EK 1001 - Why does water leak out when vascular permeability is increased?

[Monkeymaniac]

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A passage in EK 1001 bio (112) talks about a peptide called Kinin. Among many functions, it increases vascular permeability.

One of the questions for the passage was asking what would happen if our body has an excess concentrations of Kinin. One of the correct answer was severe vascular edema (water build-up between blood vessel and interstitial space).

They explain that increasing the vasuclar permeability would make water leak out of the blood because of poor tight junction mechanism. Here, since blood has higher osmorality (compared to interstitial space), shouldn't water flow into the blood vessel instead? Thanks in advance.

 

[Charles_Carmichael]

The blood pressure will force water out of the vessels until the blood pressure and oncotic pressure equalize. By the point where these two pressures equalize, there would be edema.

Remember, whether fluid is pushed out of or pulled into vessels depends on the balance between blood pressure (the hydrostatic pressure) and the oncotic pressure (the pressure plasma proteins exert). Think of it sort of like capillary exchange where fluid is pushed out at the beginning of the capillary because the blood pressure is greater than the oncotic pressure pulling fluid back in. It's only towards the end of the capillary, where the oncotic pressure is greater than the blood pressure, that the fluid is pulled back into the vessel. In the case of this question, the blood pressure would be greater than the oncotic pressure in the beginning, thus resulting in edema.

Hope this helps.