The problem of blood pressure

According to Wikipedia, blood pressure refers to the FORCE exerted by circulating blood on the walls of blood vessels.

So, in my guess, this problem is indeed related to Mechanics in Physics, somehow.

Kinetic friction, the friction we generally talk about in daily life, exists between two surfaces whenever they tend to move or move against each other, i.e. friction is a resisting force.

Think in this way,
1) The force exerted by the circulating blood on the walls of blood vessels is
--------> (direction: to the right; magnitude: [=size] 8 units)
2) The friction working against force acted by cirsulating blood is
<--- (direction: to the left; magnitude: 3 units)
3) The resultant (think it as overall) force acting by circulating blood is
-----> (direction: to the right; magnitude: 5 units [8 - 3 = 5])
4) Thus, the force acting by the circulating blood on the walls of blood vessels is said to be decreased, i.e. the blood pressure decreases.

Looking at the question from a slightly deeper perspective, force is the measurement of rate of change of momentum.

By p = m(v - u), where p = momentum; m = mass; u = initial velocity; v = final velocity, a change in velocity (speed considering direction as well, i.e. 2 m/s to the right ≠ 2 m/s to the left) leads to a change in momentum.

By F = Δp / t, where F = force; Δp = change in momentum; t = impact time (time used for the change in momentum), a change in momentum leads to a change in force, holding impact time constant (<- this assumption is not quite realistic, though).

[ooops, too long...]

2008-06-08 19:45:28 補充：
[continued]

As you have mentioned, firction decreases speed of blood flow, thus leads to a change (decrease) in momentum of circulating blood, and as a result, changes (decreases) the force acting on the walls of blood vessels, that is, changes (decreases) blood pressure.

2008-06-08 19:46:07 補充：
[continued]

(Since blood flows only in one direction, a change in blood speed is equal to a change in blood velocity.)

2008-06-08 19:46:20 補充：
[continued]



Wish that the above helps you understanding why the friction in capillary walls decreases blood pressure. =]]
(Yet I cannot guarantee that my explanation is 100% correct; after all, it is my GUESS. =P)