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)
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.
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.
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
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.
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.
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).
Medications that decrease heart rate and contractility.
Used for various cardiac conditions and work by blocking adrenergic receptors.