At sea level, what proportion of the oxygen carried by haemoglobin is actually released into tissue?

1. Tissues at high altitude still receive enough oxygen at rest, which seems surprising given the lower saturation. 2. Part of the answer lies in how much of the carried oxygen is actually handed over. 3. At sea level, saturation is high but only a quarter of the oxygen molecules are released into the tissue. 4. At high altitude a greater proportion of the carried oxygen is released instead. 5. So the number entering tissue is nearly equal in the two settings, and the fourth option is correct. 6. The remaining oxygen stays bound as the venous reserve, which is why the higher fractions in the other options cannot be right. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 22 "The Respiratory System", section 22.6 Modifications in Respiratory Functions_