List the physiological factors which increase respiratory rate and explain their
mechanism
It was expected that candidates would provide not only a list of factors which increase respiratory rate but to give physiological reasoning behind each mechanism. Providing a list of factors was not sufficient for a pass. Vague, imprecise answers attracted fewer marks. A request to explain a mechanism requires the candidates to write a comprehensive physiological reasoning. Many candidates did not demonstrate
a comprehensive understanding of these mechanisms. Most answers failed to include all the stimulants of respiratory rate, especially the non-chemical controls. Many answers were not structured or structured in a way that meant they missed many of the mechanisms or that they repeated the same information through the answer which used time but failed to gain additional marks.
This hopefully lends a structure for the next time this appears:
| 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 preceeds 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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