Let me see if i got this right. There are 2 ways (there may be more, but for the time being i am only interested in these two) by which vascular permeability can occur:
1- Vasodilation at the arteriole level increases its hydrostatic pressure and also increases downstream hydrostatic pressure (in venules) which leads to fluid filtration and hence vascular permeability at the venule level.
2- Vasoconstriction of smooth muscle cells leads to contraction of pericytes, which therefore creates a space between endothelial cells and allows fluid to pass through thereby also leading to vascular permeability at the venule level.
Now i am sure about the second way but not so sure about the first way. Your input?
J = K (Hydrostatic Art - Hydrostatic Int) - (Oncotic Art - Oncotic Int)
K: Permeability. Take your hands, palms facing you. INterlace your fingers. Smush them as close together as you can. Any gaps? Water going to get through? No. Tight seal. Now pull them apart gently, maybe an inch. Ooh. some cracks develop. Water get through? yep. Alot? Well, maybe. Now separate your hands entirely. Gap? Oh yeah. WAter goes through? Yep. Alot? Definitely.
Think of permeability as how tight the gaps are in your fingers. Vasoconstriction, clamps down permeability. Vasodilation, opens up permeability. and more stuff leaks. This usually happens as a result of inflammation (anaphylaxis, sepsis) but may simply be the way things are (the liver venules for example).
Hydrostatic Art. So you can't really change the hydrostatic pressure of the interstitium. But you can change the hydrostatic pressure of the arteries and veins. Picture a balloon. Not a round one that floats, but a long hot-dog looking one, open at both ends. Connect it to a faucet. Turn the water on. What happens? Water goes through the balloon, and spills out the other side. Still have that baloon in your head? See the side where all the water is running out into the sink? Tie it off. Leave the faucet on. What happens to the balloon? It starts to inflate. Inflate with water. It gets distended. At this point the hydrostatic pressure of the balloon is great, but the permeability is 0. Poke a hole in the middle with a needle. What happens? GUSH! water comes out the side.
The way you "fill up the balloon" is by increasing hydrostatic pressure of veins, by "blocking the outflow of the balloon." This happens in
CHF, Clots, Compression, and Cancer . Notice the alliteration. The same amount goes in from the arteries (the fauce is on), but now less can leave (becasue the veins are tied off).
oncotic pressure This one has no clever imagery. Its just basic chemistry. If you've got "stuff" on one side of a membrane, and water on the other, where does water go? To the side with more stuff. So, if you can imagine, if you lose albumin (for all intents and purposes the "stuff" that matters in your blood), water won't want to stay in your vessels. So it leaves. The way you get decreased albumin is by not taking it in (
protein losing enteropathies or malnutrition ), but spilling it out (
nephrotic syndrome), and by not being able to make protein (
cirrhosis)
Whew, that was a lot. Basically, increasing permeability has nothing to do with hydrostatic pressure, but it does "let more fluid out."Remember that whatever the veins cannot take away, the lymphatics will (unless they are blocked). Vasodilation most certainly does not lead to increased fluid reasborption. Its what gives you the rubor, dolor, rubor of inflammation - you get more fluid in the space!
