Consequences of hyperoxic vasoconstriction on pressures of oxygen in the tissues

The various effects of hyperoxia on arteries depending on their size combine with each other to limit an increase in PO2. Contractions of the 1st and 2nd order vessels decrease capillary blood flow while the small effect on 3rd order vessels causes an increase in shunted output69.

Both these phenomena account for the unchanged or moderately increased tissue PO2 in hyperoxia. Whalen & Nair81,82 showed in normal rats that cellular pressure of oxygen was approximately 6 mmHg and did not increase significantly when animals breathed NBO2. Their PvO2 increased indicating that the full potential for oxygen delivery to cells had not been realized under normobaric conditions: this was therefore not a reflection of what might be achieved under conditions of greater transcapillary oxygen diffusion as might be achieved during HBO.

These experimental effects are important because they show that cellular PO2 only undergoes limited changes in hyperoxia. Thus the decrease in blood flow due to the hyperoxic vasoconstriction really has a protective effect against the toxic effects of oxygen.

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