If the oxygen pressure inside cells is lower, is the oxygen pressure of the blood the same or greater?
Oxygen diffuses into the blood and body tissues. Thus, oxygen will always move from an area of greater concentration to an area of lower concentration. When we consider the relative oxygen pressures in the blood versus the tissue cells, the answer to this question varies depending on which part of the circulatory system we consider.
Inhaled air contains more oxygen pressure than blood, so oxygen diffuses into the blood present in the capillaries of the lungs.
Thus, the blood leaving the lungs has been oxygenated and has an oxygen pressure greater than that of the body's cells.
As the oxygenated blood passes through the body's tissues, oxygen diffuses into the body's cells. During this stage, the oxygen pressure of the body's tissues is lower than that of the blood.
The blood entering the lungs has given off as much oxygen as possible to the body, and thus has an oxygen pressure equal to the oxygen pressure of the body's cells.