The Cardiovascular System

Chapter 11: The Cardiovascular System

Intrinsic Conduction System of the Heart

  • Key Components:

    • Sinoatrial (SA) Node: Often referred to as the pacemaker of the heart, located in the right atrium.

    • Atrioventricular (AV) Node: Located at the junction of the atria and ventricles.

    • Atrioventricular (AV) Bundle (Bundle of His): (Leads to the Bundle Branches).

    • Right and Left Bundle Branches: Situated in the interventricular septum.

    • Purkinje Fibers: Spread throughout the ventricular myocardium.

  • Functionality:

    • The intrinsic conduction system initiates and regulates the cardiac rhythm, ensuring the heart contracts in a coordinated fashion.

    • Depolarization occurs in a wave that travels from the SA node through the atrial myocardium to the AV node, then to the AV bundle, the bundle branches, and finally the Purkinje fibers in the ventricular walls.

Physiology of the Heart

  • Heart Functionality:

    • The heart pumps approximately 6 quarts (6 liters) of blood over 1,000 times a day, totaling about 6,000 quarts (1,500 gallons).

  • Regulating Heart Activity:

    • Two key systems:

    1. Intrinsic Conduction System: Regulates basic heart rhythm independently of the nervous system.

    2. Autonomic Nervous System (ANS): Acts to increase (sympathetic) or decrease (parasympathetic) heart rate as needed.

Cardiac Impulses

  • Spontaneous Contraction: Unlike skeletal muscle, cardiac muscle cells can contract spontaneously and rhythmically without external stimuli.

    • Atrial cells beat at approximately 60 beats per minute while ventricular cells contract at 20-40 beats per minute.

  • Av Node Delay: The impulse from the SA node is delayed in the AV node, allowing the atria to finish contracting before the ventricles do.

Homeostatic Imbalances Related to Conduction

  • Heart Block: A condition where the AV node fails to transmit impulses, causing the ventricles to beat independently at a slower rate.

  • Pacemakers: Surgical pacemakers may be required when the heart rate is improperly regulated.

  • Ischemia & Fibrillation: Ischemia (lack of blood supply) can lead to fibrillation, characterized by uncoordinated heart muscle contractions.

  • Tachycardia and Bradycardia:

    • Tachycardia: Rapid heart rate exceeding 100 beats per minute.

    • Bradycardia: Slower heart rate under 60 beats per minute.

Cardiac Cycle and Heart Sounds

  • Phases in Cardiac Cycle:

    1. Atrial Diastole: Heart relaxed, AV valves open, and blood flows from atria to ventricles.

    2. Atrial Systole: Atria contract forcing additional blood into the ventricles.

    3. Isovolumetric Contraction: Blood pressure rises in the ventricles, all valves closed, preparing to eject blood.

    4. Ventricular Systole: Ventricles contract, semilunar valves open, blood is ejected.

    5. Isovolumetric Relaxation: Ventricles relax, valves closed, blood returns to atria from systemic circulation.

  • Heart Sounds (Lub-Dup):

    • The first sound (lub) is produced by the closing of the AV valves during ventricular systole.

    • The second sound (dup) occurs when the semilunar valves close at the end of ventricular systole.

    • Murmurs indicate abnormal blood flow, often indicative of valve problems.

Electrocardiogram (ECG)

  • Definition: An electrocardiogram traces the electrical activity of the heart.

  • Waves on ECG:

    • P Wave: Represents atrial depolarization.

    • QRS Complex: Represents ventricular depolarization and obscures atrial repolarization.

    • T Wave: Represents ventricular repolarization.

  • Clinical Implications:

    • Abnormalities in the ECG waveform can indicate issues with the cardiac conduction system or myocardial infarctions (MIs).

Cardiac Output

  • Definition: Cardiac output (CO) is the amount of blood pumped by each ventricle per minute:

    • CO = HR imes SV

    • Average Values: With an average heart rate of 75 beats per minute and stroke volume of 70 ml per beat, the average CO = 5.25 L/min.

  • Stroke Volume: The volume of blood ejected per contraction, influenced by:

    • Preload: The degree of stretch of cardiac muscle fibers before contraction.

    • Contractility: The intrinsic ability of cardiac muscle to contract, influenced by calcium levels and neural stimulation.

    • Afterload: The pressure the ventricles must overcome to eject blood, influenced by blood pressure in major arteries.

  • Factors Affecting Heart Rate:

    1. Neural Controls: Sympathetic stimulation increases heart rate; parasympathetic decreases it.

    2. Hormones and Ions: Hormones (e.g., epinephrine) affect heart rate and contractility; electrolyte imbalances can severely influence heartbeat.

    3. Physical Factors: Age, gender, exercise, and body temperature all influence baseline heart rate.

Homeostatic Imbalances and Conditions

  • Congestive Heart Failure (CHF): A progressive condition where heart pumping efficiency is inadequate, leading to insufficient circulation to meet tissue needs.

  • Left Heart Failure: Results in pulmonary congestion as blood backs up into the lungs, leading to pulmonary edema.

  • Right Heart Failure: Causes systemic congestion, marked by swelling in distal body parts.