Neural Control of Breathing
The brainstem contains specialised groups of neurons that generate the rhythmic pattern of breathing. The primary inspiratory centre is the dorsal respiratory group (DRG) located in the medulla. It initiates inhalation by activating the diaphragm and external intercostal muscles.
Complementing the DRG, the ventral respiratory group (VRG) is mainly involved in expiration. While the DRG drives inspiration, the VRG provides the motor output for active exhalation, especially during forced breathing.
Within the pons, two additional centres fine‑tune the breathing pattern:
- Apneustic centre – prolongs inspiration, allowing a deeper breath.
- Pneumotaxic centre – inhibits the apneustic centre, reducing tidal volume and respiratory rate.
The combined activity of these pontine structures is referred to as the pontine respiratory group, which modulates both the rate and depth of breathing.
Chemical Drive to Ventilation
The most potent stimulus for ventilation is an increase in arterial carbon dioxide (hypercapnia). Elevated PaCO₂ lowers the pH of the cerebrospinal fluid, triggering a rapid increase in breathing frequency and depth.
In contrast, hypoxia alone does not stimulate breathing unless the arterial oxygen pressure (PO₂) falls below roughly 8 kPa (60 mmHg). Thus, low oxygen must be severe to elicit a ventilatory response.
The relationship between arterial PaCO₂ and ventilation is approximately linear; as CO₂ rises, ventilation increases proportionally.
Peripheral Chemoreceptors
Peripheral chemoreceptors are located in the carotid bodies at the carotid bifurcation and the aortic bodies near the aortic arch. These receptors sense changes in arterial O₂, CO₂, and pH.
Within the carotid and aortic bodies, two cell types are present:
- Type I (glomus) cells – chemosensory cells that release neurotransmitters in response to hypoxia, stimulating afferent nerves.
- Type II cells – supporting, glia‑like cells that do not directly participate in the chemosensory response.
Central Chemoreceptors
Central chemoreceptors reside on the ventral surface of the medulla and monitor the pH of the cerebrospinal fluid (CSF). They are sensitive to CO₂ because CO₂ freely crosses the blood‑brain barrier, where it is rapidly converted to carbonic acid.
The enzyme carbonic anhydrase catalyses the reaction:
CO₂ + H₂O ⇌ carbonic acid ⇌ H⁺ + HCO₃⁻
The Henderson‑Hasselbalch equation for this buffer system uses a pK (pKa) of 6.1. An increase in CO₂ partial pressure therefore leads to a decrease in pH, producing an acidic environment that stimulates ventilation.
Acid‑Base Balance and Respiratory Compensation
Understanding the interplay between metabolic and respiratory disturbances is essential for interpreting blood gas results.
| Condition | Primary Change | Effect on pH |
|---|---|---|
| Metabolic acidosis | Loss of HCO₃⁻ | pH ↓ |
| Respiratory acidosis | Increase in PaCO₂ | pH ↓ |
| Metabolic alkalosis | Increase in HCO₃⁻ | pH ↑ |
| Respiratory alkalosis | Decrease in PaCO₂ | pH ↑ |
In metabolic alkalosis, the primary compensatory mechanism is hypoventilation, which raises PaCO₂ and brings pH back toward normal.
Conversely, hyperventilation reduces arterial CO₂ and raises pH, often leading to respiratory alkalosis.
Adaptations in Disease and Voluntary Control
Chronic pulmonary disease does not increase the sensitivity of central chemoreceptors; instead, prolonged exposure to high CO₂ levels reduces their responsiveness.
Voluntary control can temporarily override the medullary respiratory centres, allowing brief breath‑holding, but it cannot permanently suppress the automatic drive. When breath‑holding continues, arterial CO₂ rises (hypercapnia) and pH falls, eventually forcing the involuntary centres to resume breathing.
Key Take‑aways
- The dorsal respiratory group initiates inspiration; the ventral group handles expiration.
- Apneustic and pneumotaxic centres in the pons respectively prolong and shorten breaths.
- Hypercapnia is the dominant chemical stimulus for ventilation; hypoxia triggers breathing only at critically low PO₂.
- Peripheral chemoreceptors in carotid and aortic bodies detect O₂ and pH changes, while central chemoreceptors monitor CSF pH.
- Carbonic anhydrase converts CO₂ to carbonic acid; the system’s pK is 6.1.
- Acid‑base disturbances are corrected by altering ventilation: hypoventilation for alkalosis, hyperventilation for acidosis.
- Chronic high CO₂ diminishes central chemoreceptor sensitivity, and voluntary breath‑holding cannot permanently silence the medullary drive.
