Introduction to the Cardiovascular System

Overview of Circuits

The cardiovascular system is composed of two main circuits: the pulmonary and systemic circuits. These circuits are responsible for transporting blood throughout the body and facilitating gas and nutrient exchange.

• Pulmonary Circuit: Carries blood to and from the lungs for gas exchange.

• Systemic Circuit: Carries blood to and from the rest of the body.

• Vessels:

  • Arteries: Efferent vessels that carry blood away from the heart.

  • Veins: Afferent vessels that carry blood to the heart.

  • Capillaries: Sites of gas and nutrient exchange.

Characteristics of Cardiac Cells

Nodal and Contractile Cells

Cardiac muscle tissue consists of two main cell populations: nodal cells and contractile cells.

• Nodal Cells:

  • Pacemaker cells that spontaneously depolarize and generate action potentials without external influence.

  • Exhibit an unstable resting membrane potential.

• Contractile Cells:

  • Have a stable resting membrane potential.

  • Intercalated disks allow cells to be chemically linked, facilitating coordinated contraction.

Cardiac Action Potentials

Nodal Cell Action Potentials

Nodal cells exhibit a "slow response" action potential characterized by a slow initial depolarization phase and repolarization to an unstable, slowly depolarizing "resting" potential.

Contractile Cell Action Potentials

Contractile cells have a stable resting potential at -90mV and a plateau phase, resulting in a long absolute refractory period.

Cardiac Muscle Contraction vs Skeletal Muscle

Comparison and Role of Calcium

Cardiac muscle contraction is similar to skeletal muscle except for the role of calcium ions.

• 20% of calcium ions required for contraction enter during the plateau phase.

• Arrival of extracellular Ca2+ triggers release of calcium from the sarcoplasmic reticulum (SR).

• Cardiac muscle is highly sensitive to extracellular Ca2+ concentrations.

The Conducting System of the Heart

Components and Function

The conducting system consists of specialized cardiac muscle cells that initiate and distribute electrical impulses, stimulating contraction.

• SA Node: Depolarizes first, establishing heart rate.

• AV Node: Delays impulse, allowing atrial contraction.

• AV Bundle, Bundle Branches, Purkinje Fibers: Distribute stimulus through myocardium.

Impulse Conduction

• SA node initiates depolarization, spreading across both atria.

• AV node depolarizes after a delay.

• Impulse spreads through AV bundle, bundle branches, and Purkinje fibers to ventricles.

Abnormal Pacemaker Function

Arrhythmias and Ectopic Pacemakers

• Arrhythmias: Abnormal rhythm or beat of the heart.

• Bradycardia: Abnormally slow heart rate.

• Tachycardia: Abnormally fast heart rate.

• Ectopic Pacemaker: Abnormal cells generate high rates of action potentials, bypassing the conducting system and disrupting ventricular contractions.

The Electrocardiogram (ECG/EKG)

Recording and Interpretation

An ECG records electrical events in the heart, helping diagnose damage and arrhythmias.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization.

• T wave: Ventricular repolarization.

• P–R interval: Start of atrial depolarization to start of QRS complex.

• Q–T interval: Ventricular depolarization to ventricular repolarization.

Energy for Cardiac Contractions

Aerobic Metabolism and Oxygen Supply

The heart requires large amounts of ATP, primarily produced through aerobic metabolism.

• Energy is derived from mitochondrial breakdown of fatty acids and glucose.

• Oxygen is supplied by circulating hemoglobin and stored in myoglobin within cardiac muscle.

Coronary Circulation and Heart Disease

Coronary Artery Disease (CAD)

• Coronary circulation provides blood to heart musculature.

• CAD involves partial or complete blockage of coronary arteries, leading to ischemia and reduced cardiac performance.

• Insufficient oxygen delivery results in hypoxia and ischemia, potentially causing cell death.

The Cardiac Cycle

Phases and Pressure-Volume Relationships

The cardiac cycle is the period between the start of one heartbeat and the next, divided into systole and diastole.

• Systole: Chamber contracts and pushes blood.

• Diastole: Chamber relaxes and fills with blood.

• Blood moves from areas of higher to lower pressure.

Phases of the Cardiac Cycle

• Atrial Systole: Atria contract, AV valves open, blood ejected into ventricles.

• Atrial Diastole: Atria relax.

• Ventricular Systole: Ventricles contract, AV valves close, semilunar valves open for ejection.

• Ventricular Diastole: Ventricles relax, all valves closed, passive filling occurs.

Pressure-Volume Loop

Graphical Representation

Pressure-volume loop graphs illustrate changes in pressure and volume during a single cardiac cycle and can show alterations due to disease.

• Maximal pressure decreases with loss of contractile cells.

• Volume shifts right as blood remains in ventricle due to decreased contractility.

Heart Sounds

Valvular Events

• Lubb: Closing of AV valves.

• Dubb: Closing of semilunar valves.

• Gurgling sounds may indicate infection or heart murmur.

Cardiodynamics

Key Volumes and Formulas

Cardiodynamics refers to the movement and force generated by cardiac contractions.

• End-diastolic volume (EDV): Volume of blood in ventricle at end of diastole.

• End-systolic volume (ESV): Volume remaining after systole.

• Stroke volume (SV): Amount of blood ejected per beat.

• Ejection fraction: Percentage of EDV represented by SV.

Cardiac Output

• Amount of blood pumped by the left ventricle in one minute.

• Formula:

Factors Affecting Heart Rate and Stroke Volume

Regulation of Heart Rate

• Parasympathetic: Acetylcholine (ACH) via vagus nerve decreases heart rate.

• Sympathetic: Norepinephrine (NE) and epinephrine (EPI) increase heart rate.

• Hormones: Thyroid hormones, NE, and EPI affect rate.

• Venous Return: Bainbridge reflex increases heart rate with increased venous return.

• Medulla: Cardioacceleratory and cardioinhibitory centers regulate autonomic output.

Factors Affecting Stroke Volume

• Preload: Degree of stretch of cardiac muscle before contraction.

• Contractility: Force of contraction at a given preload.

• Afterload: Pressure the heart must overcome to eject blood.

Summary of Cardiac Physiology

• Heart anatomy and electrophysiology

• ECG interpretation

• Cardiac action potentials

• Conduction system

• Excitation-contraction coupling

• Cardiac cycle and pressure/volume loops

• Regulation of cardiac output

Practice Questions

• Cardiac output of the right side of the heart is what percentage of the cardiac output of the left side of the heart? Answer: 100% (Cardiac output of left and right sides should be equal)

• From outermost to inner, name the 3 distinct layers of the heart.

• Walk through blood flow as it would move through the heart if you start at the Vena Cava’s.

• What are the key steps in an action potential of a SA nodal cell? Ventricle cell?

Additional info:

• Academic context was added to clarify the role of calcium, the phases of the cardiac cycle, and the regulation of cardiac output.