Sun28November0333PM 46

The partial pressure of oxygen in the alveoli (PAO2) is greater in the apical compared to that of the basal lung units in the erect position.

What is the single most important reason for this?

(Please select 1 option)

The basal lung units are better perfused than apical units

The apical units are better ventilated Incorrect answer selected

The apical units have a greater volume at FRC

The basal units are better ventilated than apical units

The V/Q ratio of apical units is greater than that of basal units This is the correct answer

Explanation

The V/Q ratio influences alveolar oxygen (PAO2) and carbon dioxide tension (PACO2) in any alveolar unit.

A Ventilation-Perfusion (V/Q) ratio graph plots the partial pressure of alveolar carbon dioxide (PACO2) against that of alveolar oxygen (PAO2). The curve itself represents all of the possible values for PACO2 and PAO2 that an individual alveolus may have given a set of model assumptions.

In theory a V/Q ratio of infinity (ventilation but no perfusion or dead space), the values the PACO2 of the alveolus will equal zero whereas the PAO2 will approach that of external air (150mmmHg). The V/Q ratio is 3.3 at the apex of the lung compared with 0.67 at the base.

In theory a V/Q ratio of zero (no ventilation but perfusion) the PACO2 and PAO2 approach the partial pressures for these gases in the venous blood. The V/Q ratio is 0.67 at the base of the lung compared with 3.3 at the apex.

Typical value for PAO2 at the apex is 132mmHg and the typical PACO2 28mmHg

Typical PAO2 at the base is 89 mmHg and the typical PACO2 42 mmHg

Answer Statistics

1

14%

2

22%

3

7%

4

2%

5

58%

Times answered: 260