Questions with Answers on “Cardiac Cycle: Phases and Regulation”

1. Explain the cardiac cycle with its phases.

Answer:
The cardiac cycle consists of three main phases:

1. Atrial Systole:
   • Duration: 0.1 seconds.
   • Atria contract, pushing blood into the ventricles through open atrioventricular (AV) valves (tricuspid and mitral).
2. Ventricular Systole:
   • Duration: 0.3 seconds.
   • Subdivided into:
     • Isovolumetric Contraction: Ventricles contract with no volume change; all valves are closed.
     • Ejection Phase: Semilunar valves open, ejecting blood into the aorta and pulmonary artery.
3. Diastole:
   • Duration: 0.4 seconds.
   • Heart chambers relax, allowing blood to flow from veins into atria and ventricles.
   • Subdivided into:
     • Isovolumetric Relaxation: Ventricles relax, all valves are closed.
     • Ventricular Filling: AV valves open, allowing blood to fill the ventricles.

2. What are the roles of the atrioventricular and semilunar valves in the cardiac cycle?

Answer:

• Atrioventricular (AV) valves (tricuspid and mitral):
  • Prevent backflow of blood from the ventricles to the atria during ventricular systole.
  • Open during atrial systole to allow blood flow into the ventricles.
• Semilunar valves (pulmonary and aortic):
  • Prevent backflow of blood from arteries into the ventricles during diastole.
  • Open during ventricular systole to allow blood ejection into the aorta and pulmonary artery.

3. What is the significance of the isovolumetric phases in the cardiac cycle?

Answer:

• Isovolumetric Contraction:
  • Occurs during ventricular systole before the semilunar valves open.
  • Builds pressure in the ventricles, essential for effective blood ejection.
• Isovolumetric Relaxation:
  • Occurs at the start of diastole when all valves are closed.
  • Allows the ventricles to relax and reduce pressure, preparing for filling.

4. Describe the regulation of the cardiac cycle by the autonomic nervous system (ANS).

Answer:

1. Sympathetic Nervous System:
   • Increases heart rate (positive chronotropic effect).
   • Enhances contractility (positive inotropic effect).
2. Parasympathetic Nervous System:
   • Decreases heart rate via the vagus nerve (negative chronotropic effect).
   • Predominantly affects the SA and AV nodes.

5. What is the role of the sinoatrial (SA) node in the cardiac cycle?

Answer:

• Acts as the natural pacemaker.
• Initiates electrical impulses that spread across the atria, causing their contraction.
• Sets the rhythm of the heart, typically 60-100 beats per minute.

6. Explain the ECG (Electrocardiogram) waves and their relation to the cardiac cycle.

Answer:

1. P wave: Represents atrial depolarization, leading to atrial systole.
2. QRS complex: Represents ventricular depolarization, triggering ventricular systole.
3. T wave: Represents ventricular repolarization during diastole.

7. How does the Frank-Starling law influence cardiac output?

Answer:

• The Frank-Starling law states that the force of contraction increases with increased ventricular filling (preload).
• A greater preload stretches the myocardial fibers, resulting in stronger contractions and higher stroke volume.

8. What is cardiac output, and how is it calculated?

Answer:

• Definition: Cardiac output is the volume of blood pumped by each ventricle per minute.
• Formula: $$\text{Cardiac Output}=\text{Heart Rate}\times \text{Stroke Volume}$$
• Example: At a heart rate of 70 beats/min and stroke volume of 70 mL/beat, cardiac output = 4900 mL/min or 4.9 L/min.

9. Discuss the phases of ventricular diastole.

Answer:

1. Isovolumetric Relaxation:
   • All valves are closed; ventricular pressure drops.
2. Rapid Filling Phase:
   • AV valves open; blood rapidly fills the ventricles.
3. Diastasis:
   • Slower filling as ventricles approach maximum capacity.

10. How does exercise affect the cardiac cycle?

Answer:

• Increases heart rate, shortening diastole more than systole.
• Enhances stroke volume due to increased venous return and myocardial contractility.
• Results in higher cardiac output to meet metabolic demands.

11. What is the significance of atrial systole in the cardiac cycle?

Answer:

• Completes ventricular filling by contributing the final 20-30% of blood to the ventricles.
• Ensures efficient ventricular preload, particularly important during high heart rates.

12. How does the medulla oblongata regulate the cardiac cycle?

Answer:

• Contains the cardiac control center.
• Sympathetic stimulation increases heart rate and contractility.
• Parasympathetic stimulation via the vagus nerve reduces heart rate.

13. What are the effects of preload, afterload, and contractility on the cardiac cycle?

Answer:

1. Preload: Increases stroke volume by stretching myocardial fibers (Frank-Starling mechanism).
2. Afterload: High afterload reduces stroke volume by opposing ejection.
3. Contractility: Greater contractility enhances stroke volume and cardiac output.

14. Describe the role of calcium ions in cardiac muscle contraction.

Answer:

• Calcium ions bind to troponin, allowing actin-myosin interactions.
• Essential for generating force during contraction.
• Controlled by sarcoplasmic reticulum release and extracellular influx.

15. Explain the role of baroreceptors in regulating the cardiac cycle.

Answer:

• Detect changes in blood pressure.
• Increase vagal activity to lower heart rate during hypertension.
• Increase sympathetic activity to raise heart rate during hypotension.

16. What is the significance of the dicrotic notch in arterial pressure?

Answer:

• Represents the closure of the aortic valve.
• Occurs during isovolumetric relaxation.
• Indicates the end of ventricular systole.

17. How does the cardiac cycle adapt during dehydration?

Answer:

• Reduced preload due to lower blood volume.
• Decreased stroke volume, compensated by increased heart rate.
• Lower cardiac output if compensation fails.

18. Describe the phases of ventricular systole.

Answer:

1. Isovolumetric Contraction:
   • Ventricles contract, pressure rises, but no volume change occurs as valves remain closed.
2. Ejection Phase:
   • Semilunar valves open, ejecting blood into the aorta and pulmonary artery.

19. How does atrial natriuretic peptide (ANP) regulate the cardiac cycle?

Answer:

• Secreted by atria in response to high blood volume.
• Reduces blood volume and pressure by promoting diuresis and vasodilation.
• Decreases preload, affecting stroke volume and cardiac output.

20. What is the importance of ventricular compliance in the cardiac cycle?

Answer:

• Refers to the ventricles’ ability to stretch and fill during diastole.
• High compliance ensures adequate filling at low pressures.
• Reduced compliance (e.g., in hypertrophy) impairs diastolic filling and cardiac output.