Central chemoreceptor

Physiology

Biochemical mechanism

It is thought that H⁺ may be the only direct stimulus detected by the receptors; the receptors are thought to sense the concentration of CO₂ indirectly by detecting H⁺ formed as it reacts with water to form carbonic acid which in turn dissociates to form H⁺ and HCO₃⁻. However, because the gaseous CO₂ from blood far more readily diffuses across the blood–brain barrier to reach the medullary interstitial fluid than H⁺ ions, the chemoreceptors are far more responsive to blood CO₂ concentration changes than to H⁺ concentration changes.

Role in regulation of respiration

Blood CO₂ is the primary parameter for biological regulation of respiration because its concentration is inversely related to pulmonary ventilation; blood O₂ concentration is meanwhile normally adequate for tissue perfusion across a wide range of ventilatory circumstances (from below 50% of normal to over 2,000% of normal) and is therefore requires less stringent control.

Factors influencing ventilatory response to CO2

• Within days, the set point of the respiratory center can gradually readjust to tolerate as much as 80% lower CO₂/H⁺ concentrations. This way, sub-optimum O₂ blood concentration can become the driving stimulus for increased ventilation without an overriding inhibitory effect of reduced CO₂ concentrations. Acute exposure to hypoxia produces a 70% increase in ventilation, whereas after adaptation to reduced CO₂ concentration, the same stimulus can produce a 400%-500% increase in ventilation. This is one of the mechanisms underlying acclimatization to higher altitudes, and in part explains why mountain climbers should ascend gradually.
• Reduced arterial CO₂ pressure significantly attenuates the ventilatory drive. For example, a bout of voluntary hyperventilation appreciably attenuates the ventilatory drive for a short period; some swimmers or sprinters intentionally hyperventilate just prior to the start of the race to suppress the urge to breathe during the race. Accidental medical overventilation of an anaesthesized patient may cause the patient to cease breathing for about a minute.
• Trained athletes and divers tend to acquire an attenuated sensitivity to lowered arterial CO₂ concentration.
• Increased work of breathing (this can be demonstrated experimentally by breathing through a narrow tube). This may partially explain the reduced CO₂ response observed in some patients with lung disease; in such individuals, administration of bronchodilators (which reduce airway resistance) increase the response.
• Sleep
• Increased age
• Genetics
• Gersonality