Structure of the Heart and the Cardiac Cycle

Gross Anatomy of the Mammalian Heart

• Overview of the Four Chambers:     - Right Atrium (RA): This chamber serves as the initial collection point for deoxygenated blood returning from the systemic circulation (the body’s organs and tissues).     - Right Ventricle (RV): Receives blood from the right atrium and pumps it to the lungs through the pulmonary circulation for oxygenation.     - Left Atrium (LA): Receives oxygen-rich blood returning from the lungs via the pulmonary veins.     - Left Ventricle (LV): Described as the "powerhouse" of the heart; it features a thick muscular wall to pump oxygenated blood throughout the entire body via systemic circulation.

• Valvular Anatomy and Classification:     - Atrioventricular (AV) Valves (Inlet Valves): These separate the atria from the ventricles.         - Left AV Valve (Mitral Valve): Positioned between the left atrium and left ventricle.         - Right AV Valve (Tricuspid Valve): Positioned between the right atrium and right ventricle.         - Functions: They allow blood to flow from the atria to the ventricles while blocking backflow from the ventricles to the atria during contraction.     - Semilunar Valves (Outlet Valves): Positioned between the ventricles and the great arteries.         - Left Semilunar Valve (Aortic Valve): Located between the left ventricle and the aorta.         - Right Semilunar Valve (Pulmonary Valve): Located between the right ventricle and the pulmonary artery.         - Functions: They allow blood to flow from the ventricles into the arteries while blocking backflow from the arteries to the ventricles.

• Functional Characteristics of Atria:     - The atria are thin-walled, low-pressure chambers.     - Three Main Functions:         1. Elastic Reservoir and Conduit: They serve as a small tunnel (conduit) and storage area for blood moving from the venous bed to the ventricle.         2. Booster Pump: They enhance ventricular filling through active contraction at the end of diastole.         3. Closure Assistance: They assist in the closure of the atrioventricular valves just before ventricular systole begins.

• Functional Characteristics of Ventricles:     - Ventricles make up the majority of the heart's total weight and provide the primary pumping force.     - Right Ventricle: Propels blood through the pulmonary circulation via the pulmonary artery.     - Left Ventricle: Propels blood through the systemic circulation via the aorta.     - Ventricular Septum: A thick muscular wall that separates the right and left ventricles.

Pathway of Blood Flow

• Right Side of the Heart (Deoxygenated Pathway):     - 1. Deoxygenated blood (represented as blue) returns from the body’s organs and tissues through the Cranial and Caudal Vena Cava.     - 2. Blood enters the Right Atrium.     - 3. Blood flows from the atrium into the Right Ventricle.     - 4. The right ventricle pumps the blood through the Pulmonary Arteries to the lungs for oxygenation.

• Left Side of the Heart (Oxygenated Pathway):     - 1. Oxygenated blood (represented as red) returns from the lungs to the heart through the Pulmonary Veins.     - 2. Blood enters the Left Atrium.     - 3. Blood flows from the atrium into the Left Ventricle.     - 4. The left ventricle pumps the blood through the Aorta to be distributed to the rest of the body, where oxygen is consumed by organs and tissues.

The Cardiac Conduction System

• Electrical Components and Sequence:     - Sinoatrial (SA) Node: The primary pacemaker located in the right atrium, immediately below and lateral to the cranial vena cave opening.     - Internodal Pathways and Bachmann’s Bundle: Facilitate the spread of the impulse through the atria.     - Atrioventricular (AV) Node: A relay station that introduces a critical delay in the impulse.     - Bundle of His (AV Bundle): The only electrical bridge through the fibrous skeleton between atria and ventricles.     - Right and Left Bundle Branches: Descend through the ventricular septum toward the apex.     - Terminal Purkinje Fibers: Distribute the impulse rapidly throughout the ventricular walls.

• Pacemaker Capabilities and Discharge Rates:     - The fibers of the conduction system possess the capacity for self-excitation (automatic rhythmical discharge).     - SA Node: 70–80 times per minute (The "Conductor" setting the override rhythm).     - AV Node: 40–60 times per minute.     - Purkinje Fibers: 15–40 times per minute.

• Detailed Node Physiology:     - SA Node Characteristics:         - Specialized cardiac muscle cells with almost no contractile filaments.         - Smaller than standard atrial fibers.         - Connect directly to atrial muscle fibers via gap junctions.         - Spread velocity cell-to-cell is approximately $0.3\,m/sec$.     - AV Node Delay:         - Located in the posterior wall of the right atrium behind the tricuspid valve.         - Delay occurs due to a lower density of GAP junctions, creating increased resistance to the flow of excitatory ions.         - Purpose: Allows the atria to empty completely before ventricular contraction begins.     - AV Bundle/Bundle of His:         - Atrial and ventricular muscles are separated by a fibrous skeleton (fibrous atrioventricular ring) which acts as an insulator.         - The AV bundle is the single point of passage, ensuring forward conduction only.     - Purkinje System:         - Fast transmission velocity ($1.5$ to $4.0\,m/sec$).         - High permeability of gap junctions and an abundance of fast voltage-gated sodium channels.         - Total elapsed time from the bundle branch to the end of the Purkinje system is only $0.03\,sec$.

• Conduction Velocities Summary:     - Atrial muscle: $0.3\,m/s$     - Atrial pathways: $1.0\,m/s$     - AV node: $0.05\,m/s$ (Slowest)     - Bundle of His: $1.0\,m/s$     - Purkinje fibers: $1.4$ to $4.0\,m/s$ (Fastest)     - Ventricular muscle: $1.0\,m/s$

• Ventricular Contraction Pattern:     - Cardiomyocytes are connected by intercalated discs with gap junctions.     - Spiral Muscle Arrangement: Muscle fibers are arranged in a double helix/spiral band.     - Contraction begins at the apex and moves upward toward the base, creating a "wringing" motion to efficiently squeeze blood toward the outflow tracts.

Action Potentials in the Heart

• Pacemaker Action Potential:     - Characterized by a slow depolarization (pacemaker potential) leading to the firing of an action potential. Involves $Na^+$ and $Ca^{2+}$ influx and $K^+$ efflux.
• Ventricular Action Potential Phases:     - Phase 0 (Depolarization): Rapid influx of $Na^+$ through voltage-gated channels.     - Phase 1: Partial repolarization; $Na^+$ channels close.     - Phase 2 (Plateau Phase): Maintained by $Ca^{2+}$ influx through voltage-gated channels, balancing $K^+$ efflux.     - Phase 3 (Repolarization): Rapid $K^+$ efflux as $Ca^{2+}$ channels close.     - Phase 4 (Resting State): Return to resting membrane potential ($-90\,mV$).
• Refractory Periods:     - Absolute Refractory Period: The cell cannot be re-excited at all.     - Relative Refractory Period: A stronger-than-normal stimulus is required to trigger another action potential.

Hemodynamics: Pressure and Valvular Function

• Concept 1: Pressure Gradients:     - Fluids always move from areas of high pressure to areas of low pressure.     - Chamber volume and pressure are inversely related (based on Boyle's Law principles).     - Systole (Contraction): Volume decreases → Pressure increases → Blood ejected to lower pressure areas.     - Diastole (Relaxation): Volume increases → Pressure decreases → Blood flows in from higher pressure areas.

• Concept 2: Passive Valvular Action:     - AV Valves:         - Open: When atrial pressure > ventricular pressure.         - Close: When ventricular pressure > atrial pressure. This produces the First Heart Sound (S1), a long-lasting "lubb".         - Chordae Tendineae and Papillary Muscles: These provide anchoring to prevent valve flaps from prolapsing into the atria during high-pressure systole. They do NOT actively open or close the valves.     - Semilunar Valves:         - Open: When ventricular pressure > arterial pressure.         - Close: When arterial pressure > ventricular pressure. This produces the Second Heart Sound (S2), a rapid snap "dub".         - These valves lack chordae tendineae and have smaller openings for rapid ejection.

The Cardiac Cycle Phases

• Duration Calculations:     - Cycle duration is the reciprocal of Heart Rate (HR).     - Example: If $HR = 80\,bpm$, duration = $\frac{1}{80} = 0.0125\,minutes/beat$ (or $750\,ms/beat$).

• Step-by-Step Cycle Details:     1. Atrial Systole: (Lasts approx. $100\,ms$). SA node fires; atria contract to add the final $20\%$ of blood to ventricles (which are already $80\%$ full passively).     2. Isovolumetric Contraction: (Ventricular Systole Part 1). Ventricles begin contracting; pressure rises; AV valves close (S1). Volume remains constant because semilunar valves are still closed.     3. Ventricular Ejection: (Ventricular Systole Part 2). Ventricular pressure exceeds arterial pressure; semilunar valves open. Approx. $60\%$ of volume is ejected.     4. Isovolumetric Relaxation: (Ventricular Diastole Part 1). Ventricles relax; pressure drops; arterial blood snaps semilunar valves shut (S2). All valves are closed; volume is constant.     5. Rapid Ventricular Filling: (Ventricular Diastole Part 2). Ventricular pressure drops below atrial pressure; AV valves open. Ventricles fill quickly to approx. $80\%$ capacity.     6. Diastasis: (Ventricular Diastole Part 3). Passive, slow filling as pressure gradients equalize.     7. Atrial Systole: The cycle repeats.

• Heart Rate Impact:     - As heart rate increases, the duration of the cycle decreases.     - Diastole (the longest phase) undergoes the greatest reduction. At extremely high rates, ventricular filling may be compromised.

Quantitative Measurements of Cardiac Function

• Volume Definitions:     - End-Diastolic Volume (EDV): The volume of blood in each ventricle at the end of diastole (filling period). Normal example: $120\,mL$.     - Stroke Volume (SV): The volume of blood ejected per beat.         - Formula: $SV = EDV - ESV$         - Example: $120\,mL - 50\,mL = 70\,mL$     - End-Systolic Volume (ESV): The volume of blood remaining in the ventricle after contraction. Normal example: $50\,mL$.     - Ejection Fraction (EF): The percentage of the EDV that is ejected.         - Formula: $EF = \frac{SV}{EDV} \times 100$         - Example: $EF = \frac{70\,mL}{120\,mL} \approx 58\%$     - Cardiac Output (CO): Total volume ejected by the heart per minute.         - Formula: $CO = SV \times HR$         - Example: $70\,mL/beat \times 70\,beats/min = 4.9\,L/min$

Clinical Correlation: Sick Sinus Syndrome

• Pathophysiology: The Sinus (SA) node fails to discharge correctly.
• Manifestations:     - Sinus Pause or Arrest: Heart stops beating temporarily.     - Clinical Sign: If the pause exceeds $8\,seconds$, the patient may collapse or faint (syncope).     - Bradycardia-Tachycardia Syndrome: Periods of very low firing rates interspersed with periods of excessive tachycardia.
• Diagnosis: Accomplished via ECG or Holter monitor.
• Treatment: Pharmacological intervention or, ideally, a pacemaker implant.