Cardiac Physiology - Anatomy and Physiology 2e

Learning Objectives

• Understand the relationship between heart rate and cardiac output.
• Describe how exercise affects heart rate.
• Identify cardiovascular centers and reflexes regulating heart function.
• Discuss factors affecting heart rate.
• Differentiate between positive and negative inotropic factors affecting heart contractility.
• Summarize factors influencing stroke volume and cardiac output.
• Describe the cardiac response to blood flow and pressure variations.

Cardiac Regulation

• Autorhythmicity: Heart cells beat regularly but respond to external influences.
• Neural & Endocrine Controls: Regulate cardiac function; sensitive to electrolytes.

Resting Cardiac Output

• Cardiac Output (CO): Blood pumped by each ventricle per minute.
  • Formula: CO = HR (Heart Rate) x SV (Stroke Volume).
  • Normal SV: 55-100 mL; resting HR: 60-100 bpm.
  • Mean CO: 5.25 L/min.
• Factors Influencing CO: Includes heart rate, stroke volume, heart size, physical condition, and contractility.

Exercise and Maximum Cardiac Output

• Increased HR: May reach 150 bpm during exercise.
• Increased SV: From 70 mL to 130 mL.
• Cardiac Reserve: Difference between maximum and resting CO.

Heart Rates

• Age Variation: HR decreases from birth to adulthood, then increases with age.
• Maximum HR: Approximately 220 bpm minus individual’s age.

Disorders of the Heart: Abnormal Heart Rates

• Bradycardia: Resting HR below 60 bpm; not significant without symptoms.
• Tachycardia: Resting HR above 100 bpm; may indicate stress or other disorders.

Correlation Between Heart Rates and Cardiac Output

• Initial Increase in CO: HR initially increases SV; CO stabilizes as HR continues to rise.
• Target HR: Maintain 120-160 bpm for health benefits.

Cardiovascular Centers

• Cardioaccelerator & Inhibitory Centers: Located in medulla oblongata.
• Sympathetic & Parasympathetic Stimulation: Affect HR via neurotransmitters (NE and ACh).

Input to the Cardiovascular Center

• Receptors: Proprioreceptors, baroreceptors, chemoreceptors provide input.
• Limbic System: Emotional state impacts HR.

Disorders of the Heart: Broken Heart Syndrome

• Stress-Induced Cardiomyopathy: Weakening of myocardium due to stress.

Other Factors Influencing Heart Rate

• Hormones & Ions: Epinephrine, NE, thyroid hormones, calcium, potassium, and sodium influence HR.
• Environmental Factors: Temperature, nicotine, caffeine.

Stroke Volume

• Preload, Contractility, Afterload: Main factors affecting SV.
• Frank-Starling Law: Greater stretch leads to stronger contraction.

Factors Influencing Stroke Volume

• Preload: Dependent on ventricular filling (EDV).
• Contractility: Force of contraction affects ESV.
• Afterload: Tension needed to eject blood against resistance.

Conclusion

• Maintaining Homeostasis: Balance between sympathetic and parasympathetic activities, electrolyte levels, and hormonal balance is essential for normal cardiac function.