Question

The partial pressure of oxygen in alveoli is about 104 mmHg, after gas exchange it becomes 40mmHg. I understand that during gas exchange, the pressure gradient drives oxygen into the blood and CO₂ out. My issue is that: since the partial pressure of oxygen in the alveolus is 104 mmHg and that of blood in the pulmonary capillary is 40mmHg, why would the partial pressure of oxygen in the blood after gas exchange also equal 104 mmHg and not 72mmHg since as oxygen leaves the alveolus the partial pressure drops and and increases by the same amount in the pulmonary capillary. At equilibrium shouldn't they both be at 72mmHg?

Answer 1

The partial pressure of oxygen in the blood after gas exchange is similar to that in the alveoli because oxygen binds rapidly to hemoglobin, maintaining a steep pressure gradient that allows for continuous diffusion until blood is nearly saturated.

Step-by-step explanation:

The question you asked relates to the understanding of gas exchange and partial pressures of oxygen in the alveoli and blood during respiration. Specifically, you are asking why, after gas exchange, the partial pressure of oxygen in blood matches that of alveolar oxygen and does not equilibrate to a mid-value.

The reason for oxygen pressure equilibrating to nearly 104 mm Hg in the blood post gas exchange is largely due to the high affinity of oxygen for hemoglobin, the respiratory pigment in red blood cells. Oxygen rapidly binds to hemoglobin once it enters the blood, which allows for more oxygen to diffuse into the blood until an equilibrium is nearly reached with the alveolar oxygen pressure, slightly reduced to about 100 mm Hg due to minimal mixing with deoxygenated blood.

In other words, as oxygen binds to hemoglobin, it is effectively removed from the pool of gaseous oxygen, keeping the partial pressure gradient steep and favoring continuous diffusion from the alveoli into the blood until the blood is nearly saturated with oxygen.