Watershed Zomes

[monsoon338]

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Hey everyone,

Just wanted to ask a basic question on watershed zones. Trying to figure out the logic behind it.

From what I can understand - there are 2 things going on.

1) One is that "watersheds" have dual supply. Is this true for ALL watersheds that they have dual supplies?

2) The second is that these are end arteries. Meaning that they receive their blood last after all the other tissues upstream have received blood.

Now, what I can't understand is why watersheds are more prone to ischemia. If they have dual supply, they should be able to have a "back up" source. Other tissues might only have one source of blood and if that isn't working - they have no fall back source.

I can understand how the 2nd reason would make the watersheds susceptible because they are the zones that get blood last.

The only things I can figure out that rectifies this is either the dual supply has nothing to do with watersheds or that during BLOCKAGE the dual supply helps, but during volume loss, the watershed hurts.

 

[Stellar Clouds]

Think about it this way: a watershed area is supplied 50% by artery A and 50% by artery B. if artery A goes down, then the area only receives half of the supply it needs, therefore it becomes ischemic.

Lets also say that artery A provides a flow of 10 mL/min and artery B provides 10 mL/min, and the water shed needs 18-20 mL/min of blood supply. With hypovolemia, A + B can only supply 12 mL/min and therefore you once again have ischemia. (Numbers completely made up of course)