Question 4

List the physiological factors which increase respiratory rate and explain their

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College Answer

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 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 preceeds 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")

  • Major sensory and motor pathways:
    • White matter tracts, long bundles of axons, whereas the cell bodies reside in the grey matter.
    • Many decussate, i.e cross midline from their origins to their destination
    • Many are made up of three or more neurons
  • Motor neuron pathways:
    • First order neurons: motor cortex,  "upper motor neurons"
    • Second order neurons: grey matter of the spine, "internuncial" neurons
    • Third order neurons: grey matter of the spine, "lower motor neurons"
  • Sensory neuron pathways:
    • First order neuron cell bodies are in the dorsal root ganglia
    • Second order neurons are in the dorsal horn of the spinal cord
    • Third order neurons are in the destination organ, eg. thalamus
  • Main motor tracts of the spinal cord and their function:
    • Posterior:
      • Septomarginal fasciculus and interfascicular fasciculus: internal spinal reflex arcs
    • Lateral:
      • Lateral corticospinal tract: fine quick voluntary movement
      • Lateral reticulospinal tract: posture, flexor movements
      • Rubrospinal tract: posture, flexor movements
    • Anterior:
      • Anterior corticospinal tract: coarse voluntary movement
      • Anterior reticulospinal tract: posture, extensor movements
      • Vestibulospinal tract: posture, extensor movements
      • Tectospinal tract: reflex postural movements (visual stimuli)
  • Main sensory tracts of the spinal cord and their function
    • Posterior:
      • Dorsal column tracts: propriception, vibration, light tough
    • Lateral:
      • Lateral spinothalamic tract: pain and temperature
      • Posterior spinocerebellar tract: tendon and joint position
      • Anterior spinocerebellar tract: tendon and joint position
      • Spinoolivary tract: cutaneous and proprioceptive information
      • Spinotectal tract: afferent information necessary for the  movement of the head in response to painful stimuli
    • Anterior:
      • Anterior spinothalamic tract: coarse touch and pressure


Wolff, Christopher B. "The physiological control of respiration." Molecular aspects of medicine 13.6 (1992): 445-567.

Remmers, John E. "A century of control of breathing." American journal of respiratory and critical care medicine172.1 (2005): 6-11.

Milne, J. A. "The respiratory response to pregnancy." Postgraduate medical journal 55.643 (1979): 318-324.

López-Barneo, J., et al. "Carotid body oxygen sensing." European Respiratory Journal 32.5 (2008): 1386-1398.

Schuitmaker, J. J., et al. "Ventilatory responses to respiratory and metabolic acid-base disturbances in cats." Respiration physiology 67.1 (1987): 69-83.

Borgbjerg, F. Molke, Kurt Nielsen, and Jan Franks. "Experimental pain stimulates respiration and attenuates morphine-induced respiratory depression: a controlled study in human volunteers." Pain 64.1 (1996): 123-128.

Natalino, Michael R., Clifford W. Zwillich, and John V. Weil. "Effects of hyperthermia on hypoxic ventilatory response in normal man." The Journal of laboratory and clinical medicine89.3 (1977): 564-572.

Heistad, D., et al. "Effect of baroreceptor activity on ventilatory response to chemoreceptor stimulation." Journal of Applied Physiology 39.3 (1975): 411-416.