A functional shunt is a significant reduction in alveolar ventilation in a severely diseased part of the lung, but the blood flow in the alveoli of this part of the lesion is not correspondingly reduced, and on the contrary, there may even be an increase in blood flow in the alveoli due to some inflammation that causes alveolar congestion. The ratio of ventilation per minute to blood flow per minute in the alveoli is significantly reduced, resulting in the flow of venous blood through the lesion into the arterial blood before it is adequately combined with oxygen. This condition is clinically similar to an arteriovenous short circuit and is therefore called a functional shunt. Functional shunts can be caused by a variety of clinical conditions, such as bronchial asthma, chronic bronchitis or pulmonary fibrosis, and pulmonary edema.