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.