Intrinsic and Extrinsic Regulation of the Heart

Heart rate (HR) is regulated by both intrinsic and extrinsic factors. Intrinsic regulation is the heart's ability to control its own rhythm, while extrinsic regulation involves factors external to the heart, such as the nervous system and hormonal control. Together, these mechanisms ensure the heart rate adapts to the body's needs.

1. Extrinsic Regulation

Extrinsic regulation of heart rate involves the autonomic nervous system (ANS) and hormonal influences. These factors modify the intrinsic rhythm of the heart to meet the body’s demands, particularly during exercise or stress.

Sympathetic Nervous System

• Function:
  • The sympathetic nervous system increases heart rate during times of stress, exercise, or excitement (fight-or-flight response).
• Mechanism:
  • Sympathetic nerve fibers release norepinephrine (noradrenaline) at the sinoatrial (SA) node, causing an increase in the rate of depolarization of the heart. This leads to a faster heart rate.
  • The sympathetic nervous system also stimulates the ventricular myocardium, increasing the force of contraction (contractility) and thus stroke volume.
• Effect on Heart Rate:
  • Increases heart rate, allowing for faster blood circulation to deliver more oxygen and nutrients to muscles during physical exertion.

Parasympathetic Nervous System

• Function:
  • The parasympathetic nervous system decreases heart rate, particularly during rest or relaxation (rest-and-digest response).
• Mechanism:
  • The vagus nerve (cranial nerve X) releases acetylcholine, which binds to receptors on the SA node, slowing down the rate of depolarization and reducing heart rate.
  • Parasympathetic stimulation primarily affects the SA node and atrioventricular (AV) node, leading to a decrease in heart rate and delaying the conduction of electrical signals through the heart.
• Effect on Heart Rate:
  • Reduces heart rate, conserving energy and promoting relaxation when the body is at rest.

Hormonal Control

• Function:
  • Hormones can influence heart rate by either increasing or decreasing it, often in response to stress or physical activity.
• Key Hormones:
  • Adrenaline (epinephrine): Released by the adrenal glands during stress or exercise. It increases heart rate by stimulating the SA node, similar to sympathetic stimulation, and also increases contractility and conduction speed.
  • Thyroid hormones (T3 and T4): These hormones increase the baseline metabolic rate, which can result in a higher resting heart rate over time.
• Effect on Heart Rate: