cardiac cycle

The cardiac cycle encompasses all the events associated with blood moving through the heart during one heartbeat.
Generally described from the viewpoint of the left ventricle.

Systole: Refers to ventricular contraction.
Diastole: Refers to ventricular relaxation.
These actions are driven by changes in pressure, which cause the opening and closing of the AV (atrioventricular) and semilunar valves.

There are four main phases of the cardiac cycle, each indicating a specific activity.

The ventricle fills with blood.
Source of Blood: Blood enters the ventricle from the atrium through the open AV valve.
Valves: The AV valve is open while the semilunar valve is closed.
Nature of Filling: This process is passive due to pressure gradients; blood moves from an area of higher pressure (atrium) to an area of lower pressure (ventricle).
Pressure in Atria vs. Ventricle: Atrial pressure > Ventricular pressure.

Definition: "Isovolumetric" means that the volume of blood in the ventricle does not change during this phase.
Valves State: All valves (AV and semilunar) are closed.
Action: The ventricle begins to contract, reducing its volume but keeping the blood volume constant.
Pressure Change: As the ventricle contracts, pressure increases due to a constant volume in a decreasing space.

Blood is ejected from the ventricle into the arteries.
Condition for Ejection: When the pressure in the ventricle exceeds the pressure in the aorta, the aortic semilunar valve opens.
Flow of Blood: Blood is pushed into the aorta due to the greater pressure in the ventricle compared to the arteries.
Pressure Dynamics:
- Pressure rises initially during ejection as blood is pushed out.
- Eventually decreases as blood volume in the ventricle lowers after ejection.
End of Phase: The semilunar valve closes when the pressure in the aorta exceeds ventricular pressure.

Definition: Like isovolumetric contraction, the ventricular volume remains unchanged.
Valves State: All valves are closed, AV and semilunar.
Action: The ventricle relaxes, leading to a decrease in pressure even though the volume of blood remains constant.
Consequences: This phase prepares the heart to begin the next cycle of ventricular filling.

The four phases cycle continuously, reflecting a dynamic interplay of pressure and volume in the heart.
Key measurements such as heart sounds and EKG can help visually correlate with the various phases.

Diastolic Pressure: The lowest point of pressure in the aorta, coinciding with diastole.
Systolic Pressure: The highest point of pressure in the aorta, occurring during ventricular contraction.
Mean Arterial Pressure (MAP): Average aortic pressure between systolic and diastolic phases.

Caused by turbulent blood flow during valve closure, not the sound of the valves themselves.
First Heart Sound (Lub): Occurs when the AV valves close.
Second Heart Sound (Dub): Occurs when the semilunar valves close, generally louder because of higher ventricular pressure.

Cardiac Output (CO): The volume of blood pumped by each ventricle per minute.
Formula: $CO = ext{Stroke Volume (SV)} imes ext{Heart Rate (HR)}$
Average CO at Rest: Approximately $5 ext{ L/min}$ , indicating almost all blood circulates through the heart every minute.

Influenced by heart rate and stroke volume, which can be adjusted through intrinsic and extrinsic factors:
- Intrinsic Factors: Autoregulation mechanisms within the heart.
- Extrinsic Factors: Influences from the nervous system and hormones.

The SA node is the pacemaker, typically firing at $100$ depolarizations/min under intrinsic conditions (unaffected by the nervous system).
At rest, influenced by the parasympathetic nervous system, the heart rate may decrease to approximately $75$ beats/min.
Sympathetic Nervous System: Increases heart rate through beta-adrenergic receptor activation by norepinephrine or epinephrine.
Parasympathetic Nervous System: Decreases heart rate primarily via acetylcholine, affecting ion channels that regulate depolarization.

Ventricular Contractility: Enhanced by sympathetic stimulation leads to stronger contractions.
End Diastolic Volume (Preload): Greater volume (stretch of ventricular wall) results in more forceful contractions due to the Frank-Starling mechanism.
Afterload: The pressure the ventricle must overcome to eject blood; higher pressure in the aorta reduces stroke volume.

States that stroke volume output matches venous return; more blood ejected from the heart leads to greater pressure in veins and increased return to the heart during diastole.

Understanding the cardiac cycle and its components is crucial for grasping the fundamentals of cardiovascular physiology and its implications for health and disease management.