Cardiovascular Dynamics

Overview of Cardiac Chambers

• Ventricles: Responsible for pumping blood. They generate the highest pressure when contracting, mainly the left ventricle which pumps blood to the systemic circuit.

• Pressure Dynamics: For blood to flow from the ventricles, the pressure must exceed the pressure above the valves (semilunar valves). If the pressure in the ventricles is lower, no blood flow will occur, as blood moves from high to low pressure.

• Atria Pressure: Similar principles apply when the atria contract to fill the ventricles; pressure must rise to open the AV valves.

Phases of the Cardiac Cycle

Key Phases of Cardiac Cycle

1. Ventricular Filling (initially during quiescent period)

   • Quiescent period: Atria fill with blood, generating pressure higher than ventricles, causing AV valves to open.

   • Three Parts:

     • Rapid Ventricular Filling: Due to a steep pressure gradient, blood flows quickly from atria to ventricles.

     • Diastasis: As pressure gradient equalizes, flow slows down, reaching equilibrium.

     • Atrial Systole: Atria contract (SA node firing) to push the last portion of blood into the ventricles, completing ventricular filling.

   • End Diastolic Volume (EDV): The volume in the ventricles after filling, typically around 130 ml.

2. Isovolumetric Contraction:

   • All valves closed; ventricles begin to contract without volume change since pressure is not yet sufficient to open the semilunar valves.

3. Ventricular Ejection:

   • Ventricles contract, pressure exceeds semilunar valves, allowing blood to eject. This is known as the Stroke Volume (~70 ml at rest).

4. Isovolumetric Relaxation:

   • After ejection, ventricles relax but the volume remains unchanged until the filling cycle begins again.

Concepts to Remember

• End Systolic Volume (ESV): The volume left in the ventricles after ejection.

• Stroke Volume (SV) = EDV - ESV.

• Cardiac Output (CO): Amount of blood ejected by the ventricles per minute; can be calculated by multiplying heart rate (HR) by stroke volume (SV).

  • CO = HR x SV; normal CO ranges from 4 to 6 liters per minute at rest.

  • Can significantly increase during exercise due to elevated HR and SV.

Factors Affecting Cardiac Output

Heart Rate (HR)

• Normal resting HR varies by age:

  • Infants: ~120 bpm

  • Adults: ~70-80 bpm

  • Increased HR: Tachycardia (>100 bpm)

  • Decreased HR: Bradycardia (<60 bpm)

• Chronotropic Agents:

  • Positive agents: Increase HR (e.g., adrenaline).

  • Negative agents: Decrease HR (e.g., vagal tone from parasympathetic system).

Stroke Volume (SV)

1. Preload: Filling the ventricles; higher preload increases stroke volume (related to Frank-Starling Law).

2. Contractility: Strength of contraction influenced by inotropic agents; positive inotropes increase contractility, while negative ones decrease it.

3. Afterload: The pressure against which the heart must work to eject blood (higher afterload reduces stroke volume).

   • Examples: Hypertension increases afterload, reducing SV.

Preload, afterload, and contractility Dynamics

• Increased preload leads to increased stroke volume.

• Increased afterload reduces stroke volume.

• Increased contractility increases stroke volume but must consider the relationship with preload.

Key Concepts on Cardiac Output during Exercise

• Evaluating exercise impact highlights the relationship between HR and SV:

  • Increase in activity leads to an increased HR and potentially an increased SV.

  • CO is critical for delivering oxygen and nutrients during physical exertion.

Clinical Implications of Hemodynamics

• Unbalanced Ventricular Outflow:

  • Pulmonary edema occurs if right ventricle lags behind left; leads to fluid retention in lungs.

  • Systemic edema occurs if the left ventricle is unable to pump effectively, leading to fluid retention throughout the body.

  • Importance of maintaining equal output from both ventricles for optimal function.

Conclusion and Review Points

• Remember: All cardiac dynamics depend on pressure gradients and valve function.

• Core values to review: EDV, ESV, Stroke Volume, and Cardiac Output.

• Essential understandings include how exercise and various factors influence HR, SV, and overall cardiac function.