19.4 Cardiac Physiology

The heart's incredible ability to adapt to our body's changing needs is at the core of cardiac physiology. From adjusting heart rate during exercise to maintaining blood pressure, the cardiovascular system works tirelessly to keep us alive and thriving.

Understanding how factors like autonomic nervous system, hormones, and medications influence heart function is crucial. This knowledge helps us grasp how the heart responds to various stimuli and how we can maintain cardiovascular health through lifestyle choices and medical interventions.

Cardiac Physiology

Heart rate and cardiac output

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• Heart rate represents the number of times the heart contracts per minute
• Cardiac output measures the volume of blood pumped by the heart per minute
  • Calculated by multiplying heart rate and stroke volume $Cardiac Output = Heart Rate \times Stroke Volume$
• Increasing heart rate while maintaining stroke volume leads to a proportional increase in cardiac output (doubling heart rate doubles cardiac output)

Exercise effects on cardiovascular function

• Exercise increases the body's metabolic demands, requiring more oxygen and nutrients
• The cardiovascular system adapts by increasing heart rate and cardiac output to meet these demands
  • Sympathetic nervous system activation during exercise releases norepinephrine, which binds to beta-1 receptors in the heart
    • Increases heart rate (positive chronotropy) and contractility (positive inotropy)
• Increased venous return during exercise (due to muscle pump and respiratory pump) contributes to increased stroke volume via the Frank-Starling mechanism

Cardiovascular control mechanisms

• Medulla oblongata contains the cardiovascular center, which regulates heart rate and contractility
  • Cardioacceleratory center increases heart rate and contractility through sympathetic stimulation
  • Cardioinhibitory center decreases heart rate and contractility through parasympathetic stimulation
• Baroreceptor reflex maintains blood pressure homeostasis by detecting changes in blood pressure
  • Increased blood pressure stimulates baroreceptors (in aortic arch and carotid sinuses), triggering a parasympathetic response to decrease heart rate and contractility
  • Decreased blood pressure reduces baroreceptor stimulation, leading to a sympathetic response to increase heart rate and contractility

Factors in heart rate regulation

• Autonomic nervous system influences heart rate and contractility
  • Sympathetic stimulation (norepinephrine) increases heart rate and contractility
  • Parasympathetic stimulation (acetylcholine) decreases heart rate and contractility
• Hormones affect heart rate and contractility
  • Thyroid hormones (T3 and T4) and catecholamines (epinephrine and norepinephrine) increase heart rate and contractility
• Ions play a role in myocardial contraction
  • Calcium is essential for myocardial contraction, with increased extracellular calcium leading to increased contractility
• Medications can modulate heart rate and contractility
  • Beta-blockers (propranolol) decrease heart rate and contractility
  • Digitalis increases contractility by increasing intracellular calcium

Inotropic agents vs heart function

• Positive inotropic agents increase myocardial contractility
  • Examples include digitalis, calcium, epinephrine, and norepinephrine
  • Mechanism involves increasing intracellular calcium concentration or sensitivity of myofilaments to calcium
• Negative inotropic agents decrease myocardial contractility
  • Examples include beta-blockers, calcium channel blockers (verapamil), and barbiturates
  • Mechanism involves reducing intracellular calcium concentration or decreasing sensitivity of myofilaments to calcium

Determinants of cardiac performance

• Stroke volume represents the volume of blood ejected from the left ventricle per beat
  • Influenced by preload (ventricular filling), afterload (resistance against which the ventricle pumps), and contractility (inherent ability of myocardium to contract)
• Increased preload leads to increased stroke volume (Frank-Starling mechanism)
• Increased afterload leads to decreased stroke volume
• Increased contractility leads to increased stroke volume
• Cardiac output is determined by the product of heart rate and stroke volume $Cardiac Output = Heart Rate \times Stroke Volume$
• Ventricular pressure-volume loop illustrates the relationship between ventricular pressure and volume during a cardiac cycle

Heart's response to hemodynamic changes

• Autoregulation allows the heart to maintain relatively constant blood flow despite changes in perfusion pressure
  • Coronary circulation adapts to meet the metabolic demands of the heart muscle
• Anrep effect describes an increase in contractility in response to increased afterload
  • Helps maintain stroke volume despite increased resistance
• Bainbridge reflex involves an increase in heart rate in response to increased venous return
  • Helps accommodate increased blood volume and maintain cardiac output

Cardiac Electrical Activity and Monitoring

• Cardiac action potential is the electrical signal that triggers contraction of cardiac muscle cells
• Electrocardiogram (ECG) records the electrical activity of the heart over time
• Heart sounds are produced by the closing of heart valves and blood flow through the heart
  • Provide information about cardiac function and potential abnormalities