Answer:
D. At the end of inhalation, pO₂ in the alveolar air is the same as pO₂ in the ambient air (air surrounding the body).
Step-by-step explanation:
In order to answer the question, we need to review the respiratory physiology.
Air moves into and out of the lungs as a result of differences the in the alveolar and atmospheric (ambient air) pressures.
AT REST:
Alveolar air pressure is equal to the atmospheric air pressure. The net difference of air pressures is zero and no air moves into lungs.
The intrapleural (i.e. pressure difference between pleura/ covering of the lung and thoracic cavity, it is actually the measure of pressure of air in the lungs) pressure is negative at this stage. This is because lung tends to collapse itself and chest wall tends to expand itself resulting in net negative pressure with lungs inside the chest cavity.
INHALATION:
During inhalation, diaphragm contracts which increases the intrathoracic (within the chest wall) volume.
As Pressure is inversely proportional to volume, diaphragmatic contraction decreases the alveolar pressure and air flows into the lungs.
⇒This concept negates options B.
During inhalation, intrapleural pressure becomes more negative. This is due to the fact that as lung increases in volume, the elastic forces which tend to collapse the lung increase in magnitude. You can conceptualize that as pressure of air in the intrapleural cavity decreases (i.e. intrapleural pressure becomes more negative), the lungs are able to expand out.
EXHALATION:
During exhalation (which is a passive process unless forced), lung pressure increases due to elastic recoil forces and alveolar pressure becomes greater than ambient pressure as a result air flows out of the lungs.
⇒This concept negates option C.
Pressure in the intrapleural cavity is negative during passive exhalation but becomes positive during forced expiration. You can think that as the pressure of air in the intrapleural space (i.e. intrapleural pressure) increase in magnitude, the lungs are more collapsed.
⇒This concept negates option E.
Under normal physiological conditions, Partial pressure of 0₂ in alveoli and arterial blood equilibrate and hence both have same partial pressures.
⇒This concept negates point A.
D. At the end of inhalation, PO2 in the alveolar air is the same as PO2 in ambient air (air surrounding the body).
This is correct as at the end of inhalation, partial pressures of both alveoli and atmosphere are equal as the decreased alveolar pressure during inhalation is compensated by the air flowing into the lungs.