During internal respiration, what are the oxygen partial pressures in tissue and in the blood arriving there?

1. Tissue uses oxygen continuously for cellular respiration, so its partial pressure stays low at about 40 mm Hg. 2. Blood arriving in the systemic capillaries carries about 100 mm Hg. 3. The gradient therefore runs from blood into tissue, which is the reverse of the situation at the alveolus. 4. The first two options put the high value in the tissue, which would make oxygen flow backwards into the blood. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 22 "The Respiratory System", section 22.4 Gas Exchange_