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

List:

· Intro

· Chemical factors

· Mechanical factors

· Increased metabolic rate

· Other

Intro:

· 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:

Baroreceptors	· 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
Neonate	· ↑MR 2x (VO₂ 7-8mL/min/kg) · RR ~40 (WOB minimized at high RR due to low total compliance)
Pregnant	· ↑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

Other:

Central cause

· Anxiety

· Stress

· Pain

· Voluntary control

· Exact mechanisms unknown

Exercise-induced

· 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