Final answer:
The PO2 in the alveoli is 104 mm Hg and the PO2 in the blood is 40 mm Hg. Oxygen diffuses from the alveoli to the blood capillaries due to this significant pressure gradient.
Step-by-step explanation:
In external respiration, the PO2 in the alveoli is 104 mm Hg, while the PO2 in the blood is 40 mm Hg.
Oxygen diffuses down its partial pressure gradient from areas of higher pressure to areas of lower pressure during external respiration. In the lungs, it diffuses from the alveoli, where there is a higher partial pressure of oxygen, into the capillary blood, which has a lower partial pressure. The partial pressure of oxygen in the alveoli is approximately 104 mm Hg due to the oxygen-rich air that is inhaled. Conversely, the partial pressure in the blood of the pulmonary capillaries is only about 40 mm Hg because the blood arriving in the lungs is depleted of oxygen due to its use by bodily tissues.
Therefore, a significant gradient of 64 mm Hg exists, which facilitates the rapid diffusion of oxygen across the respiratory membrane and into the blood, where it can bind with hemoglobin in red blood cells (RBCs) and be carried to tissues in need of oxygen.