Overview:

• Heart rate varies frequently in response to body needs and external stimuli.
• Several internal and external factors influence the medulla oblongata, the brain’s cardiovascular control center to adjust heart rate by either:
  • stimulating the SNS cardioaccelerator nerve which releases Epinephrine and Norepinephrine on the SA node, binding to beta-1 adrenergic receptors, opening Ca⁺⁺ channels, causing depolarization, bringing cardiomyocytes closer to threshold.
  • stimulating the PSNS vagus nerve which releases Acetylcholine on the SA node, binding to muscarinic receptors, opening K+ channels, causing hyperpolarization, bringing cardiomyocytes further from threshold.

Factors That Can Increase Heart Rate:

1. Thyroid Hormone:

• Regulates basal metabolic rate (BMR).
• Increased thyroid hormone → higher BMR → increased heart rate.
• Conditions:
  • Hyperthyroidism: Excess thyroid hormone leads to faster heart rates.
  • Hypothyroidism: Less thyroid hormone results in slower heart rate.

2. Stress Response & Temperature:

• Biological Stressors: Exercise, Extreme heat or cold, Illness, or Emotional stress.
• • The body activates fight or flight (Sympathetic Nervous System, SNS), increasing heart rate to deliver more oxygen and nutrients.
• Heat: Blood vessels dilate for heat dissipation, requiring a faster heartbeat.
• Cold: The body may also increase heart rate to maintain core temperature.

3. Physical Activity & Exercise:

• During exertion:
  • The cardiac accelerator nerve releases epinephrine on beta-1 adrenergic receptors on the SA node.
  • This speeds depolarization, increasing both heart rate and force of contraction.
  • Supports muscle activity by delivering oxygen-rich blood more rapidly.

4. Smoking & Nicotine Use:

• Nicotine stimulates release of epinephrine.
• Results:
  • Increased heart rate.
  • Vasoconstriction and narrowing of blood vessels.
  • Elevated blood pressure, making the heart work harder.
  • Over time, can cause heart deterioration.

5. Carbon Monoxide Poisoning:

• Carbon Monoxide is a product of gas combustion (e.g., engine exhaust)
• Binds to hemoglobin, displacing oxygen.
• Causes hypoxia.
• Chemoreceptors detect low oxygen, signaling the medulla to increase heart rate to compensate.

6. Lung Conditions & Tar:

• Tar buildup damages lung tissue, reducing gas exchange surface area.
• Leads to hypoxia, which triggers reflex mechanisms to speed up the heart.
• Pulmonary fibrosis and alveolar damage due to frequent lung infections or inhalation of damaging chemicals

7. Pregnancy:

• Increases metabolic rate due to fetal growth and supporting physiological changes.
• Larger ATP requirement and oxygen demand.
• Heart rate increases to sustain the increased blood volume and metabolic activity.

Summary:

Multiple factors—hormonal, environmental, physiological, and pathological—can influence heart rate. These responses aim to ensure adequate oxygen and nutrient delivery during stress, activity, or illness. Understanding these factors helps evaluate normal physiology and the body’s adaptation to various stimuli.