List the physiological factors that increase respiratory rate. Include an explanation of the mechanism by which each achieves this increase.
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 factor | Mechanism | Physiological effect |
| PaCO2 |
Sensed by peripheral chemoreceptors: |
Increased PaCO2 increases the respiratory rate and tidal volume Decreased PaO2 increases the respiratory rate |
| PaO2 | ||
| pH |
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) |
| Temperature |
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 |
| Exercise |
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. |
| Pregnancy |
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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