Lung dead space vs shunt

[yoyohomieg5432]

I'm really having trouble differentiating these 2 conceptually. Can anybody explain how they are different?

I don't understand what causes the shunts to happen. if there is any part of the lung that is not getting oxygen doesn't it just automatically constrict so that blood can get oxygen elsewhere?

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[dorster]

So how I think of it is...

Physiologic Shunt = the alveoli is perfused but NOT ventilated (ex: Pneumonia causing an area of alveoli not to be well oxygenated but the area is still being perfused i.e. blood is still getting there) so the lung has opposite physiology to other tissues of the body where if an area is not being ventilated (i.e. decreased oxygen delivery) this causes the vessel to vasoconstrict in that area to "shunt" blood to an area of the lung where it is able to get oxygen.

Pathologic Dead space= the alveoli are ventilated but NOT perfused (ex: PE)

 

[yoyohomieg5432]

So how I think of it is...

Physiologic Shunt = the alveoli is perfused but NOT ventilated (ex: Pneumonia causing an area of alveoli not to be well oxygenated but the area is still being perfused i.e. blood is still getting there) so the lung has opposite physiology to other tissues of the body where if an area is not being ventilated (i.e. decreased oxygen delivery) this causes the vessel to vasoconstrict in that area to "shunt" blood to an area of the lung where it is able to get oxygen.

Pathologic Dead space= the alveoli are ventilated but NOT perfused (ex: PE)

So the physiologic shunt is only a transient thing? Like in your example, if it isn't being ventilated then the blood will leave that area and thus close the shunt. Is that correct?

 

[Ven0m]

So the physiologic shunt is only a transient thing? Like in your example, if it isn't being ventilated then the blood will leave that area and thus close the shunt. Is that correct?

Yes, if the blood flows through those alveoli which don't have oxygen, they won't pick up as much blood, which makes those returning vessels hypoxic. The body responds by constricting those hypoxic vessels which are returning to the heart. Basically this block downstream causes blood in other pulmonary arterioles to flow to different capillaries, hoping they would get oxygen elsewhere.

This could go wrong tho, if your whole lung is hypoxic (e.g., in emphysema), where there is perfusion but not enough ventilation, the whole lung could get vasoconstricted, which puts enormous pressure on the right ventricle, causing right ventricular dilation (called acute cor pulmonale), and a whole lot of other stuff could result including death.

To learn more about that vasoconstriction that happens when pulmonary capillaries start perfusing alveoli that dont have enough oxygen, check out wikipedia's "pulmonary hypoxic vasoconstriction" article!!