A physiological shunt can develop if there is infection or edema in the lung that obstructs an area. 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.
P hysiological dead space or physiological shunts, arise from a functional impairment of the lung or arteries. Note that this does not occur when lying down, because in this position, gravity does not preferentially pull the bottom of the lung down. As a result, the rate of gas exchange is reduced. An anatomical shunt develops because the ventilation of the airways does not match the perfusion of the arteries surrounding those airways. 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. Likewise, it takes less energy to pump blood to the bottom of the lung than to the top when in a prone position. When someone is standing or sitting upright, the pleural pressure gradient leads to increased ventilation further down in the lung. The lung is particularly susceptible to changes in the magnitude and direction of gravitational forces. 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. Anatomical dead space occurs naturally in areas of the lungs that don’t come in contact with alveoli (like the trachea). Anatomical dead space and anatomical shunts arise from anatomical deficiencies. Dead space is created when no ventilation and/or perfusion takes place. As a result, the amount of oxygen in the blood decreases, whereas the carbon dioxide level increases. Dead spaces can severely impact breathing, because they reduce the surface area available for gas diffusion. 
Both produce dead space or shunts, regions of ineffective lung tissue. This difference between the amounts of air and blood reaching the lungs is referred to as ventilation/perfusion (V/Q) mismatch.
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