Regulation of ventilation

CNS regulation

Ventilation is regulated by several groups of neurons in the brainstem:

• Medullary respiratory center (in reticular formation)
  • Dorsal respiratory group: inspiration and breathing rate
  • Ventral respiratory group: active expiration and inspiration
• Pneumotaxic center (in upper pons): regulates length of inhalation via inhibition.

Several chemoreceptors and mechanoreceptors are important in providing sensory input to regulatory groups in the CNS.

Chemoreceptors

Central

• In medulla
• Sensitive to pH changes of CSF (due primarily to pCO₂ change, also acidosis)
• Activation leads to hyperventilation within 20-30 s

Peripheral

• Carotid and aortic bodies
• Sensitive to
  • decreases in pO₂ < 10-13 kPa (baroreceptors)
  • increases in pCO₂ (baroreceptors)
  • decreases in pH (chemoceptors)
  • heat (thermoreceptors)
• stimulation leads to hyperventilation

Mechanoreceptors

• Lung stretch receptors (in smooth muscle of airways): Hering-Breuer reflex
• Irritant receptors (between airway epithelial cells): stimulated by noxious particles
• Juxtacapillary receptors (in alveolar walls by capillaries): stimulated by engorgement of capillaries, leading to tachypnea
• Joint and muscle receptors: activated to stimulate early breathing during exercise

Defensive respiratory responses to irritants

Lungs are protected from irritants via several mechanisms:

Mechanical protection

• vibrissae in the nasal cavity trap dust particles
• mucociliary escalator in conducting airways moves mucus into pharynx

Immune protection

• pulmonary alveolar macrophages
• IgA antibodies in bronchial secretion

Defensive Reflexes

Herring-Breuer

Prevents excess inspiration or exhalation

1. Pulmonary stretch receptors sense tension abnormality
2. Signal is conducted via vagus nerve to medulla (inspiration regulation) or pontine center (expiration regulation)
3. Appropriate inspiratory/expiratory reflex generated

Cough

1. Irritation of airway receptors, pharynx, pleura, diaphragm, perichondrium, stomach, auditory canal or tympanic membrane
2. Signal conducted via CNs V, IX, X and phrenic nerves to medulla
3. CN X, phrenic nerve, and spinal motor nerve generate response of respiratory muscles:
   • small inspiration followed by forced expiration against closed glottis
   • pressures in thorax and abdomen build up
   • glottis is forced open
4. air rushes out, loosens mucus and irritants (usually fine particles)

Sneeze

1. Irritation of nasal mucosa or excess fluid in airway stimulates olfactory receptors or CN V endings
2. Signal is conducted to medulla: nucleus solitarius and reticular formation
3. CNs V, VII, IX, and X generate response of pharyngeal, tracheal, and respiratory muscles:
   • deep inspiration
   • pressures in thorax and abdomen rise, but pharynx constricts
   • air is forced through nose and mouth, facial and nasal muscles also constrict
4. air rushes out, loosens mucus and irritants

Hiccup

1. Abrupt intake of excess air (ex: via eating) stimulates sympathetic, phrenic, and vagus nerves (can also be stimulated by neural damage)
2. Signal is conducted to midbrain
3. Motor fibers of phrenic nerve and accessory nerves stimulate diaphragm and intercostal muscles
4. Spasmodic contraction of diaphragm closes glottis

References

Boron, W. and Boulpaep, E., 2017. Medical physiology. 3rd ed. Philadelphia: Elsevier, p.719

Costanzo, L., 2019. Physiology - Board Review Series. 7th ed. Philadelphia: Wolters Kluwer, p.136-139