cardiac output 24

Cardiac Output and Stroke Volume

  • Cardiac Output (CO): Defined as the product of stroke volume (SV) and heart rate (HR).

    • CO = SV x HR

  • Stroke Volume (SV): The volume of blood pumped by the heart with each beat.

    • Formula: SV = End Diastolic Volume (EDV) - End Systolic Volume (ESV)

End Diastolic Volume (EDV)

  • Influenced by venous return and stretch.

  • Venous Return Mechanisms:

    • Skeletal Muscle Pump: Muscles contract, compressing veins to assist blood flow back to the heart.

    • Respiratory Pump: Changes in thoracic pressure during breathing facilitate blood flow to the heart.

    • Vasoconstriction: Constriction of large blood vessels increases venous return.

End Systolic Volume (ESV)

  • Influenced by contractility.

  • Factors Affecting Contractility:

    • Sympathetic Nervous System (SNS): Increases contractility by releasing norepinephrine.

    • Epinephrine: Enhances contractility and acts as a hormone.

    • Thyroxine: Affects contractility through metabolic actions.

Heart Rate (HR)

  • Influenced by both sympathetic and parasympathetic nervous systems.

  • Sympathetic Control:

    • Activates the Cardioacceleratory Center in the medulla.

    • Increases HR via the cardiac sympathetic plexus releasing norepinephrine.

  • Parasympathetic Control:

    • Activates the Cardioinhibitory Center in the medulla.

    • Decreases HR via the vagus nerve releasing acetylcholine.

  • Factors Influencing Heart Rate:

    • Exercise

    • Blood Pressure

    • Stress and Fear

Relationship Between Heart Rate and Cardiac Efficiency

  • Lower heart rate allows for more time to fill with blood, increasing stroke volume and overall cardiac efficiency.

    • A slower heart rate can lead to greater contractile force due to increased stretch of the heart.

Autonomic Nervous System (ANS) Review

  • Role of Cardioinhibitory Center:

    • Receives sensory inputs about blood pressure and oxygen levels.

    • Sends commands via the vagus nerve to decrease heart rate.

  • Role of Cardioacceleratory Center:

    • Increases heart rate and contractility through sympathetic activation.

    • Targets SA node, AV node, and ventricular muscle.

Medical Terminology

  • Bradycardia: Low heart rate (below 50 beats per minute).

    • Normal for some endurance athletes but concerning for the general population.

  • Tachycardia: High heart rate (above 100 beats per minute at rest).

    • Can be sinus tachycardia (less severe) or ventricular tachycardia (potentially lethal arrhythmia).

Beta Blockers

  • Medications that decrease heart rate and contractility.

  • Used for various cardiac conditions and work by blocking adrenergic receptors.

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