Why the hydrostatic pressure of blood DROPS as blood moves away from heart?

I don't know the exact answer, but it would seem logical to me:
1. The blood does compress at least somewhat and while the heart pushing out the blood increases the pressure it would get lower farther away.
2. Blood plasma can pass into cells/kidneys/other places and that would contribute to the pressure being lower.
3. Arterial walls are less permeable than capillary walls so they could sustain a higher pressure.

Probably the correct answer is a combination of 2 and 3.

I know I'm right. Look at my post

And OP, why would a narrower lumen in any way INCREASE the pressure?

Thanks for all your answers.. not as complicated as it seems. Still think it's quite contradictory because we've been told in a previous chapter that there's high blood pressure in the arteries due to the small lumen. But I guess hydrostatic pressure is different because like raceforthefishman said, the blood can leave the arteries.. didn't think of it like that.



Because there is a set volume of blood flowing through a smaller space?..There's a higher pressure than there is in the veins anyway right? Is my logic wrong?

Like I mentioned before (same idea) if you have a narrower lumen, the pressure will be greater...volume thing...

analogy time

If you have a hose pipe...normal water running out of it is fine, if you put your thumb over the whole effectively making it narrower...you can tell the pressure has increased...

Like I mentioned before (same idea) if you have a narrower lumen, the pressure will be greater...volume thing...

analogy time

If you have a hose pipe...normal water running out of it is fine, if you put your thumb over the whole effectively making it narrower...you can tell the pressure has increased...

This isn't necessarily true. If you have a set volume of liquid traveling through a pipe with a smaller diameter the liquid will travel faster, but the pressure will be lower. Something to do with Bernulli equations, which I used to know just before my Physics exam.

Exactly, but the question is why the hydrostatic pressure DROPS as it leaves the heart. That's why I'm so confused
x

Its the first thing I said...larger volume etc, because of lots of capillaries etc

oh ok

just dug out the AS text book (appreciate that) I'm getting a little muddled between hydrostatic pressure and blood stuff...

So I suppose if the hydrostatic pressure is influenced by the blood pressure, and the blood pressure decreases because its getting spread out....then there will be less hydrostatic pressure

Is it anything to do with plasma fluid and ultrafiltration and all that stuff? I find it confusing, myself, but as the hydrostatic pressure of the blood is higher than that of the tissue fluid, substances are forced out of the blood in the capillaries and into the surrounding tissue fluid. This results in the hydrostatic pressure in the arteries and capillaries dropping, until at the venular end of the capillary, substances from the tissue fluid are forced back in as the hydrostatic pressure of the blood in capillaries is less than that of the tissue fluid.

I'm not 100% on this, though.

Nope I don't think this question relates to tissue fluid formation; this comes later in the cycle of heart>arteries>arterioles>capillaries (capillaries are where all the tissue fluid jazz happens). By the time blood reachs capillaries there would have already been lots of friction decreasing blood pressure.

You are very nearly 100% right with your explanation of tissue fluid; although osmotic pressure is very important too, not just hydrostatic pressure :-)

Hope this helps

Thank you That's helped a lot