.

Phases of Ventricular Systole

Ventricular systole, the contraction phase of the ventricles, is further divided into two main phases:

• Isovolumetric Contraction: This is the initial phase where the ventricles begin to contract, but no blood is ejected.
• Ventricular Ejection: This phase marks the forceful expulsion of blood from the ventricles into the pulmonary artery and aorta.

Detailed Look at Events During Ventricular Systole

1. Isovolumetric Contraction

This phase begins with the closure of the atrioventricular (AV) valves (tricuspid and mitral valves) and ends with the opening of the semilunar valves (pulmonary and aortic valves). The term "isovolumetric" indicates that the volume of blood in the ventricles remains constant during this brief period.

• Ventricular Contraction Begins:
  • The electrical impulse that started in the sinoatrial (SA) node has now traveled through the atrioventricular (AV) node, bundle of His, and Purkinje fibers, causing the ventricular myocardium to depolarize.
  • This electrical activity triggers the contraction of the ventricular muscle fibers.
• Rising Ventricular Pressure:
  • As the ventricles contract, the pressure inside the ventricles rapidly increases.
  • This pressure quickly exceeds the pressure in the atria, which are filling with blood.
• Closure of Atrioventricular (AV) Valves:
  • The increasing ventricular pressure forces the AV valves (tricuspid on the right side and mitral on the left side) to close abruptly.
  • The closure of the AV valves produces the first heart sound, often referred to as "S1" or the "lub" sound.
  • This sound signifies the beginning of systole.
• No Change in Ventricular Volume:
  • During this phase, both the AV valves and the semilunar valves (aortic and pulmonary valves) are closed.
  • Because all valves are closed, blood cannot enter or exit the ventricles, hence the term "isovolumetric."
  • The ventricular volume remains constant as the pressure rises dramatically.
• Tension Development:
  • The ventricular muscle fibers are actively developing tension, preparing to eject blood.
  • The myocardium is contracting against a closed system, which requires significant energy.
• Duration:
  • This phase is relatively short, lasting only about 0.05 seconds.

2. Ventricular Ejection

This phase starts when the ventricular pressure exceeds the pressure in the aorta and pulmonary artery, causing the semilunar valves to open, and ends when the ventricles begin to relax. During this phase, blood is ejected from the ventricles into the great arteries.

• Semilunar Valves Open:
  • As ventricular pressure continues to rise and exceeds the pressure in the aorta (approximately 80 mmHg) and the pulmonary artery (approximately 10 mmHg), the semilunar valves (aortic and pulmonary valves) open.
  • The opening of these valves allows blood to flow out of the ventricles.
• Rapid Ejection Phase:
  • Initially, there is a rapid ejection of blood from the ventricles into the aorta and pulmonary artery.
  • The force of ventricular contraction propels a large volume of blood quickly.
  • Approximately 70% of the stroke volume (the amount of blood ejected with each beat) is ejected during this rapid ejection phase.
• Reduced Ejection Phase:
  • Following the rapid ejection phase, the rate of blood ejection slows down.
  • The ventricular pressure begins to decrease as the ventricles start to repolarize and relax slightly.
  • The remaining 30% of the stroke volume is ejected during this reduced ejection phase.
• Ventricular Volume Decreases:
  • As blood is ejected, the volume of blood in the ventricles decreases significantly.
  • The ventricles do not empty completely; a small amount of blood remains in the ventricles after ejection, known as the end-systolic volume (ESV).
• Aortic Pressure Increases:
  • The ejection of blood into the aorta causes the aortic pressure to rise.
  • The aortic pressure reaches its peak during the ejection phase, creating the systolic blood pressure.
• Pulmonary Artery Pressure Increases:
  • Similarly, the ejection of blood into the pulmonary artery causes the pulmonary artery pressure to increase.
• Aortic and Pulmonary Blood Flow:
  • Blood flows from the left ventricle into the aorta, distributing oxygenated blood to the systemic circulation.
  • Blood flows from the right ventricle into the pulmonary artery, transporting deoxygenated blood to the lungs for oxygenation.
• Duration:
  • The ventricular ejection phase lasts approximately 0.2 to 0.3 seconds.

Other Important Events During Ventricular Systole

Beyond the primary events of isovolumetric contraction and ventricular ejection, several other critical changes occur during ventricular systole:

1. Changes in Atrial Pressure

• Atrial Filling: Throughout ventricular systole, the atria are passively filling with blood returning from the systemic and pulmonary circulations.
• Increasing Atrial Pressure: As the atria fill, the pressure within the atria gradually increases.
• V Wave: The v wave in the atrial pressure tracing corresponds to the period of atrial filling during ventricular systole. This wave represents the increase in atrial pressure as blood accumulates in the atria while the AV valves are closed.

2. Changes in Ventricular Volume

• End-Diastolic Volume (EDV): At the beginning of ventricular systole (after atrial contraction and ventricular filling), the ventricles contain their maximum volume of blood, known as the end-diastolic volume (EDV).
• Stroke Volume (SV): As the ventricles contract and eject blood, the ventricular volume decreases. The amount of blood ejected during each contraction is the stroke volume (SV).
  • The stroke volume is calculated as the difference between the end-diastolic volume (EDV) and the end-systolic volume (ESV): SV = EDV - ESV.
• End-Systolic Volume (ESV): At the end of ventricular systole, the ventricles contain the remaining volume of blood, known as the end-systolic volume (ESV). The ESV represents the amount of blood that was not ejected during contraction.

3. Changes in Aortic Pressure

• Systolic Pressure: As the ventricles eject blood into the aorta, the aortic pressure rises to its peak value, known as the systolic pressure.
• Dicrotic Notch (Incisura): Towards the end of ventricular ejection, as the ventricles begin to relax and the aortic valve closes, there is a brief increase in aortic pressure, followed by a sharp decline. This brief increase is known as the dicrotic notch or incisura.
  • The dicrotic notch is caused by the elastic recoil of the aorta and the brief backflow of blood against the closed aortic valve.
• Diastolic Pressure: After the dicrotic notch, the aortic pressure gradually declines to its lowest value, known as the diastolic pressure, during ventricular diastole.

4. Changes in Coronary Blood Flow

• Reduced Coronary Blood Flow: During ventricular systole, the increased pressure within the ventricular myocardium can compress the coronary arteries, which supply blood to the heart muscle itself.
• Systolic Compression: The compression of the coronary arteries reduces coronary blood flow during systole, particularly in the subendocardial regions (the inner layer of the ventricular wall).
• Diastolic Perfusion: Coronary blood flow primarily occurs during ventricular diastole, when the heart muscle is relaxed and the coronary arteries are not compressed.

5. Metabolic Changes

• Increased Oxygen Consumption: The contracting ventricular muscle requires a significant amount of energy, which is derived from the metabolism of oxygen and nutrients.
• ATP Hydrolysis: During ventricular systole, there is increased hydrolysis of adenosine triphosphate (ATP) to provide energy for muscle contraction.
• Lactate Production: Under conditions of increased workload or reduced oxygen supply, the heart muscle may produce lactate as a byproduct of anaerobic metabolism.

6. Electrical Changes

• QRS Complex: The electrical activity of ventricular systole is reflected in the electrocardiogram (ECG) as the QRS complex.
  • The QRS complex represents the depolarization of the ventricles, which triggers ventricular contraction.
• ST Segment: The ST segment represents the period between ventricular depolarization and repolarization.
• T Wave: The T wave represents the repolarization of the ventricles, which occurs during ventricular diastole.

7. Hormonal and Neural Influences

• Autonomic Nervous System: The autonomic nervous system (sympathetic and parasympathetic branches) can influence ventricular systole by modulating heart rate, contractility, and vascular resistance.
• Sympathetic Stimulation: Sympathetic stimulation increases heart rate and contractility, leading to a more forceful ventricular contraction and increased cardiac output.
• Parasympathetic Stimulation: Parasympathetic stimulation (vagal nerve) decreases heart rate and contractility, leading to a less forceful ventricular contraction and decreased cardiac output.
• Hormones: Hormones such as epinephrine and norepinephrine can also influence ventricular systole by increasing heart rate and contractility.

Clinical Significance

Understanding the events that occur during ventricular systole is crucial for diagnosing and managing various cardiovascular conditions. Some examples include:

• Heart Failure: In heart failure, the ventricles may not contract forcefully enough to eject an adequate amount of blood, leading to reduced cardiac output.
• Valvular Heart Disease: Valvular abnormalities, such as aortic stenosis or mitral regurgitation, can affect ventricular systole by impeding blood flow or causing backflow of blood.
• Arrhythmias: Ventricular arrhythmias, such as ventricular tachycardia or ventricular fibrillation, can disrupt the normal sequence of ventricular contraction, leading to ineffective blood ejection.
• Hypertension: Chronic hypertension can increase the workload on the ventricles, leading to ventricular hypertrophy and impaired systolic function.

Summary of Events During Ventricular Systole

To summarize, ventricular systole is a complex phase of the cardiac cycle involving numerous coordinated events:

1. Isovolumetric Contraction:
   • Ventricular contraction begins.
   • Ventricular pressure rises rapidly.
   • AV valves close (S1 heart sound).
   • No change in ventricular volume.
   • Tension development in ventricular muscle.
2. Ventricular Ejection:
   • Semilunar valves open.
   • Rapid ejection of blood.
   • Reduced ejection of blood.
   • Ventricular volume decreases (stroke volume).
   • Aortic pressure increases (systolic pressure).
   • Pulmonary artery pressure increases.
3. Other Important Events:
   • Atrial filling and increasing atrial pressure (v wave).
   • Changes in ventricular volume (EDV, SV, ESV).
   • Changes in aortic pressure (dicrotic notch).
   • Reduced coronary blood flow due to systolic compression.
   • Increased oxygen consumption and metabolic changes.
   • Electrical changes (QRS complex, ST segment).
   • Hormonal and neural influences (autonomic nervous system).

By understanding these events, healthcare professionals can better assess cardiovascular function and develop appropriate treatment strategies for patients with heart disease.