Q.What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air?

Comparing the two, atmospheric air is richer in oxygen and poorer in carbon dioxide than alveolar air.

• The oxygen level is higher in the atmospheric air than in the alveoli, because oxygen is continually taken up by the blood at the alveoli.
• The carbon dioxide level is lower in the atmospheric air than in the alveoli, because carbon dioxide is continually added to the alveolar air from the blood. …

To answer this, it helps to remember why the air deep in the lungs is not the same as the air outside. Atmospheric air is fresh air that has not yet given up oxygen or picked up waste gas. Alveolar air, by contrast, has been sitting at the exchange surface where oxygen constantly leaves it to enter the blood and carbon dioxide constantly enters it from the blood.

This leads to two clear differences:

• Oxygen is at a higher pressure in the atmospheric air than in the alveoli. As soon as air reaches the alveoli, oxygen begins diffusing into the blood, so the alveolar oxygen pressure settles at a value lower than that of the incoming atmospheric air.
• Carbon dioxide is at a much lower pressure in the atmospheric air than in the alveoli. Atmospheric air carries almost no carbon dioxide, whereas the alveoli continually receive carbon dioxide brought back from the tissues, so the alveolar level is far higher. …

Method: Comparing Partial Pressures Between Two Locations in the Respiratory Pathway

Use this whenever asked to compare gas pressures at two points along the air's path (atmosphere vs alveoli, alveoli vs blood, and so on).

Steps

Step 1: Identify what each location gains or loses

Work out what happens to each gas as air moves from the first location to the second — does that location gain or lose the gas via diffusion?

Step 2: Apply the diffusion rule

A gas always moves from higher to lower partial pressure. If alveolar air is constantly losing O2 to the blood and gaining CO2 from the blood, then relative to the alveoli, freshly arrived (atmospheric) air must be higher in O2 and lower in CO2. …