In-Depth Notes on Heart Beat & Cardiac Output

The Cardiovascular System

Learning Outcomes

  • Relate the structure of the heart to its function.
  • Discuss factors affecting:
    • Cardiac Output (CO)
    • Stroke Volume (SV)
    • Heart Rate (HR)
  • Understand clinical correlations, especially heart failure.

The Heart: External Features

  • External Structures
    • Superior vena cava: Carries deoxygenated blood from the upper body to the heart.
    • Aorta: Major artery supplying oxygenated blood from the heart to the body.
    • Pulmonary trunk: Divides into left and right pulmonary arteries that carry blood to the lungs.
    • Percardium: Protective sac surrounding the heart.
    • Coronary arteries: Supply blood to the heart muscle.
    • Auricles: Flaps on the atria to increase volume capacity.

The Heart: Internal Features

  • Internal Structures
    • Fossa ovalis: Remnant of the fetal foramen ovale, a hole between atria in fetal development.
    • Pectinate muscles: Ridges in the atria that help in contraction.
    • Chamber structures: Right atrium, left atrium, right ventricle, left ventricle with valves (tricuspid and mitral) controlling blood flow.

Blood Flow through the Heart

  • Pulmonary Circulation: Blood moves from the heart to the lungs and back to the heart.
  • Systemic Circulation: Blood moves from the heart to the rest of the body and back.

Coronary Circulation

  • Vital for providing blood to the myocardium.
  • Begins at the left & right coronary arteries.
  • Anastomoses: Connections that provide alternative routes for blood supply in case of blockages.

Cardiac Muscle

  • Features:
    • Gap junctions: Allow electrical impulses to pass between cardiac cells.
    • Conducting system: Coordinates contraction of heart muscles.

Conduction Pathway

  • SA Node: Primary pacemaker, initiates heartbeats.
  • AV Node: Delays impulse to allow atrial contraction before ventricles contract.
  • His-Purkinje Network: Distributes impulses throughout ventricles to trigger contraction.

Autorhythmic Cells

  • Generate action potentials without external stimuli; main components include:
    • SA node (pacemaker)
    • Conduction system elements.

Action Potential Generation

  • Depolarization: Initiated by Na+ and Ca2+ influx.
  • Repolarization: Involves K+ outflow, resetting the resting membrane potential.

Cardiac Cycle & Electrocardiogram (ECG)

  • Cardiac Cycle: The sequence of events in one heartbeat.
  • ECG: Measures the electrical activity of the heart.

The Heart Sounds

  • "Lub": Closure of AV valves during ventricular contraction.
  • "Dubb": Closure of semilunar valves during ventricular relaxation.

Cardiac Output (CO)

  • Volume of blood ejected from the left ventricle into the aorta per minute.
  • Formula: CO = SV x HR Where:
    • CO: Cardiac Output (mL/min)
    • SV: Stroke Volume (mL/beat)
    • HR: Heart Rate (beats/min)
  • Cardiac Reserve: Difference between resting CO and maximum CO.

Stroke Volume (SV)

  • The amount of blood pumped from the ventricle per heartbeat.
  • SV Calculation: SV = EDV - ESV Where:
    • EDV: End-diastolic volume
    • ESV: End-systolic volume

Determinants of Stroke Volume

  • Preload: Degree of stretch of heart muscle before contraction.
  • Contractility: Strength of contraction independent of preload.
  • Afterload: Pressure the heart must generate to eject blood.

Regulation of Cardiac Output (CO)

  • Factors influencing CO:
    • Increased venous return leads to increased preload.
    • Positive inotropic agents enhance contractility and SV.
    • Reduced arterial pressure decreases afterload, increasing SV.

Heart Rate Regulation

  • Influenced by:
    • Chemicals: Hormones (e.g., epinephrine), ions (K+, Ca2+).
    • Nervous system: Sympathetic stimulation increases HR; parasympathetic decreases.
    • Physical factors: Age, gender, temperature.

Clinical Correlations: Heart Failure

  • Heart failure occurs when the heart cannot pump enough blood, leading to:
    • Deprivation of oxygen and nutrients to vital organs.
  • Causes: Coronary artery disease, hypertension, cardiomyopathy, arrhythmias, infections.
  • Symptoms: Pulmonary edema, peripheral edema, fatigue, shortness of breath.