Cardiac Output and Physiological Regulation

Overview of Cardiac Output

  • Definition: Cardiac output (CO) is the total volume of blood that is pumped by each ventricle of the heart in one minute, reflecting the heart's efficiency in delivering oxygen and nutrients to tissues throughout the body.

  • Formula for Calculation:

    • Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)

      • Where Heart Rate is measured in beats per minute (bpm) and Stroke Volume is measured in milliliters (ml) per beat.

Key Concepts in Cardiac Output

Heart Rate and Stroke Volume

  • Heart Rate (HR): The number of cardiac cycles (heartbeat) per minute, which varies with activity.

    • Average resting heart rate: 60 to 100 bpm; factors such as fitness level, age, and emotional state alter this.

  • Stroke Volume (SV): The amount of blood ejected by one ventricle during a single contraction, measured in ml.

    • Example of SV:

      • At rest, stroke volume is typically around 70 ml/beat.

    • Calculation Example:

      • At rest: CO = 75 bpm × 70 ml/beat = 5250 ml/min or 5.25 L/min.

Maximum Cardiac Output

  • Definition: The maximum volume of blood the heart can pump during intense physical activity, serving as a measure of cardiovascular fitness.

  • Variability:

    • Generally higher in trained athletes as their hearts adapt to efficient pumping mechanisms compared to sedentary individuals.

    • Resting cardiac output varies depending on factors such as physical fitness, hydration status, and body positioning.

Cardiac Reserve

  • Definition: Cardiac reserve refers to the difference between resting cardiac output and the maximal cardiac output, representing the heart's capacity to increase output in response to physical stress or activity.

    • A higher cardiac reserve suggests better cardiovascular health and fitness levels, allowing for greater physical performance.

Factors Affecting Cardiac Output

Physiological Responses

  • Increased Venous Return: An increase in blood returning to the heart due to factors such as enhanced venous tone and muscular contractions.

    • Leads to higher end-diastolic volume (EDV), consequently raising stroke volume through the Frank-Starling mechanism.

Increased Force of Contraction

  • Effect of Sympathetic Stimulation (e.g., hormones like epinephrine and norepinephrine):

    • These hormones increase myocardial contractility, resulting in enhanced stroke volume.

    • During diastole, a more significant reduction in end-systolic volume (ESV) is also noted due to greater ventricular emptying.

Heart Rate Contributions

  • The heart rate is influenced by:

    • Increased sympathetic nervous system activity leads to elevated heart rate and consequently higher cardiac output.

    • Decreased parasympathetic activity allows for a significant rise in heart rate, enhancing overall cardiac output during stress or physical activity.

Stroke Volume Factors

  • Definition of Stroke Volume: This is computed as:
    extStrokeVolume(SV)=extEndDiastolicVolume(EDV)−extEndSystolicVolume(ESV)ext{Stroke Volume} (SV) = ext{End Diastolic Volume (EDV)} - ext{End Systolic Volume (ESV)}

Main Factors Affecting Stroke Volume

  1. Preload: The degree of stretch of cardiac muscle fibers (Frank-Starling law) before contraction, significantly influenced by venous return to the heart.

    • An increased preload results in increased cardiac output due to optimal muscle fiber length for forceful contractions.

    • Factors that can enhance venous return include:

      • A slower heartbeat allowing longer filling time during diastole.

      • Physical activity that employs muscle contractions to assist venous blood return.

  2. Contractility: This refers to the intrinsic strength of cardiac muscle contraction, independent of preload status.

    • Greater contractility results in a lowered ESV following contraction.

    • Positive inotropic agents, including epinephrine, improve contractility by augmenting calcium availability in cardiac cells.

    • Hormones like thyroid hormone and glucagon can increase contractility levels.

    • Factors that decrease contractility:

      • Elevated extracellular potassium concentrations.

      • The use of calcium channel blockers in pharmacological treatments.

  3. Afterload: Refers to the resistance that the ventricles must overcome to eject blood during systole.

    • For example, typical aortic pressure is about 80 mmHg, while pressure in the pulmonary trunk is approximately 10 mmHg.

    • Increased afterload reduces stroke volume due to the increased workload against the resistance.

Factors Influencing Heart Rate

  • Age: Fetal heart rates are higher, with a gradual decline throughout life.

  • Gender: Females typically exhibit higher average heart rates compared to males due to differences in body size and metabolic demands.

  • Exercise: Physical training boosts heart rate and also typically results in lower resting heart rates in trained individuals.

  • Body Temperature: Higher body temperatures lead to increased metabolic rates and subsequently elevated heart rates.

Terminology Related to Heart Rate

  • Tachycardia: A condition characterized by an elevated heart rate (>100 bpm), potentially resulting in arrhythmias if sustained over time.

  • Bradycardia: A term for abnormally slow heart rates (<60 bpm), which can lead to inadequate circulation of blood and oxygen supply to vital organs.

Summary of Cardiac Output Regulation

  • Physical exercise enhances venous return through muscle contractions, ultimately leading to increases in stroke volume and cardiac output.

  • Positive influences, such as hormonal effects from thyroid hormones and catecholamines, boost myocardial contractility, further elevating stroke volume.

  • Reduced parasympathetic nervous system activity or heightened sympathetic activity results in increased heart rate, thus raising overall cardiac output.

Development and Age-related Changes in the Heart

Fetal Development Remnants

  • Fossa Ovalis: This is a remnant of the foramen ovale, which closes after birth, creating a permanent indent in the atrial septum.

  • Ligamentum Arteriosum: Formerly the ductus arteriosus during fetal life, this structure closes after birth, leaving a fibrous remnant that prevents blood from mixing between the pulmonary and systemic circulations.

Age-related Cardiac Changes

  • Sclerosis of Heart Valves: A degenerative process leading to the abnormal hardening of heart valves, which can manifest as heart murmurs and may impede blood flow.

  • Decline in Cardiac Reserve: Aging can narrow the gap between maximal and resting cardiac outputs, affecting exercise tolerance and recovery.

  • Fibrosis: The replacement of healthy cardiac muscle with fibrous scar tissue, potentially leading to impaired function and arrhythmias.

  • Atherosclerosis: The accumulation of plaques in arterial walls can prevent normal blood flow and increases the risk of cardiovascular diseases; adopting a healthy lifestyle can help mitigate this risk.