🫀 The Cardiac Cycle: Structure, Physiology, and Mechanics

Core Overview & Roadmap:
- Definition and basic timeline of the cardiac cycle
- Chamber anatomy and the dual-heart configuration (pulmonary and systemic)
- Detailed sequential stages of the cycle (Diastole, Atrial Systole, Ventricular Systole, and Isovolumic phases)
- The heart's electrical conduction system and pacemakers
- Hemodynamics, valve operations, and pressure-volume relationships (Wiggers diagram insights)

⏱️ Introduction and Fundamental Timing

The cardiac cycle represents the complete performance of the human heart from the beginning of one heartbeat to the beginning of the next.

Core Periods of the Cycle

Diastole: The period during which the heart muscle relaxes and refills with blood.
Systole: The period following diastole characterized by robust contraction and the pumping of blood.
Process Flow: After emptying its chambers, the heart relaxes and expands to receive a fresh influx of blood returning from the lungs and systemic circulation before contracting once again.

Heart Rate and Duration Metrics

Baseline Assumptions: Assumes a healthy heart operating at a typical rate of 70 to 75 beats per minute.
Cycle Duration: Each individual cardiac cycle (heartbeat) takes approximately 0.8 seconds to complete.
Inversely Proportional Rule: The duration of the cardiac cycle is inversely proportional to the heart rate (as heart rate increases, cycle duration decreases).

🏛️ Chamber Anatomy and Circulatory Divisions

The heart consists of four primary chambers paired into two functional units working in concert:

The Left Heart: Comprises the left atrium and left ventricle.
The Right Heart: Comprises the right atrium and right ventricle.

Circulatory Paths

Pulmonary Circulation:
- Managed by the right ventricle.
- Pumps oxygen-depleted blood through the pulmonary trunk and pulmonary arteries to the lungs.
Systemic Circulation:
- Managed by the left ventricle.
- Pumps newly oxygenated blood throughout the body via the aorta and all other systemic arteries.

🔄 The Four Major Stages of the Cardiac Cycle

The cardiac cycle is broken down into four foundational stages of mechanical and hemodynamic activity:

Stage Number	Stage Name	Primary Mechanical Activity	Associated Period
1	Isovolumic Relaxation	Ventricular pressure falls significantly; AV valves remain closed while chambers prepare to fill.	Ventricular Diastole
2	Inflow (Filling)	Blood returns via vena cavae and pulmonary veins, flowing through atria into relaxed ventricles (includes rapid inflow, diastasis, and atrial systole).	Ventricular Diastole
3	Isovolumic Contraction	Ventricles start contracting via SA node signals; back-pressure forces AV valves closed, halting volume changes.	Ventricular Systole
4	Ejection	Ventricular pressures exceed aortic/pulmonary pressures; aortic and pulmonary valves open to eject blood supplies.	Ventricular Systole

📋 Detailed Breakdown of Cycle Phases

1. Ventricular Diastole (Early & Late)

Early Phase: The heart relaxes and expands, receiving blood into both ventricles passively through both atria as the mitral and tricuspid valves (atrioventricular or AV valves) open. Pressure levels in both atria and ventricles remain near-zero.
Late Phase (Atrial Systole): Near the end of diastole, the two atria begin to contract, forcing a final pressurized volume of blood ("topping-off" the ventricles) just before ventricular contraction.

2. Isovolumic Contraction

Prompted by electrical signals from the sinoatrial node, the ventricles initiate contraction.
As back-pressure against the AV valves increases, the valves are forced to close.
This closure halts blood flow in or out of the ventricles, marking the isovolumic contraction stage.

3. Ejection Stage (Ventricular Systole - First and Second Phases)

Pressure Surges: Contractions cause ventricular pressures to rise rapidly, soon exceeding pressures in the trunks of the aorta and pulmonary arteries.
Valve Activation: The aortic and pulmonary valves open, resulting in separated blood volumes being ejected from the two ventricles.
Relaxation Phase: After ventricular pressures fall below their peaks and drop below the pressures in the aorta and pulmonary arteries, the aortic and pulmonary valves snap shut (producing the incisura and dicrotic notch in arterial pressure waves).

4. Isovolumic Relaxation

Ventricular pressures fall significantly.
Atria begin refilling as blood returns to the right atrium (from the vena cavae) and left atrium (from the pulmonary veins).
Mitral and tricuspid valves open again, transitioning the completed cycle back to ventricular diastole for a new "Start".

⚡ The Heart Electrical Conduction System

In a healthy heart, all activities and rests during each individual cardiac cycle are initiated and orchestrated by signals of the heart's electrical conduction system—the specialized "wiring" that carries electrical impulses throughout the network of cardiomyocytes.

Key Conduction Structures and Pathways

Cardiomyocytes: Specialized muscle cells capable of initiating internal contractions without external nerve signals (barring heart rate adjustments driven by metabolic demand).
Sinoatrial (SA) Node:
- Located in the upper wall of the right atrium.
- Acts as the primary cardiac pacemaker.
- Produces a wave of electrical impulses (creating an action potential across myocardium cells) that coordinates sinus rhythm and initiates atrial contraction.
Atrioventricular (AV) Node:
- Located in the lower wall of the right heart between the atrium and ventricle.
- Acts as an electrical gate to slow and coordinate the current before conduction below the atria.
Bundle of His & Purkinje Fibers:
- Circuits conducting signals below the AV node.
- Stimulate coordinated contractions of both ventricles.
Physiological Significance of the AV Delay: The programmed delay at the AV node ensures adequate time for blood volume to flow through the atria and fully fill the ventricular chambers before ventricular systole initiates.

Electrocardiogram (ECG) Correlation

Electrical systole initiates atrial systole at the P wave deflection of a steady ECG signal, triggering the mechanical contraction sequence.

Cardiac Cycle: Anatomy and Physiology Study Notes