2006A10 List the physiological factors which increase respiratory rate and include a brief
explanation of the mechanism by which each achieves this increase.



·         Intro

·         Chemical factors

·         Mechanical factors

·         Increased metabolic rate

·         Other



·         Respiratory rate (RR) controlled by medullary respiratory centre

·         ↑Afferents -> ↑rate of discharge of pre-Botzinger pacemaker cells -> ↑rate of efferents to respiratory muscles -> ↑RR

·         Afferents may have synergistic effects


Chemical factors (most important):

Peripheral chemoreceptors

·   Carotid bodies (sensitive to ↑PaCO2, ↓PaO2, ↓pH)

·   Aortic bodies (sensitive to ↑PaCO2, ↓PaCO2)

·   Stimulus -> K+ channel closure -> afferent via CNIX Hering’s nerves (carotid) and CNX (aortic)

·   5x faster than central – for rapid control

·   Stimuli are synergistic with each other

·   PaO2 important only if low (especially <50mmHg)

·   ↑PaCO2 1mmHg -> ↑minute ventilation 2-3L/min

Central chemoreceptors

·   Ventral medulla

·   Stimuli: ↑PaCO2, CNS acidosis

·   CO2 crosses blood-brain barrier, dissociates into H+ and HCO3- (carbonic anhydrase), decreases pH in CSF and brain ECF -> ↑afferent

·   4x more powerful than peripheral

·   Most important factor normally


Mechanical factors:


·   Aortic sinus and carotid sinuses

·   Stretch-activated mechanoreceptors

·   ↓mAP -> ↓inhibitory afferent to medulla -> indirect stimulation of resp centre

Pulmonary J receptors

·   Stretch/irritation -> ↑afferent via general visceral afferent, spinomedullary tract

·   Important in dyspnoea induced by acute pulmonary oedema

Airway smooth muscle

·   Tidal volume >1L -> ↑↑Stretch -> ↑afferent to resp centre -> transient ↓RR (Hering Breuer reflex)

Joint and skeletal muscle proprioceptors

·   ↑Limb motion -> ↑Abeta afferent via spinomedullary tract -> unknown mechanism


Increased metabolic rate:

Common mechanism

·   ↑Metabolic rate (MR) -> ↑VCO2 -> ↑PaCO2 -> ↑RR


·   ↑MR 2x (VO2 7-8mL/min/kg)

·   RR ~40

(WOB minimized at high RR due to low total compliance)


·   ↑MR 1.2x (VO2 300mL/min)

·   RR 16-20

·   ↑Lean mass, ↑cardiac and respiratory work

·   Progesterone also sensitises respiratory centre -> compensated respiratory alkalosis

Hypermetabolic disease

·   ↑Thyroid

·   Sepsis

·   Malignant hyperthermia

Specific dynamic action of food

·   Mostly due to protein deamination and transamination



Central cause

·   Anxiety

·   Stress

·   Pain

·   Voluntary control

·   Exact mechanisms unknown


·   Cause unknown

·   Moderate exercise:

o ?↑Activity of descending pathway at onset of exercise

o ?↑PaO2 oscillation during ventilatory cycle

·   Exhaustive exercise:

o ↑H+

o ↓ PaO2 via chemoreceptors




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