Physiologic shunt refers to:

If some pa of the lung does not have ventilation but has blood perfusion, blood leaving this pa will remain mixed venous blood. That is, this blood will not be oxygenated. And, it will mix with aerial blood in left ventricle. This is called physiologic shunt or venous admixture. Atelectasis is the most common cause of a physiologic shunt; ventilation: perfusion ratio (VA/Q) is zero.