Peripheral chemoreceptors respond only below about 8 kPa
Expansion
Mediated by the peripheral chemoreceptors, principally the carotid bodies, which have the highest blood flow per gram of any tissue.
The response curve is flat until about 8 kPa (60 mmHg) and then rises steeply. This threshold matches the shoulder of the oxygen dissociation curve, below which content falls rapidly.
Their importance rises in two situations:
- Chronic hypercapnia, where the central response is blunted by cerebrospinal fluid bicarbonate compensation
- Metabolic acidosis, where they contribute to the compensatory hyperventilation
The oxygen therapy debate in chronic obstructive pulmonary disease follows from this. Excess oxygen can raise carbon dioxide, though modern understanding attributes most of the rise to worsened ventilation-perfusion matching (release of hypoxic pulmonary vasoconstriction) and the Haldane effect, with loss of hypoxic drive a smaller contributor. The practical response is controlled oxygen targeting a saturation of 88 to 92 per cent, never withholding oxygen from a hypoxic patient.