Ventilation and Perfusion

In the lungs, the gas in the air must go into the liquid blood. According to Henry’s Law, the

higher the partial pressure of a gas, the greater number of gas particles that will dissolve in

liquid. The solubility of the gas in a liquid also plays a role. Nitrogen is high in the air but low in

blood because it doesn’t dissolve very well. The composition of atmospheric air is different

than alveolar gas. There is more water vapor in alveolar gas because of the humidification

process. There is also more carbon dioxide in alveolar air and less oxygen (because of exchange

of gases). The end result, however, is that oxygen will enter the bloodstream and carbon

dioxide will leave the bloodstream.

VENTILATION AND PERFUSION

Ventilation is the movement of air in and out of the lung, while perfusion is the flow of blood in

the lung’s capillaries. The ventilation must match perfusion in order to have efficient gas

exchange. Imbalances do occur naturally, however, if alveolar ducts are blocked and because of the effects of gravity on the blood flow. Disease can cause a ventilation/perfusion (or V/Q)

mismatch.

The partial pressure of oxygen in the alveoli is about 104 mm Hg, whereas the same value in the

oxygenated venous blood in the lungs is about 100 mm Hg, indicating a reasonable efficiency of

gas exchange. If it becomes less efficient, the difference between these two pressures is

greater. If ventilation is not enough in an alveolus, there is redirection of blood flow to “better”

more oxygenated alveoli. There can be increased dilation of vessels in the lungs as well,

increasing the rate of gas exchange. The pH, oxygen level, and CO2 level will affect the blood

flow in the capillaries.

The diameter of the airways determines ventilation, while the diameter of the blood vessels

determines perfusion. The greater the CO2 concentration in the bronchioles, the greater is the

bronchiolar dilation. The same is true of a low oxygen level in the bloodstream. The goal then is

to allow more CO2 to be exhaled from the body. The more oxygen in the alveoli, the more the

pulmonary vessels dilate to increase the blood flow and oxygen exchange.

There are two places where gas exchange occurs: 1) at the respiratory membrane in the alveoli,

and 2) in tissues, where CO2 enters the bloodstream and O2 enters the cells. The lung gas