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:
  • Adrenaline increases heart rate during exercise or stress, while thyroid hormones can raise resting heart rate when elevated over a long period.

Blood Pressure

• Function:
  • Blood pressure influences heart rate through a feedback mechanism. When blood pressure increases, heart rate may decrease, and when blood pressure decreases, heart rate may increase. This helps maintain homeostasis and ensure adequate perfusion to vital organs.
• Mechanism:
  • Baroreceptors located in the carotid arteries and aortic arch detect changes in blood pressure. If blood pressure is too high, baroreceptors signal the brain to activate the parasympathetic nervous system, reducing heart rate. If blood pressure is too low, the sympathetic nervous system is activated to increase heart rate.
• Effect on Heart Rate:
  • High blood pressure: Parasympathetic activation leads to a lower heart rate to reduce the workload on the heart.
  • Low blood pressure: Sympathetic activation increases heart rate to help raise blood pressure and improve blood flow.

2. Intrinsic Regulation

Intrinsic regulation refers to the heart’s ability to regulate its own rhythm, independent of external influences, through the action of specialized cells and structures.

Sequence of Excitation

• The sequence of excitation is the electrical pathway that regulates the contraction of the heart muscle. It ensures that the heart contracts in a coordinated manner.
• Key Components:
  1. Sinoatrial (SA) Node: The SA node, located in the right atrium, acts as the heart’s natural pacemaker. It generates electrical impulses that initiate each heartbeat.
  2. Atrioventricular (AV) Node: The AV node, located at the junction of the atria and ventricles, briefly delays the electrical signal to allow the atria to contract and empty blood into the ventricles before ventricular contraction.
  3. Bundle of His: After the AV node, the signal travels down the Bundle of His to the right and left bundle branches, and then to the Purkinje fibers, which distribute the electrical impulse throughout the ventricles, causing them to contract.
• Mechanism:
  • The SA node generates an electrical impulse that spreads through the atria, causing them to contract. The impulse reaches the AV node, where it is delayed before traveling down the Bundle of His and Purkinje fibers, resulting in ventricular contraction.
  • The heart’s intrinsic electrical conduction system is what allows the heart to beat rhythmically and efficiently, ensuring coordinated contractions of the atria and ventricles.
• Effect on Heart Rate:
  • The intrinsic pacemaker function of the SA node sets the heart rate under normal conditions, with the sympathetic and parasympathetic nervous systems modulating it based on external needs.