Oxygen content in the lung from top to bottom

Effect of Gravity

Notice the diameter of the arteries in the figure above (pulmonary angiogram) - much larger at the base than top. Gravity creates higher blood flow and higher air flow at the base of the lungs, but the effect is unequal.  Air flow is higher at the base because the alveoli are smaller and more compliant.  Blood flow is higher at the base because the pressure is highest.  Most of the gas exchange occurs at the base of the lungs where most of the air and blood are located.
As shown in the figure, the V/Q is 3.3 at the top of the lungs and 0.63 at the base of the lungs.  Arterial PO2 is lower at the base of the lungs because of the low V/Q ratio at the base. Low V/Q is essentially hypoventilation; i.e., it produces higher PCO2 and lower PO2.

The reason for the differences in ventilation/perfusion ratio from top to bottom is that gravity has a greater effect on blood flow than on air flow.  This is shown in the figure below:




A 3-compartment model is useful to understand the effect of V/Q on oxygen values.  In areas of lung with low V/Q; e.g., 1/10, the oxygen of air in the alveoli and in blood leaving the area (bottom of the lung) is low in oxygen.  In areas of the lung with evenly matched V/Q; e.g., 10/10 or 1, the oxygen level is high in the alveolar air and blood leaving that area (middle of the lung).  In areas of the lung with high V/Q; e.g., 10/1, the oxygen level is slightly higher in blood (due to flat upper portion of hemoglobin -O2 binding curve) and higher in alveolar air.  This would apply to the top of the upright lung.

Clincical correlate for infectious diseases: 

Mycobacterium tuberculosis is an obligate[1] pathogenic bacterial species in the family Mycobacteriaceae and the causative agent of tuberculosis.[2]The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen.  Lesions normally found in the upper lung where oxygen levels are higher.