Final answer:
Under physiological conditions, the lowest partial pressure of oxygen is found in the blood returning to the heart after delivering oxygen to body tissues, with a partial pressure of about 40 mm Hg.
Step-by-step explanation:
The student has asked where the lowest partial pressure of oxygen is found under physiological conditions. The partial pressure of oxygen will be lowest in the blood returning to the heart after it has delivered oxygen to the tissues of the body. During this process, known as systemic circulation, oxygen dissociates from hemoglobin and diffuses into tissue cells.
By the time the blood returns to the heart, the partial pressure of oxygen (Po₂) has decreased to about 40 mm Hg. During external respiration in the lungs, oxygen diffuses from the alveoli, where its partial pressure is higher (approximately 100 mm Hg), into the capillaries, where it is lower (40 mm Hg). As blood passes through body tissues, oxygen is utilized in cellular respiration, further lowering its partial pressure in the returning venous blood.