Question

(from The ECG). Explain the physiological mechanisms that are responsible for the variation in resting heart rate that occurs during inspiration and expiration.

Answer 1

The variation in the heart rate during respiratory cycle ( inhalation and exhalation) is called respiratory sinus arrhythmia

The mechanism is as follows:

• During inspiration, the stretch receptors of the lung are activated
• They send sensory information to the nucleus tracts solitarius.
• This nucleus then sends information to the nucleus ambiguous.
• On stimulation of nucleus ambiguous, there is inhibition of the vagal nerve.
• The vagal nerve is the parasympathetic nerve of the heart.
• Therefore inhibition of the vagus reduces the parasympathetic influence over the heart.
• Therefore, the sympathetic autonomic influence is unrestricted.
• This causes increase in heart rate ( as sympathetic input increases the heart rate and parasympathetic reduces the heart rate)
• The reverse happens during exhalation
• Also, during inspiration, thoracic volume increases.
• As volume increases, the intrathoracic pressure reduces.
• This causes increase in the blood flow to the heart.
• This stimulates the baroreceptors.
• These baroreceptor inhibit the vagal nerve. Thus increasing the heart rate.
• During exhalation, the thoracic volume reduces a d the pressure inside increases.
• This reduces the venous return to the heart.
• This reduces the activation of baroreceptors.
• Therefore, the vagus nerve gets actived and the heart rate reduces.