Chemoreceptors can sense oxygen that is:

1. Most oxygen in blood travels bound to a carrier. 2. That bound fraction is invisible to the sensors. 3. The chemoreceptors are only able to sense dissolved oxygen molecules, not oxygen bound to hemoglobin. 4. So the answer is dissolved. _Source: OpenStax 'Anatomy and Physiology 2e', Ch 22 'The Respiratory System'_