Chapter 17: The Cardiovascular System – The Heart (Study Notes)

Chapter 17: The Cardiovascular System – The Heart

Electrical Events of the Heart

The heart generates and conducts electrical impulses independently of nervous system stimulation, though rhythm can be influenced by the autonomic nervous system.

• Heart depolarization and contraction: Occur without nervous system input.

• Autonomic nervous system: Can alter heart rhythm.

Setting the Basic Rhythm: The Intrinsic Conduction System

The heart's coordinated contraction is regulated by a specialized conduction system composed of pacemaker cells and conduction pathways.

• Intrinsic cardiac conduction system: Network of noncontractile (autorhythmic) cells that initiate and distribute impulses for coordinated depolarization and contraction.

• Pacemaker potential: Unstable resting membrane potential in pacemaker cells, leading to spontaneous depolarization.

• Action potential initiation:

  • Slow Na+ channels open, K+ channels close, and Ca2+ channels open, causing depolarization.

  • Repolarization occurs as Ca2+ channels close and K+ channels open.

Sequence of Excitation

Electrical impulses travel through the heart in a specific sequence, ensuring coordinated contraction.

• Sinoatrial (SA) node: Primary pacemaker, initiates impulses.

• Atrioventricular (AV) node: Delays impulse (~0.1 sec) to allow atria to contract before ventricles.

• AV bundle (Bundle of His): Connects atria to ventricles.

• Bundle branches: Carry impulses through interventricular septum.

• Purkinje fibers: Distribute impulse to ventricular muscle, causing contraction.

Modifying the Basic Rhythm: Extrinsic Innervation of the Heart

Heart rate is modulated by the autonomic nervous system (ANS) via cardiac centers in the medulla oblongata.

• Cardioacceleratory center: Sympathetic signals increase heart rate and contractility.

• Cardioinhibitory center: Parasympathetic signals (via vagus nerve) decrease heart rate.

Action Potentials of Contractile Cardiac Muscle Cells

Contractile cells generate action potentials that lead to heart contraction.

• Depolarization: Fast voltage-gated Na+ channels open.

• Plateau phase: Slow Ca2+ channels open, prolonging depolarization.

• Repolarization: Ca2+ channels close, K+ channels open.

• Equation for action potential duration: $\text{Duration} = \text{Plateau phase} + \text{Depolarization} + \text{Repolarization}$

Electrocardiography

An electrocardiogram (ECG or EKG) records the electrical activity of the heart, providing diagnostic information.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization and atrial repolarization.

• T wave: Ventricular repolarization.

Mechanical Events of the Heart

The cardiac cycle describes the sequence of events during one heartbeat, including systole (contraction) and diastole (relaxation).

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Cardiac cycle: All events associated with blood flow through the heart during one heartbeat.

• Equation for cardiac output: $\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

Heart Sounds

Heart sounds are produced by the closing of heart valves.

• First sound (lub): Closing of AV valves.

• Second sound (dup): Closing of semilunar valves.

Regulation of Pumping

Cardiac output is regulated by stroke volume and heart rate.

• Stroke volume (SV): Volume of blood pumped by one ventricle per beat.

• Equation for stroke volume: $\text{SV} = \text{EDV} - \text{ESV}$ where EDV = end diastolic volume, ESV = end systolic volume

• Cardiac reserve: Difference between resting and maximal CO.

Regulation of Stroke Volume

Stroke volume is influenced by preload, contractility, and afterload.

• Preload: Degree of stretch of heart muscle before contraction (Frank-Starling law).

• Contractility: Strength of contraction at a given muscle length.

• Afterload: Back pressure exerted by arterial blood.

Regulation of Heart Rate

Heart rate is regulated by the autonomic nervous system, hormones, ions, and other factors.

• Sympathetic nervous system: Increases heart rate and contractility.

• Parasympathetic nervous system: Decreases heart rate.

• Chemical regulation: Epinephrine, thyroxine, and ion concentrations (Na+, K+, Ca2+).

• Other factors: Age, gender, exercise, body temperature.

Summary Table: Key Events in the Cardiac Cycle

Phase	Event	Valve Status	Pressure Change
Atrial systole	Atria contract, ventricles fill	AV valves open, SL valves closed	Ventricular pressure low
Ventricular systole	Ventricles contract, blood ejected	AV valves closed, SL valves open	Ventricular pressure rises
Early diastole	Ventricles relax, atria fill	AV valves open, SL valves closed	Ventricular pressure falls

Additional info:

• Frank-Starling law: Increased venous return increases stroke volume by stretching cardiac muscle fibers.

• Electrolyte imbalances (especially K+ and Ca2+) can cause arrhythmias.

• Cardiac output adapts to meet metabolic demands during exercise and stress.