<P> A small degree of shunt is normal and may be described as an' anatomical shunt' . Anatomical shunting occurs when too much of the blood supplying the lung tissues via the bronchial arteries is being returned via the pulmonary veins without undergoing gas exchange . In addition, some of the coronary veins drain directly into the left ventricle of the human heart . This is why the arterial PO2 is slightly lower than the alveolar PO2 . </P> <P> Shunt refers to the passage of dexoygenated blood from the right side of the heart to the left, without participation in gas exchange in the pulmonary capillaries . While shunt is an absolute (ventilation is zero), lung units with a V / Q ratio of less than 0.005 are indistinguishable from shunt from a gas exchange perspective . </P> <P> Pulmonary shunting is minimized by the normal reflex constriction of pulmonary vasculature to hypoxia . Without this hypoxic pulmonary vasoconstriction, shunt and its hypoxic effects would worsen . For example, when alveoli fill with fluid, they are unable to participate in gas exchange with blood, causing local or regional hypoxia, thus triggering vasoconstriction . Blood is then redirected away from this area, which poorly matches ventilation and perfusion, to areas which are being ventilated . </P> <P> Because shunt represents areas where gas exchange does not occur, 100% inspired oxygen is unable to overcome the hypoxia caused by shunting . </P>

Why does administration of higher levels of oxygen not help in shunt disorders