Heart Rate

Heart Rate

What is normal heart rate? Describe variation and regulation of heart rate.
Answer:

Heart rate:

• Normal heart rate in adults in 70-80 beats per minute.

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Variations:

1. Physiological variations:

1. Age.
   • After birth, heart rate decreases with age.
   • However, in old age it is high.
2. Sex.
   • Heart rate is slightly higher in females.
3. Surface area.
   • Heart rate is inversely proportional to surface area.
4. Body temperature.
   • For each 1oF rise in body temperature, heart rate increase by about 10 beats per minutes.
5. Emotions.
   • Excitement, fear, anger – increases heart rate.
   • Shock, grief decreases heart rate.
6. Exercise.
   • Heart rate increases with severity of exercise.
7. Respiration.
   • During inspiration, heart rate increases while during expiration, heart rate decreases.
8. Metabolic rate.
   • An increase in metabolic rate, increases heart rate.

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2. Pathological variations:

1. Increase in intracranial tension, heart rate decreases.
2. Thyrotoxicosis – increases heart rate.
3. In fever, anaemia, hypoxia – heart rate increases.
4. In jaundice, hypothyroidism- heart rate decreases.

3. Drugs:

• Epinephrine – increases heart rate.
• Nor epinephrine causes reflex brady cardia.

Regulation of heart rate:

• Heart rate is regulated by the nervous mechanism.
• It consists of three components.

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1. Vasomotor center:

Situation:

• Bilaterally in the reticular formation of medulla oblongata and lower part of the pons.

Areas:

1. Vasodilator area/depressor area.
   • Forms medial portion of vasomotor center controlled by cerebral cortex and hypothalamus.
   • Functions:
   • Decreases heart rate
   • Decreases BP
   • Decreases stroke volume
   • Decreases venous return.
   • Causes vasodilation.
2. Vasoconstrictor/pressor area.
   • Forms lateral portion of vasomotor center.
   • Controlled by cerebral cortex and hypothalamus.
     Functions:
   • Incraeses heart rate
   • Increases BP.
   • Increases stroke volume.
   • Increases venous return.
3. Sensory areas.
   • Causes vasoconstriction.
   • Forms posterior part of vasomotor center.
   • It receives sensory impulses via. glossopharyngeal and vagus nerve.
     Function:
   • Controls vasoconstrictor and vasodilator area.

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2. Motor (Efferent) Nerve Fibres:

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3. Sensory (Afferent) nerve fibers:

• These fibers pass through the inferior cervical sympathetic nerve.
  Function:
• Carries pain and stretch sensations from the heart to the brain.