Question

How much partial pressure of co2 in alveoli?

Answer 1

Since inspired air is 21% oxygen and atmospheric pressure is 760 mmHg (at sea level), the partial pressure of oxygen is 0.21 x 760 mmHg = 160 mmHg. As air moves into the alveoli, water vapour and carbon dioxide are added, and that reduces the partial pressure of oxygen to about 100 mmHg in the alveolar gas.

Answer 2

The alveolar oxygen partial pressure is lower than the atmospheric O2 partial pressure for two reasons. Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg.