Cardiovascular Physiology Quiz

Cardiovascular Physiology

• The heart has a specialized system for generating rhythmic electrical impulses and rapidly conducting them.

• This system causes rhythmical contraction of the heart muscle.

• When functioning normally, the atria contract slightly before the ventricles, allowing the ventricles to fill before pumping blood.

• This ensures efficient pressure generation in the ventricular chambers.

• The heart's system is vulnerable to damage from heart disease, particularly ischemia (poor coronary blood flow).

• Abnormal heart rhythms (arrhythmias) can severely impact the heart's pumping effectiveness, potentially causing death.

• Specialized Excitatory and Conductive System of the Heart

• The sinus node, or sinoatrial node (SA node), is a specialized cardiac muscle strip that generates normal rhythmic impulses.

• It's located in the superior posterolateral wall of the right atrium.

• SA node fibers lack significant contractile filaments and are small (3 to 5 micrometers in diameter), unlike surrounding atrial muscle fibers (10 to 15 micrometers).

• SA node fibers connect directly with atrial muscle fibers.

• This ensures immediate action potential spread to the atrial muscle wall.

• Automatic Electrical Rhythmicity of the Sinus Fibers

• Some heart fibers have inherent self-excitation capabilities.

• This leads to automatic rhythmic discharge and contraction.

• Sinus fibers exhibit this property, and the sinus node typically regulates the heart's rhythm.

• Mechanism of Sinus Nodal Rhythmicity

• The "resting" membrane potential of the sinus nodal fibers lies between -55 to -60 millivolts, differing from the ventricular muscle's -85 to -90 mV resting potential.

• Fast sodium channels are mostly inactive at this potential.

• Slow sodium-calcium channels open, promoting an inward flux of sodium and calcium ions.

• This influx neutralizes some intracellular negativity, leading to a gradual increase in the resting membrane potential.

• Potassium channels open when the potential reaches approximately -40 mV, causing an outward flux of potassium ions.

• This returns the membrane potential to the -55 to -60 mV resting level and terminates the action potential.

• Hyperpolarization briefly pushes the resting potential below -60mV before it returns to its normal voltage.

• This cycle repeats due to the inherent leakiness of the sinus nodal fibers to sodium and calcium ions.

• Specialized Excitatory and Conductive System of the Heart - Internodal Pathways and Transmission of the Cardiac Impulse Through the Atria

• Impulse transmission through the atria occurs via internodal pathways (anterior, middle, and posterior).

• These pathways facilitate rapid conduction (0.3-1 m/sec).

• Excitation spreads through the atrial muscle via these pathways, which ultimately reach the AV node.

• Specialized Excitatory and Conductive System of the Heart - Atrioventricular Node and Delay of Impulse Conduction from the Atria to the Ventricles

• The AV node, a specialized node in the posterior wall of the right atrium, is slower to conduct the signal from the atria to the ventricles.

• This delays the impulse, giving the atria time to empty their blood into the ventricles before ventricular contraction begins.

• The delay is necessary as a safety mechanism to prevent re-entry of impulses.

• Rapid Transmission in the Ventricular Purkinje System

• Special fibers, Purkinje fibers, transmit action potentials rapidly from the AV node to the ventricles (1.5-4.0 m/sec).

• These fibers are large and have reduced myofibrils.

• The high permeability of gap junctions facilitates rapid transmission throughout the ventricle.

• This rapid transmission results in near-simultaneous contraction of the ventricular fibers.

• Transmission of the cardiac impulse in the ventricular muscle

• The impulse passes from the Purkinje fibers into the surrounding ventricular muscle tissue and continues to spread throughout the ventricles.

• This propagation is slightly slower than in the Purkinje system.

• Finally, the total time for complete impulse transmission through the heart is approximately 0.06 sec.

• Summary of the Spread of the Cardiac Impulse Through the Heart

• The diagram (Figure 4) graphically displays the spread of the cardiac impulse through the heart.

• The numbers on the figure represent the time intervals (in fractions of a second).

• It details the origin in the SA node and subsequent cardiac impulse appearances in different parts.

• Control of Excitation and Conduction in the Heart - Sinus Node as the Pacemaker of the Heart

• The sinus node is the primary pacemaker as it discharges considerably faster than the AV node or any other region in the heart.

• Other areas in the heart can sometimes serve as pacemakers (namely the AV node or Purkinje fibers).

• Control of Excitation and Conduction in the Heart - Role of the Purkinje System in Causing Synchronous Contraction of the Ventricular Muscle

• The impulse reaches approximately all parts of the ventricles simultaneously.

• This nearly simultaneous arrival of the impulse is due to the high speed of transmission through the Purkinje fibers.

• This near simultaneous contraction of the ventricles allows the heart to pump effectively.

Test your understanding of cardiovascular physiology, focusing on the heart's specialized system that generates and conducts electrical impulses. Learn about the functions of the SA node, the significance of heart muscle contraction, and the implications of arrhythmias on heart health.