Pulmonary circulation pressure is very low compared to that of the systemic circulation. It is also independent of cardiac output. This is because of a phenomenon called recruitment, which is the process of opening airways that normally remain closed when cardiac output increases. As cardiac output increases, the number of capillaries and arteries that are perfused (filled with blood) increases. These capillaries and arteries are not always in use but are ready if needed. At times, however, there is a mismatch between the amount of air (ventilation, V) and the amount of blood (perfusion, Q) in the lungs.

There are two types of V/Q mismatch. Both produce dead space or shunts, regions of ineffective lung tissue. Dead spaces can severely impact breathing, because they reduce the surface area available for gas diffusion. As a result, the amount of oxygen in the blood decreases, whereas the carbon dioxide level increases. Dead space is created when no ventilation and/or perfusion takes place.

1. Anatomical dead space and anatomical shunts arise from anatomical deficiencies. Anatomical dead space occurs naturally in areas of the lungs that don’t come in contact with alveoli (like the trachea). In these spaces, the lungs are ventilated and receiving enough air, but blood is not being oxygenated in that space because the air is not reaching perfused areas. An example of an anatomical shunt is the effect of gravity on the lungs. The lung is particularly susceptible to changes in the magnitude and direction of gravitational forces. When someone is standing or sitting upright, the pleural pressure gradient leads to increased ventilation further down in the lung. Likewise, it takes less energy to pump blood to the bottom of the lung than to the top when in a prone position. Perfusion of the lung is not uniform while standing or sitting, and some of the circulating blood doesn’t make it past the lungs to be oxygenated. An anatomical shunt develops because the ventilation of the airways does not match the perfusion of the arteries surrounding those airways. As a result, the rate of gas exchange is reduced. Note that this does not occur when lying down, because in this position, gravity does not preferentially pull the bottom of the lung down.
2. Physiological dead space or physiological shunts, arise from a functional impairment of the lung or arteries. This happens when there is a lack of blood flow where the alveoli have enough air to oxygenate blood or there is a lack of air in an area where the blood flow is normal. A physiological shunt can develop if there is infection or edema in the lung that obstructs an area. This will decrease ventilation but not affect perfusion; therefore, the V/Q ratio changes and gas exchange is affected. A physiological dead space could be something like a blood clot that cuts down perfusion around well-ventilated alveoli. Emphysema, which causes a deterioration of capillaries around alveoli will also create physiological dead space.

The lung can compensate for these mismatches in ventilation and perfusion. If ventilation is greater than perfusion, the arterioles dilate and the bronchioles constrict. This increases perfusion and reduces ventilation. Likewise, if ventilation is less than perfusion, the arterioles constrict and the bronchioles dilate to correct the imbalance.