Final answer:
Lower partial pressure of oxygen in the lungs compared to external air is due to the removal of oxygen by the blood, creating a pressure gradient that facilitates the uptake of oxygen into the bloodstream.
Step-by-step explanation:
The partial pressure of oxygen is lower in the lungs than in the external air because oxygen is being continuously removed by the blood from the air in the lungs. This process occurs due to the diffusion gradient where oxygen moves from an area of high pressure (the alveoli) to an area of lower pressure (the blood in the capillaries). As such, oxygen in the alveoli has a partial pressure of about 100 mm Hg (PALV O₂), which is higher than the oxygen pressure in the blood of the capillaries at about 40 mm Hg. Additional factors contributing to the lower partial pressure include the presence of water vapor in the lungs and the mixing with carbon dioxide, which is being transported back to the lungs after cellular respiration in tissues.
Dalton's law and Henry's law explain the behavior of gases like oxygen and carbon dioxide in the lungs, where gases move down their pressure gradients during gas exchange. Consequently, during external respiration, the partial pressure of oxygen is replenished in the lungs as the blood releases carbon dioxide and takes up oxygen once again.