List the physiological factors that increase respiratory rate. Include an explanation of the mechanism by which each achieves this increase.

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College Answer

The main points candidates were expected to cover included:
· A description of the central and peripheral chemoreceptors, their predominant stimuli and
effect on ventilation.
· PaCO2 as the main influence on normal ventilation, the near-linear relationship to minute
ventilation around the normal range, and how CO2 produces this effect.
· PaO2 and pH and their sites of action.
· Other stimuli to ventilation – exercise, pregnancy, temperature, baroreceptors.
Candidates frequently confused central and peripheral receptor activities and failed to provide
any relative significance to the major factor(s). The use of a graph relating the main factors
to minute ventilation would have been helpful.

Reference: Nunn 6th edition 60-68
Kam 1st edition 92-98


This question is essentially identical to Question 6 from the first paper of 2010.

Physiological Factors which Influence the Respiratory Rate

Physiological factor Mechanism Physiological effect

Sensed by peripheral chemoreceptors:
- Carotid bodies (glossopharyngeal nerve)
- Aortic bodies (vagus nerve)

Increased PaCO2 increases the respiratory rate and tidal volume 

Decreased PaO2 increases the respiratory rate
(rapidly acting breath-to-breath control of respiration)
- this response to hypoxia is triphasic


Sensed by central chemoreceptors in the medulla

Decreased pH in the CSF increases the respiratory rate and tidal volume
(slow acting, steady state control; adjustments occur over minutes)

Increased sensitivity of periphperal chemoreceptors to O2

Increased sensitivity of central chemoreceptors to changes in pH

A rise in temperature will increase the minute volume at any given PaCO2 and PaO2 level

Responses to hypoxia and hypercapnia are
amplified by hyperthermia


Descending control of muscle activity simultaneously simulates the central respiratory control centres

The ventilatory response of exercise is increased ventilation, and because this is not a feedback mechanism the increase in ventilation is simultaneous with the beginning of exercise, or actually slightly precedes it. 

Progesterone acts directly on central integrative control of ventilation

During pregnancy, minute volume increases and the stable PaCO2  baseline progressively decreases, producing the respiratory alkalosis of pregnancy
Blood pressure

Sensed by aortic chemoreceptors and carotid sinus baroreceptors

Hypertension decreases the respiratory rate and hypotension increases it.

With a sufficient acute hypertensive event, respiration may briefly cease ("adrenaline apnea")



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