31. Heart Rate, Stroke Volume, and Cardiac Output and Their Changes During Different Physiological Conditions

Heart Rate (HR)

• Normal heart rate: 60–90 bpm

• Tachycardia: Heart rate >100 bpm (e.g., exercise, stress)

• Bradycardia: Heart rate <60 bpm (e.g., sleep, professional athletes, increased vagal tone)

• Types of heart rate origins:

  • Sinus (nomotopic)

  • Ectopic (heterotopic) – atrial, nodal, or ventricular

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Stroke Volume (SV)

• Normal SV at rest: 70 mL

• SV = EDV – ESV

• End Diastolic Volume (EDV):

  • ~130 mL

  • Blood volume in the ventricles at end of diastole

  • Left ventricular EDV: 100–300 mL

• End Systolic Volume (ESV):

  • ~60 mL

  • Blood volume remaining in the ventricles at end of systole

Factors Affecting Stroke Volume:

• Heart size

• Fitness level

• Gender

• Preload: ↑ Preload → ↑ SV

  • Venous blood flow into heart

  • Associated with myocardial fibre stretch

• Afterload: ↓ Afterload → ↑ SV

  • Resistance to blood flow depending on aortic pressure during systole

• Contractility: ↑ Contractility → ↑ SV

Starling’s Law: The more the heart fills (↑ EDV), the harder it contracts → ↑ SV

Cardiac Output (CO)

• At rest: ~5 L/min

• During exercise: 4–7 times higher

• CO = SV × HR

Factors Affecting CO:

• ↑ Heart rate (due to activity, stress) → ↑ CO

• ↑ Stroke volume (via ↑ preload, ↓ afterload, ↑ contractility) → ↑ CO

• Baroreceptors: Detect pressure changes → adjust vasoconstriction/vasodilation, contractility, and venous return (preload)

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